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The topography plays an essential role in the initiation and development of precipitating clouds, therefore has a profound effect on the ultimate spatial distribution of precipitation. This study investigates the fine‐scale characteristics of a synoptic‐induced precipitation event in Southwest China, a region characterized by a sequence of steep mountains aligned roughly north‐south. The convection‐permitting simulation successfully reproduces the observed rainband induced by a synoptic‐scale shear line. The spatial distribution of accumulated precipitation over three small‐scale mountains (named M1, M2, and M3 from east to west) exhibits distinct inhomogeneity. The accumulated precipitation is significantly enhanced on the western slope of M1, the high‐altitude area of M2, and the eastern slope of M3. The low‐level vortex generated on the western slope of M1, as well as the convergence established over M2 and the eastern slope of M3, dynamically contributes to the enhanced precipitation over the various mountain locations. As the highest of the three mountains, M2 exhibits pronounced blocking effects on the mesoscale circulations. Additional sensitivity experiment demonstrates that the mesoscale circulations and corresponding precipitation are sensitive to the highest elevation in the continuous mountains. The rainfall accumulation over M2 (M3) could decrease (increase) by 54.3% (63.4%), when the terrain of M2 is reduced to the comparable height of surrounding mountains. With more upslope wind flowing over M2, the convergence over M3 is strengthened. The more intense upward motion and stronger potential instability both contribute to the notable increase of precipitation over M3. This study implies the possible mechanisms of the inhomogeneous precipitation over Southwest China and could deepen the current understanding of the topographic effects on precipitation over complex terrains. Plain Language Summary: Southwest China is characterized by high average elevation with a succession of quasi‐north‐south‐oriented mountains. Such topography with local variance tends to generate inhomogeneous distribution of precipitation. In this study, we explore the spatial characteristics of a typical synoptic‐induced rainfall case that occurs in Southwest China and examine the role of steep topography relief in this rainfall process by using a convection‐permitting model. The simulation reasonably reproduces the distribution and evolution features of the observed rainband. We find significant enhancement of precipitation in the different locations of the small‐scale mountains. The steep topographic relief modulates the distribution of precipitation by exerting dominant influences on local circulations. The results from the sensitivity experiment, in which the terrain of Cang Mountain reduced to the comparable height as the surrounding mountains, further reveal that the highest mountain peaks in this complex terrain region plays a key role in the near‐surface flow patterns and thermal conditions. Consequently, this leads to variations in the enhancement of precipitation over the mountains. Key Points: The precipitation under strong synoptic forcing is significantly enhanced by the steep topography relief of Southwest ChinaThe mesoscale circulations modulated by the steep mountains play an essential role in shaping the distribution of precipitationThe mesoscale circulations and corresponding precipitation are sensitive to the highest elevation in the continuous mountain ranges [ABSTRACT FROM AUTHOR]

As one of the major producers of extreme precipitation, mesoscale convective systems (MCSs) have received much attention. Recently, MCSs over several hotpots, including the Sahel and US Great Plains, have been found to intensify under global warming. However, relevant studies on the East Asian rainband, another MCS hotpot, are scarce. Here, by using a novel rain‐cell tracking algorithm on a high spatiotemporal resolution satellite precipitation product, we show that both the frequency and intensity of MCSs over the East Asian rainband have increased by 21.8% and 9.8% respectively over the past two decades (2000–2021). The more frequent and intense MCSs contribute nearly three quarters to the total precipitation increase. The changes in MCSs are caused by more frequent favorable large‐scale water vapor‐rich environments that are likely to increase under global warming. The increased frequency and intensity of MCSs have profound impacts on the hydroclimate of East Asia, including producing extreme events such as severe flooding. Plain Language Summary: Mesoscale convective systems (MCSs), accounting for more than half of the total rainfall in the East Asian rainband, frequently generate high‐impact extreme weather events, such as flooding. In the summer of 2020, large regions of East Asia suffered extensive flooding and damage. Therefore, understanding the long‐term changes of MCSs is crucial to gain insights into how extreme weather may change in the context of global warming. However, compared to several other MCS hotpots, the investigation of long‐term changes of MCSs is scarce over East Asia. Here, based on a high spatiotemporal resolution satellite precipitation product and a novel MCS tracking method, we find that MCSs have become more frequent and intense in the East Asian rainband and accounted for three quarters of the total rainfall increase during 2000–2021. It is further found that increases in atmospheric total column water vapor, which is mainly due to increased temperature caused by anthropogenic forcing, leads to more frequent large‐scale water vapor‐rich environments that are responsible for the intensification of MCSs. As water vapor increases with global warming, it is very likely that MCSs will continue to intensify in this region into the future. Key Points: Mesoscale convective systems (MCSs) have become more frequent and intense in the East Asian rainband over the past two decadesThe significant increase of MCS precipitation accounted for three quarters of the total rainfall increase during 2000–2021The increase of atmospheric total column water vapor, mainly driven by anthropogenic forcing, leads to more favorable environments for MCSs [ABSTRACT FROM AUTHOR]

Using an iterative rain‐cell tracking method to identify and track mesoscale convective system (MCS) precipitation during the warm seasons (May–October) of 2015–2020, we reveal the seasonal variation in the spatiotemporal distribution, rainfall characteristics, and the life cycle of MCS precipitation in different months over North China. Notable MCS precipitation centers are located around mountain slopes and downstream areas. The diurnal peak of MCS rainfall is manifested as a southeastern delay from afternoon in mountainous regions to midnight or morning over downstream plains. In May and June, MCS rainfall is distributed on the hillside of Taihang–Yan mountains and the Shandong Peninsula. MCS precipitation gets an early evening peak on mountain slopes and a nocturnal maximum over the North China Plain (NCP). In July and August, the MCS activities extend to NCP, and another MCS precipitation center exists over the Luliang Mountains in western North China, under the seasonal march of the monsoon flow. The local diurnal peaks to the east of 114°E are generally later than in May and June, and MCS precipitation gets a peak during midnight to early morning over the NCP. MCSs in July and August feature the strongest intensity. In September and October, the MCS precipitation shifts to high terrain of the Taihang Mountains and its west, with an early afternoon peak. MCSs have the largest size and the fastest propagation speed in September and October, but with the weakest intensity. Compared with short‐lived MCSs, long‐lived MCSs have stronger precipitation intensity, larger rainfall area, and faster propagation speed. In addition, the life cycle of long‐lived MCS precipitation represents asymmetric development and decay. Long‐lived MCSs occur with more pronounced upper troposphere warming and low‐level anomalous southwesterlies. Meanwhile, higher specific humidity in the low‐ and mid‐level troposphere provides a more favorable environment for the maintenance of MCS precipitation. [ABSTRACT FROM AUTHOR]

Spring precipitation over the southeastern Tibetan Plateau (SETP) produces more than 34% of annual precipitation, which is comparable to summer precipitation. This pre‐monsoon rainfall phenomenon, influenced synthetically by atmospheric circulations and topography, makes the SETP an exception to its surroundings. Here, fine‐scale characteristics and typical synoptic backgrounds of this unique phenomenon have been investigated. The spring precipitation over the SETP is characterized by high frequency at hourly scale, with a single diurnal peak at night or early morning. Event‐based analysis further demonstrates that the spring precipitation is dominated by long‐lasting nocturnal rainfall events. From early to late spring, the dominant synoptic factor evolves from terrain‐perpendicular low‐level winds to atmospheric moisture, influencing the spatial heterogeneity and fine characteristics of the spring precipitation. The westerly dominated type, featured by lower geopotential height over the TP and enhanced westerlies along the Himalayas, produces limited‐area precipitation at those stations located at topography perpendicular to low‐level winds. In contrast, the moisture‐dominated type is featured by an anomalous cyclone over the Bay of Bengal and induces widespread precipitation around the SETP, which is the leading contributor to the spring precipitation there. Accumulated precipitation amount of long‐lasting nocturnal events is the largest under moisture‐dominated type, which has a large portion of weak precipitation due to weak thermal instability. Findings revealed in this study complete the picture of spring precipitation influenced by different dominant synoptic factors over the SETP, which deepen the current understanding of the joint influence of circulation and topography on the hydrological cycle of complex terrains. Plain Language Summary: The precipitation over the Tibetan Plateau (TP) is generally considered to be concentrated in summer. However, the southeastern TP (SETP) is an exception, where the spring precipitation is larger or comparable compared to summer precipitation. In this study, we analyzed the fine‐scale characteristics and typical synoptic backgrounds of this unique phenomenon. The spring precipitation over the SETP is featured by frequent nighttime rainfall events lasting more than 3 hr. In early spring (March), low‐level winds uplifted by topography produce the precipitation over the SETP, while atmospheric water vapor plays a more important role in late spring (May). Atmospheric backgrounds and complex terrain jointly influence the fine‐scale characteristics and the range of the spring precipitation over the SETP. Our analysis provides a more completed picture of the spring precipitation over the SETP, which typically reflects the joint influence of atmospheric circulations and complex terrain. Key Points: Spring rainfall over the southeastern Tibetan Plateau (SETP) is characterized by frequent nighttime events of long durationDominant synoptic factors evolve from low‐level wind in early spring to moisture in late spring, continuously producing rainfall over SETPSynoptic factors and complex terrain jointly shape fine features and spatial heterogeneity of the spring rainfall over the SETP [ABSTRACT FROM AUTHOR]

Besides the respective importance of the frequency, intensity and diurnal cycle of precipitation, the diurnal cycle of intensity–frequency structure could reveal more physical details about the evolution process of precipitation systems. Here, an objective method to quantitatively depict the diurnal cycle of intensity–frequency structure of precipitation is proposed, and applied to complex terrains of the southeastern Tibetan Plateau (TP) and its surroundings. Results show the diurnal cycle of precipitation over the high terrain has a unimodal night peak, while low terrain with a frequency‐dominated night peak and an intensity‐dominated afternoon peak. Both weak and intense precipitation are more frequent at night than those in the afternoon over unimodal regions compared to bimodal regions. Further analysis reveals different diurnal evolution process of intensity–frequency structure within unimodal and bimodal regions, respectively, which reflects different spatiotemporal traits and evolution process of precipitation. Among unimodal regions, the elevated southeastern TP is featured by the synchronous decrease in both weak and intense precipitation from the night peak to the afternoon minimum. While the decrease of total precipitation in regions with lower terrain is dominated by the decrease of intense precipitation, but with an increase in weak precipitation simultaneously. Regarding bimodal regions, the intense precipitation over southern Hengduan Mountains has a more prominent role in leading the diurnal cycle of precipitation amount, compared with that over the southeastern Yungui Plateau. The proposed method provides a new perspective with more physical details to investigate precipitation characteristics in complex terrains, which could be further used to reasonably evaluate and improve the performance of numerical models. [ABSTRACT FROM AUTHOR]

Compared to singular hot or wet extremes, their occurrence in close sequence might cause larger impacts, because the initial hazard either increases exposure/vulnerability of affected communities to the next or physically worsens the second hazard. Heatwave‐preconditioned hourly precipitation extremes in the Yangtze River Valley are typical of the latter situation. Fueled by soaring convective energy after heatwaves' passage, hourly precipitation extremes are heavier, intensifying faster, and more concentrated during afternoon to evening, compared to those not proceeded by heatwaves. Consequently, flash flood‐producing events (e.g., 50 mm/hr or stronger) account for larger fractions in preconditioned events than in the non‐preconditioned group, with the spectrum difference further exaggerated by warming. This drives the frequency increase in potentially high‐impact events several‐fold larger than expected from the widely‐adopted univariate perspective. These results point to heatwave‐preconditioned hourly precipitation extremes as an emerging hazard of climate change carrying greater flash flood risks. Plain Language Summary: Despite plentiful knowledge on heatwaves' impacts on human health, little has been known about heatwaves' implication for extreme rainfall and the resulting floods. After experiencing sustained sweltering weather, people are sometimes eager for cooling brought about by heavy precipitation; however, subsequent rainfall, if intensified by the heat into an exceptional level, introduces uncharted risks of flash floods. Hourly precipitation extremes in the Yangtze River Valley are a case in point. Within 3 days after the passage of a heatwave, hourly precipitation extremes in the region are more torrential than those not preceded by heatwaves. These heatwave‐preconditioned events are also intensifying faster with climate warming. In this context, heavier events, such as those stronger than 50 mm/hr, are more prevalent in preconditioned events than in the non‐preconditioned group. Namely, once an hourly precipitation extreme occurs in the wake of a heatwave, the chance of it evolving into a flood‐causing one is higher. So, to better evaluate risks of flash floods, a more holistic view is needed, by not only considering precipitation itself but also factoring into its temporal proximity to heatwaves. Key Points: Rigorously sticking to the impact‐centric principle, heatwave‐preconditioned hourly precipitation extremes are defined and analyzedIn the Yangtze River Valley, heatwave‐preconditioned hourly wet extremes are heavier and intensifying faster than non‐preconditioned onesPreconditioned and non‐preconditioned hourly precipitation extremes differ not only in intensity spectrum but also in diurnal cycle [ABSTRACT FROM AUTHOR]

The atmospheric water cycle over the Hengduan Mountains (HM) is crucial to the local and downstream ecosystem and social economy, while its characteristics and variability have not been thoroughly revealed yet. Based on a bulk method, the atmospheric water cycle over the HM and its difference between wet and dry summers is analysed in this study. Results show that the summer precipitation recycling ratio over the HM is 11.5% in climatology and is significantly and negatively correlated with precipitation there on interannual timescale. In wet summers, the precipitation recycling ratio decreases by 13% compared to dry summers. The origin of water vapour for precipitation over the HM is mainly contributed by the external water vapour transport (local evaporation) in wet (dry) summers. August is found to have the strongest climatological mean precipitation recycling ratio (15.2%) and atmospheric water cycle among 3 months in summer. Also in August, the difference in precipitation recycling ratio between wet and dry summers is largest. In wet summers, although the increase in the external moisture inflow is the largest in June (3.0 × 107 kg·s−1), the increase of precipitation is the largest in August (2.6 × 107 kg·s−1) due to its stronger atmospheric water cycle. Our study promotes the knowledge of the atmospheric water cycle over the HM and provides a new perspective to understand the interannual variations of the precipitation over the HM. [ABSTRACT FROM AUTHOR]

Based on 10 pairs of atmospheric general circulation models from the Coupled Model Intercomparison Project Phase 6 High Resolution Model Intercomparison Project, we find that high‐resolution models (HRMs, ∼50 km) perform better than low‐resolution models (LRMs, ∼100 km) in simulating the East Asian summer monsoon (EASM) rain belt. We attribute the increased rainfall to enhanced vertical advection of water vapor in the HRMs which is mainly contributed by enhanced vertical motion. Based on moist static energy analyses, the stronger vertical motion in the HRMSs is mainly balanced by enhanced net energy flux into the atmosphere column in the "Meiyu‐Baiu" (MB) region due to the increase of air‐sea turbulent latent heat fluxes along the Kuroshio and sensible heat fluxes over land. The increased net energy flux in the HRMs drives stronger ascending motion, resulting in more realistic rainfall in the MB region. Higher saturated specific humidity determined by finer sea surface temperature, less near‐surface specific humidity along the Kuroshio, and stronger near‐surface winds on western side of the western North Pacific Subtropical High strengthened by the enhanced pressure gradient together contribute to higher ventilation efficiencies and enhanced ocean forcing along the Kuroshio, leading to more turbulent latent heat fluxes in the HRMs. Additionally, drier and less cloudy atmosphere in the HRMs result in more downward shortwave radiation heating over the land, which partly contributes to the increased sensible heat flux. This understanding of the sensitivity of simulated MB to model resolution has implications for precipitation simulation and prediction over East Asia. Plain Language Summary: Improving the Meiyu‐Baiu simulation is a long‐term challenge for the climate‐modeling community. One way to achieve such improvement is to develop high‐resolution global climate models, since there are many mesoscale systems embedded in the large‐scale monsoon circulation that need small grid spacing to be resolved. By analyzing ten pairs of models, we show that the high‐resolution models (HRMs) generally show better performance in Meiyu‐Baiu simulations than the low‐resolution models (LRMs), with a reduction in bias of 31%. The increased air‐sea latent heat flux in the HRMs is a dominant energy source driving increased Meiyu‐Baiu rainfall. Stronger near‐surface winds and drier air in the HRMs favor increased evaporation, resulting in a more turbulent latent heat flux. Hence, we explain the processes that can help improve Meiyu‐Baiu simulation when improving the horizontal resolution from ∼100 to ∼50 km in General climate models. Key Points: High‐resolution models (∼50 km) perform better than low‐resolution models (∼100 km) in simulating Meiyu‐BaiuThis added value is mainly attributed to the enhanced air‐sea turbulent latent heat fluxes along KuroshioStronger near‐surface wind and drier air in high‐resolution model favor the more turbulent latent heat fluxes [ABSTRACT FROM AUTHOR]

The Tibetan Plateau (TP) is known as Asian Water Tower and its atmospheric water cycle has been a lasting challenge to climate modeling community. Here, we compare two sets of the Met Office Unified Model simulations—one is a convection‐parameterized version (large‐scale model; LSM) and the other is a convection‐permitting model (CPM) simulation. The added value of the CPM in terms of atmospheric water cycle process is analyzed, including external moisture transport, fraction of atmospheric water vapor converting to precipitation and the precipitation recycle ratio. Results show that the simulated TP precipitation and evaporation for the summer of 2009 is significantly improved in the CPM. First, the overestimation of atmospheric water cycle by LSM is improved in CPM due to a reasonable representation of the fraction of atmospheric water vapor converting to precipitation. The overestimation of precipitation recycle ratio also indicates the LSM generates excessive convection compared to the CPM and therefore has a larger wet bias over the TP. Second, a better simulation of local precipitation has feedback on the circulation. Compared with the LSM, the less moisture convergence in the CPM is dominated by the stronger outflow through the eastern edge of the TP rather than the weaker inflow, implying the upscale effects of the resolved moist convection on the moisture transport over the TP. Our results imply that the CPM is a useful tool in the reproduction of moisture transport and atmospheric water cycle process over the Asian Water Tower and other regions of the world with complex topography. Plain Language Summary: The atmospheric water cycle over the Tibetan Plateau (TP, also as known as the Asian Water Tower) has been a challenge to climate models with convection parameterization. Benefiting from the high enough resolution, convection‐permitting model (CPM) can explicitly resolve the deep convection and therefore reduce the bias related with the convection parameterization scheme. Here, we compare a CPM with its low resolution version that employs convection‐parameterization in simulating atmospheric water cycle process over the TP. We find the precipitation and evaporation over the TP is significantly improved in the CPM. There is more water vapor flowing out of the TP and less water vapor in the atmosphere converting into the precipitation in the CPM, resulting in a reduced wet bias. CPM is free from the constrains of the convection parameterization scheme and therefore depicts the atmospheric water cycle over the TP better. Key Points: The convection‐permitting model (CPM) better reproduces precipitation over the Tibetan Plateau than convection‐parameterized model (large‐scale model [LSM])The overestimation of atmospheric water cycle by LSM is improved in CPM simulationThe interaction between large‐scale circulation and convection further reduces moisture convergence and wet bias in the CPM [ABSTRACT FROM AUTHOR]

The Tibetan Plateau (TP) plays an essential role in influencing the global climate, and precipitation is one of its most important water‐cycle components. However, accurately simulating precipitation over the TP is a long‐standing challenge. In this study, a convection‐permitting model (CPM; with 4 km grid spacing) that covers the entire TP was conducted and compared to two mesoscale models (MSMs; with model horizontal resolutions of 13 and 35 km) over the course of a summer. The results showed that the two MSMs have notable wet biases over the TP and can overestimate the summer precipitation by more than 4.0 mm·day−1 in some parts of the Three Rivers Source region. Moreover, both MSMs have more frequent light rainfall; increasing horizontal resolution of the MSMs alone does not reduce the excessive precipitation. Further investigation reveals that the MSMs have a spurious early‐afternoon rainfall peak, which can be linked to a strong dependence on convective available potential energy (CAPE) that dominates the wet biases. Herein, we highlight that the sensitivity of CAPE to surface temperatures may cause the MSMs to have a spurious hydrological response to surface warming. Users of climate projections should be aware of this potential model uncertainty when investigating future hydrological changes over the TP. In comparison, the CPM removes the spurious afternoon rainfall and thus significantly reduces the wet bias simulated by the MSMs. In addition, the CPM also better depicts the precipitation frequency and intensity, and is therefore a promising tool for dynamic downscaling over the TP. [ABSTRACT FROM AUTHOR]

Changes in hydrological cycle under 1.5°C and 2.0°C warming are of great concern on the post‐Paris Agreement agenda. In particular, the annual range of precipitation, that is, the difference between the wet and dry seasons, is important to society and ecosystem. This study examines the changes in precipitation annual range using the Community Earth System Model low‐warming (CESM‐LW) experiment, designed to assess climate change at stabilized 1.5°C and 2.0°C warming levels. To reflect the exact annual range in different regions, wet and dry seasons are defined for each grid point and year. Based on this metric, the precipitation annual range would increase by 3.90% (5.27%) under 1.5°C (2.0°C) warming. The additional 0.5°C of warming would increase annual range of precipitation by 1.37%. The enhancement is seen globally, except in some regions around the subtropics. Under the additional 0.5°C of warming, a significant increase in the annual range occurs over 15% (22%) of the ocean (land) regions. The increase is associated with the enhanced precipitation during wet season. Moisture budget analysis shows that the enhancement in annual range is dominated by vertical moisture advection, which includes thermodynamic (TH, moisture) and climate dynamic (CD, circulation changes) terms. The TH term plays a dominant role, while the CD term partly offsets the effects of the TH term. The TH term dominates over most regions except for part of the tropical ocean and some of the land regions, where the CD term is also remarkable. Thus, the enhancement of the annual range of precipitation is mainly caused by the increase in moisture. Plain Language Summary: The Paris Agreement proposed a target to limit global warming to less than 2°C and pursue efforts to limit warming to less than 1.5°C. Since then, great effort has been devoted to exploring the impacts of the 1.5°C and 2.0°C warming scenarios. The changes in the seasonal cycles of precipitation have important impacts on natural and human systems but are unknown under 1.5°C and 2°C warming levels. We focus on the changes in the annual range of precipitation, which represents seasonal cycle and is defined as the difference between wet and dry seasonal rainfall in which the wet and dry seasons vary spatially and temporally. We project the changes by using CESM low‐warming experiments. In the experiments, the multiyear global mean surface temperatures will stabilize at 1.5°C and 2.0°C above the preindustrial level by the end of the 21st century. We find significant increases in seasonal cycle over both ocean and land. The increase is dominated by the enhancement of precipitation during wet season. The increased seasonal cycle is caused by the increase in water vapor over most regions except for part of the tropical ocean and some of the land regions, where the effects of circulation changes are remarkable. Key Points: The precipitation annual range, that is, the difference between wet and dry seasons, would increase by 3.90(5.27)% globally at 1.5(2)°C scenariosUnder the 2.0°C warming scenario, over 22% of the landmass will receive more precipitation in the wet season than that under the 1.5°C warmingThe increase in the precipitation annual range is due to the vertical moisture advection dominated by moisture increase with global warming [ABSTRACT FROM AUTHOR]

During the period from June 30th to July 6th, 2016, a heavy rainfall event affected the middle and lower reaches of the Yangtze River valley in eastern China. The event was characterized by high‐intensity, long‐duration (lasted more than 6 days) precipitation and huge amounts (over 600.0 mm) of rainfall. The rainfall moved eastward from the Sichuan basin to the middle Yangtze River valley during the first 2 days, then Mei‐yu front formed and circulations became more "quasi‐stationary". During the second‐phase, successive heavy rainfall systems occurred repeatedly over the same areas along the front, leading to widespread and catastrophic flooding. In this study, limited‐area convection‐permitting models (CPMs) covering all of eastern China, and global‐model simulations from the Met Office Unified Model are compared to investigate the added values of CPMs on the veracity of short‐range predictions of the heavy rainfall event. The results show that all the models can successfully simulate the accumulated amount and the evolution of this heavy rainfall event. However, the global model produces too much light rainfall (10.0 mm/day), fails to simulate the small‐scale features of both atmospheric circulations and precipitation, and tends to generate steady heavy rainfall over mountainous region. Afternoon precipitation is also excessively suppressed in global model. By comparison, the CPMs add some value in reproducing the spatial distribution of precipitation, the smaller‐scale disturbances within the rain‐bands, the diurnal cycle of precipitation and also reduce the spurious topographical rainfall, although there is a tendency for heavy rainfall to be too intense in CPMs. [ABSTRACT FROM AUTHOR]