Abstract The effects of spray on air‐sea fluxes in high‐wind environments have been studied for decades, and several models have been developed and implemented. Owing to the difficulties in making direct measurements in these conditions, however, many process‐level details of spray‐turbulence interaction remain unknown, and certain assumptions must often be made regarding droplet lifetimes and their interaction with background conditions. This work represents Part II of a study where the spray‐laden hurricane boundary layer is resolved using large‐eddy simulation. Here we focus on the effects of spray on air‐sea fluxes of heat, moisture, and momentum, and investigate the potential modifications to the bulk flux coefficients , , and . The simulations show an increase in the drag coefficient —a numerical artifact reflecting the fact that the winds must accelerate all newly formed droplets. Unresolved wave breaking and near‐surface turbulence are likely important factors not contained in the simulations. Spray impacts on thermodynamic fluxes, on the other hand, are largely controlled by background humidity and suspended spray mass, and we demonstrate a process where spray can actually limit its own influence by saturating the boundary layer. The simulations are also used to show that bulk models which rely on assuming that all spray‐mediated fluxes are caused by re‐entrant droplets might be inaccurate, since they do not account for the vertically distributed source/sink of heat and moisture caused by the potentially large number of small droplets which have escaped the surface layer.
Abstract Changes in near surface wind speeds (NSWS) over the Arctic Ocean play a critical role in regulating sea ice drift, the surface energy budget, and sea ice‐atmosphere interactions. However, the drivers behind their long‐term variability and trends remain poorly understood. Here, we use ERA5 reanalysis and CESM2 simulations to quantify the relative contributions of internal variability and external forcing to winter and summer NSWS changes since 1940. While NSWS in both seasons exhibit significant increasing trends during 1940–2014, their underlying mechanisms differ. External forcing explains about 60% of winter NSWS variations. In contrast, summer NSWS variability is largely controlled by internal climate variability. Furthermore, the dominant external forcing affecting winter NSWS shifts around 1970: anthropogenic aerosols are the main driver before 1970, whereas greenhouse gases prevail afterward. These findings underscore that the distinct physical processes govern seasonal Arctic NSWS variability, thereby improving our understanding of regional climate evolution.
Abstract Tropical cyclones (TCs) cause major economic losses in Australia due to severe winds and floods; however global climate models (GCMs) are too coarse to properly resolve them. Here, we dynamically downscale ERA5 reanalysis and 16 CMIP6 simulations from 11 GCMs using the Conformal Cubic Atmospheric Model (CCAM) to a ∼10 km resolution over the Australian region. We evaluate whether these simulations can represent TC activity over the Australian region by analyzing TC frequency, genesis, intensity, lifespan, and the role of ocean–atmosphere coupling. The downscaled simulations reproduce observed spatial and seasonal patterns of TC activity, with the highest skill in the central and eastern subregions. CCAM‐ERA5 captures the observed intensity distribution for Categories 2–4 and the relationship between lifespan and intensity. The CCAM‐CMIP6 ensemble reproduces mean TC number well in the central and eastern subregions but underestimates the most intense (Categories 4–5) and medium‐lived TCs (6–8 days) while overestimating Category 1 systems. Ocean coupling has model‐dependent effects rather than producing a systematic improvement. Coupled configurations generally simulate fewer TCs than atmosphere‐only configurations, mainly through reduced weak‐TC numbers, while differences in stronger‐TC intensity are generally small and metric dependent. Overall, the CCAM downscaled simulations provide a realistic representation of Australian TC climatology, particularly in the central and eastern subregions, but remaining biases in the western basin and in the upper tail of TC intensity should be considered in future projection and impact applications.
Abstract Precipitation variability describes fluctuations between wet and dry conditions and plays an important role in shaping hydrological extremes and their societal impacts. Using idealized carbon dioxide removal experiments from CMIP6 models, we assess how precipitation variability responds to symmetric CO 2 ramp‐up and ramp‐down forcing across time scales from day‐to‐day to intraseasonal. Precipitation variability strengthens with increasing CO 2 and remains stronger during the ramp‐down phase than during the ramp‐up phase, even at comparable CO 2 concentrations. This asymmetry is evident across time scales and is particularly pronounced over several densely populated land regions, including North America, Europe, South Asia and East Asia. Diagnostics based on a simplified moisture‐budget framework indicate that the asymmetry is primarily driven by thermodynamic moistening, which enhances variability in vertical moisture transport. These results demonstrate a pronounced asymmetric response of short‐timescale precipitation variability to CO 2 forcing, highlighting the persistence of hydroclimatic variability beyond the period of CO 2 reduction.
Abstract Using the multi‐frequency radar suite, ground‐based observations, and the Weather Research and Forecasting (WRF) model with the Predicted Particle Properties microphysics scheme with the version of two‐ice category (P3‐2ice), we study the characteristics of a cold‐air outbreak (CAO) snowfall event on 6–7 December 2024. The cloud system is shallow convection, with the microphysical characteristics differing between the first and second snowfall periods. The WRF model reproduces the synoptic‐scale weather system and mesoscale structure well, but underestimates the event‐averaged snow rate and rime mass fraction by approximately 52% and 24%, respectively. Within the riming process, the collection of cloud water by ice is the dominant pathway. However, the fixed collection efficiency decouples riming from wind shear. To study the impact of collection efficiency on snow rate, we increase the efficiency as a simplified approach to represent the potential effect of shear‐generated turbulence on riming. The results of sensitivity experiments reveal that the snow rate exhibits a positive correlation with collection efficiency, but with contrasting behaviors between the two snowfall periods. In the first period, persistent severe underestimation occurs; in the second period, overestimation occurs when efficiency is high, and the rime mass fraction shows a nonlinear response to efficiency. These results indicate that a fixed collection efficiency is inadequate and a dynamic environment‐constrained efficiency is required. The research provides new insights into how to improve the riming process parameterization, contributing to a better understanding of the dynamic and microphysical characteristics of CAO snowfall.
Abstract Drizzle, a common feature of marine boundary layer clouds formed through collision‐coalescence, plays a key role in cloud microphysics and evolution. However, simultaneously retrieving cloud and drizzle properties from remote‐sensing observations remains challenging because drizzle droplets often dominate radar signals, masking cloud contributions. To address this, we developed Ensemble Cloud Retrieval (ENCORE), a retrieval framework that combines shortwave radiometer, lidar, and cloud radar measurements to estimate cloud and drizzle properties concurrently. Evaluation against in situ and ground‐based data sets at the Atmospheric Radiation Measurement (ARM) Eastern North Atlantic (ENA) site demonstrates robust performance for cloud properties, with mean biases of 3% to 44% for droplet number concentration, −21% to 19% for effective radius, and −54% to 69% for liquid water content. Column‐integrated quantities, including liquid water path and optical depth, differ by 10% to 50% and −20% to 33%, respectively, yielding radiation closure within 15%. ENCORE also outperforms the ARM NDROP product in retrieving cloud droplet number concentration. Drizzle retrievals, however, remain more challenging, with drizzle water content biases ranging from −87% to −56%. Drizzle number concentration is especially uncertain under weak drizzle conditions, likely due to sensitivity limitations and radar thresholds used in ENCORE. Despite these challenges, process‐based evaluations, such as Z–R relationships and cloud adiabaticity, are consistent with in situ observations, demonstrating ENCORE's ability to capture cloud–drizzle covariability that is critical for understanding warm‐rain processes and aerosol–cloud–precipitation interactions.
Abstract Based on daily station observations and ERA5 reanalysis data during warm seasons of 1961–2024, the characteristics of regional compound dry‐hot events (RCDHEs) and associated mechanisms over Yangtze River Basin (YRB) have been systematically explored. Results show more than 45% of the total occurrence days (OD) of compound dry‐hot events (CDHEs) over most YRB in warm season is contributed by the RCDHEs, which show distinct sub‐seasonal variations and concentrate in mid‐June to mid‐July. The coverage rate (CR), intensity and OD of RCDHEs all show significant increasing trends during 1961–2024, especially as the intensity and OD increase at much faster rates after 2000. Moreover, the occurrences of RCDHEs are mainly driven by two synoptic patterns named P1 and P2, respectively. The P1 (P2) pattern features that the intensified Western Pacific Subtropical High and South Asian High with southerly (northerly) location relative to the climatology move toward each other. Meanwhile, the intensity, CR and OD of RCDHEs under both P1 and P2 patterns all show increasing trends during 1961–2024, and the accelerated increasing rates of the intensity and OD after 2000 are mainly contributed by those under the P1 pattern. Mechanism analysis reveals that the decreased cloud cover and precipitation in southern (central‐eastern) YRB due to the subsidence induced by the abnormal high pressure under the P1 (P2) pattern enhance the surface net shortwave radiation and heating there, further leading to much higher occurrence probability of CDHEs in southern (central‐eastern) YRB during the RCDHEs under the P1 (P2) pattern.
Abstract Building on a previous study showing that NetCAPE0C—formulated as CAPE calculated up to the 0C isotherm, after subtracting CIN and correcting for water loading—is effective in predicting hail occurrence in maritime tropical (mT) environments, we examined the microphysics to understand why , particularly for ordinary pulse storms. A suite of idealized Cloud Model 1 (CM1) simulations, created by systematically varying CAPE and CIN to generate different NetCAPE0C values, was analyzed as hydrometeors progressed from condensate to hail. As NetCAPE0C increases, the maximum hail size near the ground correspondingly increases. Our results show that NetCAPE0C is effective because the buoyancy accumulated between the surface and the 0C isotherm governs parcel ascent into the mixed‐phase layer, where maximum supercooled liquid water content (SLWC) peaks near −5C, while also controlling updraft width and depth. Larger NetCAPE0C values promote earlier raindrop formation, accelerating riming and hail growth. In these simulated mT pulse storms, hail growth is constrained by three mechanisms: (a) the rapid warm‐rain process, which in weaker updrafts leads to earlier precipitation fallout and reduces the likelihood that raindrops are lifted and frozen as hail embryos; (b) an imbalance between updraft strength, abundant SLWC, and the shallow hail growth zone (HGZ), which limits the vertical distance available for riming growth and produces relatively simple hail trajectories dominated by wet growth and shedding; and (c) substantial melting during descent through the deep layer below the 0C isotherm. Together, these results show that NetCAPE0C plays a dual role by controlling both hail production and growth aloft and hail survival through its influence on sub‐cloud melting.
Abstract Accurate characterization of temperature extremes is essential for climate risk assessment, particularly in regions highly sensitive to climate change like the Mediterranean Basin. This study compares near‐surface temperatures from E‐OBS and ERA5‐Land, focusing on warm (>90th percentile) and cold (<10th percentile) extremes. Our findings indicate substantial temporal agreement across most of the domain, with event overlaps in the period 1991–2020 (last decade) up to 74% (86%) for warm extremes and 64% (80%) for cold extremes, though certain sub‐regional sectors emerge as structural outliers with weaker coherence. For warm extremes, a systematic bias is prevalent: E‐OBS is generally 1.1°–1.3°C warmer than ERA5‐Land at the 90th percentile, though this divergence narrows at higher intensities. Distribution analyses reveal a pronounced negative skewness in certain areas, reflecting localized overestimations by the reanalysis. Regarding decadal warming trends, ERA5‐Land generally exhibits steeper rates for warm extremes, reaching up to 0.86°C/decade compared to 0.68°C/decade in E‐OBS, despite lower absolute temperatures. For cold extremes, both data sets capture coherent warming signals across most areas, maintaining high correlations (0.94–0.98), whereas the outlier sector shows a clear trend inconsistency with a weak or negative warming signal in the reanalysis. Comparative spatial analyses confirm that complex topography amplifies data dispersion and intensifies the reanalysis positive differences in mountainous terrains, as shown by wider probability density functions in elevated areas. Crucially, regional station density modulates trend coherence, as high‐density areas show uniform warming shifts across all percentiles. Overall, both data sets confirm their ability to reproduce seasonality and exhibit a regional warming signal.
Abstract Sub–micron particulate matter (PM 1 ) in the New York (NY) metropolitan area impacts air quality and human health. We characterized refractory black carbon (BC) and non–refractory (NR) PM 1 in Mineola, NY during winter 2024 and NR‐PM 1 during summer 2023. This study investigated seasonal differences in PM 1 , drivers of wintertime PM 1 elevated events, and potential respiratory exposure based on measured PM properties. Organic aerosol (OA) dominates both winter (63%) and summer (86%) NR–PM 1 while BC comprised 6% of winter PM 1 . Primary OA dominates winter PM 1 (57%) with cooking organic aerosol (COA) contributing on average 29%, but up to 81%, of elevated PM 1 events. In summer, OA was impacted by wildfire smoke and biogenic sources and COA averaged only 9% of OA, but sporadically enhanced OA. In winter, COA drove several PM 1 events, during which coating thickness increased. Modeled deposition rates allowed us to explore potential impacts of aerosol size on the human respiratory tract. Simulated deposition indicated urban PM 1 primarily deposited to the sensitive alveolar region. BC–containing particles exacerbated this effect relative to cores in the same size range. Canadian wildfire events during summer 2023 enhanced total deposition to the lungs when weighted by mass, with relative deposition favoring the nose, throat, and associated head airways more than other summer periods. Our observations demonstrate that cooking is an important local source of PM 1 in urban regions throughout the year, and that BC and NR‐PM 1 from multiple sources potentially pose a threat to respiratory deposition and community health.
Abstract In this study, we characterized cold air outbreak (CAO) snowfall over the Shandong Peninsula, East China, using gauge and weather radar observations from 2020 to 2024. We developed a gauge‐radar matched data set and determined the coefficients in Z = aS b every 30 min using a differential evolution algorithm. The results show that the exponent b is relatively stable at 1.72 whereas the prefactor a shows large variations. Then, we fixed b at 1.72 and classified a values into three groups. Our analysis suggests that a larger prefactor a is characterized by smaller peak reflectivity, larger differential reflectivity, larger specific differential phase, larger mean volume diameter, smaller number concentration and smaller snowfall rates and vice versa. We present two cases with contrasting values of a (109.0 vs. 567.8). Surface disdrometer observations suggest that the first event is characterized by significant riming, whereas the latter exhibits signatures of unrimed snow aggregates. Further spectral analysis of the radar mosaic for the two events reveals wavelengths of 29.5 and 141.1 km, with corresponding periods of 0.6 and 3.1 hr, respectively. Multi‐event 2DFFT wavelength analyses suggest that the microphysics of CAO snowfall are associated with the spatial structure of the snowstorm system.
Abstract Anthropogenic warming is expected to substantially modify the hydroclimate of South America, a region in which low‐level jets (LLJs) and mesoscale convective systems (MCSs) are key components of moisture transport and precipitation. However, the responses of these systems to future climate change, including the relative roles of thermodynamic and dynamic forcing, remain poorly understood. Using convection‐permitting model projections under a climate warming of approximately 3°C above preindustrial levels, this study examines future changes in the characteristics of LLJs and MCSs. The results reveal a widespread thermodynamic intensification of MCS precipitation, with maximum rainfall rates surging at magnitudes consistent with thermodynamic expectations. However, the structural evolution of MCSs exhibits a sharp dynamic bifurcation between the tropics and subtropics. In Southeastern South America, the strengthening of the background northerly flow enhances vertical wind shear during Central and Andes LLJ events. This kinematic support facilitates the upscale growth of MCSs into larger, longer‐lived complexes, elevating the potential for high‐volume hydrometeorological extremes. Conversely, the projected circulation anomaly across the western Amazon basin weakens the Northern LLJs, reducing the dynamic organization of historical squall lines and favoring a transition toward widespread, thermodynamically driven, loosely organized convective clusters. These regional shifts are consistent with a stationary Rossby wave train resembling the Pacific‐South American pattern, which enhances meridional moisture transport into the subtropics. These findings highlight a transition toward a complex hydrometeorological hazard profile, in which future extremes are defined not merely by total rainfall, but by regime‐specific changes in storm morphology, duration, and peak intensity.
Abstract This study investigates the impact of assimilating dynamic and thermodynamic atmospheric boundary layer (ABL) observations on the simulation of tropical cyclone (TC) rapid intensification (RI) using the atmosphere–ocean coupled Hurricane Analysis and Forecast System (HAFS) with the Joint Effort for Data assimilation (DA) Integration (JEDI) framework. Despite the recognized importance of ABL processes for TC evolution, the influence of assimilating ABL observations on hurricane prediction remains insufficiently quantified. To address this gap, fully self‐cycled DA experiments were conducted for Hurricane Idalia (2023), which underwent RI from tropical storm to Category 4 TC under favorable oceanic but dry atmospheric conditions. The All ABL experiment assimilated all available observations, including dropsondes sampling the ABL, whereas No ABL withheld those data to isolate their contribution. During DA cycling, All ABL substantially reduced background and analysis errors in ABL temperature and winds, recovering high‐entropy air and strengthening inflow near the inner‐core region. Forecasts initialized from these analyses captured the observed RI (Category 4), whereas No ABL remained mostly below Category 2 intensity. The assimilation of ABL observations enhanced entropy, radial inflow, and tangential wind, supporting stronger secondary circulation and a pronounced warm‐core structure consistent with observations. Air–sea interaction diagnostics further showed that All ABL maintained higher entropy through enhanced wind‐induced enthalpy fluxes, sustaining continuous RI and compact storm organization. These results demonstrate that realistic ABL representation is essential for predicting RI. The findings highlight the need for strongly coupled atmosphere–ocean DA systems to improve future hurricane forecasting.
Abstract Accurate high‐resolution precipitation information is essential for hydrologic modeling, extreme‐event monitoring, and climate assessment, yet, conventional satellite precipitation products are often constrained by the relatively coarse spatial resolution of infrared and microwave observations. We present a quantitative precipitation estimation (QPE) framework that exploits 2‐km visible‐band observations from the Himawari‐8 geostationary satellite within a U‐Net convolutional neural network architecture. The model uses six consecutive daytime multispectral visible images to encode cloud‐motion and cloud‐evolution information for hourly precipitation retrieval. Evaluation against the China multi‐source merged precipitation analysis shows that multispectral and temporally continuous visible‐band information improves precipitation‐area delineation and false‐alarm control compared with single‐time or single‐band visible configurations. The VIS‐based retrieval shows performance comparable to IR QPE in selected light‐to‐moderate rain‐rate ranges, and the VIS+IR QPE configuration provides a more balanced performance by combining VIS‐derived cloud texture and evolution with selected IR thermal and water‐vapor information. Case studies further demonstrate the value of temporal continuity, showing that consecutive visible imagery helps capture fine‐scale cloud structures and associated precipitation patterns during daytime events. These results highlight the potential of high‐spatial‐resolution visible imagery as a complementary daytime source for fine‐scale precipitation monitoring, whereas infrared and microwave observations remain essential for more physically complete precipitation retrieval across the diurnal cycle.
Abstract Typhoon peripheral conditions modulate the formation and atmospheric fate of ozone precursors, yet the vertical distribution and the underlying physical‐chemical drivers remain less constrained. Here, we examine the vertical profiles of ozone precursor proxies, including formaldehyde (HCHO), glyoxal (CHOCHO), and nitrogen dioxide (NO 2 ), in the Pearl River Delta (PRD) in China during the 2023–2024 typhoon seasons using ground‐based Multi‐Axis Differential Optical Absorption Spectroscopy. Under peripheral conditions, tropospheric columns of HCHO and CHOCHO increase by 30.8% and 34.4%, respectively, with the most pronounced enhancements occurring within 0.5–1.0 km. GEOS‐Chem budget analysis indicates that such shift upward of vertical peaks is driven by intensified enhanced vertical transport that enhances net production by 14.0%–25.0% at 0.5–1.0 km. In contrast, NO 2 profiles remain stable, due to a rebalance between strengthened upward convection and accelerated chemical loss. Such vertical redistributions shift the ozone sensitivity regime from volatile organic compound‐limited to NO x ‐limited within 0.5–1.0 km. Our findings underscore the necessity of height‐resolved monitoring for refining ozone mitigation strategies in coastal cities.
Abstract Extreme precipitation has increased over many regions in recent decades, but the simulation and projection remain particularly challenging over complex terrains, such as the Tibetan Plateau (TP). Here, we investigate the future changes in precipitation extremes and persistence over the TP using a kilometer‐scale (3.3 km) convection‐permitting ICON (Icosahedral Nonhydrostatic Weather and Climate Model) model, focusing on comparisons with coarser‐resolution CMIP6 models. Evaluated against station observations, ICON systematically improves the simulation of daily precipitation characteristics, including dry‐day frequency, extreme precipitation and precipitation persistence, reducing root mean square errors by up to ∼10%–94% compared to ERA5 and the CMIP6 ensemble. To project future changes, the pseudo‐global warming (PGW) method is used for ICON, driven by large‐scale climate change signals from the CMIP6 ensemble mean. Both ICON PGW and CMIP6 project qualitatively consistent signals, including increasing extreme precipitation over almost the entire TP and, over the southeastern TP, more dry days and more frequent but shorter precipitation events. Despite consistent signs, ICON PGW suggests a drier southeastern TP than CMIP6, with larger increases in dry days, smaller increases in extreme precipitation and event frequency, and a larger reduction in event duration. The systematic drier future in ICON compared to CMIP6 are linked to projected weakened low‐level summer mean southwesterlies south of the TP, which suppress moisture transport into the interior southeastern TP and thus, reduce both daily and extreme precipitation. As water from southeastern TP affects downstream populations closely, these results provide more reliable projections for future risk assessments.
Abstract Dust, sea salt, and anthropogenic and biogenic particles in total atmospheric deposition are sources of essential elements that can significantly impact biogeochemical cycles in the Atlantic Ocean and in islands of the Caribbean region. This unique 41‐month data set records direct sampling of total atmospheric deposition from March 2015 to August 2018 in Guadeloupe (North Tropical Atlantic Ocean). It quantifies the total atmospheric deposition fluxes of 33 trace elements. Results on deposition samples for Sr and Nd isotopic signatures are consistent with previous studies conducted in the Caribbean region based on aerosol samples, confirming that trans‐Atlantic Saharan dust originated predominantly in the Libya‐Algeria‐Mali region of North Africa. The REE composition is stable for the majority of samples, though some slight inter‐annual variation was observed. Samples collected in 2016 present slightly different REE patterns compared to those collected in 2015, 2017, and 2018. A large fraction of some trace elements (Se, Sb, Cu, Mo, As, and Pb) may come from biogenic sources or from atmospheric pollution occurring at the global level. Lead isotope ratios reveal a mixture of transatlantic dust from the Libya‐Algeria‐Mali region, and anthropogenic sources with no clearly defined region of origin. This result highlights the need for updated reviews of aerosol lead isotopic signatures worldwide. Backward trajectory analysis suggests that atmospheric pollution from North America and Europe may be a significant source of Sb, whereas biogenic gas from the Sargasso Sea may contribute to high levels of Se.
Abstract The impact of aviation induced contrail cirrus on Earth's radiation budget depends on the optical depth (OD) and the life time of these clouds. Their tendency to enhance global warming may be mitigated by replacing conventional kerosene with fuels that form fewer soot particles, such as sustainable aviation fuels (SAF). The resulting reduction in ice nucleation leads to lower contrail OD, shorter contrail life times and, hence, a lower radiative effect (RE). Here we investigate the regional and seasonal variability in the relationship between the reduction in soot number emissions and the associated reduction in the contrail cirrus RE. We use the atmospheric general circulation model ECHAM5‐CCMod to study the impact of replacing kerosene with SAF. The resulting reduction in contrail ice nucleation is largest in winter, when contrails form at ambient temperatures well below the contrail formation threshold. The reduction in contrail cirrus OD and RE, however, is small in winter. Hence, a large reduction in the initial ice crystal number cannot be directly translated into a large reduction of the RE. Instead, reducing ice nucleation is most effectively lowering the RE in seasons and areas in which contrails attain a large ice water content. Our model indicates, that the largest decline in the contrail cirrus RE per used SAF can be achieved over Europe or the North Atlantic flight corridor in autumn and over USA‐Mexico and the North Atlantic in summer.
Abstract Molecular chlorine (Cl 2 ) is a key source of chlorine atoms (Cl∙) in the atmosphere, which can alter local oxidation chemistry and enhance both ozone (O 3 ) and particulate matter. While Cl 2 levels in some coastal cities can be substantial and are enhanced by particulate nitrate, the presence and impact of Cl 2 in US cities is poorly understood. In summer 2023, we measured gas phase Cl 2 mixing ratios (4 and 32.5 m above ground level, a.g.l) and fluxes (32.5 m a.g.l) in Mineola, New York. Cl 2 mixing ratios and fluxes consistently peaked during the afternoon, with averages (± standard deviation) of 24 ± 10 ppt and 170 ± 100 ppt·cm·s −1 , respectively, at 32.5 m from 12 to 4 p.m. local time. Strong correlations between Cl 2 emissions, solar radiation, hypochlorous acid, and O 3 suggest photochemical multiphase chemistry is a prominent source of Cl 2 to the region. Positive fluxes indicate emission of Cl 2 from the surface to the atmosphere and are consistent with sources exceeding the expected multiphase production rate of Cl 2 on aerosol surfaces. We hypothesize that rooftops, fences, roads and other urban surfaces act as reservoirs for chloride (Cl − ) from deteriorating building materials, industrial processes, and dry deposition of sea spray particles and chlorine‐containing gases, enabling heterogeneous formation of gas phase Cl 2 upon interaction with atmospheric pollutants such as O 3 . Deterioration of building materials may also enable surface chemistry that could impact urban chlorine chemistry, even in areas without strong coastal sea spray influence.
Abstract Pronounced interannual variability of the late‐summer (July–August) East Asian summer monsoon exerts strong influences on flood and drought events across East Asia, yet the underlying processes governing this variability remain poorly understood. Using reanalysis data, we identify the dominant mode of circulation variability that captures the dominant interannual variability of the late‐summer East Asian monsoon system. The mode features a troposphere‐deep anticyclonic/cyclonic circulation, tilting northwestward with height, within the westerly jet exit region. This mode is dynamically maintained by extracting kinetic and potential energy from the background flow. The jet exit region enables the mode to gain eddy kinetic energy from the background field. Meanwhile, its northwestward‐tilting vertical structure facilitates the extraction of available potential energy from the meridional temperature gradient over East Asia. Vorticity budget analysis reveals that this northwestward‐tilting vertical structure arises from the vertical shear of climatological northerlies on the eastern flank of the summer South Asian High. In turn, the mode drives vertical motion anomalies via temperature advection, regulating precipitation anomalies of the late‐summer East Asian monsoon.
