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Precipitation is a critical part of the global hydrological cycle that determines the distribution of water resources. It is also an essential meteorological variable used as input for hydroclimatic models and projections. However, precipitation data frequently lack complete series, especially at daily and sub‐daily precipitation stations, which are usually large, bulky, and complex. To address this, gap filling is commonly used to produce complete hydrometeorological data series without missing values. Several gap‐filling methods have been developed and improved. This study seeks to fill the gaps of 201 daily precipitation time series in Central Italy by localizing the approach used to generate the Serially Complete dataset for the Planet Earth (SC‐Earth). This method combines the outcome of 15 strategies based on four various gap‐filling techniques (quantile mapping, spatial interpolation, machine learning, and multi‐strategy merging). These strategies employ the daily dataset of the neighbouring stations and the matched ERA5 data to estimate missing values at the target stations. Both raw data and the final serially complete station datasets (SCDs) underwent comprehensive quality control. Many accuracy indicators have been utilized to evaluate the performance of the strategies' estimations and the final SCD, such as Correlation Coefficient (CC), Root mean square error (RMSE), Relative bias (Bias %), and Kling‐Gupta efficiency (KGE″). Multi‐strategy merging strategy based on the Modified Kling‐Gupta efficiency (MS1) shows the highest performance as an individual precipitation gap‐filling strategy. However, the machine learning strategy using random forest (ML3) has the most outstanding share in the final estimates among all other strategies. In the end, the temporal–spatial performance of the final SCD is promising and depends on the pattern of the missing values (MV%). The mean values of KGE″, CC, variability (α), and bias term (β) are 0.9, 0.93, 1.064, and 4.98 × 10−7, respectively. [ABSTRACT FROM AUTHOR]

Quantifying leaf photosynthetic response to nitrogen (N) deposition under contrasting water conditions is important for reliably modelling terrestrial carbon and water cycles, a topic that has not been well understood. Here, we analysed 737‐paired observations from 102 publications to assess the response of 11 leaf photosynthesis‐related properties to N addition under different water conditions. Our research includes global experiments, with 19 conducted in the field and 83 in greenhouses. Treatments without water reduction were classified as ‘no water change’, while those with reduced water or precipitation causing physiological drought were categorized as ‘drought’. We found that, compared to the control group, N addition significantly increased leaf photosynthetic rate (Pn; 20.9%), leaf transpiration (E; 8.3%) and stomatal conductance (gs; 14.1%). However, the decrease in Pn (−11.6%), E (−24.7%) and gs (−23.9%) under the combination of N addition and drought indicated that N addition could not offset the negative effects of drought. Furthermore, N addition significantly enhanced water‐use efficiency (WUE) by 19.8% under no water change conditions and by 21.1% under drought conditions. Within plant functional groups, herbaceous species exhibited greater susceptibility to N addition than woody species, especially under drought conditions. The observed patterns of increase in Pn with longer experimental duration and WUE with higher N rate under drought conditions showed that plants would gradually adapt to long‐term water stress in the context of N deposition. Furthermore, our results showed that drought could strengthen the correlations between leaf photosynthetic properties. Lastly, our study demonstrated that N addition and drought significantly impacted leaf nitrogen content and SPAD, respectively, and further affected gs, Pn and WUE. Synthesis. Our results emphasize the crucial role of water conditions in shaping the response of leaf photosynthesis to nitrogen (N) enrichment and also acknowledge the significance of leaf functional traits in regulating the dynamics of leaf photosynthetic processes. [ABSTRACT FROM AUTHOR]

Horizontal and vertical moisture advection in the lower troposphere of the Arctic under progressing global warming is examined using a large‐scale ensemble model data set. Advection is decomposed into terms related to the basic state of the atmosphere and transient eddies and compared against a non‐warming experiment. During summer, horizontal moisture advection increases mainly by transient eddies advecting moisture from the lower latitudes. During winter, enhanced evaporation due to reduced sea ice becomes a source of moisture diminishing the role of transient eddies moistening the atmosphere. This effect intensifies under extreme global warming, turning the change in total horizontal advection in the lower troposphere negative. Diminished horizontal advection during winter is counteracted by vertical advection accompanied with enhanced evaporation and upper‐level horizontal advection maintaining the increase in column moisture. These results improve our understanding of how the water cycle in the Arctic responds via atmospheric processes under global warming. Plain Language Summary: This study uses a data set with a large number of members to investigate how the atmosphere is moistened in the Arctic (north of 70˚N) under various stages of global warming. The movement of moisture is summarized in terms of horizontal and vertical components of advection, a measure of how the atmosphere is moistened in a given specific humidity gradient and wind, which is decomposed into parts related to the average state of the atmosphere and parts related to high‐frequency phenomena, such as cyclones. It is found that (a) during summer, moisturization of the Arctic is mainly driven by horizontal movement of moisture from a remote source in the lower latitude, (b) during winter, this moisturization due to horizontal movement of moisture is suppressed and is driven more by vertical movement of moisture due to reduction of sea ice and increased evaporation. As global warming progresses, the role or horizontal movement of moisture is taken over by the local process of evaporation and vertical movement of moisture. This research helps us understand how the water cycle in the Arctic changes with severe global warming and improves our knowledge of the atmospheric processes that govern the climate system through various feedbacks. Key Points: Moisture advection in the Arctic under progressing global warming is examined using a large‐scale ensemble model simulation data setMoistening effect of horizontal advection in the lower troposphere is enhanced for summer and diminished for winter under extreme warmingEnhanced horizontal advection in the upper layer and surface evaporation are consistent with increased column moisture during winter [ABSTRACT FROM AUTHOR]

The dynamical environment of the stratosphere, during the summer over North America, provides a natural integrator of the impact of convection in the lower stratosphere, as air can be confined for periods of a few days to more than a week. In situ data obtained during the NASA Dynamics and Chemistry of the Summer Stratosphere (DCOTSS) field campaign show increasing water vapor mixing ratios in background air as a function of time the air parcel spent within the North American Monsoon Anticyclone region. We find that water vapor added to the stratosphere by convection decreases with altitude and tends to drop below detectable limits by the 415 K isentrope in 2021 and the 430 K isentrope in 2022. Integrating between potential temperatures of 380 and 460 K we find that convection added between 20 and 32 Tg per summer to the stratosphere in 2021 and 2022. While the total amount is only 1%–4% of the amount ascending in the tropics across the tropical tropopause, small changes in the annual flux of water can have a significant effect on the radiation budget of the atmosphere. Locally, over North America we find that convection increased the water vapor mixing ratio at 380 K by as much as 40%. Tropopause‐penetrating convection is part of the yearly cycle of stratospheric water vapor and we suggest that it must be included in extratropical models to accurately predict future trends in stratospheric water vapor. Plain Language Summary: Water vapor in the stratosphere is important for regulating Earth's climate and also influences the amount of ozone in the stratosphere. Most air rises into the stratosphere through the tropical tropopause where the excess water freezes out leaving very dry air. But air and water can also be injected into the stratosphere via strong convective storm systems. During the summer months, strong storms over the Sierra Madre Occidental and the central United States are known to produce many overshoots that can be seen with radar and satellite. A recent NASA airborne campaign called Dynamics and Chemistry of the Summer Stratosphere (DCOTSS) aimed to study this region. The aircraft was equipped with 12 different sensors measuring gases, aerosols, and meteorological variables. We use these measurements to look at the cumulative effect these storms have on the background air in the stratosphere and specifically calculate the amount of water vapor added by convection as a function of altitude above the tropopause. We find that water added by storms over North America is 1%–4% of the total amount ascending in the tropics. Over North America convection may account for as much as a 40% increase in lower stratospheric water vapor. Key Points: Deep convection over North America lofts significant amounts of water to the lower stratosphereThe North American Monsoon Anticyclone (NAMA) provides a natural integrator of convective perturbations to the stratosphereConvective detrainment of water vapor and ice is quantified by looking at changes in background air as a function of time spent in the NAMA [ABSTRACT FROM AUTHOR]

Precipitation δ18O has offered valuable insights into the evolution of the Asian monsoon. Recent researches focusing on precipitation Δ′17O has enhanced our understanding by offering new perspectives beyond those of δ18O, revealing insights into vapor sources and continental recycling. Nevertheless, there remains a lack of interannual triple oxygen isotope data, particularly in the Asian monsoon region. In this study, we analyzed the triple oxygen isotopes and hydrogen isotopes in monthly precipitation samples collected from Chongqing in Southwest China between 2019 and 2022 A.D. Seasonal variations in δD, δ18O, δ17O, and d‐excess values were observed, with lower values during the rainy season and higher values during the dry season, highlighting the impact of changes in moisture sources and local meteorological conditions on seasonal shifts in δD, δ18O, and δ17O. While, mean Δ′17O values were higher in rainy season and lower in dry season. Notably, during rainy season, there is a negative correlation between monthly Δ′17O values and the RH of the vapor source area, as well as a positive correlation with d‐excess. Recalculated Δ′17O values based on RH of oceanic moisture source, are higher than the measured values for this period, indicating the contribution of terrigenous moisture to precipitation in SW China. Precipitation Δ′17O values provide a more precise reflection of changes in moisture source, continental recycling, and evapotranspiration processes that drive water cycling compared Integrating modeling works in future will facilitate the use of precipitation Δ′17O values to quantify the impact of different moisture source on precipitation. Plain Language Summary: In the context of climatic warming, changes in water vapor source areas have substantial implications for the stability of regional water resources. Despite the importance of these changes, few studies have analyzed them using precipitation data from SW China. We conducted monthly measurements of isotopic compositions in precipitation samples collected in monsoonal SW China between 2019 and 2022 A.D. Seasonal variations in δD, δ18O, δ17O, and d‐excess revealed low values during the rainy season (May–October) and significantly higher values during the dry season (November–April). These variations are primarily attributed to different water vapor sources in distinct seasons. Moreover, we observed a negative correlation between precipitation Δ′17O and relative humidity in the source area of the water vapor during the rainy season, suggesting that Δ′17O contains valuable information about the water vapor's source area. Additionally, the relatively higher Δ′17O values observed in the rainy season imply the influence of continental recycling and evapotranspiration processes. Key Points: Characteristics of triple oxygen and hydrogen isotopes in precipitation were analyzed based on a 4‐year monitoring period in Southwest ChinaTriple oxygen isotopes in precipitation serve as indicators of the extent of isotopic fractionation in water vapor between different seasonsChanges in precipitation Δ′17O show a negative correlation with relative humidity from the moisture source area during the rainy season [ABSTRACT FROM AUTHOR]

Summary Water use efficiency (WUE) represents the trade‐off between carbon assimilation and water loss in plants. It remains unclear how leaf stomatal and photosynthetic traits regulate the spatial variation of leaf WUE in different natural forest ecosystems. We investigated 43 broad‐leaf tree species spanning from cold‐temperate to tropical forests in China. We quantified leaf WUE using leaf δ13C and measured stomatal traits, photosynthetic traits as well as maximum stomatal conductance (Gwmax$$ {G}_{{\mathrm{w}}_{\mathrm{max}}} $$) and maximum carboxylation capacity (Vcmax$$ {V}_{{\mathrm{c}}_{\mathrm{max}}} $$). We found that leaves in cold‐temperate forests displayed ‘fast’ carbon economics, characterized by higher leaf nitrogen, Chl, specific leaf area, and Vcmax$$ {V}_{{\mathrm{c}}_{\mathrm{max}}} $$, as an adaptation to the shorter growing season. However, these leaves exhibited ‘slow’ hydraulic traits, with larger but fewer stomata and similar Gwmax$$ {G}_{{\mathrm{w}}_{\mathrm{max}}} $$, resulting in higher leaf WUE. By contrast, leaves in tropical forests had smaller and denser stomata, enabling swift response to heterogeneous light conditions. However, this stomatal configuration increased potential water loss, and coupled with their low photosynthetic capacity, led to lower WUE. Our findings contribute to understanding how plant photosynthetic and stomatal traits regulate carbon–water trade‐offs across climatic gradients, advancing our ability to predict the impacts of climate changes on forest carbon and water cycles. [ABSTRACT FROM AUTHOR]

Variations in evapotranspiration and their sensitivity to controlling variables are pivotal for comprehending water balance dynamics and climate change, particularly in high‐altitude regions such as the Qilian mountains. Environmental shifts are bound to disrupt local water cycles and balance, with significant implications for these alpine areas. To enhance our understanding of evapotranspiration variability across different altitudes within the Qilian Mountains' high‐elevation region and to assess the model's adaptability and responsiveness to environmental factors, our study involved measuring actual evapotranspiration at three distinct elevations. This was achieved using meteorological stations and continuous data from a weighing‐type microlysimeter at the Shaliu River basin's gradients of 3797, 4250 and 4303 m, spanning the growing seasons from June 2020 to October 2022. We utilized 10 models to calculate the value of reference evapotranspiration, which were then matched against actual evapotranspiration data to identify the most appropriate model. Our research found that across the three elevation gradients, the daily average evapotranspiration were 3.663, 3.845 and 4.317 mm day−1, respectively. Across the three elevations, with consistent intra‐annual fluctuations. Notably, August experienced the highest monthly evapotranspiration at 4.750 mm day−1, and reach peak at 10:00 and 15:00 on the three elevation gradients. The results from the simulation of the 10 models indicate that the Dalton model is more suitable for our study area compared with the other models, showing the best R2, root mean square error and percentage error values. Partial least squares regression analysis, coupled with an enhanced regression tree model, identified precipitation as the most critical factor, with a variable importance in projection score of 2.079, contributing 52.6% to evapotranspiration. Collectively, precipitation were identified as key factors influencing evapotranspiration variability within our research area. Our study's insights are valuable for anticipating the impacts of future climate change. This conclusion is instrumental for refining water budget projections in Alpine regions under climate change scenarios. [ABSTRACT FROM AUTHOR]

As a key factor influencing the cloud life cycle and radiative properties, liquid‐ice mass partitioning remains a major source of uncertainties in modeling mixed‐phase clouds. One of the unresolved problems is that liquid‐ice mixing is highly inhomogeneous, but it has not been well understood and quantified for parameterization. In this study, the liquid‐ice mixing homogeneity (χ) is quantified using the information‐theoretic entropy based on airborne measurements. It is demonstrated that χ is positively correlated with the condensed water content (CWC). Cloud regions with low χ are consist of liquid and ice clusters. With the increase in χ, the size and frequency of continuous mixed‐phase clusters increase. For a given CWC, χ is lower at relatively warm temperatures as sedimentation of large ice crystals can enhance the inhomogeneity. The strong positive relationship between CWC and χ indicates CWC should be considered when parameterizing the liquid‐ice mixing in models. Plain Language Summary: Mixed‐phase clouds consist of water vapor, supercooled droplets, and ice particles. The complex liquid‐ice interactions have a substantial influence on the macrophysical and microphysical characteristics of the clouds, which ultimately affect the global water cycle and radiative balance. Observations and model experiments have shown that liquid and ice are not uniformly mixed, and the liquid‐ice mixing homogeneity also strongly affects the cloud glaciation processes. However, the liquid‐ice mixing homogeneity is still not well understood and is poorly represented in numerical models. In this study, based on airborne measurements, the liquid‐ice mixing homogeneity is quantified. It is found that there is a positive correlation between mixing homogeneity and the condensed water content, while the latter is a predictable value in models. These results provide a new idea to constrain the liquid‐ice mixing in models and are potentially helpful to improve the model simulation of mixed‐phase clouds. Key Points: Liquid‐ice mixing homogeneity is quantified using information‐theoretic entropy and airborne measurementsCondensed water content is positively related to liquid‐ice mixing homogeneity in mixed‐phase cloudsCondensed water content and temperature should be considered in parameterization of liquid‐ice mixing homogeneity [ABSTRACT FROM AUTHOR]

Climate models indicate that a warmer environment will increase low‐level moisture, potentially intensify extreme precipitation. However, its impact on different rainfall types remains unclear. Using satellite data, we examined changes in light (0‐95th percentile, ≤5.28 mm hr−1) and heavy (95‐100th percentile, >5.28 mm hr−1) precipitation in the tropics from 1998 to 2019. Our findings show a −9 ± 2% (23 ± 2%) change in heavy (light) rain intensity and a 13 ± 2% (−24 ± 1%) change in heavy (light) rain frequency. These changes link to warmer sea surface temperatures, increased atmospheric stability and water vapor, and weakened upward velocity. These insights shed light on how heavy and light precipitation patterns respond to changing climate, emphasizing the complexities within the hydrological cycle. Plain Language Summary: The hydrologic cycle depends greatly on precipitation. With the expectation of climate warming causing substantial changes to the global water cycle, comprehending these shifts becomes pivotal in foreseeing how climate changes will affect society. Climate models suggest that as temperatures rise, low‐level moisture will increase. However, the specific impacts of this heightened moisture on the intensity and frequency of precipitation events remain uncertain. Analyzing data from the Tropical Rainfall Measuring Mission (1998–2019), we found notable changes: a 23 ± 2% increase in light rain intensity, a −9 ± 2 % decrease in heavy rain intensity, a 13 ± 2 % rise in heavy rain frequency, and a significant −24 ± 1% decrease in light rain frequency. These changes result diverse factors like warming oceans, stabilized atmospheric conditions, increased water vapor, and weakened upward velocity. This underscores the intricate interplay between different rainfall types and how they respond to the evolving climate. Key Points: In the tropics from 1998 to 2019, heavy rain intensity decreased and light rain intensity increasedIn the tropics from 1998 to 2019, heavy rain frequency increased and light rain frequency decreasedRain shifting patterns linked to increased SST, atmospheric stability, and water vapor and decreased upward velocity [ABSTRACT FROM AUTHOR]

Efforts to detect long‐term changes in global mean evaporation minus precipitation over the ocean remain ambiguous. Here we define an ad hoc sea surface salinity index to assess the observed and simulated intensification of the freshwater flux pattern over the global ocean and, thus, of the overall water cycle. A recent salinity reconstruction shows a long‐term amplification of the climatological patterns, thereby supporting the popular "fresh gets fresher, salty gets saltier" paradigm. Unlike in a previous study, no systematic underestimation of this amplification is found in the latest generation of global climate models. Yet, the "fresh gets fresher" paradigm is much more robust than its "salty gets saltier" counterpart and the proposed salinity index does not yet provide a strong constraint on the model‐dependent projected intensification of the global water cycle intensification along the 21st century. Plain Language Summary: Recent changes in continental mean precipitation and evaporation remain poorly observed and thus poorly constrained in both atmospheric reanalyzes and global climate models. Here we propose a new index based on global sea surface salinity contrasts as a surrogate of changes in metric of the water cycle intensity. Overall, both observed and simulated indices support the "fresh gets fresher, salty gets saltier" paradigm, widely used to depict the overall intensification of the water cycle. Unlike in a previous study, no systematic underestimation of the observed amplification in salinity patterns is found in the latest generation of global climate models. The "salty gets saltier" response is however less robust than the "fresh gets fresher" and the proposed salinity index is not yet very useful to constrain the future intensity of the global water cycle. Key Points: Both observations and climate models evidence an amplification of the global ocean climatological contrasts in sea surface salinity (SSS)Yet, the "fresh gets fresher" paradigm is more robust than its salty counterpart, which is more sensitive to the ocean domain's definitionGlobal SSS observations do not yet provide a strong constraint on the projected intensification of the global water cycle [ABSTRACT FROM AUTHOR]

Surface solar radiation is fundamental for terrestrial life. It provides warmth to make our planet habitable, drives atmospheric circulation, the hydrological cycle and photosynthesis. Europe has experienced an increase in surface solar radiation, termed "brightening," since the 1980s. This study investigates the causative factors behind this brightening. A novel algorithm from the EUMETSAT satellite application facility on climate monitoring (CM SAF) provides the unique opportunity to simulate surface solar radiation under various atmospheric conditions for clouds (clear‐sky or all‐sky), aerosol optical depth (time‐varying or climatological averages) and water vapor content (with or without its direct influence on surface solar radiation). Through a multiple linear regression approach, the study attributes brightening trends to changes in these atmospheric parameters. Analyzing 61 locations distributed across Europe from 1983 to 2020, aerosols emerge as key driver during 1983–2002, with Southern Europe and high elevations showing subdued effects (0%/decade–1%/decade) versus more pronounced impacts in Northern and Eastern Europe (2%/decade–6%/decade). Cloud effects exhibit spatial variability, inducing a negative effect on surface solar radiation (−3%/decade–−2%/decade) at most investigated locations in the same period. In the period 2001–2020, aerosol effects are much smaller, while cloud effects dominate the observed brightening (2%/decade–5%/decade). This study therefore finds a substantial decrease in the cloud radiative effect over Europe in the first two decades of the 21st century. Water vapor exerts negligible influence in both sub‐periods. Plain Language Summary: This study explores the recent increase in surface solar radiation in Europe, commonly referred to as "brightening," which began in the mid‐1980s. Surface solar radiation is crucial for life on Earth, fostering warmth, shaping wind patterns, and fueling the hydrological cycle and photosynthesis. Utilizing data derived from satellites, we simulated surface solar radiation under different atmospheric conditions regarding clouds, aerosols, and water vapor. This allowed us to separate the individual effects and analyze their contributions to the observed brightening across 61 locations in Europe from 1983 to 2020. We find that the decrease in aerosols was the dominant driver for the brightening during the initial period from 1983 to 2002. Southern Europe and high‐elevation areas showed modest aerosol effects, while Northern and Eastern Europe experienced higher values. Cloud effects were predominantly negative (decreasing surface solar radiation) at most locations. However, between 2001 and 2020 the aerosol effect resulted much smaller and the brightening was mostly attributed to a decrease in cloudiness during that sub‐period. Water vapor was found to have a negligible impact on surface solar radiation trends. Key Points: Satellite application facility on climate monitoring (CM SAF) Land Flux v1.0 surface solar radiation data agree well with station‐based measurements over Europe, also at high‐altitude locationsThe direct aerosol effect was the main driver of the European brightening from 1983 to 2002. Changes in cloudiness rather counteracted itFrom 2001 to 2020 the brightening in Europe was dominated by the cloud effect and the aerosol effect resulted much smaller [ABSTRACT FROM AUTHOR]

The Tibetan Plateau (TP) provides vital water resources for downstream regions, with spring precipitation contributing considerably to the annual totals over the southeastern TP. The added value of convection‐permitting modeling in simulating the spring climate over the TP is uncertain. Here, we conducted and compared decade‐long regional convection‐permitting (3.3 km) and convection‐parameterized (13.2 km) Icosahedral Nonhydrostatic Weather and Climate Model (ICON) simulations to reproduce the atmospheric water cycle in spring over the TP. Results indicated that 3.3 km mesh ICON (ICON_3.3 km) exhibited notable added value in simulating the spring atmospheric water cycle over the TP. ICON_3.3 km reduced the wet biases of precipitation in the ERA5 reanalysis and 13.2 km mesh ICON (ICON_13.2 km) simulations, and improved the simulation of surface evaporation over the central and eastern TP. The reduction in the simulated precipitation in ICON_3.3 km was primarily followed by a decrease in surface evaporation from March to May, second by a reduction in water vapor flux convergence in May due to decreased water vapor inflow from the southeastern TP. Furthermore, compared to ICON_13.2 km, ICON_3.3 km alleviated the "drizzling" bias, leading to drier surface soils and decreased evaporation, and lead to 3% decrease in the fraction of evaporation converted into precipitation. Sensitivity experiments conducted at resolution of 13.2 km but turning off the convection parameterization demonstrated that both explicit representation of convection and enhanced horizontal resolution were crucial for accurately representing the spring atmospheric water cycle over the TP. Our results highlighted the need to develop kilometer‐scale models for successfully reproducing the climate characteristics across the TP. Plain Language Summary: Climate modeling is evolving toward kilometer‐scale or convective‐permitting model (CPM). The added value of regional CPM in simulating the spring climate over the Tibetan Plateau (TP), known as "Asian Water Tower," is unknown. Here, we have performed and compared two decade‐long regional climate simulations at two horizontal resolutions: convection permitting (3.3 km) and convection parameterized (13.2 km), to reproduce the atmospheric water cycle in spring over the TP. Results indicated that the CPM reduced the wet biases in the latest reanalysis data and the convection‐parameterized model over the central and eastern TP. The reduction of wet biases was mainly followed by the decreased surface evaporation, which was associated with the alleviation of "drizzling" bias in CPM. We further showed evidences that both explicit representation of convection and enhanced horizontal resolution were crucial for accurately representing the spring climate over the TP. Our findings highlight the importance of developing high‐efficiency kilometer‐scale (<4 km) models for reproducing all the climate characteristics across the TP region. Key Points: Regional convection permitting model exhibited evident added value in simulating the spring atmospheric water cycle over the Tibetan Plateau (TP)Reduced precipitation by convective‐permitting model was mainly followed by decrease in surface evaporation, associated with the alleviation of drizzling biasesBoth explicit convection and enhanced resolution were crucial for well representing the spring atmospheric water cycle over the TP [ABSTRACT FROM AUTHOR]

The current‐state of polymer‐based humidity sensors faces numerous limitations, including energy‐costly synthesis, low sensitivity, and slow response times. This study presents innovative approach to overcome these challenges, based on a robust all‐water‐borne in situ miniemulsion polymerization. The use of water throughout the entire process mitigates the negative environmental impact. Thiol‐ene polymers reinforced with reduced graphene oxide (rGO) with concentrations ranging from 0.2–1.0 wt% are selected to fabricate these chemoresistive sensors. The selected thiol‐enes present high hydrophobicity and a semicrystalline nature, suggesting resistance to early delamination even under prolonged exposure to humidity. Incorporating rGO not only imparts electrical conductivity but also enhances mechanical and water resistance of the composite films. The 0.6% rGO composite exhibits optimal resistance for humidity sensing, demonstrating rapid and consistent responses across three exposure cycles to water vapor concentrations ranging 800–5000 ppm. Moreover, the sensor exhibits remarkable selectivity toward water vapors over these of toluene, propanol, and 4‐methyl‐2‐pentanol, attributed to the high surface hydrophilicity and inherent porosity of the waterborne film, and network structuring of rGO platelets within the matrix. In summary, this study pioneers a novel approach to polymer‐based humidity sensing, addressing key limitations while offering enhanced sensitivity, rapid response times, and superior selectivity. [ABSTRACT FROM AUTHOR]

Assisted tree migration has been proposed as a conceptual solution to mitigate lags in biotic responses to anthropogenic climate change. The rationale behind this concept is that tree species currently growing under warmer and drier climates will be more resistant and resilient to the new climatic conditions than tree species naturally growing in currently wetter and colder climates. However, we hypothesize that, by being more stress‐tolerant to warmer and drier conditions, translocated species should exhibit different functional attributes, which could induce important ecological and societal costs and overcome the desired benefits of maintaining wood production and other ecosystem services. We used principal component analysis (PCA) to analyze variation in seven traits of 106 tree and tall shrub species from contrasting latitudinal distributions in western North America and Europe to predict the potential functional changes of forest ecosystems due to the translocation of tree species from low to high latitudes. We show that species from both continents differed primarily by their position on the leaf economy spectrum (LES) and their size traits. Even though, in Europe, differences in LES were significantly correlated to species southern latitudinal positions, in both continents differences in size traits were significantly correlated to latitude. These results suggest that assisted migration by translocating more conservative species of shorter stature in currently cooler climates should decrease the buffering capacity of forest canopies, decrease facilitation for understory species, and increase wildfire risks, whose effects have the potential to accelerate climate warming through negative atmospheric feedback processes. As an alternative solution to assisted migration that may accelerate rather than mitigate climate change, we recommend that foresters gradually diversify the vertical structure and layering of the existing forest canopy to maintain a sustainable water cycle and energy balance between the soil, the tree and the atmosphere without increasing the wildfire risk. [ABSTRACT FROM AUTHOR]

Central Asia (CA) is among the world's most vulnerable regions to climate change. Increasing anthropogenic greenhouse gas concentrations (GHGs) are the primary forcing of the current and future climate system for the time scale of a century. By analysing observation datasets, we show that a warming of 1.2°C led to a decrease of 20% in snow‐depth CA during the last 70 years, especially over the mountains. In recent decades, longer summer times and fewer icing days (more than 20 days·year−1) have exposed unprecedented shock to CA's climate system's components. Furthermore, we analyse 442 model simulations from Coupled Model Inter‐comparison Project Phase 5 and 6 (CMIP5, CMIP6) and show that CMIP6 simulations are generally warmer and wetter than the CMIP5 ones in CA. For instance, under the highest emission scenarios (RCP8.5 and SSP5‐8.5), CMIP6 projects a 6.1°C increase, while CMIP5 projects a 5.3°C increase, suggesting CMIP6 anticipates greater warming with high emissions. In contrast to CMIP6, the CMIP5 precipitation trends suggest a potential nonlinear relationship between increased greenhouse gas emissions and changes in precipitation, though the impact is much less pronounced than the temperature changes. Our analysis shows that CMIP6 models are more sensitive to temperature rise than CMIP5 ones. Both simulation sets' ensemble means capture well the observed warming trend. The imposed snow‐melting leads to an increase in the run‐off in the vicinity of glaciers. Such climatic shifts lead to more flooding events in CA. Given the projected warming range of 2–6°C in CA at the end of the century in various scenarios and models, such warming trends might be catastrophic in this region. The seasonal cycle of the temperature change indicates an extension of the glacier's melting period under future scenarios with fossil‐fueled development. The models' uncertainty increases for the far‐future time‐slice, and warming larger than 4°C in CA is very likely among all the models and during all the seasons if no sustainable action is taken. This study also incorporates a detailed Köppen climate classification analysis, revealing significant shifts towards warmer climate categories in Central Asia, which may have profound implications for regional hydrological cycles and water resource management, particularly in the Amu Darya and Syr Darya river basins under warmer scenario by the end of the century. The Tundra and ice cap climate categories will lose more than 60% of their coverage at the end of the century compared to the historical period in the Amu Darya and Syr Darya river basins. [ABSTRACT FROM AUTHOR]

Climate change will impact mountain watershed streamflow both directly—with changing precipitation amounts and variability—and indirectly—through temperature shifts altering snowpack, melt, and evapotranspiration. To understand how these complex processes will affect ecosystem functioning and water resources, we need tools to distinguish connections between water sources (rain/snowmelt), groundwater storage, and exit fluxes (streamflow/evapotranspiration), and to determine how these connections change seasonally and as climate shifts. Here, we develop novel watershed‐scale approaches to understand water source, storage, and exit flux connections using a dynamic‐flux particle tracking model (EcoSLIM) applied in California's Cosumnes Watershed, which connects the Sierra Nevada and Central Valley. This work develops new visualizations and applications to provide mechanistic understanding that underpins the interpretation of isotopic field data at watershed scales to distinguish sources, flow paths, residence times, and storage selection. In our simulations, streamflow comes primarily from snow‐derived water while evapotranspiration generally comes from rain. Most streamflow starts above 1,000 m while evapotranspiration is sourced relatively evenly across the watershed and is generally younger than streamflow. Modeled streamflow consists primarily of water sourced from precipitation in the previous 5 years but before the current water year, while ET consists primarily of water from precipitation in the current water year. ET, and to a lesser extent streamflow, are both younger than water in groundwater storage. However, snowmelt‐derived streamflow preferentially discharges older water from snow‐derived storage. Dynamic‐flux particle tracking and new approaches presented here enable novel model‐tracer comparisons in large‐scale watersheds to better understand watershed behavior in a changing climate. Plain Language Summary: Climate change will alter the hydrologic cycle: storms will be more intense and higher temperatures will result in less snow, earlier melt, and more early‐season water use by plants. To understand the combined effect of these changes, we need models to simulate water flows and precisely study the effects of rain, snowmelt, subsurface storage, streamflow, and plant water uptake. These models can show us how water flows between different components of the landscape, and how this flow changes seasonally and in response to climate change. This study develops new modeling tools that simulate the Cosumnes River in the Sierra Nevada (California, USA) and will help interpret field data at watershed scales. The majority of simulated streamflow originates from snow while plants rely on rainfall for evapotranspiration (ET). Most streamflow starts as precipitation above 1,000 m while ET comes from water that falls evenly across the watershed. Water used for ET is generally younger than streamflow: most streamflow is 1–5 years old, while most ET is less than 1 year old. Both ET and streamflow are younger than water stored in the subsurface. These new modeling tools, called dynamic‐flux particle tracking, will help us predict how watersheds will behave as climate changes. Key Points: Dynamic‐flux particle tracking (DFPT) is a novel technique to investigate watershed function and connectionWater source wedge plots illustrate connectivity between source water elevation, precipitation phase, and water exit pathways and locationDFPT shows how source phase and watershed conditions influence age‐ranked storage selection behavior [ABSTRACT FROM AUTHOR]

Humidification and dehumidification (HDH) desalination units compared with the other thermal desalination systems have relative advantages due to working under ambient pressure and low temperatures and are more attractive. In this research, the application of an HDH desalination unit with indirect‐contact of air and water streams in a steam power plant is investigated. To increase the energy efficiency of the system, a closed air cycle has been considered. Also, by application of the closed water cycle in the humidifier, the recovery rate of the desalination unit increases according to the concentration of discharge flow. The flow rate of boilers blowdown in the studied steam power plant is around 2.2 kg/s and can be used as a unique source of required thermal energy in the desalination system to produce freshwater from power plant chemical effluents. On the basis of the available cooling water (30 m3/day), this stream is considered as the limiting flow in the modeling. The detailed design of different parts of the desalination unit, including the humidification tower, condensers, wet air circulation fans, the required pumps, and the heat exchanger, is presented in this study. The maximum production of freshwater and the minimum energy intensity are obtained for the circulating air flow rate of 4 kg/s. By 14% recovery of lost steam in the power plant, 20.8 m3/h of desalinated water is produced. The gain output ratio and the energy intensity of the system are 1.025 and 2201 kJ/L, respectively. The production cost with the proposed indirect‐constant HDH system is equal to 0.56 $/m3 of freshwater. [ABSTRACT FROM AUTHOR]

To bridge the knowledge gap between (a) our (instantaneous‐to‐seasonal‐scale) process understanding of plants and water and (b) our projections of long‐term coupled feedbacks between the terrestrial water and carbon cycles, we must uncover what the dominant dynamics are linking fluxes of water and carbon. This study uses the simplest empirical dynamical systems models—two‐dimensional linear models—and observation‐based data from satellites, eddy covariance towers, weather stations, and machine‐learning‐derived products to determine the dominant sub‐annual timescales coupling carbon uptake and (normalized) evaporation fluxes. We find two dominant modes across the Contiguous United States: (1) a negative correlation timescale on the order of a few days during which landscapes dry after precipitation and plants increase their carbon uptake through photosynthetic upregulation. (2) A slow, seasonal‐scale positive covariation through which landscape drying leads to decreased growth and carbon uptake. The slow (positively correlated) process dominates the joint distribution of local water and carbon variables, leading to similar behaviors across space, biomes, and climate regions. We propose that vegetation cover/leaf area variables link this behavior across space, leading to strong emergent spatial patterns of water/carbon coupling in the mean. The spatial pattern of local temporal dynamics—positively sloped tangent lines to a convex long‐term mean‐state curve—is surprisingly strong, and can serve as a benchmark for coupled Earth System Models. We show that many such models do not represent this emergent mean‐state pattern, and hypothesize that this may be due to lack of water‐carbon feedbacks at daily scales. [ABSTRACT FROM AUTHOR]

Sea‐level rise (SLR) poses a severe threat to the coastal environment through seawater intrusion into freshwater aquifers. The rising groundwater table also exacerbates the risk of pluvial, fluvial, and groundwater flooding in coastal regions. However, current Earth system models (ESMs) commonly ignore the exchanges of water at the land‐ocean interface. To address this gap, we developed a novel land‐ocean hydrologic coupling scheme in a state‐of‐the‐science ESM, the Energy Exascale Earth System Model version 2 (E3SMv2). The new scheme includes the lateral exchange between seawater and groundwater and the vertical infiltration of seawater driven by the SLR‐induced inundation. Simulations were performed with the updated E3SMv2 for the global land‐ocean interface to assess the impacts of SLR on coastal groundwater under a high CO2 emission scenario. By the middle of this century, seawater infiltration on the inundated areas will be the dominant component in the land‐ocean coupling process, while the lateral subsurface flow exchange will be much smaller. The SLR‐induced seawater infiltration will raise the groundwater levels, enhance evapotranspiration, and increase runoff with distinct spatial patterns globally in the future. Although the coupling process is induced by SLR, we found topography and warming temperature have more control on the coupling impacts, probably due to the relatively modest magnitude of SLR during the selected future period. Overall, our study suggests significant groundwater and seawater exchange at the land‐ocean interface, which needs to be considered in ESMs. Plain Language Summary: As ocean levels rise, seawater threatens to intrude into coastal freshwater aquifers that millions of people depend on for drinking water and irrigation. While regional studies have examined the impacts of sea‐level rise (SLR) on coastal groundwater systems, current Earth system models (ESMs) overlook the exchange of water between ocean and groundwater. Our work addresses this gap by developing a water exchange process between ocean and land components in a state‐of‐the‐science ESM. This coupling scheme includes lateral exchanges between seawater and groundwater, as well as vertical seawater infiltration resulting from oceanic inundation. We used this new coupled model to assess SLR impacts on the global coastal groundwater table under a higher CO2 emission scenario. We found that SLR will raise groundwater levels and intensify the hydrological cycle by midcentury mainly due to increased seawater infiltration. Further, while SLR triggers this increased seawater infiltration, topography and warming temperature play more significant roles in determining its magnitude. Key Points: A novel land‐ocean coupling scheme is developed and implemented in Energy Exascale Earth System Model version 2 to evaluate the hydrologic exchange at the land‐ocean interfaceThe impact of land‐ocean coupling on the groundwater table is dominated by seawater infiltration, while lateral exchange is negligibleFuture sea‐level rise will induce significant seawater intrusion into the coastal groundwater system [ABSTRACT FROM AUTHOR]

Hydrology has a long history, but is still considered a young science due to its lack of a solid scientific foundation as a natural science. Field experimentation is crucial when investigating hydrological processes and mechanisms, and is essential if hydrology is to have a solid, science‐based foundation. Professor Wei‐Zu Gu (1932–2022) was an internationally renowned scientist in the field of hydrology and is recognized as the greatest pioneer of experimental hydrology and isotope hydrology in China. He created the Hydrohill experimental catchment, which serves as both a great public facility for experimental hydrology and a valuable legacy for researchers that will enable them to conduct advanced hydrological experiments in the future. This legacy consists of innovative infrastructure that bridges the gap between natural watershed experiments and artificial physical models. The Hydrohill is an intensively instrumented experimental catchment that allows different elements of the hydrological cycle and their tracing indicators to be comprehensively measured. To provide an in‐depth understanding of the Hydrohill, this paper presents a short history of the site, its experimental objectives, a site description (including location, construction and instrumentation), site conditions (such as soil, hydrological and meteorological properties), and its contributions to hydrological science. We acknowledge Professor Gu for creating the Hydrohill experimental hydrology facility and enhancing our understanding of hydrological processes and mechanisms. Finally, we hope that Chuzhou Scientific Hydrology Laboratory, along with support from Professor Gu's friends, will ensure the continued growth of the Hydrohill so that it can address unsolved problems in hydrology. [ABSTRACT FROM AUTHOR]

Evapotranspiration (ET) stands as a pivotal element in the terrestrial‐atmospheric energy interchange, modulated by a complex array of factors including land use dynamics and climate change. The elucidation of regional and temporal patterns, alongside the mechanisms underpinning ET and its components, amidst environmental shifts, has emerged as a focal point in contemporary hydrological discourse. The Han River catchment, under the influence of the subtropical monsoon, presents an exemplary case study for hydrological inquiry due to its distinct catchment characteristics. This research probes the evolution and influencing mechanisms of ET within the catchment from 2000 to 2018, employing the improved Shuttleworth–Wallace model (i.e., SWH model), multivariate statistical techniques and additional methodologies. Findings reveal that (1) the annual mean ET, evaporation (E) and vegetation transpiration (T) within the Han River catchment from 2000 to 2018 were quantified at 1156.77, 784.21 and 372.56 mm, respectively. The overall spatial pattern showed a gradual decrease from the Chaoshan Plain area identified as having higher values compared to other regions, which may be attributed to the weakened vegetation cooling effect and the indirect effect of the heat island effect brought about by construction land expansion. (2) The significant decrease of E may be attributed to the optimization of vegetation growth conditions in the catchment, resulting in more solar radiation intercepted by the vegetation canopy. (3) Climatic alterations exerted a notable influence on ET, E and T than land use changes. Temperature, Normalized Difference Vegetation Index (NDVI), net radiation and wind speed were identified as the most consequential factors affecting ET. This study lays a scientific groundwork for subsequent exploration into the spatio‐temporal dynamics and mechanisms influencing evapotranspiration and its elements in the Han River catchment, contributing to a broader understanding of hydrological cycling. [ABSTRACT FROM AUTHOR]

3H enters the hydrologic cycle after oxidizing in the 3H1HO molecule and it constitutes a very useful tracer for hydrological studies. One of these applications is streamflow component separation, which provides useful information to understand the hydrological cycle. This application is based on the fact that the contents in precipitation (runoff) tend to be markedly higher than those in groundwater (baseflow) because of decreasing activity in the last as a consequence of radioactive decay. The main objective of this paper is to test 3H for hydrograph separation in sub‐tropical South America, where it is favoured by high values in precipitation. The catchment of the Quequén Grande River, in Argentina, was selected. Total flow in surface water is a mixing between the baseflow and the event flow portion; the separation was done in three sections of the drainage network, and the proportion of baseflow were 36%, 88% and 47%. [ABSTRACT FROM AUTHOR]

The detection of hydrogen chloride (HCl) in the atmosphere of Mars was among the primary objectives of the ExoMars Trace Gas Orbiter (TGO) mission. Its discovery using the Atmospheric Chemistry Suite mid‐infrared channel (ACS MIR) showed a distinct seasonality and possible link to dust activity. This paper is part 2 of a study investigating the link between HCl and aerosols by comparing gas measurements made with TGO to dust and water ice opacities measured with the Mars Climate Sounder (MCS). In part 1, we showed, and compared, the seasonal evolution of vertical profiles of HCl, water vapor, temperature, dust opacity, and water ice opacity over the dusty periods around perihelion (solar longitudes 180°–360°) across Mars years 34–36. In part 2, we investigated the quantitative correlations in the vertical distribution between each quantity, as well as ozone. We show that there is a strong positive correlation between HCl and water vapor, which is expected due to fast photochemical production rates for HCl when reacting with water vapor photolysis products. We also show a strong positive correlation between water vapor and temperature, but are unable to show any correlation between temperature and HCl. There are weak correlations between the opacities of dust and water ice, and dust and water vapor, but only very low correlations between dust and HCl. We close with a discussion of possible sources and sinks and that interactions between HCl and water ice are the most likely for both, given the inter‐comparison. Plain Language Summary: The ExoMars Trace Gas Orbiter (TGO) has observed three Martian dusty seasons (summer in the southern hemisphere). In mid‐2018, we made the first detection of hydrogen chloride (HCl) in the Martian atmosphere using the Atmospheric Chemistry Suite (ACS). Finding this gas, among others, was a priority of ExoMars because its primary source may be volcanic. Since then, we have observed two more dusty periods with the reappearance of HCl each time. In part 1, we presented the climatology of HCl over these three dusty periods (in Mars years 34, 35, and 36) and investigate their relationships with temperature and water vapor measured by ACS, and with airborne dust and water ice measured with the Mars Climate Sounder (MCS) on the Mars Reconnaissance Orbiter (MRO). In this paper, part 2, we quantify the correlations between HCl, water vapor, ozone, dust, temperature, and water ice. We show that HCl is closely correlated to water vapor, which is, in turn, correlated to temperature. There is only a very low correlation between HCl and dust or water ice, which both impact water vapor. Possible mechanisms for seasonal HCl production and destruction are discussed, concluding that the freeze‐thaw cycle of atmospheric water is most likely. Key Points: Comparison of trace gases and aerosol opacity between ACS on ExoMars TGO and MCS on MROLow correlations between HCl and the opacities of dust or water ice, but very strong correlation with water vapor and the formation of icesHCl source and sink possibilities are discussed, concluding that frost activity may be the most likely possibility for both [ABSTRACT FROM AUTHOR]

Drought intensity is commonly characterized using meteorologically‐based metrics that do not provide insight into water deficits within deeper hydrologic systems. In contrast, global positioning system (GPS) displacements are sensitive to both local and regional hydrologic‐storage fluctuations. While a few studies have leveraged this sensitivity to produce geodetic drought indices, hydrologic drought characterization using GPS is not commonly accounted for in drought assessment and management. To motivate this application, we produce a new geodetic drought index (GDI) and quantify its ability to characterize hydrologic drought conditions in key surface and sub‐surface hydrologic reservoirs/pools across California. In northern California, the GDI exhibits a strong regional association with surface‐reservoir storage at the 1‐month time scale (correlation coefficient: 0.83) and groundwater levels at the 3‐month time scale (correlation coefficient: 0.87), along with moderate associations with stream discharge at the daily (instantaneous) time scale (correlation coefficient: 0.50). Groundwater in southern California is best characterized with a 12‐month GDI (correlation coefficient: 0.77), and surface‐reservoir storage is optimized with the 3‐month GDI (correlation coefficient: 0.72). Two sigma uncertainties are ±0.03. Differences between northern and southern California reveal that the GDI is sensitive to unique aquifer and drainage basin characteristics. In addition to capturing long‐term hydrologic trends, rapid changes in the GDI initiate during clusters of large atmospheric river events that closely mirror fluctuations in traditional hydrologic and meteorological observations. We show that GPS‐based hydrologic drought indices provide a significant opportunity to improve drought assessment, in California and beyond, by improving our understanding of the hydrologic cycle. Plain Language Summary: Although quantifying the total volume of water loss is of critical importance during periods of drought, drought intensity is often characterized using meteorologic observations, such as precipitation, rather than using more holistic hydrologic observations, such as reservoir levels and groundwater. While precipitation is a good measure of the amount of water entering a region, precipitation models do not determine the amount of water retained in a watershed or the amount lost due to runoff and evapotranspiration, which are important factors for drought management. We address this need by producing a hydrologically based drought index that captures changes in both surface and subsurface hydrologic reservoirs/pools using surface‐loading geodesy, which quantifies changes in water volume based on how the shape of the Earth changes under the weight of the water. In this study, we use three‐dimensional global positioning system data to develop a geodetic drought index (GDI). Comparison with independent hydrologic observations indicates strong regional and temporal correlations with reservoir storage, groundwater fluctuations, and stream discharge observations, suggesting the GDI can effectively characterize variations in total hydrologic storage. The GDI provides an opportunity to improve hydrologic models for drought management and to advance our understanding of the water cycle. Key Points: Current drought assessment methods rely primarily on meteorologic drought indices that do not characterize total water storageThe geodetic drought index quantifies hydrologic drought and is especially sensitive to groundwater and surface‐reservoir storageDrought metrics based on geodetic data improve characterization of total water storage, providing unique insight for drought management [ABSTRACT FROM AUTHOR]

Precipitation changes in full response to CO2 increase are widely studied but confidence in future projections remains low. Mechanistic understanding of the direct radiative effect of CO2 on precipitation changes, independent from CO2‐induced SST changes, is therefore necessary. Utilizing global atmospheric models, we identify robust summer precipitation decreases across North America in response to direct CO2 forcing. We find that spatial distribution of CO2 forcing at land surface is likely shaped by climatological distribution of water vapor and clouds. This, coupled with local feedback processes, changes in convection, and moisture supply resulting from CO2‐induced circulation changes, could determine North American hydroclimate changes. In central North America, increasing CO2 may decrease summertime precipitation by warming the surface and inducing dry advection into the region to reduce moisture supply. Meanwhile, for the southwest and the east, CO2‐induced shift of subtropical highs generates wet advection, which might mitigate the drying effect from warming. Plain Language Summary: How precipitation changes in full response to increased CO2 remains unclear. Mechanistic understanding of how increasing CO2 alone changes precipitation can help us better predict future precipitation in full response to CO2 increase. We find that the precipitation responses are much stronger during summer than winter in North America. During summer, precipitation significantly decreases in central North America in response to the direct CO2 radiative effect, while in the southwest and the east, precipitation changes are small. There might be stronger longwave radiation forcing near the surface induced by CO2 increase in arid regions due to weaker masking effect of water vapor and clouds. This, along with local feedbacks, significantly warms the central region. Meanwhile, the direct forcing of CO2 induces northward shifts in subtropical highs, consequently generating anticyclonic anomalies. These wind anomalies result in drier air advected into central North America, leading to moisture divergence and hence reducing moisture supply. The reduced water supply, coupled with the warming, may decrease precipitation in the region. For the southwest and the east, the anticyclonic anomalies cause wetter air advected into the regions and generate moisture convergence, which increases moisture supply and might mitigate the drying effect caused by CO2‐induced warming. Key Points: Robust precipitation decreases during summer in response to CO2 direct radiative effect are identified in central North AmericaClimatology and changes of hydrological environment may collectively modulate the regional CO2 radiative effect on summer precipitationCO2‐induced circulation changes could alter the moisture supply to reshape the regional hydrological cycle [ABSTRACT FROM AUTHOR]

This study investigates the specific circulation anomalies that have sustained the low Antarctic sea ice state since 2016. Firstly, we find a significant strengthening and southward shift in the Ferrel Cell (FC) during 2016–2022, resulting in a marked increase in southward transport of heat and moisture. Secondly, this enhanced FC is closely associated with a stronger mid‐latitude wave pattern. This pattern is zonally asymmetric and greatly amplifies the poleward advections of heat and moisture, leading to the increased downward longwave radiation, more liquid precipitation and sea ice retreat in specific regions, including the western Pacific and Indian Ocean sectors, Ross and northern Weddell Seas. The mechanism deduced from the short‐term period is further supported by the results of 40 ensemble members of simulations. The southward expansion of the FC and sea ice decline are closely linked to La Niña‐like conditions but may also be driven by anthropogenic global warming. Plain Language Summary: Following the sudden decline in 2016, the Antarctic sea ice extent has persisted at historically low levels. In 2023, it reached unprecedented record lows. However, the specific atmospheric circulation anomalies that have sustained the Antarctic sea ice at low levels are still unknown. It is well‐established that the Ferrel Cell, a mid‐latitude atmospheric meridional circulation, plays a pivotal role in the energy exchange between the high‐ and mid‐latitudes. Our findings indicate that the enhanced Ferrel Cell zonally intensified southward transport of heat and moisture over the sea ice regions, which sustains the overall low Antarctic sea ice state. Additionally, in the horizontal plane, the enhanced mid‐latitude wave pattern is responsible for the regional sea ice retreat over the western Pacific sector, Ross Sea, Indian Ocean sector, and northern Weddell Sea, and is also closely associated with the enhanced Ferrel Cell. The effects of the enhanced Ferrel Cell on Antarctic sea ice decline are further supported by the results of large ensemble simulations. Therefore, this study suggests that concurrent with the southward shifting of the Ferrel Cell, the stronger warm and moist air intrusions, and the increased liquid precipitation, restrict the Antarctic sea ice expansion following its sudden decline. Key Points: Since 2016, the low Antarctic sea ice extent has persisted, consistent with heat and moisture accumulation over the sea ice edgesThe Ferrel Cell was enhanced and shifted southward, leading to the increased southward heat/moisture advection, and liquid precipitationThe effects of the enhanced Ferrel Cell on Antarctic sea ice decline are further supported by the results of large ensemble simulations [ABSTRACT FROM AUTHOR]

Given the important role of Atmospheric River precipitation (ARP) in the global hydrological cycle, accurate representation of ARP is significant. However, general circulation models (GCMs) demonstrate bias in simulating ARP. The target of this study is to quantify the performance of ARP intensity/frequency for CMIP6 simulations, and further to improve ARP estimation of CMIP6 using Cycle‐Consistent Generative Adversarial Networks (CycleGAN) with highlighting the more accurate ARP features under the global warming background. The findings of this study are as follows: (a) although ARP intensity/frequency in reserved‐optimal CMIP6 overall reproduces the observation, it is still underestimated at the stronger Atmospheric river (AR) scales, particularly for the AR highly active mid‐latitude regions. (b) The CycleGAN‐based bias correction approach markedly diminishes the bias of the CMIP6 simulations within most of the AR scales among both global and the four AR highly active regions. Moreover, the performance of the ARP in AR highly active regions is significant improvement, which is mainly due to the reduction of the bias at the strongest scale. (c) Relative to reference period (1986–2005), ARP intensity/frequency at the strongest scale increase notably under 3°C warming level, with an average value of 373.3% in intensity and 415.9% in frequency for global and the four key regions before correction, and the value is 451.9% and 492.5% after bias correction. The results illustrate that CycleGAN can effectively improve the ARP simulations of GCMs, and an early warning implies that future strong extreme ARP should potentially surpass the current expected. Plain Language Summary: Despite the important role of Atmospheric River precipitation (ARP) in global hydrological cycle, accurately simulating global ARP remains a great challenge. Here, the content of our study consists of three main parts. Firstly, we quantify the performance of CMIP6 general circulation models (GCMs) on ARP intensity/frequency simulations and find that CMIP6 GCMs underestimates ARP at the Atmospheric river (AR) highly active regions, especially in the strongest AR intensity scale. Secondly, we utilize a deep learning method named CycleGAN to improve global ARP intensity/frequency simulations of CMIP6 GCMs. The results indicate that CycleGAN effectively corrects the ARP intensity/frequency bias within most of the AR intensity scales among both global and the AR highly active regions. Moreover, the performance of ARP in the AR highly active regions is significant improvement, which is mainly due to the reduction of the bias in the strongest scale. Finally, the trained CycleGAN model is used to evaluate ARP under global warming of 1.5, 2, and 3°. The bias correction results show that ARP in the strongest AR intensity scale surpasses the current expected, especially under 3° warming level. Key Points: CMIP6 models underestimates Atmospheric River precipitation (ARP) at the Atmospheric river (AR) highly active regions, especially in the strongest AR scaleThe CycleGAN‐based deep learning method reduces model bias notably for ARP intensity and frequency, respectivelyARP in the strongest AR scale potentially surpasses the current GCM‐based projections expected in different degrees of warming after bias correction [ABSTRACT FROM AUTHOR]

Accurately simulating moisture transport in summer over the TP is uncertain for current numerical models with one important factor being horizontal resolution. In this study, in order to investigate the moisture transport across scales, three experiments are conducted for summer of 2015 using a global variable‐resolution model (MPAS‐A), including one with globally quasi‐uniform resolution of 60 km (U60km) and two with regional refinements over the TP at resolutions of 16 km (V16km) and 4 km (V4km). The wet bias of summer rainfall within the TP increase from U60km to V16km but is significantly improved in V4km. One important source of rainfall bias is the moisture transport across scales. The differences in moisture transport among three simulations are significantly influenced by the changes in wind fields through the Himalayas and eastern TP in two layers, 700–600 and 600–400 hPa, which is largely modulated by their difference in large‐scale circulations particularly monsoon depression. At convection‐parameterized scale (from 60 to 16 km), the scale‐aware Grell‐Freitas convection scheme produces more rainfall and latent heat due to its large sensitivity to the integrating timestep. This sensitivity along with further resolved dynamical processes, collectively strengthen the monsoon depression to the south of TP and make it shift northward in conjunction with the mid‐latitude westerlies. With resolution increasing to convection‐permitting scale (from 16 to 4 km), the resolved moist convection releases significantly less latent heat and then reproduces a weaker monsoon depression. This causes a discrepancy that exceeds the resolution‐related difference at convection‐parameterized scale. Plain Language Summary: Summer moisture transport is crucial for the water cycle over the TP. Three experiments are conducted for summer of 2015 using a global variable‐resolution model, which can use smaller grid‐spacing over focus regions to get more details of atmospheric processes. The numerical experiments vary in the grid‐spacing, including one with a globally uniform grid‐spacing of 60 km, and the other two with smaller grid‐spacing of 16 and 4 km over the TP. The summer rainfall within the TP at 4 km shows the closest agreement with observation. The differences among three experiments are strongly linked with wind through the Himalayas and eastern TP in the lower and middle troposphere. Atmospheric systems especially monsoon depression vary in simulations with different grid‐spacing and hence change wind fields. With grid‐spacing at tens of kilometers, convection parameterization generates more latent heat and air motion in the model becomes more resolved with smaller grid‐spacing so that monsoon depression is stronger and shifts northward along with mid‐latitude westerlies. With grid‐spacing at a few kilometers, modeling air motion can directly simulate convective processes without convective parameterization, which releases less latent heat and generates weaker monsoon depression to the south of TP compared to coarser grid‐spacing. Key Points: Moisture transport is influenced by difference in simulated large‐scale circulations particularly monsoon depression across various scalesAt convection‐parameterized scale, monsoon depression is enhanced with increasing resolution due to sensitivity of GF scheme to timestepAt convection‐permitting scale, the resolved moist convection and its associated less latent heat lead to weaker monsoon depression [ABSTRACT FROM AUTHOR]

Weekly cycles (WCs) in cloud properties have been reported and linked to aerosol effects. Yet the extent to which human activities contribute to their occurrence remains unclear. Here, we revisit aerosol–cloud interactions from the WCs over central Europe using long‐term satellite and reanalysis data. Significant WCs in aerosol and cloud droplet number concentration (Nd) are detected with minima/maxima on Monday/Friday, indicating a clear signal of the Twomey effect. Notably, Nd–to–aerosol sensitivity from WCs is found to decrease at larger aerosol concentrations, confirming the nonlinear behavior of the aerosol–Nd relation (in log–log space) reported previously, but from a distinct perspective. Nevertheless, no discernible WCs in liquid water path are found. The pronounced WCs in cloud cover are demonstrated to be driven by natural variability. Our results indicate that the WCs offer a useful pathway for investigating the Twomey effect, but are not as effective for detecting cloud adjustments. Plain Language Summary: Aerosol–cloud interactions are the largest uncertainty in the anthropogenic forcing of climate. Specifically, an increase in aerosols increases cloud droplet number concentration (the Twomey effect), which further changes liquid water path and cloud cover (cloud adjustments), ultimately alters radiations. Weekly cycles would be a useful tool for such study, assuming no 7‐day periodicity in meteorological dynamics. In this study, we revisit aerosol–cloud interactions from weekly cycles using long‐term satellite observations and reanalysis data. The analysis is restricted to central Europe—a region with strong weekly cycles in anthropogenic emissions. We find significant weekly cycles in aerosol and cloud droplet number concentration with minima/maxima on Monday/Friday. Importantly, the sensitivity of cloud droplet number concentration to aerosol is found to decrease in polluted conditions, confirming the behavior of the nonlinear cloud response to aerosol as reported previously. It is demonstrated that the weekly cycle in liquid water path is negligible; though a pronounced cycle in cloud cover detected, it is predominately caused by natural variability. Hence, caution is warranted when attributing observed weekly cycle in cloud cover to aerosol effects. Conclusively, weekly cycles are useful for detecting the Twomey effect but less effective for cloud adjustments. Key Points: The analysis of weekly cycles reveals a reduced sensitivity of cloud droplet number concentration to aerosol at larger aerosol loadingThe strong weekly cycle in cloud cover is mainly a result of natural variability rather than being attributed to aerosol effectsWeekly cycles offer a useful pathway to investigating the Twomey effect, but are not as effective for detecting cloud adjustments [ABSTRACT FROM AUTHOR]

Precipitation is one of the main components of the hydrological cycle and its precise quantification is fundamental to providing information for the understanding and prediction of physical processes. Precipitation observations based on ground‐based devices (manual and automatic rain gauges) are highly accurate but have limited spatial coverage. On the other hand, remote sensing products cover large areas but with lower accuracy. In this context, this study aims to provide a more accurate monthly precipitation estimating product, with lower latency than other products but without directly relying on field data. The methodology consists of applying a machine learning method (k‐nearest neighbours algorithm) to satellite‐based remote sensing data (IMERG Early Run product) and re‐analysis‐based (MERRA‐2) variables with a particular connection to precipitation. The method was applied over the Brazilian territory, which features a large range of precipitation regimes. This methodology resulted in the development of an adjusted IMERG product (IMERG BraMaL). Compared with the original IMERG products (Early Run and Final Run), IMERG BraMaL has improved the evaluated metrics between ground‐based and satellite data in almost all analyses. For instance, KGE (Kling‐Gupta efficiency) went from lower values (0.70 and 0.82 for Early and Late Run, respectively) to values above 0.86 in the IMERG BraMaL. The adjusted product also presented superior performance statistics compared with other global precipitation products (CHIRPS, PERSIANN‐CDR, and MSWEP). The main advantages of IMERG BraMaL compared with IMERG Final Run are (i) much faster availability to the end‐users; (ii) non‐dependency on any field data, allowing its application in areas where rain gauge data is unavailable or of low quality; (iii) the non‐relationship of errors to local features; and (iv) the much‐improved estimations in regions in Brazil where, historically, satellite‐based products usually underestimate the observed data. [ABSTRACT FROM AUTHOR]

Climate change has an adverse effect on the global hydrological cycle and hence climatic extremes are becoming more persistent. Therefore, the present study analyses the characteristics of the compound extremes in India using the Godavari River basin as a test bed. The study examines the individual return period (IRP) and pairwise joint return period (JRP) of several climate extreme indices such as consecutive dry days (CDD), consecutive wet days (CWD), percentage rainfall of very dry days (R5Tot) and very hot days (TX95) considering historical and future period of two scenarios, SSP245 and SSP585, from the CMIP6 consortium. Both SSPs indicates an upward trend in the indices for CWD and TX95, whilst showcasing a declining trend for CDD and R5Tot at 5% significance level. Under SSP585, the values of JRPs TR5Tot,TX952, TR5Tot,TX955, TR5Tot,TX9510 and TR5Tot,TX9525 ranges from 3 to 5 years, 14–55 years, 35–274 years and 104 to >1000 during epoch‐1 (2023–2060). During epoch‐2 (2061–2100), the JRPs values are generally higher. Regions displaying lower JRPs values suggest a higher likelihood of simultaneous occurrences of extremes such as floods and droughts, as well as very dry days and warm days within the same year. Under SSP585, the maximum (minimum) value of change rate (CR) of JRP for TCDD,CWD10 and TCDD,CWD25 during epoch‐1 is more than 900% (−80%) and 1000% (−90%), whereas during epoch‐2, the corresponding CR values are greater than 700% (−80%) and 1000% (−90%). The future possibility of co‐occurrence of 'CDD and CWD' and 'R5Tot and TX95', both exceeding their 10‐ and 25‐year IRP, in a single year, is highly likely for the regions with low CR values and unlikely for the regions with high CR values. The study would help in understanding the spatial and temporal variability of compound climate extremes over the Godavari River basin. [ABSTRACT FROM AUTHOR]

Satellite observations of column‐averaged water isotopes are relatively new retrieval products that are in need of further in situ evaluation. Such evaluation studies are generally difficult to perform due to the wide mismatch in temporal and spatial scales between the satellite observations based on instantaneous pixel averages during an overpass and airborne in situ measurements ranging up to several hours over a km‐scale. In addition, topography, weather conditions and in particular cloudiness impose severe constraints on an exact collocation between satellite and airborne in situ measurement platforms. Here we present a new method that allows a comparison between in situ measurements and satellite observations of δD on a broader statistical basis. We use regional isotope‐enabled model simulations as intermediate information to identify the area for best comparisons. Applying our methodology to TROPOMI total column δD retrievals for the L‐WAIVE campaign in Annecy, France, during June 2019 increases the number of satellite pixels for comparison despite widespread cloudiness on average by a factor of 20. In addition, the comparison of simulated and observed δD revealed a dependency of the satellite evaluation on the structure of the middle and upper troposphere. We conclude that our method provides a more robust statistic basis for in situ evaluation of δD satellite retrievals. The method will thus be useful in planning and executing forthcoming validation and evaluation campaigns, and can potentially be used for the evaluation of other satellite products. Plain Language Summary: A characteristic of atmospheric water vapor is the concentration of stable heavy hydrogen and oxygen isotopes. Isotopic concentrations are observed by diverse techniques such as remote sensing from satellites and land‐based instruments, and direct measurements from aircrafts, ships and stations on land. These measurements lead to a variety of data sets which span different distances (from a few to hundreds of km) and time periods (from seconds to days). While remote measurements provide data of a large spatial and temporal coverage at a coarse resolution, direct measurements are often obtained during research campaigns over a limited time period with high spatial and temporal resolution. Especially the later data sets are of high value as they describe the atmospheric state in high detail. In this study we develop a method to extrapolate direct, campaign based high‐quality measurements of water vapor to the largest possible representative area using output from a numerical weather simulation. This allows us to compare remotely sensed and direct isotope measurements. This comparison illustrates how to interpret total column measurement and to identify where the model has difficulties to correctly simulate the vertical isotope distribution. This knowledge is of use for future application of remotely sensed data sets and model development. Key Points: New, expanded collocation criterion provides larger data sets for comparison of δD observations during a field campaignWe evaluate TROPOMI δD column retrievals with in situ airborne measurements using a COSMOiso‐based collocation criterionThe combination of subcolumn and total column averaged δD is used to characterize vertical δD gradients and reveal model biases [ABSTRACT FROM AUTHOR]

The changing climate and intensifying human activities have made an impact on the hydrological processes in the upper Yangtze River (UYR), but quantifying their effects remains uncertain. This study used the Budyko framework to investigate the response of runoff (Q) to climate change and human activities during 1956–2017 and evaluate the impacts of human activities, including land use/cover change, water use, dam construction, and vegetation change, on watershed characteristic. Results show that climate change is the dominant driver of Q variations in the Wujiang River (WJR), Jialing River (JLR), and Jinsha River (JSR) watersheds, with contributions of 58.6%, 66.9%, and 67.6%, respectively. However, in Mingjiang River (MJR) and UYR watersheds, human activities contribute more to Q variations with 55.2% and 51.2%, respectively. Human activities play important roles in variation of watershed characteristics, and they can explain 22%, 26%, 36%, 25%, and 53% of the watershed character change in UYR, WJR, JLR, MJR, and JSR, respectively. This study conducts a comprehensive analysis of the causes of Q change in UYR, and provides a new perspective to explore the effects of specific human activities on watershed characteristics. Plain Language Summary: With climate change and intensifying human activities, the water cycle has been changed. Runoff is an essential component of the water cycle, and its response to changing environments has drawn broad attention. This study aims to quantify the contributions of climate change and human activities to runoff changes in the upper Yangtze River (UYR) and its four sub‐watersheds over the past half‐century. According to the results, human activities and climate change have been responsible for approximately 51.2% and 48.8% of the observed runoff changes in UYR, respectively. It suggests that both human activities and climate change have played significant roles, and their contributions are remarkably comparable. In addition, the impacts of human activities on watershed environment changes are analyzed. The results suggest that increased settlement area and industrial water use are the dominant influencing factors. Key Points: Human activities and climate change have made comparable contributions to the runoff reduction in the upper Yangtze RiverWater use, land use/cover change, vegetation change, and dam construction together explain 22% of the watershed environment changesHuman settlements and increased industrial water use have been the principal factors contributing to the changes in watershed environment [ABSTRACT FROM AUTHOR]

Transpiration plays a vital role in determining the watershed water cycle. However, we still have little knowledge of the characteristics of tree transpiration in the Hanjiang River Basin, which is the water source for the middle route of South‐to‐North water diversion project. Here, we measured sap flux density of oak trees (Quercus, the dominant species here) at the 10‐min resolution for 2 years and explored its response to the environmental conditions. The incoming short‐wave radiation and vapour pressure deficit well explained the variation of daytime sap flux density, and a statistical model was then proposed to calculate the daytime sap flux density correspondingly; then a nighttime sap flux density module was proposed based on the daytime sap flux density calculation. Sap flux density showed clear counter‐clockwise hysteresis response to incoming short‐wave radiation, and clockwise hysteresis response to vapour pressure deficit. Our sap flux density model can well reproduce the corresponding hysteresis response to incoming short‐wave radiation and vapour pressure deficit. This study unravelled the environmental controls of sap flux density of the oak trees in the Hanjiang River Basin, proposed an efficient model for the sap flux density simulation, provided important knowledge for understanding the corresponding forests' water use, which is of critical significance in determining the water availability for the middle route of the South‐to‐North water diversion project. [ABSTRACT FROM AUTHOR]

Forest trees greatly influence both the routing of water downward into the subsurface and the re‐routing of water upward through water uptake and transpiration. To reveal how the subsurface soil water pools used by trees change across seasons, we analysed 2 years of stable isotope ratios of precipitation, soil water from different depths (using both bulk sampling and suction‐cup lysimeters), and xylem in a mixed beech and spruce forest. Precipitation as well as mobile and bulk soil waters all showed a distinct seasonal signature; the seasonal amplitude decreased with depth, and mobile soil waters fluctuated less than bulk soil waters. Xylem water signatures in both tree species were similar to the bulk soil water signatures and rather different from the mobile soil water signatures. The beech and spruce trees had different isotope ratios, suggesting the use of different water sources, and these differences were larger under dry antecedent conditions than wet antecedent conditions. Despite these differences, both species predominantly transpired waters with a winter‐precipitation isotopic signature throughout the summer, including during wet conditions when more recent precipitation was available. Over most of the sampling dates, the fraction of recent precipitation (i.e. from the preceding 30 days) in xylem water was low, despite both species typically demonstrating the use of both shallow and deeper soil waters. These results provide evidence that the soil water storages used by these trees are largely filled in winter and bypassed by recent precipitation, implying long residence times. [ABSTRACT FROM AUTHOR]

Large spatio‐temporal gradients in the Congo basin vegetation and rainfall are observed. However, its water‐balance (evapotranspiration minus precipitation, or ET − P) is typically measured at basin‐scales, limited primarily by river‐discharge data, spatial resolution of terrestrial water storage measurements, and poorly constrained ET. We use observations of the isotopic composition of water vapor to quantify the spatio‐temporal variability of net surface water fluxes across the Congo Basin between 2003 and 2018. These data are calibrated at basin scale using satellite gravity and total Congo river discharge measurements and then used to estimate time‐varying ET − P over four quadrants representing the Congo Basin, providing first estimates of this kind for the region. We find that the multi‐year record, seasonality, and interannual variability of ET − P from both the isotopes and the gravity/river discharge based estimates are consistent. Additionally, we use precipitation and gravity‐based estimates with our water vapor isotope‐based ET − P to calculate time and space averaged ET and net river discharge within the Congo Basin. These quadrant‐scale moisture flux estimates indicate (a) substantial recycling of moisture in the Congo Basin (temporally and spatially averaged ET/P > 70%), consistent with models and visible light‐based ET estimates, and (b) net river outflow is largest in the Western Congo where there are more rivers and higher flow rates. Our results confirm the importance of ET in modulating the Congo water cycle relative to other water sources. Plain Language Summary: Rainfall and vegetation vary substantially across the Congo Basin. However, the spatial variations, seasonality, and interannual variability of the net water balance, (the difference between evapotranspiration and rainfall) is not well quantified. Atmospheric observations of the isotopic composition of water vapor are sensitive to the balance of evapotranspiration (ET) and precipitation (P). We calibrate new observations of the isotopic composition of water vapor to ET − P that is based on satellite gravity measurements and ground‐based river discharge measurements to quantify ET − P across four quadrants of the Congo basin. When combined with satellite measurements of rainfall, we show that ET is the largest source of Congo basin water vapor. As ET is about 70% of observed rainfall, vegetation therefore plays an outsized role on the Congo water cycle. Additionally, when combined with satellite measurements of gravity, we show that river discharge is higher in the western part of the basin, where there are more rivers and stronger flows. Key Points: Water vapor isotopes provide a reliable proxy for evapotranspiration (ET) minus precipitation (P) in the Congo BasinET contributes over 70% to P within the four quadrants of the Congo BasinRiver discharge is highest in the western part of the basin, where there are more rivers and higher flow [ABSTRACT FROM AUTHOR]

Changes in precipitation and its causes under future warming scenarios in Northwest China are of great concern. Based on the dynamical downscaling results of the Weather Research and Forecasting model (WRF), this study quantitatively identified the contribution of local evaporation and external moisture transport to future precipitation changes in western and eastern Northwest China (WNWC and ENWC) by calculating the precipitation recycling ratio (PRR). The results showed that the WRF could reasonably reproduce the observed spatial and temporal distribution of precipitation and evaporation during the historical reference period of 1985–2014. Under the 1.5 and 2.0°C stabilized warming scenarios from 2071 to 2100, there is a stronger precipitation growth in WNWC than in ENWC, and the PRR generally increases in WNWC and decreases in ENWC. Further analysis indicates that external moisture transport is the dominant contributor to the increased precipitation in all cases with its contribution exceeding 70%, while the difference of precipitation growth in two sub‐regions arises from the summer moisture transport at the southern boundary as the inflow in WNWC and outflow in ENWC. Compared to the negligible contribution of evaporation in ENWC (less than 7%), evaporation contributes more than 23% to the precipitation increase in WNWC under future warming scenarios, which may be related to the strong land‐atmosphere coupling there. Furthermore, an additional warming of 0.5°C leads to an increase in precipitation for both sub‐regions, which is mainly due to the contribution of external moisture transport. Plain Language Summary: In this study, we discuss the relative contributions of evaporation and moisture transport to precipitation changes in Northwest China under 1.5/2.0°C stabilized warming scenarios. With Northwest China (NWC) dividing it into two sub‐regions of western and eastern parts, we find that although the western NWC has less precipitation during the reference period of 1985–2014, the increase in precipitation is about twice that of the eastern NWC under the warming scenarios. The contribution of evaporation to the increased precipitation is greater in the western NWC compared with the reference period, which may indicate the accelerating hydrological cycle in western NWC. Moisture transport is the main contributor to the increase in precipitation and is also the main reason for the difference in precipitation changes between the two sub‐regions, mainly due to differences in summer moisture transport across the southern boundary. This work provides a scientific basis for assessing water resources in Northwest China under future warming scenarios. Key Points: External moisture transport contributes more than 70% of precipitation in Northwest China, both in terms of mean and variabilityContribution of evaporation to the precipitation increase is stronger in western Northwest China, due to its strong land‐atmosphere couplingThe difference in moisture transport between the two regions is mainly due to changes in southern border moisture transport in summer [ABSTRACT FROM AUTHOR]

Mixed‐phase stratiform clouds contain numerous liquid, mixed‐phase, and ice clusters, quantifying the cluster scales is potentially helpful to improve the parameterizations of microphysics and radiation models. However, the scales of hydrometeor clusters at different levels of stratiform clouds are not well understood. In this study, using airborne measurements and a large eddy simulation, we show that turbulence plays an important role in controlling the clusters with length of a few hundred meters, while the scales of larger clusters have stronger vertical variations from cloud base to top. The liquid clusters are the largest near the cloud top, while the lengths of ice clusters decrease from cloud base to top. The lengths of mixed‐phase clusters depend on the glaciation process, a faster glaciation results in smaller mixed‐phase clusters. The results improve our understanding on how the liquid and ice are mixed at different levels in stratiform clouds. Plain Language Summary: Mixed‐phase clouds are vital to the global radiative balance and water cycle. The coexistence of liquid and ice, and the way they mix have significant impacts on the cloud microphysical properties. Observations suggest hydrometeors organize in clusters in mixed‐phase stratiform clouds, while such inhomogeneity is still poorly represented in models. In this study, using airborne measurements and model simulation, we analyzed the scales of liquid, mixed‐phase, and ice clusters at different levels in a stratiform cloud observed in Northeast China. The results suggest turbulence controls the scale of clusters with length of a few hundred meters, while larger clusters have stronger vertical variations. The results improve our understanding on the cluster scales of hydrometeors at different levels in stratiform clouds, and are potentially helpful to improve the parameterizations of microphysics and radiation. Key Points: Turbulence plays an important role in controlling the scale of hydrometeor clustersThe scales of liquid, mixed‐phase, and ice clusters have different height dependencies from cloud top to baseA faster glaciation process results in smaller mixed‐phase clusters [ABSTRACT FROM AUTHOR]

Accurate estimation and attribution of large‐scale changes in plant transpiration are critical to understand the impacts of vegetation dynamics on the terrestrial hydrological cycle. However, these aspects remain poorly understood due to the limited reliability of global transpiration products. Here we compile data from 101 site‐based transpiration measurements across the globe and use them to constrain three biophysically based data‐driven transpiration products. The constrained transpiration reveals a prominent increasing trend of 0.61–0.79 mm yr−2 during 1980–2021, which is overestimated by 8%–32% in unconstrained transpiration. We further find that the global transpiration increase is mainly driven by leaf area index increase (40%), followed by climate change (19%), though offset partly by CO2‐induced stomatal closure (−38%) and land use and cover change (−3%). Our refined estimates indicate a less substantial increase of global transpiration than previously thought, improving the understanding of transpiration change impact on global hydrological cycle. Plain Language Summary: As a major component of terrestrial evapotranspiration, plant transpiration plays a critical role in global hydrological cycle and climate. However, transpiration and its changes are far from constrained due to the lack of field observations. By compiling a large number of in situ transpiration measurements, we correct three widely used transpiration products using the in situ data (i.e., constraints). We find that the original transpiration trend reflects an overestimation by 8%–32% compared to our constrained results, indicating that cautions should be taken when using the unconstrained products to detect transpiration trend. By combining five land surface models, we further find that the transpiration increase is mainly driven by leaf area index increase and climate change, whereas CO2‐induced stomatal closure and land use and cover change offset a considerable part of this increase. Our results improve the understanding the transpiration changes and the underlying drivers over the past four decades. Key Points: We compile data from 101 site‐based transpiration measurements across the globe and use them to constrain three transpiration productsConstrained transpiration reveals an increasing trend of 0.61–0.79 mm yr−2, while original transpiration overestimates this trend by 8%–32%Global transpiration increase is mainly driven by LAI increase and climate change, through offset partly by stomatal closure and LUCC [ABSTRACT FROM AUTHOR]

Summary: Plant water uptake from the soil is a crucial element of the global hydrological cycle and essential for vegetation drought resilience. Yet, knowledge of how the distribution of water uptake depth (WUD) varies across species, climates, and seasons is scarce relative to our knowledge of aboveground plant functions. With a global literature review, we found that average WUD varied more among biomes than plant functional types (i.e. deciduous/evergreen broadleaves and conifers), illustrating the importance of the hydroclimate, especially precipitation seasonality, on WUD. By combining records of rooting depth with WUD, we observed a consistently deeper maximum rooting depth than WUD with the largest differences in arid regions – indicating that deep taproots act as lifelines while not contributing to the majority of water uptake. The most ubiquitous observation across the literature was that woody plants switch water sources to soil layers with the highest water availability within short timescales. Hence, seasonal shifts to deep soil layers occur across the globe when shallow soils are drying out, allowing continued transpiration and hydraulic safety. While there are still significant gaps in our understanding of WUD, the consistency across global ecosystems allows integration of existing knowledge into the next generation of vegetation process models. [ABSTRACT FROM AUTHOR]

SUMMARY: The grapevine industry is of high economic importance in several countries worldwide. Its growing market demand led to an acceleration of the entire production processes, implying increasing use of water resources at the expense of environmental water balance and the hydrological cycle. Furthermore, in recent decades climate change and the consequent expansion of drought have further compromised water availability, making current agricultural systems even more fragile from ecological and economical perspectives. Consequently, farmers' income and welfare are increasingly unpredictable and unstable. Therefore, it is urgent to improve the resilience of vineyards, and of agro‐ecosystems in general, by developing sustainable and environmentally friendly farming practices by more rational biological and natural resources use. The PRIMA project PROSIT addresses these challenges by characterizing and harnessing grapevine‐associated microbiota to propose innovative and sustainable agronomic practices. PROSIT aims to determine the efficacy of natural microbiomes transferred from grapevines adapted to arid climate to commonly cultivated grapevine cultivars. In doing so it will test those natural microbiome effects on drought tolerance. This multidisciplinary project will utilize in vitro culture techniques, bioimaging, microbiological tests, metabolomics, metabarcoding and epigenetic analyses. These will be combined to shed light on molecular mechanisms triggered in plants by microbial associations upon water stress. To this end it is hoped that the project will serve as a blueprint not only for studies uncovering the microbiome role in drought stress in a wide range of species, but also for analyzing its effect on a wide range of stresses commonly encountered in modern agricultural systems. Significance Statement: This Perspective Review manuscript focuses on innovative and sustainable solutions to drought stress in viticulture, representing the core research of the "Plant Microbiome in Sustainable Viticulture (PROSIT)" project funded by the PRIMA Foundation with the support of the Horizon 2020 Program. [ABSTRACT FROM AUTHOR]

Observations and models show that near‐surface relative humidity is nearly constant at ∼80% over the ocean in the current climate, and almost invariant in the global mean in projected future climates. Here, this behavior is investigated through the development of a simple theoretical model for near‐surface relative humidity by considering the moisture balance above a uniform ocean surface. The relative humidity is predicted to depend on only the near‐surface wind speed, air‐surface temperature difference, surface wetness and large‐scale moisture convergence. Although developed in the context of moist over‐ocean convection, the theory is able to determine the relative humidity in a suite of idealized simulations over both wet and dry surfaces with a root‐mean‐square error of less than 3%. The theory also predicts the climatology of relative humidity over the ocean with a root‐mean‐square error of less than 3%. The theory thus provides a theoretical basis for investigating changes in relative humidity over the ocean, water vapor feedbacks and the water cycle in current and future climates. Plain Language Summary: Relative humidity is the water content of air relative to its saturation value. Observations and climate models show that global mean near‐surface relative humidity is almost invariant in a changing climate, an observation which underpins the enhancement of the water cycle with increasing global mean temperatures and surface radiative feedbacks. Furthermore, relative humidity over the ocean, whether in the tropics or polar regions, takes a near uniform value of ∼80%. However, a theoretical basis for this remarkable invariance of relative humidity has not been previously established. Here we show that near‐surface relative humidity is controlled by the geometry of moist convection over the ocean surface. Its value is set by a non‐dimensional geometric constant and is independent of the absolute air temperature, with weak dependence on near‐surface wind speeds, air‐surface temperature difference and large‐scale moisture convergence. Hence, the over ocean relative humidity is largely climate and latitude invariant. Key Points: Convection above the ocean surface exhibits a characteristic geometry that controls the value of near‐surface relative humidityA quadratic equation is derived to predict near‐surface relative humidity in terms of known surface properties and a geometric constantThe equation predicts the relative humidity in idealized simulations and ocean observations with less than 3% error [ABSTRACT FROM AUTHOR]

Ecosystem water use efficiency (WUE) is a crucial parameter for understanding the interaction between carbon and water cycles. However, the spatio–temporal evolution and drivers of WUE remain unclear. This study utilized global annual scale global land surface satellite gross primary productivity and evapotranspiration data from 1982 to 2018 to estimate WUE and analyze its spatio–temporal characteristics. Additionally, the study investigated the response of WUE changes to five environmental factors (precipitation [PRE], soil moisture, temperature [TEM], palmer drought severity index, and vapor pressure deficit [VPD]) on WUE changes using partial correlation and structural equation modeling. The results suggested that the global annual WUE increased markedly over the study period, at an average rate of 0.0016 gC m−2 mm−1 H2O year−1. In contrast to the existing knowledge on the drivers of WUE change, climate change was found to have a larger contribution to WUE changes at the global and regional scales, especially in terms of TEM and VPD. A positive correlation between TEM and WUE was observed, but extreme TEM could lead to a decrease in WUE. VPD had the most significant direct effect on WUE, and its negative effect offset the positive influence of TEM especially in hyper‐arid, semi‐arid, and arid regions. These findings offer new insights into the impact of VPD and global warming on WUE. Plain Language Summary: Water use efficiency (WUE), defined as the ratio of gross primary productivity to evapotranspiration, is one of the key indicators for measuring the carbon‐water coupling in ecosystems. Analyzing the temporal and spatial evolution of global WUE and its influencing factors can offer valuable insights into addressing the challenges posed by global climate change. In this study, we examined the spatio‐temporal dynamics of global WUE over an extensive time span and utilized structural equation modeling to identify the primary drivers of WUE variation. Our findings reveal a consistent increasing trend in annual global WUE from 1982 to 2018, with an average annual increase of 0.0016 gC m−2 mm−1 H2O. Temperature and vapor pressure deficit (VPD) emerged as the key drivers of WUE changes, with the positive effect of temperature on WUE being counteracted by its negative influence through VPD (approximately 75%). This research contributes to our understanding of how climate change impacts carbon and water cycles within ecosystems and lays a foundation for enhancing carbon budget assessments. Key Points: Global water use efficiency (WUE) increased at an average rate of 0.0016 gC m−2 mm−1 H2O year−1 between 1982 and 2018Extreme temperatures that either excessively high (>26°C) or low (<2°C) can cause WUE to decreaseThe positive effect of temperature on WUE was offset by its negative effect on WUE through vapor pressure deficit [ABSTRACT FROM AUTHOR]

Twenty first century faces unprecedented challenges for the management of global waterways. The Himalayan waterways in Asia are exposed to unpredictable climatic warming together with anthropogenic perturbations caused by population growth, land use change and socio‐economic development. Given the increased public concerns on the Himalayan Mountain development programmes including the hydropower and tourism, there has been a growing need of the use of interdisciplinary scientific approaches to address water resources challenges that the Himalayan region has faced during the 21st century. Ecohydrology is an emerging scientific tool that explores key hydrological processes regulating structure and function of ecosystems, as well as assessing the impact of biological processes on water cycle variables under rapidly changing environment. The International Geosphere‐Biosphere Programme and the International Hydrological Programme hosted by the United Nations Educational, Scientific and Cultural Organization have adopted ecohydrology as a sustainable development tool by linking water resources and poverty eradication and ecosystem restoration, irrigation, energy and sanitation. However, ecohydrology tool needs to be reconfigured for sustainable development of rapidly changing Himalayan waterways. Here we propose the advancement of ecohydrology by developing various integrated frameworks of ecology, hydrology, hydraulics and sociology for resilient waterways in the Asian Himalayas. [ABSTRACT FROM AUTHOR]

Non‐perennial streams play a crucial role in ecological communities and the hydrological cycle. However, the key parameters and processes involved in stream intermittency remain poorly understood. While climatic conditions, geology and land use are well identified, the assessment and modelling of groundwater controls on streamflow intermittence remain a challenge. In this study, we explore new opportunities to calibrate process‐based 3D groundwater flow models designed to simulate hydrographic network dynamics in groundwater‐fed headwaters. Streamflow measurements and stream network maps are considered together to constrain the effective hydraulic properties of the aquifer in hydrogeological models. The simulations were then validated using visual observations of water presence/absence, provided by a national monitoring network in France (ONDE). We tested the methodology on two pilot unconfined shallow crystalline aquifer catchments, the Canut and Nançon catchments (Brittany, France). We found that both streamflow and stream network expansion/contraction dynamics are required to calibrate models that simultaneously estimate hydraulic conductivity K$$ K $$ and porosity Φ$$ \varPhi $$ with low uncertainties. The calibration allowed good prediction of stream intermittency, both in terms of flow and spatial extent. For the two catchments studied, Canut and Nançon, the hydraulic conductivity is close reaching 1.5 × 10−5 m/s and 4.5 × 10−5 m/s, respectively. However, they differ more in their storage capacity, with porosity estimated at 0.1% and 2.2%, respectively. Lower storage capacity leads to higher groundwater level fluctuations, shorter aquifer response times, an increase in the proportion of intermittent streams and a reduction in perennial flow. This new modelling framework for predicting headwater streamflow intermittence can be deployed to improve our understanding of groundwater controls in different geomorphological, geological and climatic contexts. It will benefit from advances in remote sensing and crowdsourcing approaches that generate new observational data products with high spatial and temporal resolution. [ABSTRACT FROM AUTHOR]

Despite the recent improvements, there is still a problem of access to WASH services. This problem access is linked to poverty and inequality, which in turn cause difficulties in accessing water, thus creating a vicious cycle. This article analyses this vicious cycle using data from international organisations related to these issues. These data show how the lack of water access leads to limitations for households, which are deprived of employment or/and education due to the difficulty in obtaining water. This opportunity cost is the main mechanism through which the feedback between lack of access to water and economic poverty occurs. Not being able to obtain employment or/and education makes it impossible to get out of a precarious situation, which prevents the wider society to benefit from any economic gain, which in turn slows down the achievement of the goal of guaranteeing access to water for all. The responsibility lies with national and international institutions, which should not only focus on obtaining the necessary financial resources for infrastructure improvements, but also on having adequate governance to ensure water sustainability and equity. [ABSTRACT FROM AUTHOR]

Nanotechnology‐inspired small‐sized water‐enabled electricity generation (WEG) has sparked widespread research interest, especially when applied as an electricity source for off‐grid low‐power electronic equipment and systems. Currently, WEG encompasses a wide range of physical phenomena, generator structures, and power generation environments. However, a systematic framework to clearly describe the connections and differences between these technologies is unavailable. In this review, a comprehensive overview of generator technologies and the typical mechanisms for harvesting water energy is provided. Considering the different roles of water in WEG processes, the related technologies are presented as two different scenarios. Moreover, a detailed analysis of the electrical potential formation in each WEG process is presented, and their similarities and differences are elucidated. Furthermore, a comprehensive compilation of advanced generator architectures and system designs based on hydrological cycle processes is presented, along with their respective energy efficiencies. These nanotechnology‐inspired small‐sized WEG devices show considerable potential for applications in the Internet of Things ecosystem (i.e., microelectronic devices, integrated circuits, and smart clothing). Finally, the prospects and future challenges of WEG devices are also summarized. [ABSTRACT FROM AUTHOR]

The Amazon River mobilizes organic carbon across one of the world's largest terrestrial carbon reservoirs. Quantifying the sources of particulate organic carbon (POC) to this flux is typically challenging in large systems such as the Amazon River due to hydrodynamic sorting of sediments. Here, we analyze the composition of POC collected from multiple total suspended sediment (TSS) profiles in the mainstem at Óbidos, and surface samples from the Madeira, Solimões and Tapajós Rivers. As hypothesized, TSS and POC concentrations in the mainstem increased with depth and fit well to Rouse models for sediment sorting by grain size. Coupling these profiles with Acoustic Doppler Current Profiler discharge data, we estimate a large decrease in POC flux (from 540 to 370 kg per second) between the rising and falling stages of the Amazon River mainstem. The C/N ratio and stable and radiocarbon signatures of bulk POC are less variable within the cross‐section at Óbidos and suggest that riverine POC in the Amazon River is predominantly soil‐derived. However, smaller shifts in these compositional metrics with depth, including leaf wax n‐alkanes and fatty acids, are consistent with the perspective that deeper and larger particles carry fresher, less degraded organic matter sources (i.e., vegetation debris) through the mainstem. Overall, our cross‐sectional surveys at Óbidos highlight the importance of depth‐specific sampling for estimating riverine export fluxes. At the same time, they imply that this approach to sampling is perhaps less essential with respect to characterizing the composition of POC sources exported by the river. Plain Language Summary: The Amazon River transports one of the largest quantities of freshwater organic carbon into the Atlantic Ocean. In this study, we collected suspended particles at different depths within a cross‐section of the Amazon River mainstem during the rising and falling stages of the river's hydrological cycle. We analyzed the organic carbon, nitrogen, grain size, and leaf‐derived compounds in these particles, and integrated water velocity measurements to calculate the quantity of carbon in particle form moving through the river at these two stages. The analyses showed that large, dense particles concentrate with depth in the Amazon River mainstem. The composition of these particles is relatively homogenous, but slight variations in metrics like carbon‐to‐nitrogen ratio, age derived from carbon‐dating, and leaf waxes imply that less degraded sources of organic carbon are found in the deeper and coarser grained particles. Overall, the data suggest that the majority of Amazon River particulate organic carbon comes from a mixture of soil organic carbon washing in from different landscapes and soil depth horizons. A globally significant quantity of this carbon will get buried in the Atlantic Ocean, forming a long‐term carbon sink. Key Points: Amazon River suspended sediments show little variation in organic carbon composition with depth despite hydrodynamic sortingEstimated particulate organic carbon fluxes range from 370 to 540 kg per second during the falling and rising stages, respectivelyThe majority of Amazon River particulate organic carbon exported from the mainstem at Óbidos is soil‐derived [ABSTRACT FROM AUTHOR]

In this research, a conditional nonlinear optimal perturbation related to parameters (CNOP‐P) method is employed to propose an ensemble prediction method titled as the conditional nonlinear optimal perturbation related to parameters ensemble prediction (CNOP‐PEP) method. Within the CNOP‐PEP method, all ensemble members are generated by the CNOP‐P method. To explore the operability and validity of the CNOP‐PEP method, long‐term evapotranspiration (ET) hindcast skill is evaluated at 13 sites on the Tibetan Plateau (TP) during the period of 2001–2018 with a land surface model. Two traditional ensemble prediction methods (the stochastically perturbed parameters (SPP) method and the one‐at‐a‐time (OAT) method) are also applied to assess the ET hindcast skills. The numerical results indicate that the CNOP‐PEP method shows the best hindcast skill for estimated ET at 13 stations on the TP among the three methods (CNOP‐PEP, OAT, and SPP methods) during the period of 2001–2018. This suggests that the CNOP‐PEP method is helpful and effective for long‐term and interannual hydrological predictions, such as long‐term and interannual ET predictions. The numerical results also indicate that ensemble members with certain special properties of fast‐growing types should be selected to implement ensemble prediction compared to these ordinary and traditional samples. The CNOP‐PEP method could supply special samples to improve the ensemble prediction and hindcast skill of hydrological cycle. Plain Language Summary: To enhance the ensemble prediction skill of evapotranspiration (ET) on the Tibetan Plateau, physical parameters in a numerical model are perturbed using ensemble techniques. Ensemble members about the physical parameters are generated by three ensemble techniques. The numerical results show the ensemble hindcast skills of ET are improved by using ensemble techniques. And, ensemble members with high hindcast skill are a rapidly evolving type of parametric perturbations, which could cause large hindcast uncertainties compared to the reference state, using the CNOP‐PEP method. Hence, the CNOP‐PEP method could be employed to improve the hindcast or prediction skill of hydrological cycle to reduce the uncertainties in model physical parameters. And, the uncertainties in model physical parameters also play a key role in ensemble prediction of hydrological cycle. The model physical parameters errors should be paid attention to enhance the forecast and hindcast skill of hydrological cycle. Key Points: The ensemble hindcast skills of evapotranspiration (ET) using three ensemble prediction methods are evaluatedHigher ensemble forecasting skills with a conditional nonlinear optimal perturbation related to parameters ensemble prediction method are foundPhysical parameter errors with maximum impact play a key role in ensemble prediction of hydrological cycle [ABSTRACT FROM AUTHOR]

Central Asia and its southern neighbors are being ravaged by more frequent and intense droughts and floods. While research on droughts is abundant, their interaction with floods and monsoons, especially when originating in the south, has been understudied. This study employed temperature, precipitation, standardized precipitation evapotranspiration index (SPEI), and agricultural drought indices such as water deficit index (WDI), cumulative water deficit (CWD), and Palmer drought severity index (PDSI) to investigate the role of monsoons in increasing drought and flooding in the southern part of central Asia, including Afghanistan and Pakistan, over a 31‐year period. Findings revealed increasing severity and intensity of droughts over time, along with their duration and peaks, contrasting with irregular fluctuations in flood intensity. Rising regional temperatures coincide with declining precipitation. During summer monsoon (SM), average precipitation ranged from 0.03 mm/day (2004) to 0.05 mm/day (2010), with variations across the region. The temperature varied between −14.35°C (2009) and 37.05°C (1991), with a sharp increase up to 0.114°C in Turkmenistan and Uzbekistan, as well as in northern Pakistan, Afghanistan, and western Tajikistan. A temperature drop during SM was revealed on the Mountains range of Himalaya, Hindu Kush, Karakoram, Parmir, Kunlun Shan and Tian Shan, and in Pakistan, down to −0.054°C. WDI showed a strong positive connection (0.71 and 0.64) with temperature (Tmax and Tmin) and a weak negative correlation (−0.5) with precipitation during normal time. However, during the monsoon season, it demonstrated a negative correlation with both within a no significant (−0.08) range for maximum temperature. Plain Language Summary: This study examines the influence of the monsoon on Central Asia's subsequent periods of drought and flooding. Central Asia, a region is located at a considerable distance from the main monsoon track. However, the monsoon's impact on the region is mediated through teleconnections and atmospheric circulation. The Indian summer monsoon brings significant rainfall to the Indian subcontinent and neighboring areas. During strong monsoon seasons, this excess moisture can propagate westward, resulting in increased rainfall in parts of Central Asia, particularly in the southern and southeastern regions. Conversely, a weak or failed monsoon can lead to reduced precipitation in Central Asia, contributing to drought conditions that affect agriculture, water resources, and ecosystems. Additionally, variations in the monsoon influence the melting of mountain snowpack and glaciers, impacting water availability in rivers crucial for irrigation and drinking water. Understanding the monsoon's influence on Central Asia's climate patterns and subsequent drought and flooding events provides insights into the complex interactions between the monsoon system and Central Asia's hydrological cycle. Additionally, the findings imply that crop productivity will continue to be impacted in the future because temperature was found to be rising and precipitation was found to be fluctuating in a decreasing manner. Key Points: The combined impact of monsoons, droughts, and floods in Central Asia [ABSTRACT FROM AUTHOR]