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This paper examines the empirical relationship between the Madden–Julian oscillation (MJO), the quasi‐biennial oscillation (QBO), and atmospheric river (AR) activity and precipitation in California on subseasonal time scales. We introduce an experimental forecast tool that uses observed anomaly patterns during a 38 yr period to predict the probability of above‐ and below‐normal AR activity and precipitation at lead times of 1–6 weeks based on the phase and amplitude of the MJO and QBO. The hindcast prediction skill of probabilistic AR activity and precipitation forecasts is evaluated for Northern, Central, and Southern California, as well as two sets of smaller geographical domains. These smaller domains are more relevant for water resource management and allow us to investigate the sensitivity of prediction skill to domain size. Consistent with previous studies, our results demonstrate that subseasonal AR activity and precipitation in California are strongly modulated by the MJO and QBO. The anomaly patterns of AR activity and precipitation vary considerably throughout the cool season, with a tendency toward below‐normal AR activity and precipitation during easterly QBO and above‐normal AR activity and precipitation during westerly QBO in JFM. The opposite patterns are generally observed in OND, but the anomaly signals are weaker and less coherent for AR activity. Certain combinations of MJO phase, QBO phase, lag time, and season yield notably higher skill scores, reinforcing the notion of "windows of opportunity" for skillful subseasonal‐to‐seasonal predictions. In California, these forecasts of opportunity are predominantly associated with easterly QBO in JFM and FMA. Plain Language Summary: This paper examines the relationship between the Madden–Julian oscillation (MJO), the quasi‐biennial oscillation (QBO), and atmospheric river (AR) activity and precipitation in California. We introduce an experimental forecast tool that shows the likelihood of above‐normal and below‐normal AR activity and precipitation based on the phases of the MJO and QBO. Consistent with previous studies, our results demonstrate that AR activity and precipitation in California are strongly influenced by the MJO and the QBO. There is a tendency for below‐normal AR activity and precipitation during easterly QBO conditions in January–March. The opposite pattern (above‐normal AR activity and precipitation) generally occurs during westerly QBO conditions. The results also suggest that our forecast tool has some potential to improve the prediction of AR activity and precipitation. The reliability and usefulness of this forecast tool depend on multiple factors, including the MJO phase, the QBO phase, and the time of year. In California, this forecast tool is likely to provide the most beneficial value during easterly QBO conditions in mid‐winter through early spring. Key Points: The modulation of atmospheric river (AR) activity and precipitation in California by the Madden‐Julian oscillation and quasi‐biennial oscillation is quantifiedA hindcast skill assessment of probabilistic AR activity and precipitation forecasts is presentedAn experimental subseasonal AR activity and precipitation forecast tool is introduced [ABSTRACT FROM AUTHOR]

The Western North Atlantic Ocean (WNAO) is a complex land‐ocean‐atmosphere system that experiences a broad range of atmospheric phenomena, which in turn drive unique aerosol transport pathways, cloud morphologies, and boundary layer variability. This work, Part 2 of a 2‐part paper series, provides an overview of the atmospheric circulation, boundary layer variability, three‐dimensional cloud structure, and precipitation over the WNAO; the companion paper (Part 1) focused on chemical characterization of aerosols, gases, and wet deposition. Seasonal changes in atmospheric circulation and sea surface temperature explain a clear transition in cloud morphologies from small shallow cumulus clouds, convective clouds, and tropical storms in summer, to stratus/stratocumulus and multilayer cloud systems associated with winter storms. Synoptic variability in cloud fields is estimated using satellite‐based weather states, and the role of postfrontal conditions (cold‐air outbreaks) in the development of stratiform clouds is further analyzed. Precipitation is persistent over the ocean, with a regional peak over the Gulf Stream path, where offshore sea surface temperature gradients are large and surface fluxes reach a regional peak. Satellite data show a clear annual cycle in cloud droplet number concentration with maxima (minima) along the coast in winter (summer), suggesting a marked annual cycle in aerosol‐cloud interactions. Compared with satellite cloud retrievals, four climate models qualitatively reproduce the annual cycle in cloud cover and liquid water path, but with large discrepancies across models, especially in the extratropics. The paper concludes with a summary of outstanding issues and recommendations for future work. Key Points: Atmospheric circulation and sea surface temperature drive large seasonal changes in precipitation, surface fluxes, and cloud typesSynoptic activity in winter yields the highest seasonal rain rates, low‐cloud occurrence, and cloud droplet number concentrationsClimate models simulate a wide range of low‐cloud properties, with improved results for models with more sophisticated turbulence schemes [ABSTRACT FROM AUTHOR]

Providing skilful predictions of high‐impact weather up to 2 weeks ahead is on the agenda of international weather centers. Evaluating the capabilities of current numerical systems in predicting past events can bring extremely valuable contributions to the assessment of the information content available today with operational models. In the framework of the activities for the fiftieth anniversary of the extreme precipitation event that occurred in Italy in November 1966, this paper investigates its predictability using state‐of‐the‐art global and regional ensemble simulations. The first goal is to assess if and how many days in advance, this event can be predicted by current European Centre for Medium‐Range Weather Forecasts (ECMWF) global ensembles. A second goal is to evaluate the potential added value of running nested higher‐resolution and convection‐permitting ensembles. It is shown that ECMWF ensembles are able to provide valuable information up to 3 days before the event. Within this forecast range, convection‐permitting simulations can provide more accurate estimations of precipitation maxima. However, the results indicate also a strong underestimation of rainfall amounts with both global and regional models even at short forecast range. To partially explain this shortcoming, we discuss how the scarcity of observations available in 1966 for the analysis process limits the quality of the ensemble initial conditions and we adopt a method to obtain more reliable ensemble forecasts. The paper concludes with a comparison with previous similar works; results indicate a gain in predictability of up to 12 hr with respect to numerical revisitations performed to mark the fortieth anniversary of the event. Key Points: Current state‐of‐the‐art global ensembles are able to provide valuable information up to 3 days before the Arno River flooding event (Italy, November 1966)Convection‐permitting ensembles provide more accurate estimations of precipitation maximaAdvances in numerical modeling occurred in the last 10 years, determine a gain in predictability of up to 12 hr with respect to similar works [ABSTRACT FROM AUTHOR]

This paper focuses on the interdecadal variation in drought in Northeast China (NEC) in recent decades. Two dry and wet periods have occurred in the past 61 years (1950–2010). The mechanism underlying the interdecadal variation in drought in NEC is further analyzed; the results indicate that the direct cause is the interdecadal change of the Okhotsk High (OH). A strong (weak) OH is conducive (detrimental) to the convergence of cold air transported by northeast cold vortex and water vapor from the east of NEC, eventually leading to an increase (decrease) in precipitation. Furthermore, we seek to understand why OH displays such an interdecadal variation. The interdecadal increases and decreases in sea ice in the Barents Sea and the Kara Sea explain most of the interdecadal change of the OH. When more sea ice is present in this area, more energy is transferred eastward from the sea ice area to Northeast Asia, weakening the OH; in contrast, less sea ice strengthens the OH. Sensitivity simulations by Community Atmosphere Model, version 4 (CAM4) using sea ice forcing in the Barents Sea and the Kara Sea yield similar results to the observed data, further confirming our conclusion. The results of this paper provide a new understanding of the interdecadal variation in drought in NEC, and it may help to improve our ability to predict drought and provide early warning in the future. Key Points: The drought over Northeast China has significant interdecadal variability during the past decadesThe Okhotsk High acts as the direct factor causing interdecadal changes in NEC drought by affecting atmospheric circulationThe interdecadal increases and decreases in sea ice in the Barents Sea and the Kara Sea can modulate the Okhotsk High changes [ABSTRACT FROM AUTHOR]

Coupled ocean‐atmosphere models still suffer from systematic biases in simulating the sea‐surface temperature (SST) in the equatorial Pacific. Like in many current generations of climate models, the Chinese Academy of Sciences (CAS) Earth System Model (CAS‐ESM2‐0) simulated warmer SST in the eastern Pacific and colder SST in the central to western Pacific at the equator. We analyzed the physical processes causing these biases. We find that model biases in surface heat flux are a consequence, not the cause, of the SST bias. Ocean currents are implicated as the cause. The biases in ocean currents come from both the ocean model itself and the bias in surface winds in the atmospheric model. We show that the bias of ocean currents in the standalone ocean model causes warming in the region of positive precipitation bias in the standalone atmospheric model. When the two models are coupled, the precipitation bias is greatly amplified. Anomalous equatorial winds as atmospheric Rossby‐gravity wave response to precipitation induce additional bias in ocean currents that transport heat in the upper ocean. The cold equatorial SST bias in the central and western Pacific is found to be due to temperature advection by the anomalous zonal currents, while the warm bias in the eastern Pacific is due to advection by the anomalous zonal and meridional currents as well as downwelling. Results from the analysis offer insights into the causes of the SST biases in coupled models in the equatorial Pacific. Plain Language Summary: The systematic bias of simulated sea‐surface temperature (SST) in the equatorial Pacific is a well‐known problem in coupled ocean‐atmosphere models. This paper uses CAS‐ESM2‐0 as a case study to investigate such a problem. We found that the SST bias in this model is caused by ocean dynamics rather than the surface heat flux. The bias in ocean currents comes from both the standalone ocean model and amplification by bias in the atmospheric model. The precipitation bias in the atmospheric model causes the Rossby‐gravity wave response of the surface wind to force additional bias in ocean currents. The cold equatorial SST bias in the central and western Pacific in CAS‐ESM2‐0 is found to be due to temperature advection by the anomalous zonal currents, while the warm bias in the eastern Pacific is due to advection by the anomalous zonal and meridional currents as well as downwelling. Key Points: The annual equatorial Pacific SST bias in CAS‐ESM2‐0 is caused by ocean dynamics rather than surface heat fluxBias in ocean currents is contributed by both the standalone ocean model and atmospheric surface windsBias in atmospheric precipitation impacts surface winds as equatorial Rossby‐gravity wave response to latent heating [ABSTRACT FROM AUTHOR]

The melting layer of precipitation has a major impact on remote sensing and telecommunications. However, there is a shortage of observational studies to validate and constrain the melting layer models especially for high‐frequency radar bands. In this paper, we report how multifrequency radar Doppler spectra can be used to retrieve the melting layer attenuation at Ka‐ and W‐bands. The presented analysis is based on identifying Rayleigh scattering regions in radar Doppler spectra measurements where dual‐wavelength spectral ratios can be related to differential attenuation. We show that the estimated attenuation at Ka‐ and W‐bands agrees reasonably well with previously reported studies, but there are indications of differences at higher rain rates. We advocate that this technique can be applied to long‐term observations to advance our knowledge of the melting process. The parameterizations of melting layer attenuation as a function of rain rate and radar reflectivity are also presented. Plain Language Summary: While the melting layer is a relatively narrow layer in precipitation systems, it has a significant impact on telecommunication and remote sensing applications. For spaceborne and ground‐based radar measurements, unaccounted attenuation in the melting layer may cause significant errors in retrievals of rainfall rate and ice cloud properties, respectively. There are two approaches to quantify attenuation in the melting layer, namely, through modeling or observations. Modeling studies have been carried out and are reported in the scientific literature; however, it is acknowledged that there is a shortage of observational studies that could help us to constrain the models. In this paper, we present a study that utilizes radar Doppler spectra recorded at X‐, Ka‐, and W‐bands to retrieve melting layer attenuation. We show that the estimated attenuation at Ka‐ and W‐bands agrees reasonably well with previous studies, but there are indications of differences at higher rain rates. With the utilization of this technique, long‐term observations are expected to advance our knowledge of the melting process, as well as modeling of the melting layer attenuation for applications in millimeter‐wavelength radars, passive microwave remote sensing, 5G/6G commercial links, and space‐Earth telecommunications. Key Points: Ka‐/W‐band melting layer attenuation is derivedThe melting layer attenuation is derived by analyzing X‐, Ka‐, and W‐band radar Doppler spectraPresented results are in good agreement with previously reported melting layer modeling results for rain rates smaller than 3 mm/hr [ABSTRACT FROM AUTHOR]

As the largest mode of coupled climate variability, the El Niño Southern Oscillation (ENSO) carries consequences for weather patterns worldwide. Because of its impacts, and the subsequent importance of predicting when ENSO might occur, there has been lengthy research into precursor mechanisms that initiate ENSO events. In this paper, thanks to the length of the SODAsi.3 data set, we study the relation between ENSO and a subset of known precursors over 140 years (1871–2011). We uncover that the influence of North Pacific Oscillation (NPO)‐related precursors—namely the Trade Wind Charging and North Pacific Meridional Mode (TWC/NPMM)—upon ENSO is nonstationary. The TWC/NPMM‐ENSO coupling is strong from 1871 to 1920, then weakens before regaining significance from 1960 onward. Importantly, in the intervening period between 1920 and 1960, not only does the TWC/NPMM‐ENSO connection disappear, there are also no other wind‐related drivers preceding ENSO events during this period. We find that in the absence of wind‐driven precursors during this intervening period the temporal characteristics of ENSO variability itself change, as the signal oscillates within a relatively narrow 6–7‐year periodicity band. These features set this intervening period apart from what we see during the first and last periods when the ENSO signal is noisier, and its power is distributed over a wider range of periodicities spanning from 2 to 6 years. These results lead us to hypothesize that, during the last 140 years, ENSO shifted between a stochastically forced interannual mode of variability, to a multiannual, quasi‐regular one with a self‐sustained oscillation. Key Points: North Pacific Oscillation (NPO)‐related wind‐stress anomalies are the predominant stochastic precursor of ENSO over the last 140 yearsENSO's response to NPO‐induced precursors is nonstationary and weakens considerably during the mid‐20th centuryThe nonstationarity in ENSO's response is mirrored by a change in ENSO's frequency and oscillatory behavior [ABSTRACT FROM AUTHOR]

This first paper of the two‐part series focuses on demonstrating the accuracy of a hyper‐resolution, offline terrestrial modeling system used for the High Mountain Asia (HMA) region. To this end, this study systematically evaluates four sets of model simulations at point scale, basin scale, and domain scale obtained from different spatial resolutions including 0.01° (∼1‐km) and 0.25° (∼25‐km). The assessment is conducted via comparisons against ground‐based observations and satellite‐derived reference products. The key variables of interest include surface net shortwave radiation, surface net longwave radiation, skin temperature, near‐surface soil temperature, snow depth, snow water equivalent, and total runoff. In the evaluation against ground‐based measurements, the superiority of the 0.01° estimates are mostly demonstrated across relatively complex terrain. Specifically, hyper‐resolution modeling improves the skill in meteorological forcing estimates (except precipitation) by 9% relative to coarse‐resolution estimates. The model forced by downscaled forcings in its entirety yields the highest skill in model output states as well as precipitation, which improves the skill obtained by coarse‐resolution estimates by 7%. These findings, on one hand, corroborate the importance of employing the hyper‐resolution versus coarse‐resolution modeling in areas characterized by complex terrain. On the other hand, by evaluating four sets of model simulations forced with different precipitation products, this study emphasizes the importance of accurate hyper‐resolution precipitation products to drive model simulations. Key Points: The skill of a hyper‐resolution, off‐line terrestrial modeling system used for the High Mountain Asia region is presentedThe study emphasizes the importance of using hyper‐resolution versus coarse‐resolution modeling in areas characterized by complex terrainThe study emphasizes the importance of an accurate hyper‐resolution precipitation product used to drive model simulations [ABSTRACT FROM AUTHOR]

This paper is the first to document the development of a type of bow echo, termed merger‐formation bow echo (MFBE), in southeast China evolving from a subtropical squall line (SL) merging with a preline convective cell (CC). Although this MFBE did not produce damaging surface winds, its intense rain rate resulted in local flooding. The evolution of the kinematic, thermodynamic, and microphysical structures is investigated using the variational Doppler radar analysis system (VDRAS) analysis and polarimetric radar observations. Key factors of this MFBE event including the rear‐inflow jet (RIJ) and cold pool exhibited different characteristics from those in classical bow echoes and limited MFBE cases. As the SL propagated towards the coast, a CC was triggered along a sea breeze front. The SL did possess a RIJ, but a bow‐shaped reflectivity did not appear until the SL‐CC merger. The RIJ weakened and became elevated during the merger, while the leading edge of SL cold pool accompanied by a weak diverging outflow did not advance with the reflectivity field. The updraft was strengthened due to the merger, resulting in enhanced precipitation falling ahead of the original SL. The subcloud evaporation locally cooled the air ahead of the SL and merged with the original SL cold pool. This combined cold pool advanced forward rapidly and caught up with the bowing radar reflectivity to form this MFBE. This study illustrates the processes of a SL‐CC merger leading to the formation of a different type of MFBE that did not produce damaging surface winds. Key Points: The first VDRAS analysis combines polarimetric radar to document microphysical and dynamic processes of a merger‐formation bow echo in ChinaRear‐inflow jet weakens during the formation of this nonclassical bow echoEvaporative cooling from intensified rainfall leads to the surge of cold pool [ABSTRACT FROM AUTHOR]

Following the description of the basic climatic characteristics of Meiyu in East Asia, we present a comprehensive review of multi–time scale variabilities of the Meiyu together with the long‐term change of extreme precipitation during the Meiyu in this paper. The dynamic prediction skills of the Meiyu on subseasonal to seasonal time scales are further described and illustrated by using present numerical prediction models. For the multi–time scale variabilities of the East Asian Meiyu, they are closely related with the impacts of both quasi‐biweekly and 30–60 day oscillation from both the tropics and the middle and high latitudes on intraseasonal time scale, the influences of El Niño–Southern Oscillation and the Tropospheric Biennial Oscillation on interannual time scale, the variations of East Asian summer monsoon on decadal time scale, and the continued impacts of global warming, Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation on interdecadal scale and long‐term trend. Under the background of the global warming and the influences of urbanization and aerosol effects, the Meiyu shows some specific characteristics including significantly weakened continuous rain, uneven spatial distribution of precipitation, and increased (decreased) number of strong (weak) precipitation days. Improvements of prediction on rain belt location, total precipitation amount, persistent heavy precipitation events, and low‐frequency variability oscillation during Meiyu season have been achieved with the improved dynamic models of the Beijing Climate Center since 2010. Key Points: Meiyu's variabilities are related with intraseasonal oscillation, interannual to multidecadal oscillation of ocean, and global warmingMeiyu impacted by global warming, urbanization, and aerosol effects exhibits stronger rainfall eventsImprovements of prediction of East Asian Meiyu on multi‐time scale have been achieved with the present operational climate models [ABSTRACT FROM AUTHOR]

This paper investigates the performance of 10 Regional Climate Models (RCMs) hindcasts from the Coordinated Regional Climate Downscaling Experiments (CORDEX) over Central Africa, covering the period 1998–2008 and performed over a common model grid spacing 0.44° (∼50 km). Multiple observational data sets are used to evaluate model performances over four targeted subregions. Throughout the work, a measure of observational uncertainty is made and we discuss whether or not the models are found within or outside the range of observational uncertainty. Results indicate that RCMs generally capture rainfall and temperature basic features, though important biases exist and vary for models and seasons. Dry (wet) biases are common features over the Congo basin (northern and southern part of the domain). In terms of precipitation and temperature in both seasonal and annual scale, most RCMs along with their ensemble mean generally fall in the range of observational uncertainty. Furthermore, most RCMs show a good spread of grid points where the added value of RCMs is found although the added value in temperature is not as great as with precipitation. UC‐WRF is among models adding less value on ERAINT and this could explain why whatever the time scale of variability, UC‐WRF outputs are generally out from the observational uncertainty. The multimodel ensemble mean is generally found within observational range when most models are there as well. This highlights the fact that the ensemble mean, built from the equal treatment of RCMs, does not generally outperform individual RCMs realization as it is reported in several previous studies. Key Points: Most RCMs along with their ensemble mean generally fall in the range of observational uncertaintyThe ensemble mean of models is generally found within the observational range when most models are there as wellThe ensemble mean of models does not generally outperform individual RCMs realization as it is reported in several previous studies [ABSTRACT FROM AUTHOR]

Southern African tropical lows are synoptic‐scale cyclonic vortices that propagate westward across southern Africa in the Austral summer. They strongly influence local rainfall and aggregate in the climatological December, January, and February mean to form the Angola Low. In this study, tropical lows are identified and tracked using an objective feature tracking method. The statistics of tropical low tracks over southern Africa are presented and compared across three reanalysis products. Findings are compared to the literature of tropical low‐pressure areas elsewhere in the world, where it is found that most tracking statistics compare well but that the tendency of tropical lows to become semistationary over Angola is unique to southern Africa. The hypothesis that tropical lows in Angola have a causal relationship with Tropical Temperate Troughs is tested, and a correlation between occurrence frequencies is found at interannual but not daily time scales. Precipitation is attributed to the tropical lows, and it is found that tropical lows are associated with 31% of rainfall across tropical southern Africa, based on gridded precipitation products. The interannual variability of the number of tropical lows that form per year (σ=6 events/year) is linked to El Niño–Southern Oscillation (ENSO) and the tropical easterly jet. The mean latitude of tropical lows is shifted northward during El Niño and southward during La Niña. Much of the interannual precipitation variability maximum in Angola is attributed to rainfall associated with tropical lows. These results provide insights into the southern African response to ENSO and into the mechanisms of rainfall in the southern African tropical edge. Plain Language Summary: Tropical lows are storms that are a bit like very weak tropical cyclones. They occur in summer in tropical regions. This paper tracks individual tropical lows across southern Africa in weather records from the last 39 years. We then look at how they behave, how much they rain, whether they interact with other storms, and how they vary from year to year. About 33 tropical lows are found each year, and they tend to cluster in eastern Angola. We find that 31% of rainfall in the region surrounding Angola and Zambia comes from tropical lows. The number of tropical lows that form in a summer is really important for whether that summer will be unusually dry or wet, particularly along the northern border of Namibia. The number of tropical lows that form in each year is related to the El Niño–Southern Oscillation and also to the winds high up in the atmosphere. In the El Niño phase of El Niño–Southern Oscillation cycle, which usually causes drought in southern Africa, tropical lows form less frequently and further north. In the La Niña phase, which is usually wet in southern Africa, tropical lows form more frequently and further south. Key Points: Vorticity tracking and rainfall attribution algorithms are applied to reanalysis tropical lows in southern AfricaTropical lows track north (south) in El Niño (La Niña); this behavior dominates the ENSO rainfall signal west of 30°EOn a synoptic time scale tropical temperate troughs and Angola tropical lows have statistically independent timing [ABSTRACT FROM AUTHOR]

The interaction between sand particles and sand bed is a key part for the study of wind‐blown sand. The splash functions obtained from experimental observations are of great significance to help us understand relevant physical processes. Due to the adoption of substitute material in experiment and immoderate assumptions in numerical simulation, the results of previous studies are still debatable. This paper experimentally studies on splash functions of natural eolian sand particles by using a self‐developed particle emission system to simulate the splash process. Based on the high‐speed photography technique, the distribution of the angle, speed, and number of the liftoff particles and its quantitative characterization are obtained. The mathematical expectations of relevant variables from this work are comparable with existing results but are quantitatively different from the measured data of substitute particles or the numerical simulated results. More importantly, the detailed statistical characteristics of the natural eolian sand splash function are also extracted from our experimental data, which are really significant for elaborative wind‐blown sand study in the future. Key Points: A particle emission system is designed and employed to simulate the splash process of natural eolian sand particlesA total of 1,024 collision events are investigated, and the pertinent splash functions are obtainedThe results indicate that the splash functions of natural sand particles are different from those of substitute materials [ABSTRACT FROM AUTHOR]

Active observations from profiling instruments such as cloud radar or lidar have proved invaluable in improving our understanding of clouds and their interaction with the atmosphere. However, while they undoubtedly contain a wealth of information about the atmospheric state, the assimilation of such observations into NWP models has so far had mixed success. One reason is the mismatch of scales between the relatively coarse model resolution and the narrow beam‐width of the instruments. To obtain an optimal analysis, this representativity error due to this mismatch in scales must be included within the observation error covariance matrix. In this paper, we demonstrate how a relatively straightforward approach that uses the local variance of the measurement combined with a global estimate of correlation provides a useful estimate of the representativity error. We also show how the method can be used to estimate the correlation in representativity error between observations in both the horizontal and vertical, potentially removing the need to thin the observations, or inflate their observation error. The method is evaluated using 2‐D and 3‐D fields of simulated and observed radar reflectivity before an example application to profiling observations is shown using Cloudsat data. Key Points: Cloud profiling observations can have large sampling errors relative to NWP model gridboxesSampling errors can be predicted using along‐track variability and climatological correlationCorrelations in sampling error between vertical levels can be significant [ABSTRACT FROM AUTHOR]

Vb cyclones typically emerge in the Western Mediterranean and propagate to the Northeast into Central Europe. This paper explores the temporal characteristics of Vb cyclone occurrence based on cyclone tracks identified at the atmospheric levels of Z700 and sea level pressure, using JRA‐55 reanalysis data for the period 1959–2015. The risk of Vb occurrence was significantly high in the 1960s and has remained at a lower level since then. Vb cyclones do not occur fully randomly according to a Poisson point process. Eleven well‐separated and distinct clusters as well as 11 hiatus periods are identified, with average occurrence rates of 21.5 and 5.2 yrea−1, respectively. During the event of Vb, the large‐scale atmospheric circulation is changed into a state favoring the development of successive Vb cyclones. Clustering is very prominent in the case of Genoan Vb cyclones in summer as well as those Vb cyclones developing over the Iberian Peninsula or the North African Coast in winter. Superposition of the polar and the subtropical jet stream over the Western Mediterranean is identified as a main feature at the onset of Vb cyclones. Vb cyclone occurrence appears to be synchronized with the Northern Atlantic Oscillation (NAO; at Z500) and Arctic Oscillation (AO; at Z1000). Clusters have occurred when both NAO and AO were negative. This relation applies to Western Mediterranean cyclones not following a Vb track as well, however to a much weaker extent. In contrast, Vb cyclone frequency was particularly low from 1988 to 1997 during a sustained positive phase of both NAO and AO. Key Points: In the past 50 years, the frequency of Vb cyclones was high in the 1960s and lower since thenVb cyclones were strongly synchronized with NAO and AO; high frequency clusters occurred when NAO and AO were negativeThe coupling of the polar and the subtropical jet stream over the Western Mediterranean is a driving mechanism of the onset of Vb cyclones [ABSTRACT FROM AUTHOR]

Midlevel Cloud‐base Turbulence (MCT) caused by ice/snow precipitation from midlevel clouds falling into dry air below the cloud base and sublimating is investigated. MCT phenomena in the lower troposphere as revealed by the Middle and Upper atmosphere (MU) radar in Shigaraki, Japan, during the Shigaraki unmanned aerial vehicle and radar Experiment campaigns in the spring‐summer of 2015 and 2016 are described. The MU radar was operated in a high‐resolution (~20‐m) range imaging mode and hence revealed the structure of these MCTs in great detail. These MCT layers grew to hundreds of meters in thickness, often reaching nearly 2,000 m in depth, lasting often for as much as 24 hr. A simple analytical model and a second‐moment closure‐based turbulent mixing model are used to estimate the levels of turbulence kinetic energy and its dissipation rate in the MCT layer and compare them to those measured by the MU radar. The study shows that MCT can give rise to moderate levels of turbulence of potential interest to aviation, confirming conclusions reached by earlier pioneering studies. However, to our knowledge, this important process has not received the attention it deserves by the atmospheric community so far, and therefore, it is the principal goal of this paper to not only present some observations and preliminary modeling results, but also redraw the attention of atmospheric scientists to this important process. Plain Language Summary: The free atmosphere above the planetary boundary layer is stably stratified, and therefore, turbulent mixing there is shear driven and occurs generally under the influence of stable density stratification. However, convective mixing can also occur there under certain conditions. When precipitating snow/ice particles fall below the cloud base of midlevel clouds, they can sublimate if the air column below the cloud is dry enough. The resulting cooling of the air column drives vigorous convection. This midlevel cloud‐base turbulence process is the subject of this article. Key Points: Midlevel cloud‐base turbulence observed by the MU radar in Shigaraki, Japan, during 2015 and 2016 is presented to show its ubiquitous natureA simple analytical model is presented for approximate estimation of turbulence parameters inside a MCT layerA one‐dimensional second‐moment closure‐based turbulent mixing model is applied to an MCT event observed during 2015 and results compared to observations [ABSTRACT FROM AUTHOR]

Prolific winter (December‐January‐February) snowfall occurs over northwest Japan due to frequent sea‐effect precipitation that develops during cold‐air outbreaks over the Sea of Japan (SOJ). Knowledge of sea‐effect clouds and precipitation across the SOJ region has historically been constrained, however, by limited offshore in situ observations and remote‐sensing limitations. This paper uses sensors from National Aeronautics and Space Administration (NASA)'s A‐Train Satellite Constellation to examine winter sea‐effect properties in the SOJ region. The analysis shows that cloud and precipitation occurrence generally increases across the SOJ from Asia to Japan, as potential sea‐effect periods with an along‐orbit mean sea surface to 850‐hPa temperature difference ≥13 °C comprise a majority of the total clouds and precipitation. Sea‐effect clouds and precipitation occur most frequently in an arc‐shaped area that extends from the western SOJ, where the Japan‐Sea Polar‐Airmass Convergence Zone (JPCZ) is common, to the coast of Honshu, and then northward to Hokkaido. Radar, lidar, and column water path statistics along A‐Train orbital tracks show that sea‐effect precipitation is deepest along the central Honshu coast and becomes shallower but more frequent with northward extent. Precipitation amount and frequency maximize along the coast and adjacent mountains but decline with inland extent, most abruptly downstream of higher mountain barriers. This work illustrates that air‐sea interactions, coastal geometry, and regional topography strongly modulate cloud and precipitation patterns during sea‐effect periods in the SOJ region. Key Points: The majority of winter clouds and precipitation over the Sea of Japan region occur during sea‐effect periodsThe distribution of sea effect varies in the region and is modulated by land‐sea interactions, coastline geometry, and orographic effectsSea-effect precipitation is shallower and more common (deeper and less common) in the northern (southern and central) Sea of Japan region [ABSTRACT FROM AUTHOR]

The tropospheric water vapor profile record from the Atmospheric Infrared Sounder (AIRS) now spans over a decade, making it a valuable resource for climate studies. To be considered as a Climate Data Record, it is key that the ultimate performance of these observations are understood. The Global Energy and Water cycle Exchanges (GEWEX) water vapor assessment has been tasked with characterizing the current state of the art in water vapor products currently available for climate analysis. Within the scope of this exercise, water vapor profiles from AIRS have been assessed using collocated characterized in situ measurements of tropospheric water vapor between 2007 and 2012. We first show how previously published methods for correcting radiosondes can be applied to global records, which show high correlations to Global Climate Observing System Reference Upper‐Air Network (GRUAN) performance at pressures between the surface and 250 hPa. We go further and show the first comparison of uncertainties from both the newly created Characterized Radiosonde Measurement (CRM) and GRUAN data sets. Global estimates of AIRS water vapor profile (wet/dry) biases relative to GRUAN and CRM are within 6 ± 0.3% ppmv and 15 ± 0.1% ppmv below 300 hPa, respectively. The CRM record allows latitudinal analysis for the first time, which when examined shows sensitivity to changes in absolute concentration due to large‐scale circulation in the International Tropical Convergence Zone. This paper advances the use of state‐of‐the‐art in situ records for characterizing absolute performance, recognizing that long‐term stability needs further research. Key Points: Altitudes below 250‐hPa corrected operational radiosondes show similar uncertainty performance to GRUANAIRS tropospheric water vapor biases relative to GRUAN are within 6 ± 0.3% ppmv below 300 hPaEstimated collocation uncertainty in the Northern Hemisphere shown to reduce to below 1% ppmv for calculated yearly biases [ABSTRACT FROM AUTHOR]

This paper presents a multimodel assessment of fidelity in representation of the diurnal cycle of precipitation over the Maritime Continent (MC). Daily mean precipitation rate and amplitude and phase of the diurnal cycle are utilized to validate General Circulation Model (GCM) performance with respect to multiyear annual mean and seasonal cycle. The analysis shows that models' 6‐hourly temporal resolution and 2.5° spatial resolution is sufficient to depict key characteristic of diurnal precipitation. Results show that most models are sensitive to the existence of the MC and show different characteristics of subdaily precipitation. However, 19 out of 20 models underestimate the daily mean precipitation over the eastern Indian Ocean and most of them underestimate both daily mean precipitation and amplitude of the diurnal cycle, especially over islands within the region. Observations show that within the MC the diurnal cycle is higher over land than over ocean. Many models decrease this land‐sea contrast. Most models recognize difference in phase of the diurnal cycle between MC islands and surrounding ocean, but some of them show opposite phasing of the diurnal cycle. Results show that modern models perform better than past generation, sometimes comparably to regional models' run in much higher resolution. Although models perform well with respect to the seasonal cycle of the daily mean precipitation, they fail to realistically represent the seasonal evolution of the diurnal cycle amplitude. Additionally, models show no coherence in land‐ocean contrast derived from a multiyear average of daily mean precipitation and amplitude of the diurnal cycle. Key Points: GCMs are sensitive to the existence of the Maritime Continent in the Indo‐Pacific warm poolMost models underestimate daily mean precipitation and amplitude of the diurnal cycle, especially over landMost GCMs underestimate land‐sea contrast in precipitation characteristics [ABSTRACT FROM AUTHOR]

Analyses of the thermodynamics of precipitating clouds are mostly based on localized in situ campaigns or, more globally, weather analyses and reanalyses. This work presents a comparison of weather analyses to satellite observations and a radiometeorological method that shows how precipitating layers inside clouds coincide with weather analyses underestimates of the amount of water vapor. The thermodynamic information from inside precipitating clouds is extracted using observed radio occultation (RO) refractivity profiles without requiring weather analysis input, thus reducing analysis‐induced biases. The radiometeorological method is described by first identifying the differences between adiabatically dry, mixing‐ratio conserving, and saturated pseudoadiabatic refractivity profiles. These reference profiles are then compared to observed RO refractivity profiles within precipitating and nonprecipitating clouds to infer changes in their height‐dependent thermodynamic states and in stability to convection. Precipitation is found to start below layers close to pseudoadiabatic and precipitation layers coincide with changes into conditional stability against convection. A statistical comparison between observed profiles and the gradients predicted for a saturated pseudoadiabatic profile is made and finds that on the global average, precipitation separates clouds from the Clausius‐Clapeyron law and profiles are close to a saturated pseudoadiabat. The results (a) help constrain the physical processes associated to precipitation inside clouds and (b) validate the potential of graphical RO techniques to analyze observations without ancillary temperature data from weather analyses. Plain Language Summary: Predicting how precipitation responds to changes in atmospheric thermal profiles remains a challenge identified by the Intergovernmental Panel on Climate Change. Solving this challenge requires comparing models with the observed thermodynamics of precipitating clouds. Only radio occultation (RO) can constraint globally the thermodynamics inside clouds with high vertical resolution. Infrared and microwave can measure the thermodynamics outside and only partially clouds, and radar instruments that measure inside clouds do not provide thermodynamic information.This paper develops a method to use RO for distinguishing the thermodynamics inside precipitating versus nonprecipitating clouds. The identification of rain is made using coincidences of RO soundings with Tropical Rainfall Measuring Mission and CloudSat observations. Using the method, we show that globally, (1) the European Center for Medium‐Range Weather Forecast ERA Interim reanalysis and the Global Forecast System weather analysis have a global bias in precipitating clouds caused by misrepresentation of the thermodynamics inside clouds; (2) the nature of the bias is associated to a misrepresentation of the lapse rates by the analyses; (3) there are changes in stability to moist convection associated to the height where precipitation originates, and (4) the transition from midtropospheric lapse rates to boundary layer shows more frequent sharp inversions in nonprecipitating than in precipitating scenes. Key Points: Model‐independent analyses of radio occultation, RO, are developed to constrain lapse rates and stability inside cloudsRO observes how precipitation changes layerwise the lapse rates that define stability to convection in ways missed by weather analysesTop of boundary layer inversions leading to refractivity gradients is sharper in nonprecipitating than precipitating scenes [ABSTRACT FROM AUTHOR]

Enhancement of precipitation processes aloft over complex terrain is documented using reflectivity data from an S‐band scanning radar (NPOL) that was deployed on the west coast of Washington State during the Olympic Mountains Experiment (OLYMPEX). From November 2015 through mid‐January 2016, NPOL obtained high‐resolution data within sectors over the ocean and over the windward slopes of the Olympic Mountains. Contoured Frequency by Altitude Diagrams of radar reflectivity highlight a higher frequency of occurrence of larger reflectivities for all heights between 2 and 8 km over land compared to ocean, with the largest difference in the 4‐ to 6‐km range indicating a robust signature of enhancement aloft over the windward slopes. This enhancement pattern is found to some degree under all environmental conditions considered but is especially pronounced during periods of high vapor transport, high melting level height, southwest low‐level winds, and neutral stability. These conditions are generally associated with warm sectors of midlatitude cyclones and atmospheric rivers. Past studies have postulated that a secondary enhancement in reflectivity aloft was an intrinsic part of atmospheric river type systems. However, these results show that further significant enhancement of this signature occurs as deep moist‐neutral, high water vapor content flow is lifted when it encounters a mountain range. Reflectivity data from the dual‐precipitation radar aboard the Global Precipitation Measurement satellite also documents this reflectivity increase aloft over the Olympic Mountains compared to the adjacent ocean, showing the potential for Global Precipitation Measurement to provide reliable estimates of precipitation structure over remote mountainous regions. Plain Language Summary: When frontal cyclones pass over a mountain range, modification of flow often results in precipitation enhancement on the windward slopes. Most studies of this phenomenon rely on the precipitation patterns deduced from surface networks of rain gauges or through numerical modeling. Little attention has been given to orographic modification of the precipitation processes occurring in the middle and upper layers of clouds. This paper uses high‐resolution vertical cross sections of radar reflectivity data taken during the Olympic Mountains Experiment. Measurements made over the ocean are compared to those over the windward slopes of the Olympic Mountains and a clear signature of enhancement aloft over the windward slopes is found at all heights from 2 to 8 km strongest between 4 and 6 km. This enhancement of reflectivity aloft is especially pronounced under environmental conditions of large water vapor transport, high melting level, strong onshore‐directed low‐level winds and neutral stability. Those environmental conditions are commonly found in warm sectors of midlatitude cyclones and in atmospheric river‐type events. This same signature of higher reflectivity aloft over windward slopes is found from satellite‐derived radar reflectivity measurements implying that satellites have the potential to provide reliable estimates of precipitation structure over remote mountainous regions. Key Points: Enhancement of precipitation processes is found in radar reflectivity data above the melting level over complex terrainEnhancement aloft is especially pronounced during periods of high vapor transport, onshore flow, and neutral low‐level static stabilitySatellite‐borne radar detects this enhancement so that reliable estimates of precipitation over remote mountain regions are possible [ABSTRACT FROM AUTHOR]

This paper presents the results of airborne pollen and spore trapping in Nanjing city, eastern China, using a Burkard pollen trap during two consecutive years (2013–2014). A total of 103 pollen and spore taxa were identified. Two concentration peaks are observed in the annual cycle, a spring peak dominated by arboreal pollen types (Morus, Cupressaceae, Pinus, Pterocarya, and Quercus) and a fall peak dominated by upland herbs (Compositae, Poaceae, Humulus, and Cruciferae). Wetland herbs and ferns dominate summer assemblages and winter assemblages are characterized by sporadic records of Artemisia, Chenopodiaceae, and Pinus. Strong year‐to‐year differences in measured pollen concentration are seen, probably in response to interyear differences in weather. Compared to long‐term means, 2013 was comparatively hot and dry and 2014 had a higher than average number of rain days during the flowering periods. Rising temperatures in early spring are connected with the timing of flowering and therefore pollen release, while rainfall during the flowering period appeared to remove pollen from the air, leading to lower recorded pollen concentration values. Four taxa, Cupressaceae, Quercus, Pinus, and Humulus, were considered in more detail. Each has a different pattern of variation in pollen concentration between the studied years. Cross correlation between pollen concentration and daily temperature, relative humidity, and precipitation at lags from 0 to −30 days also showed different responses for each taxon, suggesting that pollen signal responses to weather conditions have to be considered at a taxon level rather than at the assemblage level. Key Points: Two pollen concentration peaks were recorded, a spring peak dominated by arboreal pollen types and a fall peak dominated by upland herbsPollen concentration values were markedly different between the two years, probably reflecting differences in weatherPollen signal responses to weather conditions have to be considered at a taxon level rather than at the assemblage level [ABSTRACT FROM AUTHOR]

Afforestation is an appropriate approach to control soil erosion, but consumes extra water resources. It also exerts feedbacks on local precipitation, affecting the water availability for water‐limited areas. The Chinese Loess Plateau (LP) has implemented the world's largest afforestation program to mitigate severe soil erosion. However, it remains unclear how the atmospheric water cycle responds to large‐scale afforestation under climate change. Here we show that afforestation elevates precipitation in rainy season by intensifying the atmospheric water cycle over the LP. Results show the upward trend of vegetation coverage fraction increased by 3.15% decade−1 and climatological precipitation increased by 54.62 mm after launching the large‐scale afforestation in 1999. Such afforestation also enhanced evapotranspiration by 22.56 mm. We found the moisture inflow and outflow continuously decreased, but more atmospheric moisture was retained since the rate of decline of inflow was less than that of outflow. Using a dynamic precipitation recycling method, we explored changes in precipitation recycling and components of the atmospheric water budget. The precipitation recycling ratio increased from 9.16 ± 0.35 to 10.18 ± 0.28% after the afforestation. Both external and internal branches of the atmospheric water cycle have been strengthened, and their contributions to the increased precipitation are 84.93% and 15.07%, respectively. The afforestation caused a more humid atmosphere, heightened soil moisture‐vegetation‐precipitation feedback, stronger moisture convergence, and more clouds; these changes favored precipitation generation. This study advances the understanding of how afforestation affects regional climate from a precipitation recycling perspective, further assisting decision‐makers to establish sustainable afforestation strategies. Key Points: More atmospheric moisture is gained by the Loess Plateau after large‐scale afforestationPrecipitation recycling ratio increases by 1.02% and more advected and evaporated moisture is converted to precipitation after afforestationContributions of external and internal branches of the atmospheric water cycle to increased precipitation are 84.93% and 15.07%, respectively [ABSTRACT FROM AUTHOR]

The relationships between the El Niño‐Southern Oscillation (ENSO), Asian‐Pacific region summer precipitation and its corresponding atmospheric circulations exhibit interdecadal changes. From the early 1990s to the early 2000s, these relationships were significant. Therefore, we divided this time frame into three periods for analysis: 1979–1991 (P1), 1992–2005 (P2), and 2006–2019 (P3). The possible mechanism underlying these relationships is as follows: the southern Indian Ocean sea surface temperature (SST) over the east of Madagascar exhibits interdecadal variation. During P2, the SST was relatively cold, which induced an anomalously high mascarene high (MH). The strengthened MH enhanced the Somali jet and Indian monsoon westerlies, which intensified the strong center of the Indian Ocean Walker circulation. Therefore, owing to the strength and location changes of the Indian Walker circulation and the Walker circulation, the connection between the ENSO and the western Pacific vertical motions strengthened, resulting in the close relationship of the ENSO with the atmospheric circulation over the Asian‐Pacific region. Hence, ENSO can influence a north‐south tripole pattern of precipitation over the Asian‐Pacific monsoon region through local vertical activities and meridional Hadley circulation over western Pacific. Numerical experiments using an atmospheric general circulation model, with prescribed three times southern Indian Ocean SST anomalies of 1995–2005 relative to 1979–2019, also lend support to the southern Indian Ocean SST's contribution to modulating the relationship between ENSO and summer precipitation over the Asian‐Pacific monsoon region. Key Points: The linkages of the El Niño‐Southern Oscillation (ENSO), Asian‐Pacific summer precipitation and its corresponding atmospheric circulations exhibit interdecadal changesThe southern Indian Ocean sea surface temperature (SST) is the key role to modulate the relationship between ENSO and Asian‐Pacific climateThe extratropical SST of the southern Indian Ocean can also modulate the relationship between ENSO and Asian‐Pacific climate [ABSTRACT FROM AUTHOR]

Paleoclimatic records reveal spatially asynchronous changes of East Asian summer monsoon (EASM) precipitation during the Holocene, but the underlying mechanisms are not fully understood. This study explores the role of the zonal shift of the Asian westerly jet (AWJ) core on the spatial‐temporal variability of EASM precipitation based on Holocene climate simulations using the KCM and CESM forced by orbital variations. We analyzed the composite differences of summer precipitation and associated atmospheric circulations between the early (middle) and middle (late) Holocene. Simulations suggest a dipole pattern of summer precipitation with increasing (decreasing) precipitation in central (southern) China from the early to late Holocene, which was shaped by the eastward shift of the AWJ core. The eastward shift of the AWJ core and the accompanied eastward movement of the upper‐level convergence over the AWJ's right exit region weakened the low‐level downward motions over central China. Meanwhile, the low‐level upward motions over southern China were weakened by the concurrent eastward (westward) migration of the South Asian High (western Pacific subtropical High) during the Holocene. Consequently, summer precipitation increased (decreased) in central (southern) China from the early to late Holocene. The eastward shift of the AWJ core was primarily driven by amplified meridional insolation gradient at middle latitudes via strengthening upper‐level westerly intensity during the Holocene. In addition, land cover difference between East Asia and central Asia caused a greater enhancement of westerlies intensity in East Asia, further modulating the eastward movement of the AWJ core during the Holocene. Key Points: Our simulations reveal a dipole pattern of summer precipitation between central and southern China during the HoloceneThe dipole precipitation pattern was strongly modulated by zonal shift of the Asian westerly jet coreInsolation variation and land cover caused the eastward shift of the Asian westerly jet core during the Holocene [ABSTRACT FROM AUTHOR]

The hydroclimate of the Tibetan Plateau (TP) and Central Asia (CA) plays a crucial role in sustaining surface water reservoirs and thus water resources in the respective regions. In this study, we investigate the changes in Asian hydroclimate and its driving forces during specific time intervals in the last 3 Ma. We conduct high‐resolution (∼0.75° per grid cell) general circulation model ECHAM‐5 experiments with boundary conditions for the mid‐Pliocene (∼3 Ma), the Last Glacial Maximum (LGM; ∼21 ka), the mid‐Holocene (∼6 ka), and the pre‐industrial. Results suggest that seasonally relatively high precipitation rates (>1 mm day−1) were longer in the mid‐Pliocene and shorter in the LGM, relative to the pre‐industrial. We calculate different monsoon indices to detect changes in the intensity, strength and duration of the East Asian summer monsoon (EASM), South Asian summer monsoon (SASM), and the Indian summer monsoon (ISM), and construct climatologies of mid‐latitude high‐level westerly jet (WJ) stream occurrences based on the ECHAM5 wind fields. Our results suggest that in warm periods (e.g., mid‐Pliocene or interglacial), the WJ migrates northward earlier in the year (April) and reaches higher latitudes than in the pre‐industrial, resulting in a wetter TP and CA. During cooler periods (e.g., LGM or glacial), the WJ migrates northward later in the year (June) and remains over lower latitudes, resulting in a drier TP and CA. Increased/decreased local precipitation in TP and CA for the mid‐Pliocene/LGM experiments correlates strongly with (a) intensity, strength and duration of the EASM, SASM, and the ISM and (b) WJ latitudinal position. Key Points: High‐resolution general circulation model (ECHAM5) is used to study past changes in hydroclimate in the Tibetan Plateau and Central AsiaSeasons with high precipitation rates were longer in the mid‐Pliocene and shorter in the Last Glacial Maximum compared to the pre‐industrialLate Cenozoic hydroclimate of the region is linked to the dynamics of the mid‐latitude jet stream and the Indo‐Asian monsoon circulation [ABSTRACT FROM AUTHOR]

A 1‐year blended surface precipitation data set using Parsivel disdrometer, surface W‐band radar, and tipping bucket measurements is produced for the Macquarie Island Cloud and Radiation Experiment (MICRE) and compared with retrievals from CloudSat (spaceborne 94 GHz radar). Surface precipitation was observed 44% ± 4% of the time between April 2016 and March 2017. Precipitation composed primarily of small particles (diameter <1 mm) occurred about 36% ± 2% of the time, constituting 10% of total accumulation. Remaining precipitation contained enough large particles such that the disdrometer could be used to identify the precipitation type as rain, ice, snow or wet snow. Seasonal and annual statistics on frequency of occurrence and accumulation for each precipitation type observed during MICRE are presented. Most ice and mixed phase precipitation was shallow, originating at a height of 3 km or lower, and occurred most often when Macquarie Island was to the northwest of the nearest cyclonic low‐pressure center. In contrast, rain was more often deep and occurred most frequently when the island was to the southeast of cyclonic lows. A weak diurnal cycle in frequency and mean rate was present with a minimum between 12:00 and 14:00 local time and maximum between 03:00 and 06:00 local time. The CloudSat 2C‐Precip‐Column product missed the lightest precipitation (because the near‐surface reflectivity is <−15 dBZ) and overestimated total liquid precipitation and occurrence of mixed phase precipitation, but captured reasonably well the distribution of rain rates for rates >0.5 mm/hr. Plain Language Summary: Understanding the nature of precipitation over the Southern Ocean (SO) is crucial to understanding cloud properties, which climate models struggle to simulate correctly with significant impacts on global climate sensitivity (how much the Earth is likely to warm due to increases in greenhouse gases). Studies assessing the performance of climate simulations rely primarily on satellite measurements and reanalysis of precipitation in remote regions like the SO due to the lack of available surface measurements. However, there are large disagreements between and among satellite and reanalysis products regarding SO precipitation characteristics. This study examines precipitation characteristics obtained from a recent field campaign located at Macquarie Island, in the middle of the SO. The analysis presented here focuses on SO precipitation type (liquid, frozen or mixed) and accumulation, as well as seasonal and storm‐related variability. Ground observations from Macquarie Island are also compared to CloudSat satellite precipitation estimates that are a crucial data source for precipitation in the region. Key Points: Macquarie Island Cloud and Radiation Experiment data are used to study seasonal, diurnal, and synoptic variation in surface precipitationRain, ice/mixed, and small particle precipitation occurred year‐round, comprising 74%, 16%, and 10% of total accumulation, respectivelyCloudSat 2C‐Precip‐Column overestimated liquid accumulation and mixed phase occurrence but underestimated light precipitation occurrence [ABSTRACT FROM AUTHOR]

Polar lows (PLs) are small, intense cyclones that form at high latitudes during the winter. Their high wind speeds and heavy precipitation can have substantial impacts on shipping, coastal communities and infrastructure. However, low‐resolution climate models poorly simulate PLs, which reduces the confidence in their future projections. In this study, Northern Hemisphere (NH) PLs are assessed for the first time in a high‐resolution (25 km) global atmosphere‐only climate model, N512 HadGEM3‐GA3, for both present‐day and future RCP8.5 climate scenarios. The representation of PLs is found to agree reasonably well with the NCEP‐CFS reanalysis. The number of NH PLs are projected to substantially decrease (by over 60%) by the end of the 21st century, which is largely due to an increase in atmospheric static stability. Large decreases in PL activity are found in the Norwegian Sea, north‐east Atlantic and north‐west Pacific Oceans. Smaller changes are found in regions of current PL activity, such as the Barents and Labrador Seas, while new regions of PL activity along the northern Russian coastline are found where the Arctic sea ice is projected to disappear. The spatial differences found in future PL activity could have a substantial impact on human activity in the Arctic region. Plain Language Summary: Polar lows (PLs) are small but intense storms that form during winter at high latitudes and affect regions such as Scandinavia, Northern Russia and Japan. Because of their strong winds and precipitation, PLs can cause damage to infrastructure, shipping and coastal communities. In this study, PLs are assessed for the first time in a high‐resolution (25 km) global climate model for both present climate conditions and a future climate scenario in the Northern Hemisphere. The number of PLs is found to substantially decrease by the end of the 21st century, mainly due to a reduction in vertical temperature differences in the atmosphere. However, new regions of PL activity are found over the Arctic Ocean where the sea ice is projected to retreat. Key Points: The present representation of Northern Hemisphere PLs in a high‐resolution global climate model is found in agreement with previous studiesThe number of PLs is found to decrease by 60% by the end of the 21st century in the RCP8.5 scenario, but new PL locations emergeThe decrease of PL activity in the future is attributed to an atmospheric static stability increase in a baroclinic instability reduction [ABSTRACT FROM AUTHOR]

Suppressing spurious precipitation is one of the key issues in numerical weather prediction. To thoroughly suppress spurious convection, it is necessary to both inhibit the development of convection in false convection areas and weaken the heat and momentum exchange between false convection areas and other surrounding regions. In this study, a scheme to suppress spurious convection through cyclically assimilating the "zero" column maximum vertical velocity (wmax) using the ensemble square root filter (EnSRF) method is proposed. The "zero" wmax is the average wmax over the no‐rain echo region in the background field which is normally close to 0 m s−1. Two convective events that exhibit different characteristics are studied. The results show that the assimilation of "zero" wmax can suppress the upward motion and reduce the water vapor and hydrometeors of the background field in the false convection region, which can effectively suppress the development of spurious convection. In addition, the scheme can reduce the water vapor transport from surrounding areas to the spurious convection region and weaken the cold pool on the ground, which is not conducive to the regeneration and maintenance of false convection. The forecasts of reflectivity and precipitation show that the assimilation of "zero" wmax can effectively suppress either the strong false convection band or the weak spurious convection surrounding the true rain band and can decrease the false alarm rate of reflectivity and precipitation. Key Points: This study proposed a new scheme to suppress spurious convection by assimilating the "zero" column maximum vertical velocityThe scheme inhibits spurious convection in both dynamic and thermal aspectsThe scheme can effectively suppress spurious convection and reduce the false alarm rate of reflectivity and precipitation [ABSTRACT FROM AUTHOR]

The 2017 North Atlantic tropical cyclone season was among the most active in the last two decades, with 17 named storms, of which six reached the major hurricane (MH) intensity: Harvey, Irma, Jose, Lee, Maria, and Ophelia. In this study, the water vapor sources for precipitation for these six MHs were examined using a Lagrangian approach. The particle dispersion model, FLEXPART, was used to identify moisture sources. Overall, the North Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico were identified as the main moisture sources, supplying ∼75%–85% of the atmospheric humidity gained by tropical cyclones, which resulted in precipitation associated with the MHs. However, the South Atlantic Ocean also contributed considerable humidity (∼14%–20%), and the remaining ∼1%–5% originated from the tropical eastern Pacific Ocean. The accumulated moisture uptake higher than the 90th percentile generally appeared within approximately 3° to 5° of the TC trajectory. Key Points: The tropical North Atlantic Ocean was the main moisture source for precipitation associated with major hurricanes in 2017The South Atlantic moisture source is not negligible, with contributions of ∼14%–20%The highest moisture uptake generally occurred within approximately 3° to 5° of tropical cyclone trajectories [ABSTRACT FROM AUTHOR]

The U.S. Great Plains warm season climate is inextricably linked to the frequency and structure of the region's southerly low‐level jet. In the present‐day climate (1977–2009), low‐level jets are shown to occur on 26%, 46%, and 62% of May–September days in the northern (NGP), central (CGP) and southern (SGP) Great Plains, respectively, and account for at least 26%, 25%, and 36% of those region's precipitation during the same period. A shortcoming of previous research has been a failure to treat upper‐level dynamically coupled, or cyclone‐induced jets, separately from jets that are relatively uncoupled from synoptic flow. Differentiating between jet types is essential to proper mechanistic diagnosis and attribution of jet‐related wind, precipitation, and temperature changes to their local land or remote oceanic forcing. Using a new CERA‐20C objective dynamical jet classification dataset, this study achieves the first quantitative assessment of changes in coupled and uncoupled jets between 1901 and 2010 for NGP, CGP, and SGP. Declines in warm season jet frequency are pinpointed to July–September jets. In the NGP and CGP, both jet types have undergone significant increases in speed and height with concomitant decreases in CAPE and precipitation. NGP uncoupled jet and CGP coupled jet precipitation has decreased by 0.5 and 0.8 mm day−1, respectively, which accounts for 41%–44% of total May–September precipitation decreases between 1905–1937 and 1977–2009. A dynamic situation in which synoptic and local soil moisture changes drive opposite jet responses is discussed. Key Points: Great Plains low‐level jets have decreased in frequency but increased in strength over the past centuryDecreases in jet precipitation explains 41%–44% of present‐day May–September precipitation shortfalls relative to 1905–1937 normsIntensification of Pacific and Atlantic subtropical highs and enhanced Rockies ridging opposes localized drying in the jet response [ABSTRACT FROM AUTHOR]

Urban flooding is a global water disaster resulting from the expansion of urban impervious surfaces and the strengthening of extreme precipitation events, especially in China. Nonetheless, few studies have focused on the spatial distributions of urban flooding characteristics and their variations in the context of climate change. In this study, eight critical metrics (i.e., maximum flooding volume, total overloaded manholes with different flooding volumes or durations, total flooding volume, mean and maximum flooding durations, maximum inundation area, and depth) are adopted to characterize the urban flood events. The impacts of climate change on these metrics are assessed for two periods, the 2030s (2020–2049) and 2070s (2060–2089), and compared with those in the baseline period (1976–2005). The Future Science City Park in Beijing, China, is selected as our study area. The results show that all four flood events are well simulated, with both efficiency coefficients and correlation coefficients being over 0.8. The number of overloaded manholes and the total flooding volume are projected to increase 19.3%–44.8% and 171%–716% under 20‐year rainfall events due to climate change in the two future periods. The spatial distribution of overloaded manholes with different increased flooding volumes is projected to expand to almost the whole area from the region with lowland and limited drainage capacity. Furthermore, the maximum inundation area and depth are projected to increase obviously. This study will be helpful for designing and improving the drainage system, controlling urban flooding, and adapting to climate change. Key Points: Flood events related to volume, duration, and inundation metrics are well simulated by integrated SWMM and NewFlood model with a GA algorithmFlooding volume and duration, inundation area, and depth are projected to increase in the futureThe overloaded manholes are projected to expand to the whole area except the region with low impact development applications [ABSTRACT FROM AUTHOR]

A merger‐formation bow echo (MFBE) in southeast China during the pre‐summer rainy season (PSRS) was simulated using three microphysics schemes including Thompson (THOM), Morrison (MORR), and Weather Research and Forecasting Double‐Moment 6‐Class (WDM6) schemes to compare against polarimetric observations and Variational Doppler Radar Analysis System (VDRAS) analyses. The three schemes captured the basic kinematic structures for this MFBE event after assimilating radar radial velocities, but all underpredicted the cold pool strength by ∼25%. Particularly, THOM produced the best raindrop size distributions (DSDs) and precipitation pattern compared with observed retrievals; further analyses indicated the larger raindrop size bias and the weak cold pool strength in THOM were owing to the relatively low rain breakup efficiency and inefficient rain evaporation, respectively. By decreasing the cutoff diameter of rain breakup parameterization from the default 1.6–1.2 mm (i.e., increasing breakup efficiency) and increasing evaporation efficiency by threefold in THOM, the simulated DSDs and precipitation were greatly improved, and the cold pool strength was significantly increased from 77% to 99% compared to that in VDRAS analyses. This study illustrated a plausible approach of combining polarimetric radar retrievals and VDRAS analyses as bases to adjust THOM default settings in simulating a MFBE event in southeast China with physical characteristics more consistent with observations. Since microphysical processes vary from convective organizations and climate regions, it is recognized more cases studies are needed in the future to examine the validity and approach in this study to improve simulations and predictions of MFBEs in southeast China. Key Points: Thompson (THOM) simulates the best system structure and microphysical processes in a merger‐formation bow echo event in southeast ChinaIncreasing rain breakup and evaporation efficiency in THOM greatly improves the simulated DSDs, precipitation and cold pool strengthThe application of assimilation technology and polarimetric retrieval bridges the direct comparison of observations and simulations [ABSTRACT FROM AUTHOR]

As anthropogenic carbon emissions continue, it is important to predict how vegetation will change due to the warming climate, increasing atmospheric CO2, and changing precipitation patterns. Some projections show decreases in vegetation either regionally in the tropics (e.g., Falloon et al., 2012; Port et al., 2012) or globally (e.g., Bastin et al., 2019) due to high temperatures and drying, and there is large uncertainty surrounding the response of global vegetation to climate change (Arora et al., 2020; Friedlingstein et al., 2006; Walker et al., 2020). Here we use simulations with the NCAR Community Earth System Model (CESM) with dynamic vegetation to explore how increasing CO2 may affect vegetation and, through feedbacks, the climate. We find that globally, warming only decreases vegetation, but when we include the effects of CO2 on plant physiology, vegetated area increases by ∼23% relative to the preindustrial simulation. This greening occurs even without significant increases in precipitation, as high CO2 increases plant water use efficiency, allowing for more vegetation in arid and hot regions. Our results show that CO2‐induced vegetation changes affect climate by amplifying high latitude warming, cooling some areas of the tropics, and shifting global precipitation patterns. Our results highlight the importance of including climate‐vegetation feedbacks in Earth system modeling. Plain Language Summary: As anthropogenic carbon emissions continue to increase, it is important to learn how vegetation will respond and how, in turn, changes in vegetation may affect climate through various feedbacks. Here, we use an Earth system model with dynamic vegetation to compare the vegetation and climate responses to warming scenarios with and without CO2‐induced changes to plant physiology. We find that increased CO2 and vegetation responses expand where plants can grow. Increased water use efficiency allows vegetation to increase in dry regions and also in hot regions, and a longer growing season allows vegetation to expand poleward. While global vegetated area decreases with warming only, relative to the preindustrial simulation, it is increased ∼23% over the preindustrial simulation when plant responses to high CO2 are included in the model. The expanded vegetation affects climate by amplifying Arctic warming, cooling some areas of the tropics, and shifting the equatorial rain belt northward. Our results are of interest both for understanding our future climate under anthropogenic climate change and for understanding past warm, high CO2 periods. Key Points: High CO2 allows plants to increase water use efficiency, permitting more vegetation in arid and hot regionsGreening affects climate by amplifying high latitude warming, cooling some areas in the tropics, and shifting global precipitation patternsPlant responses to high CO2 shift climate‐vegetation relationships in biome space, relative to model scenarios with warming only [ABSTRACT FROM AUTHOR]

Based on an object‐based tracking (OT) algorithm, the precipitation systems in the convection‐permitting (CP, ∼4 km) regional climate model (RCM) are evaluated from the Lagrangian viewpoints. The characteristics of precipitation systems over eastern China during 1998–2007 derived from the CP RCM are compared with the satellite‐derived precipitation data set CMORPH. It is found that, with the cumulus parameterization switched off and reasonable representation of complex terrain, the precipitation systems in the CP RCM can propagate in the same direction as observation, but CP RCM tends to under‐/overpredict the precipitation systems with slow/fast propagation speed. Nevertheless, CP RCM can reasonably capture the average duration and eccentricity of all precipitation systems. The major deficiency in CP RCM is that the simulated precipitation systems are significantly weaker with larger coverage area. Over‐/underrepresentation of precipitation systems driven by large‐scale circulations/intense thunderstorms in CP RCM leads to wet and dry biases in the total precipitation of the longer‐duration (≥48 hr) and shorter‐duration (<48 hr) systems, respectively. It is suggested that assessment of the lifecycles of propagating precipitation systems in the Lagrangian framework is complementary to the traditional point‐to‐point Eulerian validation method. Key Points: Precipitation systems in long‐term convection‐permitting simulation over eastern China are assessed in the Lagrangian frameworkConvection‐permitting simulation captures the duration, eccentricity, and propagation direction of the tracked systems wellConvection‐permitting simulation over‐/under‐represents systems dynamically/thermally driven by large‐scale circulations/thunderstorms [ABSTRACT FROM AUTHOR]

The northeast China cold vortex (NCCV) refers to a closed low‐pressure system over northeast China extending from China‐Siberia to the northwest Pacific coast region. It is cut off from the westerly jet belt in the middle and upper troposphere, has a cold core, lasts several days, and could bring heavy rain events in late spring and summer that are of great challenge to weather forecasters. This study investigates the possibility of observing temperature structures of NCCVs by two microwave temperature sounders, that is, the Microwave Temperature Sounder‐2 (MWTS‐2) and the Advanced Microwave Sounding Unit‐A (AMSU‐A) onboard the two polar‐orbiting operational environmental satellites (POES) Fenyun‐3 satellite FY‐3D and Meteorological Operational satellite MetOp‐B, respectively. The MWTS‐2 and AMSU‐A together provide 3D atmospheric temperatures in the middle and upper troposphere, as well as the lower stratosphere four times daily. Both limb‐corrected direct observations of brightness temperature and retrievals of atmospheric temperature within NCCVs are characterized by cold and warm cores below and above the tropopause, respectively, comparing favorably with the European Centre for Medium‐Range Weather Forecasts ERA5 reanalysis. Compared with the ERA5 reanalysis in June 2019, the MWTS‐2 retrieved atmospheric temperatures have no scan‐dependent biases, the biases are smaller than ±0.2K, and root‐mean‐square errors (RMSEs) are smaller than and 2.0K at all altitudes. This study suggests the possibility of using multiple POES microwave‐temperature‐sounding observations for near‐real‐time monitoring of NCCVs' 3D temperature structures. Plain Language Summary: Near‐real‐time monitoring of the northeast China cold vortex (NCCV)'s 3D temperature structures by the polar‐orbiting satellite microwave observations. Both limb‐corrected direct observations of brightness temperature and temperature retrievals within NCCVs are characterized by cold and warm cores below and above the tropopause, respectively, comparing favorably with ERA5 reanalysis. Key Points: This study investigates the possibility of observing temperature structures of NCCVs by two microwave temperature sounders MWTS‐2 and AMSU‐A [ABSTRACT FROM AUTHOR]

The Tibetan Plateau (TP) is often called the "Water Tower of Asia," which contains the largest amount of snow and glaciers outside the polar regions. As an important and variable feature of the land surface, snow coverage on the TP has great impacts on regional climate. However, the commonly used ERA5 reanalysis in dynamical downscaling largely overestimates the snow depth for the TP. To improve the representation of snow cover in ERA5, a new ERA5‐driven downscaling data set (High Asia Refined analysis version 2, HAR v2) was generated by the Weather Research and Forecasting (WRF) model with the bias‐corrected snow depth. This study aims to identify and better understand the impact of bias‐corrected initial snow conditions on simulated regional climate, by comparing the HAR v2 with a 5‐year ERA5 forced WRF simulation without bias correction of initial snow depth (referred to as WRF_ERA5). The results show that the bias correction significantly improves the simulation of 2 m air temperature (T2), with regional mean cold bias reduced by 0.2°C–2.4°C, but no significant improvement in precipitation simulation is found. Further comparative analysis reveals that higher snow depth in WRF_ERA5 leads to T2, mean daily precipitation, summer extreme precipitation, and contributions of convective precipitation to summer mean daily precipitation decrease by 0°C–4°C, 0%–60%, 0%–40%, and 0%–10%, respectively, in most areas of the TP. In addition, the bias‐corrected initial snow depth also has impacts on simulated diurnal cycles of precipitation and T2 and leads to peak hours one hour earlier. Overall, this study confirms the importance of snow cover for the climate in the TP. Key Points: Bias correction of ERA5 initial snow depth (BCISD) can significantly improve the simulation of 2 m air temperature (T2) over the TPBCISD (i.e., less snow) has a great effect on thermodynamic variables, resulting in increased precipitation (prcp) and higher T2BCISD also has impacts on simulated diurnal cycle of prcp and T2 and leads to peak hours one hour earlier [ABSTRACT FROM AUTHOR]

Mesoscale convective systems (MCSs) have been identified as an important source of precipitation in the Tibetan Plateau (TP) region. However, the characteristics and structure of MCS‐induced precipitation are not well understood in this location. Infrared (IR) satellite imagery has been used for MCS tracking, but cirrus clouds or cold surfaces can lead to false MCS classification over mountain regions. Here, we combine brightness temperatures from IR imagery with satellite precipitation estimates from GPM IMERG and track MCSs over the TP, at the boundary of the TP (TPB), and in the surrounding lower‐elevation plains (LE), between 2000 and 2019. In most parts of LE and TPB, MCSs produced 50%–80% of the total summer precipitation (60%–90% of summer heavy precipitation), whereas MCSs over the TP account for below 10% of the total summer precipitation (10%–30% of summer heavy precipitation). Our results also show that MCSs that produce the largest amounts of heavy precipitation are characterized by longevity and large extents rather than by high intensities. These are mainly located in the populous areas south and east of the TP. A tracking of meso‐β convective systems over the TP shows that small‐scale convection makes a large contribution to total and heavy precipitation. This suggests that more localized convective systems are important for the regional water cycle over the higher terrain and highlights the importance of convective‐scale modeling to improve our understanding of precipitation dynamics in the TP region. Plain Language Summary: Storm systems that extend over several 100 km can represent a risk to people's lives and livelihoods, as they may lead to flooding, extreme winds and heavy rainfall. The Tibetan Plateau (TP) has received increasing attention over the last few decades because it has experienced drastic changes in the water cycle as a response to global warming. Although it is known that large storm systems develop in the populous surrounding regions of the TP, the rainfall characteristics from these storms are not well understood. It is difficult to identify storm systems in satellite images over high mountain regions, because high clouds and low surface temperatures can give signals similar to those of storm clouds. We therefore used a new method to track large storm systems in satellite images over the TP to clarify their role in the water cycle. Our results show that most of the storms that produce heavy rainfall occurred in the regions south and east of the TP. Storm systems over the TP are generally smaller in size and shorter in duration, which means that climate model simulations at high spatial resolution are needed to further investigate them. Key Points: Mesoscale convective systems (MCSs) were tracked in the Tibetan Plateau (TP) region for the past two decadesCo‐locations of brightness temperatures with precipitation indicate reduced numbers of MCSs over the TPPrecipitation and large‐scale environments associated with MCSs were compared between different subregions [ABSTRACT FROM AUTHOR]

An urban heat island (UHI) is a well‐known urban climatic feature; however, an opposite effect, i.e., an urban cool island (UCI), was recently observed at locations where high‐rises were concentrated. Here, we analyzed the impact of two urbanization factors (anthropogenic heat flux (AH) and building height (ZR)), which could affect the formation of UHIs and UCIs, on urban precipitation over the Seoul Metropolitan area. AH caused an increasing precipitation in urban and downwind areas, especially during daytime. AH directly contributed to the sensible heat flux, resulting in surface warming associated with an UHI. Surface heat was transferred within the planetary boundary layer by turbulence, which increased the low‐level instability, resulting in increased precipitation. In the experiments with the ZR, both surface temperature and precipitation increased (decreased) during the nighttime (daytime). The diurnal variation of ground heat flux intensified with an increasing ZR, inducing a smaller variation of sensible heat flux for surface energy balance. Changes in the ground heat flux could also explain occurrence of UCIs in cities with high‐rises. Tall buildings reduced daytime surface temperature, thereby facilitating atmospheric stabilization, and reducing precipitation. Also, the surface friction increased with a higher ZR, resulting in enhanced convergence in the western part of urban areas where dominant westerlies first met land, which was favorable for increased precipitation. In summary, a study of variations in AH and ZR demonstrated how UHIs and UCIs significantly altered precipitation, respectively. In particular, an UCI alleviated an increment in thermo‐driven daytime precipitation caused by an UHI. Key Points: Anthropogenic heat caused increasing precipitation in urban and downwind areas, especially during the daytimeBuilding height increased (decreased) both surface temperature and precipitation during the nighttime (daytime) in urban areasThe increment in thermo‐driven daytime precipitation caused by an urban heat island was alleviated by an urban cool island [ABSTRACT FROM AUTHOR]

This study examines August droughts in northern China (north of 35°N) at interannual and interdecadal timescales. During the period from 1968 to 1992, the precipitation in the western and eastern regions of northern China showed an out‐of‐phase pattern. During the period from 1994 to 2018, the climate in the Tarim Basin became wetter, but there was no significant connection with drought in North China. Further studies demonstrated that the Atlantic sea surface temperature (SST) and regional soil moisture anomalies were related to drought changes in northern China. Before the early 1990s, the east‐west standardized precipitation index dipole mode in northern China was physically linked with the wet soil in central Asia in the preceding June; moreover, the wet land surface could persist until August and cause anticyclonic and cyclonic systems over Mongolia and the subtropical northwestern Pacific, thus contributing to precipitation surplus in northwest China and a deficit in North China. After the early 1990s, however, wet conditions over the Tarim Basin were related to wet soil in the East European Plain in the preceding July: the wet soil could persist into August and generate anomalous circulation over the Tarim Basin, leading to abnormal humidity. Additionally, the SST conditions in the North (South) Atlantic are key sources of climate predictability for northern China drought at the interseasonal scale after (before) the early 1990s. Key Points: The spatial distribution of the standardized precipitation index in northern China shows a marked zonal dipole patternThe drought in northern China underwent a remarkable interdecadal change in approximately 1992The Atlantic sea surface temperature and regional soil moisture anomalies were responsible for the drought changes in northern China [ABSTRACT FROM AUTHOR]

Understanding the cloud impact on forest evapotranspiration (ET) is crucial for studying the interaction of vegetation‐cloud‐atmosphere. Combining long‐term (2003–2010) satellite passive microwave observations and in‐situ measurements, a non‐linear response of canopy‐scale ET to cloud increase was found at a temperate forest in Northeast China. As cloud increased, an initial enhancement (4%–10%) in ET occurred under less cloudy sky, while a significant reduction (>20%) in ET occurred under more cloudy sky. The phenomenon existed under both high and low vegetation water content (VWC) indicated by satellite microwave emissivity difference vegetation index (EDVI). Analysis showed that this was the combined effect from the enhancement (5%–30%) in evaporative fraction (EF) and the reduction (5%–50%) in net radiation under cloud increase. Decoupling analysis based on coefficients (ρ) of path analysis model showed that enhanced EF (ρ > 0.61) rather than radiation (ρ < 0.47) dominated the ET enhancement under less cloudy sky, while the control of reduced radiation became stronger (ρ > 0.63) and could not be compensated by increased EF (ρ < 0.48) under more cloudy sky. EF enhancement under clouds was strongly correlated with the decline in canopy resistance (Rs) which was dominated by vapor pressure deficit (VPD). Higher VWC increased ET via reducing Rs and enlarging EF. This positive effect of VWC was more noticeable under less cloudy sky. Associated mechanisms could be related to the dynamic controls of plant physiology and environmental conditions induced by VWC and clouds. This study highlighted the dynamic effect of clouds and VWC on forest ET and improved our knowledge of vegetation‐cloud interactions. Key Points: The non‐linear response of ET to cloud increase results from increase of evaporative fraction and reduction in net radiationVegetation water content (VWC) significantly affects ET status under various cloud conditions, particularly under clear skyLower VWC can restrain ET under clouds by increasing canopy resistance and lowering evaporative fraction [ABSTRACT FROM AUTHOR]

California experienced a sharp transition from wet to historically dry conditions during the 2012/13 winter. Such transitions in the precipitation regime have strong consequences for water management, but predicting them at the seasonal time scale remains very challenging as little is known about what drives them. The 2012/13 winter was characterized by a neutral El Niño Southern Oscillation, strong Madden‐Julian Oscillation (MJO) activity, and sudden stratospheric warming. This study examines the potential influence of these different drivers in atmospheric global climate model experiments with prescribed sea surface temperature (SST) and sea ice concentration and constrained tropical and/or Arctic variability. Our model results are compared to reforecasts from the North American Multi‐Model Ensemble (NMME) and Subseasonal Experiment (SubX)/Subseasonal to Seasonal (S2S) projects to determine which important processes may have been missed by the seasonal prediction systems. Our simulations suggest that the tropical warm‐west, cool‐east SST anomaly played an active role in the wet conditions of November–December 2012 through a typical tropical‐extratropical teleconnection. The tropical SST anomalies were not accurately predicted by the NMME models, and none of them predicted the wet anomalies in November–December. In contrast, the sudden transition toward dry conditions in January is found to be independent of tropical SST variability, and rather influenced by strong MJO activity over the western Pacific. The SubX/S2S models with a better MJO prediction tend to predict the dry conditions in January better. Our study highlights how intraseasonal processes superimpose on more persistent seasonal anomalies and induce regime shifts such as the wet‐to‐dry transition of the 2012/13 winter. Key Points: California experienced a wet‐to‐dry transition during 2012/13 winter, but it was not predicted in the seasonal prediction modelsThe wet conditions in November–December were driven by the tropical sea surface temperature (SST), which was missed in the seasonal prediction modelsThe subseasonal prediction models that predicted the Madden‐Julian Oscillation (MJO) were also able to predict the dry conditions in January [ABSTRACT FROM AUTHOR]

This study investigates the possible relationship between changes in monsoonal (May–June) rainfall diurnal variations over the south China coast and urban surface processes using hourly rain gage data during 1981–2014 and semi‐idealized numerical simulations. The continuous landward penetration of regional climatic rainband observed prior to urbanization is blocked by the city. The morning precipitation initiated at the coast "jumps" to the downstream area of city in the late afternoon, causing a 2 hr delay of the rainfall peak time. Simulations clearly reproduce this change by turning off/on the urban land use, and demonstrate a direct connection between the change of rainfall diurnal variations with a magnitude‐enhancing but slower‐penetrating sea breeze as well as the thermodynamic conditions modified by the city. Individual impacts of sensible heating, evapotranspiration, and friction from urban surface are examined using various sensitivity experiments. Urban sensible heat is found to act as the energy source to enhance the sea breeze and induce the asymmetric low‐level urban inflow that blocks the penetration of sea breeze and changes the rainfall location. Evapotranspiration and latent heat flux suppressed by the impervious urban surface causes a drier boundary layer and lower CAPE over the city. The "jumping" of the climatic rainband may result from the competing interaction of the dynamic and thermodynamic factors. Urban friction has little impact on the daytime rainfall, but could slow down seaward wind speed and enhance coastal convergence in the early morning if no strong urban heat island is present. Key Points: The afternoon rainfall peak time during May–June over the PRD is delayed by 2 hr after urbanizationSimulations reproduce these changes of rainfall diurnal cycle and also local circulations as the result of urban surface expansionImpacts of urban sensible heat and evapotranspiration are critical to the changes of dynamic and thermodynamic conditions for precipitation [ABSTRACT FROM AUTHOR]

It is important to understand the hydroclimate history of water‐deficient regions, such as the Alxa desert of northwest China. However, there are few high‐resolution proxy records for the hydroclimate changes over the past centuries in the Alxa desert. Fortunately, there are many tree‐ring hydroclimate reconstructions from the nearby mountains. However, the hydroclimate correlations between the Alxa desert and adjacent mountains are unclear. Here, we collected high‐quality data from 43 meteorological stations in northwest China (1960–2017). We found increasing mean temperatures in the study area, both on seasonal and annual timescales. We also found that while relative humidity had mostly decreased in the Mt. Qilian and Alxa desert, precipitation and the standardized precipitation evapotranspiration index (SPEI) increased mostly. In the Mt. Helan and Hasi ranges, relative humidity, precipitation, and SPEI all showed decreasing trends. Empirical orthogonal function analysis indicated that precipitation, relative humidity, and SPEI in the Mt. Qilian were similar to the corresponding conditions in the central and western Alxa desert. Hydroclimate conditions in the Mt. Helan and Hasi ranges were correlated with data from the eastern desert. Tree‐ring records of precipitation and SPEI from the mountains were used to validate the hydroclimate correlations during the instrumental period. We found strong similarities between hydroclimatic changes in the Mt. Qilian and those in the central and western Alxa desert, on both annual and decadal timescales, from 1877 to 2006. Tree‐ring width chronologies from mountainous areas were shown to faithfully represent the hydroclimatic history of the desert area. Plain Language Summary: Northwestern China has faced serious ecological challenges over the past five decades, including intensified desertification and declining lake levels. Understanding the regional hydroclimate history is necessary to manage water resources in this water‐deficient region. However, there are few climate proxies for these desert areas. In contrast, high‐resolution tree‐ring climate proxies have been studied intensively in the nearby mountains. This study attempts to find reliable correlations between mountain and desert hydroclimate. We found increasing mean temperatures over the entire study area. We also found that precipitation, relative humidity, and drought index data from the western and central desert were strongly correlated with data from Mt. Qilian, while data from the eastern desert were correlated with those from the Helan and Hasi mountain ranges. When relative humidity decreased mostly in the Mt. Qilian, precipitation and SPEI mostly increased; in the Mt. Helan and Hasi areas, relative humidity, precipitation and SPEI mostly decreased. Tree‐ring records from the mountains were compared to validate these correlations. Significant correlations were found between the precipitation reconstructions from the central desert and Mt. Qilian from 1877 to 2006. We also used tree‐ring width chronologies to show that they could represent the hydroclimatic history of the Alxa desert. Key Points: Precipitation and SPEI indicate wetter trends in Mt. Qilian but drier trends in Mt. Helan and Hasi ranges from 1960 to 2017Precipitation, relative humidity, and SPEI in western and central (eastern) Alxa desert resemble conditions in Mt. Qilian (Helan and Hasi)Tree‐ring proxies from nearby mountains can faithfully represent the hydroclimate history of the Alxa desert [ABSTRACT FROM AUTHOR]

This study compares the impacts of global warming and intense urban anthropogenic heat (AH) on extreme hourly precipitation over the Pearl River Delta (PRD) megacity, located in coastal South China. Using the cloud‐resolving Weather Research and Forecasting (WRF) model coupled with the single‐layer urban canopy model (SLUCM), three downscaling experiments were carried out: the first (second) having zero (300 W/m2) as diurnal maximum AH values prescribed over PRD urban grids, under the current climate conditions. The third experiment with diurnal maximum AH of 300 W/m2 under future projected climate representative concentration pathway (RCP) 8.5. Boundary conditions were derived from PRD extreme rainfall episodes, identified from the Geophysical Fluid Dynamics Laboratory Earth System Model (GFDL‐ESM2M) historical and RCP8.5 runs. Global warming forcing leads to ∼20% to more than 100% increase in the probability of hourly precipitation with the magnitude of 20–100 mm/hr over urban locations. The enhancements from intense AH forcing were similar. However, two forcings have distinct signatures in modulating the thermodynamic environment. Warming due to AH is limited to the lowest 1 km above ground, while global warming warms up the whole troposphere. Intense AH results in enhanced convective available potential energy (CAPE) and reduced convective inhibition (CIN) within the megacity, allowing convection to be triggered more easily and with more vigor. On the other hand, global warming enhances CAPE and CIN, over both urban and rural areas. Our results highlight the different mechanisms of AH and global warming in exacerbating extreme urban rainfall, despite the similar impacts on the rainfall intensity. Plain Language Summary: Human activities can generate extra heat in cities, called anthropogenic heat. This heat can exacerbate the heavy rainfall in the urban area over the Pearl River Delta urban area, where located at the south China coast. We used computational models to simulate and compare the situation of the presence and absence of anthropogenic heat during a list of extreme rainfall events under a current and global warming environment. Results show that anthropogenic heat enhances the heavy rainfall in the urban area, while global warming enhances heavy rainfall over both urban and nonurban area in the Pearl River Delta. Both enhancements are comparable to each other. Anthropogenic heat causes a localized urban temperature increment, while global warming causes a global temperature increment. The different temperature response creates the differences of rainfall response. Key Points: Anthropogenic heat creates higher convective available potential energy (CAPE) and lower convective inhibition (CIN) in urban locations within the Pearl River Delta and intensifies urban extreme rainfallGlobal warming results in higher CAPE, which exacerbates extreme rainfall in both urban and surrounding rural areasImpacts due to intense anthropogenic heat and global warming on extreme rainfall can be comparable over highly urbanized coastal locales [ABSTRACT FROM AUTHOR]

Variations in the water vapor that atmospheric rivers (ARs) carry toward North America within Pacific storms strongly modulates the spatiotemporal distribution of west‐coast precipitation. The "AR Recon" program was established to improve forecasts of landfalling Pacific‐coast ARs and their associated precipitation. Dropsondes are deployed from weather reconnaissance aircraft and pressure sensors have been added to drifting ocean buoys to fill a major gap in standard weather observations, while research is being conducted on the potential for airborne Global Navigation Satellite System (GNSS) radio occultation (ARO) to also contribute to forecast improvement. ARO further expands the spatial coverage of the data collected during AR Recon flights. This study provides the first description of these data, which provide water vapor and temperature information typically as far as 300 km to the side of the aircraft. The first refractivity profiles from European Galileo satellites are provided and their accuracy is evaluated using the dropsondes. It is shown that spatial variations in the refractivity anomaly (difference from the climatological background) are modulated by AR features, including the low‐level jet and tropopause fold, illustrating the potential for RO measurements to represent key AR characteristics. It is demonstrated that assimilation of ARO refractivity profiles can influence the moisture used as initial conditions in a high‐resolution model. While the dropsonde measurements provide precise, in situ wind, temperature and water vapor vertical profiles beneath the aircraft, and the buoys provide surface pressure, ARO provides complementary thermodynamic information aloft in broad areas not otherwise sampled at no additional expendable cost. Plain Language Summary: Aircraft are deployed to make extraobservations and improve forecasts of the atmospheric rivers that carry moisture toward the coast of North America in northeast Pacific storms. Dropsondes are released from the aircraft to measure moisture, temperature and winds as they descend. We have increased the coverage of the aircraft measurements by making simultaneous measurements of the atmosphere using Global Positioning System (GPS) signals, and for the first time, signals from the European Galileo satellite system which provides 50% more radio occultation measurements. These airborne radio occultation (ARO) measurements of the refractive index of the atmosphere are based on travel time delays of the satellite signals, and are used to provide refractivity, moisture, and temperature profile information to the sides of the aircraft. The observations are assimilated into a mesoscale weather model to improve the initial moisture conditions used for the weather forecast. Cross sections through the updated model volume show variations in the refractivity that correspond to the atmospheric river features at low levels and cold front features at upper levels. Key Points: Flights over northeast Pacific atmospheric rivers provide dense airborne radio occultation and dropsonde data for assimilation in modelsThe first Galileo RO profiles are compared with nearby Global Positioning System (GPS) profiles to assess accuracy in the troposphereThe model refractivity anomaly distinguishes key characteristics of the atmospheric river including the low‐level jet and tropopause fold [ABSTRACT FROM AUTHOR]

Inland frontal heavy rainfall (FR) and coastal warm‐sector heavy rainfall (WR) are two typical types of precipitation in South China during the pre‐summer rainy season. FR and WR are found to be generated by different mechanisms, but their differences in microphysical features and relevant possible reasons are not well understood. In the present study, dual‐polarization observations merged from 10 radars are utilized to investigate microphysical characteristics of two coexisting convective systems producing FR and WR. Overall, the WR has a stronger convection intensity, larger raindrop size, and higher liquid water content than the FR. The larger raindrops might be partly attributed to the deeper warm‐sector convection evidenced by stronger convective updrafts, which could potentially promote mixed‐phase processes. Raindrops in the WR are still larger even when the WR has a similar convection intensity with the FR, which might be explained by the active warm rain processes associated with higher low‐level humidity and more evident low‐level rotation in the WR. In addition, the FR was originally developed near the front and then vacated from the front into the warm sector, exhibiting an obvious temporal evolution in microphysical characteristics. Despite weaker convection intensity, the FR away from the front manifests a larger mean raindrop size than the FR near the front possibly because of more abundant environmental moisture in the warm sector. Key Points: The warm‐sector convection has a stronger convection intensity, larger raindrop size, and higher liquid water contentThe larger raindrop size in the warm‐sector heavy rainfall is possibly due to higher low‐level humidity and more evident low‐level rotationThe frontal heavy rainfall exhibits a temporal evolution in microphysical characteristics [ABSTRACT FROM AUTHOR]

We investigate the relationship between convective available potential energy (CAPE), precipitation, the number and size of storms and overshooting tops, and lightning stroke density (f) over the Central America region. While f increases almost linearly with CAPE1/2 over land, f is nearly muted over the ocean when CAPE is small. In the high‐CAPE regime, on the contrary, oceanic storms produce as many lightning flashes as land storms. We show that individual oceanic storms are smaller and contain fewer overshooting tops compared to land storms, although the difference exists across low‐ and high‐CAPE regimes. While f increases as individual storm size increases, the storm size required to produce lightning appears to be disproportionately high in the low‐CAPE regime, likely due to the stronger entrainment effect. The entrainment effect on f in the low‐CAPE regime appears to be much weaker over land. Applying the CAPE threshold for lightning over the ocean to the CAPE‐based lightning parameterization scheme of Romps et al. (2014), https://doi.org/10.1126/science.1259100 improves its performance, in particular, at representing the land‐sea contrast in f. Key Points: Over the tropical ocean, storms seldom produce lightning when convective available potential energy (CAPE) is smallThe inhibition of lightning in the low CAPE environment is likely due to the storm size being too small to produce a strong updraftImplementing a CAPE threshold for lightning over the ocean improves the performance of a CAPE‐based lightning parameterization scheme [ABSTRACT FROM AUTHOR]

In the Budyko framework, the landscape parameter (n in the Choudhury‐Yang equation) controls precipitation (P) partitioning to evapotranspiration (ET) and runoff (R). Systematically investigating the underlying influence of catchment characteristics on n is critical for understanding the partitioning of P. Using two runoff datasets with more than 30,000 stations around the world, we found that the variability in n was significantly correlated with a series of catchment characteristics, but varied across climate zones. Hence, several empirical models based on catchment characteristics were developed to estimate n for different climate zones. The developed empirical models applied to Budyko framework explained 92% of the spatial variability in the global mean R and provided more reasonable ET than that from Global Land Evaporation Amsterdam Model (GLEAM) in terms of long‐term water and energy balances. Empirical models were further applied to predict and attribute future R under the high‐end Shared Socioeconomic Pathways (SSP5‐8.5). The changes in P, potential evaporation (PET), and n are projected to alter global R by 61.5%, −24.0%, and 14.5% during 2081–2100. Notably, compare with the historical period, the influence of n was reversed in arid zones under the future scenario, where n leads to a significant decrease in R. The developed empirical models n enhances the applicability and capability of the Budyko framework for both historical and future hydrological assessments over the globe. Key Points: This study for the first time developed empirical models of the Budyko parameter for different climate zones using thousands of stationsThe developed empirical models applied to the Budyko framework show more reasonable results than the complicated process‐based modelThe factor contributing the most to future runoff changes is precipitation changes, instead of potential evapotranspiration [ABSTRACT FROM AUTHOR]

Extreme precipitation events are expected to increase in frequency and intensity in the future climate, but the magnitude of these changes remains uncertain. The relationship between extreme precipitation and the surface temperature has been investigated to more robustly assess projected increases in extreme precipitation considering that projected temperature is more adequately simulated. Relationships between extreme precipitation (daily and sub‐daily) and surface air temperature (SAT) or surface dew‐point temperature (SDPT) are analyzed in this study using the 50‐members ensemble from the fifth version of the Canadian Regional Climate Model covering the Northeastern North America region over the period 1956–2099. Temperature‐precipitation scaling rates (TPSRs) were estimated using local SAT and SDPT seasonal anomalies as covariate over both periods for 2–100‐year extreme precipitation events and durations ranging from 1 to 24 h. Contrasting responses were obtained when using SAT or SDPT, especially in the southern part of the domain. Median scaling rates over the entire domain for SDPT were close to the Clausius‐Clapeyron scaling (≈7%/°C) while they were much smaller for SAT and even negative in southern regions, showing that moisture availability is a key factor for these regions. TPSR based on SDPT is also more robustly constrained and can be used to estimate changes in short‐duration extreme precipitation in a future period from TPSR in the historical period over a large part of the domain. Plain Language Summary: Extreme precipitation events are expected to be more intense and frequent in the future. The magnitude of these changes remains, however, uncertain. It is also possible to relate extreme precipitation changes to changes in surface temperature. The basic idea is that a warmer atmosphere can hold more moisture and therefore trigger more intense precipitation. Such a relationship can estimate future changes in extreme precipitation since simulated temperature from climate models is more reliable than precipitation. The response of extreme precipitation events to changes in surface air temperature and the surface dew‐point temperature was analyzed over the Northeastern North America region using a large ensemble of 50 climate simulations covering the period 1956–2099. It was shown that more robust estimates of the temperature‐precipitation relationship are obtained when using the surface dew‐point temperature, confirming that available moisture can be a limiting factor in increasing extreme precipitation for some regions. Those estimates were consistent between the historical and future climate over a large part of the study region for the more extreme events and confirm that future extreme precipitation changes can be estimated from changes in dew‐point temperature. Key Points: Temperature‐precipitation scaling using surface dew‐point temperature provides more robust estimates compared to surface air temperatureThe link between extreme precipitation and surface dew‐point temperature (SDPT) changes is more robustly constrained for short‐duration events and the future periodThe scaling using SDPT in historical period can be used to estimate changes in rainfall extremes in the future over most of the domain [ABSTRACT FROM AUTHOR]