Moisture Flux Convergence Research Articles

Effect of offshore troughs on the South India erratic summer monsoon rainfall in June 2017

The onset and advance of southwest monsoon are accompanied by the appearance of the offshore trough along the southwest coast of India. This offshore trough escorts a deluge of rainfall to the southwest coast, and sometimes rainfall band moves eastward further into south India. These broad observations were noticed during the summer monsoon of June 2017. Meteorological agencies and media had reported a huge amount of rainfall over the southwest coast of India during the month. But, in the far interior of south India, rainfall was less. Due to the less rainfall, water resources depleted, which affected local farmers and common man of south India. The confused views of the common man on southwest coast rainfall could be due to lack of understanding related to various factors affecting rainfall over the same region. This article is an endeavor to address the preliminary understanding of the southwest coast rainfall during June 2017, with more stress on offshore troughs. The study begins with area-averaged rainfall statistics over south, southwest, and southeast India by employing satellite and rain gauge merged rainfall datasets. Area averaged analysis revealed offshore trough contributed 80 % of rainfall over the South West India, 68 % over South East India, contributing to an overall 75 % over south India in 2017. To identify offshore trough position and strength in the reanalysis and model simulations, a new method called VSV (Vertical Shear of Vorticity) method was introduced. The computed offshore troughs were categorized into Active, Normal, and Feeble based on the strength of meridional gradient of mean sea level pressure and 850 hPa horizontal winds. The contribution due to each category of the offshore trough over different sub-regions was investigated to find out the effect of the offshore trough to total rainfall. Dynamic and thermodynamic features of these categories of the offshore trough were investigated by using proxies like equivalent potential temperature and moisture flux convergence. We found that during active offshore trough an eastward propagation of rain bands persists, which was explained by using moisture flux convergence and equivalent potential temperature at different levels of the atmosphere.

Impacts of convectively coupled equatorial waves on rainfall extremes in Java, Indonesia

AbstractRainfall extremes cause significant socioeconomic impacts in Indonesia, as they are often followed by disastrous events, such as floods and landslides. Of particular interest is Java Island, the most populated region in Indonesia, which is prone to damaging flooding as a result of heavy rainfall. The prediction of rainfall extremes in this region has mainly been focused on the effects of seasonal and intraseasonal variability, such as monsoons and the Madden–Julian Oscillation. Here, using an extensive station database from 1987 to 2017 and the gridded Asian Precipitation‐Highly Resolved Observational Data Integration Toward Evaluation of Water Resources (APHRODITE) product from 1980 to 2007, we show that severe weather conditions associated with rainfall extremes in Java during the rainy season (November to April) can also be attributed to convectively coupled equatorial waves (CCEWs) that occur on a shorter time scale.Evidence is presented that CCEWs, including Kelvin, equatorial Rossby (ER), and mixed Rossby‐gravity (MRG) waves, significantly modulate daily rainfall extremes over Java Island. Of these three types, the Kelvin waves have the greatest influence on heavy rainfall over Java Island. The convectively active (suppressed) phases of Kelvin waves increase (decrease) the probability of extreme rain events over land regions by up to 60% (50%) of the baseline probability. On the other hand, the convectively active phases of ER (MRG) waves increase the probability by up to 45% (40%), while the suppressed phases decrease this by up to 40% (30%). In terms of the mechanism of rainfall extremes, CCEWs modulate moisture flux convergence, leading to the enhancement of local convection over the region. In addition, the analysis of multiple wave events indicates that positive (negative) interferences of the CCEWs lead to an amplification (suppression) of extreme rainfall probability. Overall, the results suggest that equatorial waves provide an important source of the predictability for daily extreme rainfall events over Java Island.

Atmospheric driving mechanisms of extreme precipitation events in July of 2017 and 2018 in western Japan

The occurrence of extreme precipitation events is now a serious concern in recent years in Japan. This study explores the atmospheric driving mechanisms of two extreme precipitation events occurred during 5–6 July 2017 and 5–8 July 2018 over western Japan. We identified that the atmospheric transport of large amounts of moisture and wind streams with wind speed of minimum 15 m s−1 from south of Japan towards north on the days before these torrential precipitation events are mainly responsible for the July 2017 and July 2018 floods over western Japan. However, the contributions from the moisture advections (both vertical and horizontal) to the atmospheric water budget plays key roles to intensify the precipitations during the said torrential events. We also find that the prominent moisture flux convergence and well-developed moist conditions mainly maintain these heavy precipitation events over the downpour affected area. Our overall analysis suggests that the atmospheric factors driving to all these two heavy precipitation events are qualitatively robust and vital to explain the mechanism of extreme precipitations.

Kenyan Long Rains: A Subseasonal Approach to Process-Based Diagnostics

AbstractThe interannual variability, trends, and the mean climatology of East African long rains are difficult for models to simulate. This is in part because long rains do not respond in a simple way to large-scale modes of variability such as ENSO and because of interactions with complex topography. Here we focus on the Kenyan regional climate in the ERA-Interim dataset during the long rains to create a set of atmospheric diagnostics that can be applied to the evaluation of climate models. Subseasonal observed rainfall and reanalysis reveal that very wet seasons and very dry seasons develop differently at the beginning of the season. Subseasonal aggregation periods (days 60–80, 80–100, 90–120, 120–150) highlight local (e.g., midtropospheric ascent, moisture flux convergence in the lower to midtroposphere, and midtropospheric moisture) and large-scale (e.g., midtropospheric zonal winds over central Africa, upper-tropospheric velocity potential) diagnostics that are useful to evaluate model atmospheric circulation affecting Kenyan rainfall in mean and wet or dry extremes.

Impacts of the Madden–Julian oscillation on precipitation extremes in Indonesia

AbstractThe influence of the Madden–Julian oscillation (MJO) on the precipitation extremes in Indonesia during the rainy season (October to April) has been evaluated using the daily station rain gauge data from 1987 to 2017 and the gridded Asian Precipitation–Highly Resolved Observational Data Integration Towards Evaluation of Water Resources (APHRODITE) from 1998 to 2015 for different phases of the MJO. The results show that MJO significantly modulates the frequency of extreme precipitation events in Indonesia, with the magnitude of the impact varying across regions. Specifically, the convectively active (suppressed) MJO increases (decreases) the probability of extreme precipitation events over the western and central parts of Indonesia by up to 70% (40%). In the eastern part of Indonesia, MJO increases (decreases) extreme precipitation probability by up to 50% (40%). We attribute the differences in the probability of extreme precipitation events to the changes in the horizontal moisture flux convergence induced by MJO. The results indicate that the MJO provides the source of predictability of daily extreme precipitation in Indonesia.

Assessing changes in the atmospheric water budget as drivers for precipitation change over two CORDEX-CORE domains

This study evaluates the projected changes in the atmospheric water budget and precipitation under the RCP8.5 scenario over two CORDEX-CORE domains: South America (SAM) and Europe (EUR). An ensemble of five twenty-first century projections with the Regional Climate Model version 4 (RegCM4) and their driving Global Climate Models (GCMs) are analyzed in terms of the atmospheric water budget terms (precipitation, P; evapotranspiration, ET; and moisture flux convergence, C). Special focus is on four subregions: Amazon (AMZ), La Plata basin (LPB), Mid-Europe (ME) and Eastern Europe (EA). The precipitation change signal in SAM presents a dipole pattern, i.e. drier conditions in AMZ and wetter conditions in LPB. Over the two European regions a seasonality is evident, with an increase of ~ 25% in precipitation for DJF and a decrease of ~ 35% in JJA. The atmospheric water budget drivers of precipitation change vary by region and season. For example, in DJF the main drivers are related to the large-scale moisture flux convergence, while in JJA over the AMZ atmospheric moisture flux convergence plays only a minor role and local processes dominate. For JJA in the GCMs the high values of the residual term do not allow us to assess which mechanisms drive the precipitation change signal over the AMZ and LPB, respectively. Same conclusions are found for the RegCM4 JJA simulations over the LPB and EA. This points to the importance of the spatial resolution of climate simulations and the role of parameterization schemes in climate models. Our work illustrates the usefulness of analyzing regional water budgets for a better understanding of precipitation change patterns around our globe.

Features and Possible Causes of the Extreme Precipitation Anomaly in China During Winter 2019/2020

In this paper, the features and possible causes of extreme precipitation in the winter of 2019/2020 in China are investigated based on the observational station daily dataset provided by the China Meteorological Administration and the ECMWF ERA5 reanalysis dataset. Record-breaking precipitations are found in most areas of China in the winter of 2019/2020, particularly for the north of China. The magnitude of precipitation in south (north) of China in 2019/2020 increase 0.39mm/day (0.15mm/day), about 39.0% (105.5%) compared with the climatological mean. Moisture budget analysis shows that contributions of atmospheric circulation anomaly on horizontal moisture advection and total moisture flux convergence are primarily responsible for the positive anomalies of precipitation in China. Further analysis suggests that the weakened East Asian Trough and the anti-cyclone circulation anomaly over the Sea of Japan favor the transportation of low-level warm and humid airflow from the Northwest Pacific and the East China Sea to the north of China. Meanwhile, the westward movement of the Subtropical High and the Southern Branch Trough anomalies transport low-level warm and humid air from the Bay of Bengal and the South China Sea to south of China. The warm moist air flows northeastward from the East China Sea, Bay of Bengal and the South China Sea while the climatological cold wind flows southeastward, which causes the increase of local precipitation in the winter of 2019/2020. In consideration of the fact that stratospheric polar vortex anomaly has a process of downward propagation to the troposphere, it is of great significance to study the changes of the stratosphere and the relationships between stratosphere and troposphere.

The Observed Water Vapor Budget in an Atmospheric River over the Northeast Pacific

AbstractCombined airborne, shipboard, and satellite measurements provide the first observational assessment of all major terms of the vertically integrated water vapor (IWV) budget for a 150 km × 160 km region within the core of a strong atmospheric river over the northeastern Pacific Ocean centered on 1930 UTC 5 February 2015. Column-integrated moisture flux convergence is estimated from eight dropsonde profiles, and surface rain rate is estimated from tail Doppler radar reflectivity measurements. Dynamical convergence of water vapor (2.20 ± 0.12 mm h−1) nearly balances estimated precipitation (2.47 ± 0.41 mm h−1), but surface evaporation (0.0 ± 0.05 mm h−1) is negligible. Advection of drier air into the budget region (−1.50 ± 0.21 mm h−1) causes IWV tendency from the sum of all terms to be negative (−1.66 ± 0.45 mm h−1). An independent estimate of IWV tendency obtained from the difference between IWV measured by dropsonde and retrieved by satellite 3 h earlier is less negative (−0.52 ± 0.24 mm h−1), suggesting the presence of substantial temporal variability that is smoothed out when averaging over several hours. The calculation of budget terms for various combinations of dropsonde subsets indicates the presence of substantial spatial variability at ~50-km scales for precipitation, moisture flux convergence, and IWV tendency that is smoothed out when averaging over the full budget region. Across subregions, surface rain rate is linearly proportional to dynamical convergence of water vapor. These observational results improve our understanding of the thermodynamic and kinematic processes that control IWV in atmospheric rivers and the scales at which they occur.

Reversed impacts of the Arctic oscillation on the precipitation over the South China Sea and its surrounding areas in October and November

The reversed impacts of the Arctic oscillation (AO) on precipitation over the South China Sea and its surrounding areas (SCSA) in October and November during 1979–2014 are investigated. The correlation coefficients between AO and the precipitation in October and November are 0.44 and − 0.31, which are statistically significant at the 99% and 90% confidence levels, respectively. In October (November), the specific humidity exhibits obvious positive (negative) anomalies in the SCSA, and an upward (downward) airflow moving from ground to the upper troposphere (1000–150 hPa) between 10°N and 30°N (10°N and 20°N) is observed with more (less) cloud cover. Moisture budget diagnosis suggests that the precipitation’s increasing (decreasing) in October (November) mainly contributed by zonal moisture flux convergence (divergence). Furthermore, the Rossby wave guided by westerlies tends to motivate positive geopotential height in the upper troposphere over approximately 20°–30°N, 40°–80°E in October, which is accompanied by a stronger anticyclone in the Arabian Sea region. However, in November, the wave train propagating from the Arabian Sea to the Bay of Bengal is observed in the form of cyclones and anticyclones. Further analysis reveal that the AO in October may increase precipitation through the southern wave train (along the westerly jet stream from North Africa to the Middle East and South China). Moreover, air-sea interactions over the North Pacific might also generate horseshoe-shaped sea surface temperature (SST) anomalies characterized by positive SST in the central subtropical North Pacific surrounded by negative SST, which may affect the precipitation in the SCSA. Ensemble-mean results from CMIP6 historical simulations further confirm these relationships, and the models that can better simulate the observed positive geopotential height in the Arabian Sea present more consistent precipitation’s increasing over the SCSA in October.

Role of atmospheric horizontal resolution in simulating tropical and subtropical South American precipitation in HadGEM3-GC31

Abstract. We assess the effect of increasing horizontal resolution on simulated precipitation over South America in a climate model. We use atmosphere-only simulations, performed with HadGEM3-GC31 at three horizontal resolutions: N96 (∼130 km; 1.88∘×1.25∘), N216 (∼60 km; 0.83∘×0.56∘), and N512 (∼25 km; 0.35∘×0.23∘). We show that all simulations have systematic biases in annual mean and seasonal mean precipitation over South America (e.g. too wet over the Amazon and too dry in the northeast). Increasing horizontal resolution improves simulated precipitation over the Andes and northeast Brazil. Over the Andes, improvements from horizontal resolution continue to ∼25 km, while over northeast Brazil, there are no improvements beyond ∼60 km resolution. These changes are primarily related to changes in atmospheric dynamics and moisture flux convergence. Over the Amazon Basin, precipitation variability increases at higher resolution. We show that some spatial and temporal features of daily South American precipitation are improved at high resolution, including the intensity spectra of rainfall. Spatial scales of daily precipitation features are also better simulated, suggesting that higher resolution may improve the representation of South American mesoscale convective systems.

Impact of recent vegetation greening on temperature and precipitation over China

Changes in land-use management and natural climate variability has led to significant vegetation greening in China over the past decades, as evidenced by satellite observations. In this study, we used a high-resolution land-atmosphere coupled regional climate model to assess the impacts of recent greening on China's regional summer climate. Our results inferred a country-averged surface cooling of 0.11 °C and a precipitation increase of 0.02 mm/d during summer in response to China's widespread greening. We also identified significant regional variability in the climatic responses to vegetation change. The regions of northern China showed significant cooling, including the Northeast China Plain, Loess Plateau, and the eastern areas of the Northern arid and semi-arid region; however, no significant temperature changes were simulated in southern China. Summer precipitation decreased in southern China and increased in the Huang-Huai-Hai Plain and the northern Loess Plateau. Variations in moisture flux convergence had a stronger influence on the precipitation patterns relative to evapotranspiration. Furthermore, our results suggested that vegetation greening would influence the occurrence of extreme climate events by decreasing the number of heatwave days (by 1–3 days, particularly in northern China), reducing the number of continuous drought events in northern China, and minimizing the risks of floods in southern China.

The Moisture Budget of Tropical Cyclones in HighResMIP Models: Large-Scale Environmental Balance and Sensitivity to Horizontal Resolution

AbstractPrevious studies have shown that the number, intensity, and structure of simulated tropical cyclones (TCs) in climate models get closer to the observations as the horizontal resolution is increased. However, the sensitivity of tropical cyclone precipitation and moisture budget to changes in resolution has received less attention. In this study, we use the five-model ensemble from project PRIMAVERA/HighResMIP to investigate the systematic changes of the water budget of tropical cyclones in a range of horizontal resolutions from 1° to 0.25°. Our results show that, despite a large change in the distribution of TC intensity with resolution, the distribution of precipitation per TC (i.e., averaged in a 5° radial cap) does not change significantly. This result is explained by the fact that low- and high-resolution models represent equally well the large-scale balance that characterizes the moisture budget of TCs, with the radius of the moisture source extending to ~15° from the center of the TC (i.e. well beyond the TC edge). The wind profile is found to converge in the low and high resolutions for radii > 5°, resulting in a moisture flux convergence into the TC of similar magnitude at low and high resolutions. In contrast to precipitation per TC, TC intensity does increase at higher resolution and this is explained by the larger surface latent heat flux near the center of the storm, which leads to an increase in equivalent potential temperature and warmer core anomalies, although this extra latent heat represents a negligible contribution to the overall moisture budget. We discuss the complication arising from the choice of the tracking algorithm when assessing the impact of model resolution.

Process‐Based Analysis of the Added Value of Dynamical Downscaling Over Central Africa

AbstractIn this study, nine global climate models (GCMs) and corresponding downscaled runs by means of the regional climate model (RCM) RCA4 are used to investigate added value (AV) in precipitation and its some drivers over Central Africa (CA). By employing a process‐based analysis approach, we intercompare abilities of RCM to those of driving GCMs in representing the total atmospheric moisture flux convergence (TMFC), moisture transport, and African Easterly Jets (AEJs). Results indicate that simulations with highest AVs in the precipitation climatology also show improvements in the representation of the TMFC and AEJs. Degraded precipitation due to the downscaling is associated with deterioration of at least two of three analyzed mechanisms, and sometimes there is inconsistent AVs between precipitation and related drivers. This sustains that a realistic representation of the moisture transport and atmospheric circulation is of great importance for the correct simulation of present (and, consequently, future) precipitation over CA.

Australian Rainfall Anomalies in 2018–2019 Linked to Indo‐Pacific Driver Indices Using ERA5 Reanalyses

AbstractThe 2019 and 2018–2019 periods had record low All‐Australia rainfall in both observations and the ECMWF's Reanalysis 5, or ERA5, data set over 1979–2019. An analysis of the relationships between interannual variability of rainfall and atmospheric circulation, vertically integrated moisture flux, and temperature anomalies using ERA5 alone is undertaken. Both standard driver indices and those that combine the Pacific and Indian Ocean influences are used. Regression fields for low annual Australian rainfall show a widespread negative rainfall anomaly extending into the east Indian Ocean or IND region and a positive anomaly in the western Pacific PAC region. A moisture flux anomaly takes moisture eastwards toward PAC. This pattern largely persists in the four seasons. In March–May 2019 it can be partially linked to a positive Niño 3.4 anomaly and in June–August to a positive Indian Ocean Dipole. For the annual case, the detrended Niño 4 index explains 24% of the 2019 ERA5 Australian rainfall deficit. This rises to 38% for the 2018–2019 deficit (46% for observations), for the Pacific‐Indian Dipole index that combines the PAC and IND regions, whose sea surface temperature anomalies have opposite effects. The correlation of the index with 2‐year rainfall is −0.7. There is much variability, in the Australian monsoon rainfall especially, that is not linked to simple indices, although rainfall is well matched with moisture flux convergence. This extends to the equatorial zone where the climate of the Maritime Continent can be better examined with ERA5's 0.25° data.

Analysis of Coupled Oceanic and Atmospheric Preconditioning for Primary Madden‐Julian Oscillation Events Across ENSO Phases

AbstractThe Madden‐Julian Oscillation (MJO) is the dominant mode of air‐sea interaction over intraseasonal timescales. The effects of the MJO are well understood, but the initiation of the MJO remains less conclusive, particularly under El Niño Southern Oscillation (ENSO) conditions. Primary MJO events are those not immediately preceded by existing MJO activity of sufficient strength. As they are rare by definition, primary MJOs remain difficult to study, especially so when observations of events are scarce and of low spatiotemporal resolution. The advent of satellites allows for more expansive observations to be made more frequently than in situ methods, thus improving the observational capabilities of pre‐primary MJO conditions in the ocean and atmosphere. We examined oceanic and atmospheric intraseasonal signals preceding two primary MJO events during contrasting ENSO events in an attempt to bridge the connection between oceanic and atmospheric observations as potentially coupled trigger mechanisms. Satellite observations and model simulations of the central and western Indian Ocean show that intraseasonal peaks in absolute dynamic topography (ADT) and sea surface temperature (SST) upward of 1 to 2 weeks prior to the observed outgoing longwave radiation (OLR) minimum. Surface ocean warming moistens the near surface through anomalous surface fluxes, which destabilizes the lower atmosphere to deep convection. Low‐level moisture flux convergence (MFC) moistens the lower atmosphere prior to convective initiation, thus forcing an increase of total column moist static energy (MSE). Coupled midtropospheric cooling is observed that further destabilizes the atmosphere. Zonal shifts in contributing initiating parameters are observed during ENSO phases.

Impacts of Protected Area Deforestation on Dry‐Season Regional Climate in the Brazilian Amazon

AbstractRainforest in protected areas in the Brazilian Amazon is at risk due to increasing economic pressures and recent weakening of environmental agencies and legislation by the federal administration. This study examines the impacts of deforestation in protected areas on dry‐season precipitation in the Brazilian state of Rondônia located in the southwestern Brazilian Amazon. Regional‐climate model simulations indicate that clearing protected forests in Rondônia would result in substantial changes to the surface energy balance, including increased sensible and decreased latent heat flux. Consequent changes to low‐level wind circulation would enhance moisture flux convergence and convection over the newly deforested areas, leading to enhanced rainfall in those areas. However, deforestation of protected areas would decrease dry season rainfall up to 30% in the existing agricultural region, with potentially important negative impacts on agricultural production. Additionally, our results indicate that following deforestation, the newly degraded areas will experience warmer and drier afternoons that could place the remaining natural vegetation under vapor deficit stress.

A New Lens for Evaluating Dynamic Controls on Shallow Convection

AbstractRising temperatures and changing dynamics can both moisten the air, making it difficult to disentangle these interrelated drivers of water cycle change. However, work by Camille Risi and colleagues presents a new way to distinguish their effects. Using large‐eddy simulations with water isotopic tracers, they show that while warming the sea surface increases the ratio of isotopically heavy‐to‐light water in the tropical marine subcloud layer, strengthening the moisture flux convergence decreases it. This divergent response provides a new framework for examining the complex mechanisms that regulate the development of convection and, ultimately, cloudiness—a target of the 2020 international field campaign EUREC4A (ElUcidating the RolE of Clouds‐Circulation Coupling in ClimAte). Moreover, their findings provide a clearer picture of why water isotopes recorded in tropical paleoproxies are a valuable lens through which to view changes in moisture transport in the past.

Ensemble Variability in Rainfall Forecasts of Hurricane Irene (2011)

AbstractTropical cyclones (TCs) moving into the midlatitudes can produce extreme precipitation, as was the case with Hurricane Irene (2011). Despite the high-impact nature of these events, relatively few studies have explored the sensitivity of TC precipitation forecasts to model initial conditions. Here, the physical processes that modulate precipitation forecasts over the Northeast United States during Irene are investigated using an 80-member 0.5° Global Forecasting System (GFS) ensemble. The members that forecast the highest total precipitation over the Catskill Mountains in New York (i.e., wet members) are compared with the members that predicted the least precipitation (i.e., dry members). Results indicate that the amount of rainfall is tied to storm track, with the wetter members forecast to move farther west than the dry members. This variability in storm track appears to be associated with variability in analyzed upper-tropospheric potential vorticity (PV), such that the wetter members feature greater cyclonic PV southwest of Irene when Irene is off the Carolina coast. By contrast, the wetter members of a 3-km Weather Research and Forecasting (WRF) Model ensemble, initialized from the same GFS ensemble forecasts, show little sensitivity to track. Instead, the wetter members are characterized by stronger lower-tropospheric winds perpendicular to the eastern face of the Catskills, allowing maximum upslope forcing and horizontal moisture flux convergence during the period of heaviest rainfall. The drier members, on the other hand, have the greatest quasigeostrophic forcing for ascent, implying that the members’ differences in mesoscale topographic forcing are the dominant influence on rainfall rate.

Impacts of soil moisture and atmospheric moisture transport on the precipitation in two typical regions of China

The effects of soil moisture and atmospheric moisture flux convergence (MFC) on the precipitation are studied in two typical regions in China, one contains both arid and semi-arid regions and the other encloses humid and semi-humid regions. Eight simulations using the weather research and forecasting (WRF) model are conducted by varying the soil moisture and MFC. It is found that in both regions, MFC has stronger effects on precipitation than soil moisture. By decomposing the precipitation change into sub contributing factors, it is found that the contribution of MFC to precipitation differ greatly between different regions. MFC affects precipitation indirectly through changing the precipitation efficiency and directly by changing the moisture influx in the arid and humid regions, whereas, in the semi-arid regions it affects the precipitation mainly by changing the precipitation efficiency and in the semi-humid regions, mainly through changing the moisture influx. Soil moisture affects the precipitation mainly through modifying the precipitation efficiency. Wet soil conditions can increase the surface pressure and reduce the moisture flux in humid regions. The mechanisms of MFC change are explored by decomposing δMFC into thermodynamic (δTH), mean circulation dynamic (δMCD), and transient eddy (δTE) components. MFC increases mainly through the positive contribution of δMCD, while the δTH and δTE mostly contribute negatively. Both δMCD and δTH contribute to the changes in MFC through the convergence/divergence term, rather than through the advection term. δTE is negative over the relatively flat regions and positive over the complex terrain with sharp terrain height gradients.

Warm‐season precipitation drivers in northeastern Argentina: Diurnal cycle of the atmospheric moisture balance and land–atmosphere coupling

AbstractSoutheastern South America is influenced by moisture transport from lower latitudes, sustains intense convective storms and is a land–atmosphere coupling hotspot, but the interconnection between these processes is still not well understood. We present the warm‐season diurnal cycle climatology of the atmospheric water balance components in the South American Low‐Level Jet (SALLJ) exit region in northeastern Argentina during 1998–2012. Different precipitation‐based types of events (clear‐sky and rainy days) were explored together with processes tied to the land–atmosphere coupling at the daily scale. Our research was based on simulations with and without soil moisture–atmosphere coupling with the RCA4 regional climate model. A control simulation was compared with a sensitivity simulation where the soil moisture was prescribed with the daily climatological values from the control run. The ERA5 reanalysis and the satellite precipitation products TRMM‐3B42 v7 and CMORPH v1.0 bias‐corrected were used for comparative purposes. From the diurnal water balance analysis, we found that moisture flux convergence in the region is the main driver for nocturnal precipitation while local evapotranspiration feeds afternoon rain events. Rainy afternoons do not show differences between simulations, but rainy nights seem to be affected. Moreover, daily correlations between surface and boundary‐layer variables showed that the local coupling is weaker during rainy days than during clear‐sky days. Therefore, we suggest that changes in non‐local drivers, such as the moisture flux through the SALLJ, are more relevant for rainy nights than the local coupling.