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As an important component of the global climate systems, ocean-continent interaction influences hydrological cycles at regional and local scales. Eastern China is a typical region where oceanic moisture (OM) has significant influences on summer precipitation. Due to its simplicity and accuracy, physical water vapor tracer method based on the stable isotope has been widely employed to estimate the contributions of different sources to precipitation. However, dense and long-term isotopic observations are limited by technical and economic reasons. Isotope-equipped global climate models (iGCMs) provide potential solutions to to improve our knowledge of hydrological processes. Accordingly, this study investigates the potential of using iGCMs to quantify the contributions of the OM to precipitation, which aim at better understanding of the hydrologic cycle processes. A new bias correction method is proposed to correct biases of precipitation isotopes simulated by five iGCMs (i.e. CAM2, GISS-modelE, LMDZ4, MIROC32, and HadAM3). A modified two-component mixing model taking 18O as a tracer is employed to assess the spatiotemporal contributions of summer OM for eastern China. Results show that iGCMs are biased in simulating preciptiation isotopes at local and regional scales. The bias correction method performs reasonable well in terms of reducing the biases. Based on biased-corrected precipitation isoscapes, the influence of OM on summer precipitation is investigated and found that OM plays an important role in summer precipitation, and spatially decreases along the transport route and the passage of time in summer. In addition, the OM contribution to precipitation contain a considerable uncertainty associated with the choice of a given iGCM for each sub-region and sub-period. Overall, bias correction is necessay before applying iGCMs to study hydrological cycles at the regional scale and the application of multi-iGCMs is critical to delineate uncertainty in studying oecanic-land interaction. [ABSTRACT FROM AUTHOR]

Climate impact of the Atlantic Multidecadal Variability (AMV) on the Northern Eurasia is poorly documented in literature due to the lack of observations in space, limited by the local fighting weather conditions, and in time, i.e. the analysed timescale is too long compared to the observational record (~120 years). To overcome this limitation, an idealized modelling approach designed in CMIP6 Decadal Climate Prediction Project is applied, based on the CMCC-CM2-SR5 coupled model: the North Atlantic sea surface temperature are continuously nudged towards the positive (negative) AMV anomalies to simulate the warm (cold) AMV phase. An ensemble of 32 members is performed for each experiment, in which all the forcings are set at the pre-industrial values for a 10-year integration time.Model results shows that, during Boreal summer, a positive phase of the AMV induces a circumglobal east-west wave train across the entire Northern Hemisphere. Enhanced westerly winds alters the climatological upper-troposphere circulation, advecting moisture from the North Atlantic Ocean along the northern part of the Eurasian continent. In particular, over Siberia, an anomalous geopotential dipole promotes the poleward movement of specific humidity from the Pacific Ocean, converging at high latitudes with the aforementioned westerly flux. Moisture anomalies regionally contribute to May-to-August rainfall, intensifying multidecadal precipitation over Scandinavia and Eastern Russia. Interestingly, rainfall rises by 10% of its climatology over Siberia, where the AMV-induced moisture fluxes converges.Furthermore, this study also points out that the AMV has a role in modulating the freshwater balance of the Arctic Ocean, due to the anomalous precipitation over the Siberian rivers' watersheds (Ob', Yenisei and Lena). Siberia is a key region for the hydrological cycle of the Arctic Ocean and the catchment areas of these three large rivers accounts for about 46% of the Arctic freshwater input provided by terrestrial sources. We find a decrease of sea surface salinity (about 12% of model climatology, broadly consistent with the anomalous river discharge) in the proximity of the river mouths during positive-AMV years.These results disclose the potential predictability of precipitation over northern Eurasian via tropical-extratropical pathway, associated with multidecadal fluctuations of the North Atlantic sea surface temperatures, in agreement with recent observational studies.This work is supported by EUCP (European Climate Prediction system) under Horizon 2020, grant agreement 776613. [ABSTRACT FROM AUTHOR]

Extreme value (EV) analysis of remotely-sensed precipitation fields is a challenging problem due to the short record length of the datasets available, and the inevitable presence of measurement errors especially for intense rainfall rates. Despite their limitations, remote sensing estimates offer unique insight into the hydrological cycle at the global scale, and their space-time resolution makes available unprecedented information for hydrological applications. Here we develop and apply new statistical tools specifically designed to improve the estimation of extreme rainfall frequency based on short datasets from remote sensing sources. In particular we focus on precipitation estimates obtained by merging retrievals from multiple satellite-borne sensors, such as those in the Tropical Rainfall Measuring Mission (TRMM) and the Global Precipitation Measurement (GPM) mission. We describe a downscaling technique that can be used to connect key statistical properties of the rainfall fields obtained from satellite measurements at the daily aggregation time scale to those observed at the rain-gauge scale. The spatial correlation function, the intermittency structure, and the probability distribution function of rainfall intensities are downscaled from the satellite grid scale to the point scale, such that they can be meaningfully compared with the same quantities from observational records at single rain gauges at the ground. The donwscaled quantities are then used to define the Metastatistical Extreme Value distribution, and thus estimate average recurrence intervals of extreme events at the point scale. Here we present the application and testing of this combined methodology, and show that it can improve the estimation of extreme events in locations where long rainfall records at the ground are not available, which is the case for vast poorly gauged areas around the globe. [ABSTRACT FROM AUTHOR]

Precipitation is an important component of the water cycle of High Asia and here particularlycrucial for the glacier mass balance and the seasonal dynamics of snow cover. To successfullymodel (i) past and future response of glaciers to climate forcing and (ii) the development ofsnow cover, reliable precipitation data are needed. However, this is especially difficult in highmountain areas where observational data, particularly at higher and complex terrain, arescarce. Beyond station data, other, spatial explicit data sources for precipitation are (i)simulated rainfall from models and (ii) retrievals from weather satellites. While precipitationpatterns are highly heterogeneous in both, time and space they are difficult to simulatecorrectly. With satellite data on the other hand, the assignment of rates of liquid andespecially solid precipitation is yet uncertain. To assess the differences among theseveral precipitation data sets, we compare selected accessible gridded data setsin High Mountain Asia. We present a comparison of five precipitation products(ERA-Interim, ERA5, HAR, JRA-55 and MERRA-2) and one satellite-based precipitationproduct (IMERG) for the period of April 1st to October 31st, 2014. The study area(80-87˚ E and 28-31˚ N) includes parts of India, a part of the Himalayas and partsof the Tibetan Plateau. The main statistical metrics used for comparison are thecorrelation coefficients r of different data sets and the corresponding coefficients ofdetermination r2 for different methods of temporal and spatial data aggregation. Wefurther compare the data sets to existing observational data available to the public. [ABSTRACT FROM AUTHOR]

Climate change is expected to increase precipitation in northern latitudes. Indeed, global observational data sets show statistically significant trends in precipitation averaged over land between 30°N and 60°N for the period 1901-2008. These changes would cause reduced salinity in the Baltic Sea, a semi-enclosed marginal sea with a large catchment area located in northern Europe. With the help of ocean simulations forced by historical atmospheric and hydrological reconstructions and local observations, we analyze the variability of sea surface salinity (SSS) of the Baltic Sea during 1850-2008. In line with other variables of the Baltic Sea water cycle, also the time series in SSS show low-frequency oscillations with a period of about 30 years. Hence, trends in absolute SSS are not significant on centennial scales. However, we found a statistically significant positive trend in the North-South gradient during 1900-2008 enhancing the sharp horizontal salinity gradient. With the help of sensitivity simulations, this change is attributed to local changes in the river runoff; the total annual river runoff to the Baltic Sea did not change. A detailed analysis of the individual rivers reveal increasing freshwater supply from the northernmost catchment indicating a footprint of the anthropogenic impact on salinity with consequences for the marine ecosystem and species distributions. [ABSTRACT FROM AUTHOR]

While future climate simulations with Regional Climate models are becoming increasingly common, there is increased interest about the effects of climate change on the hydrological cycle at small spatial scales.We present a new hydrological RCP8.5 scenario simulation over Italy using the high resolution CHyM model, driven by the RegCM Regional Climate Model at 12km. The simulation spans the period 1976-2100, with a resolution ranging from 300 to 900m (depending on the region). Future changes both in terms of mean and extreme discharges under the RCP8.5 climate scenario are presented and discussed.Average discharges show a shift towards increased fall and winter flows by the end of the century, especially for the northern regions fed by the Alpine mountain range. The patterns follow changes in the average precipitation provided by the driving RCM.Several flood proxy metrics are analysed, including mean annual peak discharge, peak over threshold events and 100-year return period projected flow (Q100).In particular, the Q100 metric shows a strong increase in future extreme discharge, on average doubling the intensity of such events by the end of the century. Extreme flow changes are uniform across most of the Italian territory, but are particularly marked over the eastern coast of central Italy, where Q100 change values can exceed +200%. [ABSTRACT FROM AUTHOR]

The variability of terrestrial water cycle, i.e., precipitation (P), evapotranspiration (E), runoff(Q) and water storage change (ΔS) is the key to understand the hydro-climateextremes. However, a comprehensive assessment for the partitioning of inter-annualvariability in P into E, Q and ΔS is still not available. In this study, we adopt therecently released global monthly hydrologic product known as the Climate DataRecord (CDR) to conduct an initial investigation of variance partitioning of global Pinto the variances (covariances) of (between) E, Q, ΔS. We also examined thelong-term mean and inter-annual variability in P, E, Q and ΔS, and further relatedthe variability to aridity and evaluated the empirical theory of Koster and Suarez.The empirical Koster and Suarez framework was found to overestimate σE/σPand underestimate σQ/σP. The biases of the empirical framework are shown to becaused by ignoring the variability in ΔS and the covariances between E, Q, ΔS,particularly in semi-arid/humid environments. We also find a great divergence of variancepartitioning under extremely hot and cold conditions. That divergence implies significantimportance of global warming in changing the variability of global water cycle. [ABSTRACT FROM AUTHOR]

Mega lakes in Central Asia have an important role in society as water reservoirs, yet the prediction of their response to ongoing climate change remains uncertain. In the Neogene, mega lakes were able to sustain themselves despite the Plio-Pleistocene aridification trend in Asia. The study of the underlying mechanism to this behavior will likely improve our prediction capabilities for similar, modern mega lakes. In collaboration with geoscientists working on lake sediments of Qaidam Basin, Gaxun Nur Basin and Orog Nuur Basin, we therefore investigate the causes of the persistence of mega lakes in dry climates and identify tipping points leading to their disappearance. Here, we present a first investigation into large scale climatic boundary conditions for regional moisture availability, specifically changes in moisture transport and precipitation, at different times in the Late Cenozoic using the atmospheric general circulation model ECHAM5. High-resolution palaeoclimate simulations (T159L31, ca. 0.8°x0.8° and 31 vertical levels, 6 hour output frequency) for the present-day (PD, control simulation), Mid-Holocene (MH, ca. 6.5 ka), Pleistocene (LGM, ca. 21 ka) and Pliocene (PLIO, ca 3 Ma) are complemented by calculations of vertically integrated moisture transport over the region to study the contribution of large scale climate to the hydrological budget over the regions formerly covered by mega lakes. LGM meridional and zonal moisture transport over the Indian Ocean and India is less than during the PD, MH and PLIO, whereas MH and PLIO zonal moisture transport is similar to PD, and PLIO meridional moisture transport is enhanced in the Arabian Sea, the Bay of Bengal and eastern China. In the regions of the aforementioned basins north of the Tibetan Plateau, modelled MH precipitation and precipitation seasonality are similar to PD conditions, whereas the LGM simulation estimates lower (ca. 8 mm/a) and the PLIO simulation higher (ca. 5 mm/a) precipitation values than PD. Furthermore, precipitation seasonality is weakened in the LGM simulation, which estimates lower precipitation esp. in summer; PLIO precipitation seasonality is enhanced by higher precipitation values esp. in spring and summer. The impact of the weakened and strengthened meridional moisture transports on the hydrological budget in the region covering the three basins will have to be investigated further with the aid of regional climate models, in which the hydrological cycle is better presented. [ABSTRACT FROM AUTHOR]

Water is a key factor in the climate system and it is important to understand development and impacts of changes in the atmospheric water transport. In our study we focus on investigating the source regions for precipitation at 7 high latitude cities (Bergen, Oslo, Stockholm, Helsinki, St. Petersburg, Magadon and Anchorage) in a time period from 1900-2070. For identifying the source regions of precipitation we implemented a new online tracking method into the Lagrangian particle transport FLEXPART. This Method tracks humidity changes of particles released over a target location by following the particles backward in time over a course of 20 days. The humidity is taken from underlying meteorological data, where we account for evaporation (E) if there is a humidity gain and precipitation (P) if there is an humidity loss. All those humidity changes are attributed to the respective grid cell to obtain monthly global E-P fields for each target location (e.g. a city).We run FLEXPART based on the coupled climate reanalysis for the 20th century (CERA-20C) data from European Center for medium range weather forecast (ECMWF) for 1900-2010 and for 2006-2070 we use data from a climate model simulation of Norwegian Earth System Model (NorESM) based on the RCP4.5 scenario. The patterns for E-P for the overlapping period are very similar in the two datasets. We clearly see, that E dominates over marine sources, but during summer the continents gain importance for contributing to precipitation. Most of the stations show a distinct precipitation trend over the 170 years, which is also reflected in intensification of certain areas of E-P. [ABSTRACT FROM AUTHOR]

Oceans are the major source of atmospheric moisture that results in precipitation over land. The moisture originating from the ocean through evaporation, leaves an imprint on sea surface salinity. The variation in sea surface salinity, with its associated atmospheric moisture flux enables us to identify the oceanic source of moisture and its trajectory for precipitation over land. In this study we demonstrate that sea surface salinity is a good indicator for locating the ocean sources of moisture and as an example we use Australian precipitation data during El Niño-Southern Oscillation/Indian Ocean dipole (ENSO/IOD) events. These events are ideal for this application because they provide a large signal-to-noise ratio. The moisture transport during the selected ENSO/IOD events from ocean to land is traced by using atmospheric moisture flux divergence. Composite maps of sea surface temperature, sea surface salinity, moisture flux divergence and Australian precipitation during ENSO/IOD events shows, the oceanic signals in precipitation over land. As ENSO/IOD events evolve, a characteristic sea surface salinity signal also emerges, accompanied by moisture flux divergence and transport. Our results also suggest that the characteristic sea surface salinity signatures occur prior to the peak of ENSO/IOD events and raises the prospect that tracking of sea surface salinity can reveal the source of moisture for terrestrial precipitation. Further, our study (for Australia) along with previous study (for US mid-west) show, during extreme precipitation events, moisture originating from ocean (Indo-Pacific for Australia and north Atlantic for US mid-west) leaves an imprint on sea surface salinity in prior seasons. This suggest that preseason sea surface salinity could also aid the improvement of seasonal prediction of extreme precipitation events across the globe. [ABSTRACT FROM AUTHOR]

The Arctic water cycle is changing dramatically as evidenced by marked shifts in Arctic seaice conditions, atmospheric processes, and hydrological regimes. Understanding thesecomplex interactions across a wide spatial and temporal scale is necessary to assess how thewhole Arctic water cycle is responding to changes today and in the future, including itspotential to feedback into the global climate system. Yet, an empirical and process-basedunderstanding of the interactions between these individual components is currently lacking inthe Arctic and further limits our ability to respond, adapt and develop resilient communitiesin the future. Measurements of oxygen and hydrogen stable isotopes in precipitation (δ18O, δ2H,d-excess) are a valuable and increasingly applied tool for tracing hydrological processes. Weare using a new coordinated network of 20 EU-INTERACT stations across the Arctic tocollect, analyze, and synthesize how precipitation isotope geochemistry varies in space andtime across the north. These new event-based data are combined with observed andmodelled analyses of storm tracks, cyclones, and sea ice traits to determine howisotopic variability is controlled by synoptic-scale atmospheric processes and sea icepatterns. Our goal is to produce weekly maps of precipitation δ18O and δ2H values across theArctic (e.g., "Isoscapes") that will provide a coherent framework to the communityfor understanding both modern and past changes in the Arctic water cycle. Forexample, initial δ18O and δ2H data from summer 2018 show that divergent stormtracks along the west coast of Greenland can lead to distinct isotopic gradients withlatitude. During mid-July we observed a 10 ‰ depletion in δ18O over a 1500 kmlatitudinal span (64˚ N to 76˚ N) at our stations in Nuuk (SW), Disko Island, andThule (NW), from -11‰ -16‰ to -21‰ respectively. This south to north depletingδ18O trend can be reversed within weeks depending upon the prevailing synopticatmospheric circulation pattern. These precipitation isotope fractionation processesalong Greenland’s west coast occur when relatively enriched moisture is transportedinto the Arctic from the North Atlantic; the reverse trend occur when moisture istransported south from the Arctic Basin into the lower latitudes along the west coast ofGreenland. This is the first coordinated network to quantify the spatial patterns of isotopes inprecipitation, simultaneously, across the entire Arctic, and our process-level studies willinform us about atmospheric transport processes within the Arctic, and between the Arcticand mid-latitude regions. [ABSTRACT FROM AUTHOR]

Water isotopes are natural tracer for hydrological cycle and potential indicator for paleoclimate changes in the Indian monsoon region. Yet the understanding of water stable isotope variations in different forms of water and their controlling are important in the regional hydrological cycle and paleoclimate rebuilding as well. The continuous observation of seasonal vapor δ18O at Lhasa, the southern Tibetan Plateau, indicates a combined effect of temperature effect in the broad region in non-monsoon season, and a strong convection precipitation influence in the source region in monsoon season. In summer monsoon season, the high frequency fluctuation of near surface vapor δ18O lasting for a few days, is inversely related to precipitation, and vapor d-excess show corresponding change, reflecting the significant influence of raindrop evaporation. Interannual variations of precipitation δ18O in the Asia monsoon region, is significantly related to the large regional cloud top height, with the underlying mechanism that the higher precipitation in the source region depleted the vapor isotope and hence the subsequent precipitation δ18O region, and therefore related to the ENSO cycle. An ice core drilled in the middle Tibetan Plateau in 2014, preserved an preciously dated interannual δ18O record. The interannual δ18O in the past decades is significantly related to Southern Oscillation Index, rather than the local climate, highlighting the large reginal control of monsoon type precipitation isotope and potentially in isotope record in varies paleoclimate proxies. [ABSTRACT FROM AUTHOR]

The Mediterranean region has been identified as one of the main climate change hotspots: its sensitivity to global change is high and its evolution remains uncertain. The region experiences many interactions and feedbacks at the oceanic, atmospheric, and hydrological levels, while facing high anthropogenic activities. Analysing the water cycle over the Mediterranean region is of major importance to environmental and socio-economic aspects. The satellite monitoring of the Mediterranean water cycle represents one of the key challenges for the hydrological community.The presentation will show recent results on using satellite soil moisture and precipitation products for hydrometeorological prediction in the Mediterranean region, and particularly for the prediction of floods and landslides. Specifically, we will show the comparison of multiple satellite precipitation products for predicting flood in 100+ basins over the Mediterranean Basin by also using different hydrological models. Moreover, the assessment of satellite precipitation products for predicting the occurrence of landslides in Italy is carried out. Among the investigated satellite products, we have firstly considered state-of-the-art products such as TMPA (TRMM Multisatellite Precipitation Analysis), GPM (Global Precipitation Measurement), and H SAF (EUMETSAT Satellite Application Facility on Support to Operational Hydrology and Water Management). Secondly, we have tested the innovative products using the SM2RAIN algorithm for rainfall retrieval from multiple satellite soil moisture products including ASCAT (Advanced SCATterometer), SMOS (Soil Moisture and Ocean Salinity mission) and SMAP (Soil Moisture Active and Passive mission). [ABSTRACT FROM AUTHOR]

Space-borne microwave radiometers can be used to observe global snow precipitation, which plays an important role in energy balance and water cycle. However, lack of knowledge on complex microphysics of snow particles hampers snowfall parameterization in numerical weather prediction and climate models. Dual polarized microwave radiometer could offer specific information on snow micro-physical parameters, compared to traditional microwave radiometers with one-single polarization. Therefore, microwave polarimetry will be beneficial to snowfall detection. This study focuses on polarized signals originating from snow precipitation at millimeter-wave band, in order to gain a deeper insight into snow microphysics. The relationship between observed brightness temperatures and polarized signals (TB-PD) is built up with explanations from radiative transfer simulations. The existing space-borne microwave radiometric observations, including both China meteorological satellite FengYun-3 microwave payloads and Global Precipitation Mission (GPM) Microwave Imager (GMI), will be investigated in this study. The two spaceborne sensors were designed with dual polarized channels at 150 GHz and 166 GHz, respectively. The results presented in this study will provide information for the design of microwave radiometer on-board China second-generation global precipitation mission. [ABSTRACT FROM AUTHOR]

Evapotranspiration, an important process representing a considerable amount of moisture lostto the atmosphere from the soil both by evaporation and by transpiration from the plantsgrowing thereon, is the link between water and energy cycles of the Earth, is the direct effectto local available precipitation. With the global warming, higher temperature is likely tolead to more evaporation, then lead to more precipitation. Tibetan-Plateau, as thethird pole in the world, is improved to be the amplifier of global change, where theclimate changes much sharper than the other regions. It is necessary to make clear theevapotranspiration change with the global warming over TP, and clarify the contribution oflocal evapotranspiration to precipitation. In our study, weather research and forecasting model(WRF) coupled with NOAH land surface model, being supplemented by MODISevapotranspiration remote sensing product, TRMM 3B42 V6 precipitation product andTRACE remote sensing product, is used to illustrate the water and energy cycle over TPconsidering the heterogeneities of elevation, landuse, and landscape, to quantify thecontribution of evapotranspiration to precipitation over entire TP, to explore theeffect of global warming on atmospheric environment over TP region. Based on thelong-term WRF simulation and remote sensing products, the trends of energy cycleelements including net radiation, latent heat flux, sensible heat flux, ground heat flux,and water cycle elements including evapotranspiration, rainfall, snowfall, runoff,soil moisture, ground water are analyzed, and then precipitation recycling over TPis disclosed. The results show that with global warming, land surface is feedingmore moisture to the atmosphere by increased evapotranspiration and a growingrecycling ratio, the contribution of evapotranspiration to local precipitation will bestrengthen. Evapotranspiration yielded by WRF model agrees well with MODISproduct, regional climate models can be considered a powerful complement to scarceobservational networks, especially for forecasting over complex terrain regions. [ABSTRACT FROM AUTHOR]

Precipitation isotopes preserve information about the histories from evaporation in the sourceregions, moisture transport, and to precipitation events, and hence was used in revealing thedynamics of regional hydrological cycle and paleoclimate rebuilding. In the monsoon region,quite a few studies emphasized the strong inverse impact of convective activity onprecipitation isotope ratio, envoke the debate about the interpretation of these ice core recordsas temperature proxies on the Tibetan Plateaus. Recent study also showed statistically theinverse influence of the proportions of stratiform to convective precipitation on water isotopein tropical and mid-latitude regions, highlighting the important role of precipitation type onisotope hydrological cycle. The influence mechanism on seasonal and interannual scaleremain highly uncertain and poorly understood, which is crucial for the utilization ofisotopes in paleoclimate studies. To further address the influence of precipitationpattern, in particular the large regional cloud cover, on water isotopes, here we use adecade of precipitation isotopes from southern Tibetan Plateau (TP) to explore thethe close relationship with both the large scale cloud cover and local climate. Wecalculated the correlation between local precipitation δ18O with different levelcloud data archived in ECMWF. Sensitive zone is identified in the northern Indiasubcontinent to the seasonal precipitation isotope in southern TP, and high negativecorrelation with total cloud cover (TCC) and high cloud cover, indicates high-levelconvection in the upper stream of moisture transport is among some of the importantfactors in controlling the seasonal precipitation isotope in southern TP. However, thecorrelation with local cloud cover in the sampling basin is less significant. Themechanism underlying with the finding is that strong convection activity in themoisture source region and on the route significantly depletes heavy isotopes in thevapor, and subsequently, produces much decreased precipitation δ18O values inthe study region. On the interannual scale, there are robust negative precipitationδ18O-TCC correlations, large positive correlations between TCC and SouthernOscillation Index over the Arabian Sea, where is different from the most significantcorrelative region of the seasonal variation. It indicates that the processes influencethe seasonal variations of the precipitation isotopes is different from those whichare important for the interannualy means. The interannual to decadal interannualprecipitation δ18O record monsoon intensity and upstream rainout resulting from ENSO.The specific study site is located in the Yamdruk-tso basin at an elevation over4400m, and thus the results from this elevation are of more significance for the higherelevation ice core records in that region. Moreover, these findings throw light on theinterpretation of paleoisotope records obtained from the India Summer Monsoonregion. KeywordsPrecipitation isotopes; Cloud cover; Convective activity; Local and regionalcontrol; Yamdruk-tso basin; Southern Tibetan Plateau [ABSTRACT FROM AUTHOR]

The isotopic composition of atmospheric water vapour is influenced by evaporation at the source and cloud processes. Due to the advection of water vapour, it is immediately coupled to the atmospheric circulation, from large-scale weather systems to vertical mixing in the turbulent atmospheric boundary layer. Combined measurements of water vapour and precipitation isotopic compositions can provide valuable insight into local and remote processes affecting the hydrological cycle. Here, we present an analysis of the influence of atmospheric circulation on the isotopic composition of near surface water vapour and precipitation continuously monitored at Bergen in Southern Norway (60.38°N, 5.33°E), located downstream of the North Atlantic storm track, between December 2016 and March 2019. The precipitation was sampled manually per precipitation event on a daily basis on the rooftop platform (54 m a.s.l, 25 m above ground) of the Geophysical Institute at Bergen and analysed for δ¹⁸O, δD, and d-excess on a laser spectrometer (L2140-i, Picarro Inc.). Ambient air was continuously pumped from a heated inlet at the same location to a laser spectrometer (L2130-i, Picarro Inc.). In addition, temperature, relative humidity, wind speed and direction, and precipitation rate were measured at a nearby automatic weather station.The two-year measurement period covered several contrasting seasons. The summer of 2017 was the wettest summer of last 50 years, in contrast to the summer of 2018 which was extremely dry and warm with long heat waves recorded. The winter of 2017/2018 was cold with unusually high snow accumulation, whereas the winter of 2018/2019 was relatively warm with mainly rain events. The strong inter-annual contrasts are associated with clear differences in seasonal circulation patterns, which are reflected by substantial differences in both surface water vapour and precipitation isotopic compositions. Furthermore, synoptic and seasonal variations in the surface water vapour and precipitation isotopic compositions coincide with the shifts in the moisture sources and transport distance, obtained from a Lagrangian moisture source diagnostic. Finally, we discuss the long-term observations at Bergen in the context of measurements from a wider network of water vapour isotopic composition monitoring in the region of the North Atlantic and the Nordic Seas. [ABSTRACT FROM AUTHOR]

Understanding the changing Arctic water cycle has new urgency as sea ice, moisturesources, and precipitation patterns are rapidly shifting due to climate change. InJuly 2018, we installed a Picarro Continuous Water Sampler and isotopic analyzerto a second-order stream in Pallas-Yllästunturi National Park, Finland. This newintegrated system enables us to continuously measure the stable isotope geochemistry(δ18O and δ2H) of stream water in-situ with exceptionally high-frequency (10/min)allowing us to reveal patterns and processes previously out of reach with standardcampaign sampling. Coupling these data with standard measurements of streamwater discharge, temperature, DOC and pH, as well as event-based precipitation andcontinuous real-time water vapor isotope measurements places watershed and regionalisotope hydrology into a new realm of understanding. Initial results indicate thatduring the prolonged mid-summer 2018 drought, mean stream water δ18O andδ2H values were -14 ± 1 ‰ and -99 ± 2 ‰ respectively. These values reflectthe fundamental recharge of the basin being mostly driven by snow melt water(typical snow δ18O ∼ -17 ‰). Our baseline drought measurements allow us topartition and quantify hydrologic threshold for enrichment of stream water isotopes asprecipitation was detected in the system later in fall 2018, and further for depletion asisotope values decreased when moving towards winter. Continuous high-frequencymeasurements of stream isotope geochemistry will provide insight into how water ismobilized in the catchment on different seasons and an assessment of how futurechanges in precipitation patterns and seasonality of flow regimes will affect Arcticwatersheds. These data will further allow more accurate hydrograph separation andprovide the basis to investigate changing watershed scale ecohydrological pathwaysthrough seasons, including in-stream biogeochemical and biological processes. [ABSTRACT FROM AUTHOR]

Precipitation recycling over the Tibetan Plateau (TP) is essential for regional land-atmosphere interactions and hydrological cycle processes. In this study, the characteristic and mechanism of moisture source variation in recent years (1982-2011) are assessed using the quasi-isentropic backward trajectory method and ERA-Interim dataset. The contribution percent of moisture source inside and outside the TP to the total precipitation in 1982-2011 is 39% and 61% respectively. Local evaporative source over the TP, evaporative source from Arabian Sea and over the southeast TP are the sub-regions that with moisture source increasing contribution. Monthly precipitation change for the TP coherence better with the moisture source change outside the TP than inside. Moisture source change inside the TP is mainly because of the latent heat change in 1998-2011 relative to 1982-1997. [ABSTRACT FROM AUTHOR]

The source of moisture is an important part of the hydrological cycle that affects climate system. Potentially, moisture sources are important controls of the isotope composition of precipitation, but their studies in the continental mid- and low-latitudes are still scarce. We identify moisture uptake locations of precipitation over Postojna (Slovenia) for period from 2009 to 2013. By using HYSPLIT trajectory model of NOAA, we did 5-day reconstruction of air mass history for the days with precipitation and determined the moisture uptake locations along back trajectories. Moisture uptake locations were identified along each trajectory using HYSPLIT output data and standard equations for saturation humidity mixing ratio, saturation vapour pressure and specific humidity. Our analysis showed that during studied period around 45% of the precipitation over Postojna originated from Mediterranean and south Atlantic area, with majority of locations originated in the Adriatic Sea. On the other hand, 41% of precipitation originated from moisture recycled over continents, predominantly from Pannonian basin.Multivariable analyses of source regions do not explain any variability of the oxygen isotope composition of precipitation over Postojna. The large proportion of recycled moisture is originated from transpiration rather than evaporation, which produced water vapour with less negative δ18O values. Thus, recycled moisture has and isotope signature undistinctable from the oceanic moisture sources. Climate parameters partly control δ18O values of precipitation, being highly important in paleoclimate studies. In case of Postojna δ18O values of precipitation, surface temperature is the main climate control, whereas amount effect was not recorded, and winter North Atlantic Oscillation (NAO) does not impact the δ18O values of precipitation. We corelated Western Mediterranean Oscillation (WeMO) with oxygen stable isotope composition and found small, but significant correlation. However, multivariate analyses correlation of WeMO and temperature du not explain more variability than temperature alone. Therefore, our interpretation of δ18O values of precipitation in terms of climate is limited to surface temperature, although at least half of the variability observed still depends on unknown controls of the hydrological cycle.Speleothems from Postojna Cave are under investigation and have the potential to record a δ18O signal related to the δ18O values of precipitation. Therefore, these speleothems may be used to reconstruct temperature in the region. [ABSTRACT FROM AUTHOR]

Tree transpiration in humid tropical forests modulates the global water cycle and is a key driver of climate regulation. Yet, our understanding and predictions of how tropical trees regulate water use in response to climate variability remain elusive. With a progressively warming climate, atmospheric evaporative demand (i.e., vapor pressure deficit, VPD) will be increasingly important for plant functioning, becoming the major control of plant water use in the 21st century. Using measurements in 34 tree species at seven sites across a precipitation gradient in the neotropics, we determined how the VPD threshold at which sap flux levels-off at maximum values (VPD-FD) and maximum sap flux (FDmax) vary with precipitation regime (long-term annual precipitation, MAP; seasonal drought intensity, P-DRY) and two functional traits related to foliar and wood economics spectra (leaf mass per area, LMA; wood specific gravity, WSG). We show that, even though VPD-FD and FDmax are highly variable within sites, they follow a negative trend in response to increasing MAP and P-DRY across sites. LMA and WSG exerted little detectable effects on VPD-FD and FDmax, suggesting that these widely-used integrative traits provide limited explanatory power of dynamic plant responses to environmental variation within hyper-diverse tropical forests. This study demonstrates that long-term precipitation and thus, soil moisture, play a critical role in the transpiration response of humid tropical forests to VPD. Moreover, our findings suggest that under predicted higher evaporative demand, trees growing in wetter environments in humid tropical regions may be subjected to reduced water exchange with the atmosphere relative to trees growing in drier climates. [ABSTRACT FROM AUTHOR]

Sub-grid orographic drag directly acts on wind and impacts the regional water cycle through control of atmospheric water vapor (AWV) transport. The effect of turbulent orographic form drag (TOFD) on wind and precipitation is investigated in this study using the WRF model for a winter month over the western Tibetan Plateau (TP), where solid precipitation supplies large amounts of water resources. The diurnal cycle of wind components and atmospheric circulation simulated with TOFD are consistent with observations and ERA-Interim data, whereas stronger westerlies exist in the simulation without the TOFD scheme. The latter results in more zonal AWV transport from the west and more precipitation over the western TP and surroundings. The implementation of the TOFD scheme leads to reduced biases, when evaluated with two observation-based precipitation products. It is therefore concluded that this scheme has a clear dynamical control on the regional atmospheric water recharge and thus the parameterization of the small-scale orographic drag in the model helps to improve the prediction of wintertime precipitation in the western TP region. [ABSTRACT FROM AUTHOR]