Midtropospheric Moisture Research Articles

Extreme rainfall in Dakar (Senegal): a case study for September 5, 2020

West African countries frequently experience extreme rainfall events during the monsoon season. On September 5, 2020, a significant event occurred in the Dakar region of Senegal with daily rainfall totals exceeding 90 mm, causing widespread flooding and displacing 1,000's of people. Despite the severity of this event, the physical mechanisms driving such extreme rainfall remain unexplored. This study aims to investigate the physical mechanisms associated with this event using multiple data sources, including satellite rainfall estimate products (GPM-IMERG, CHIRPS) and reanalysis data (ERA-5). By analyzing wind fields and mid-tropospheric moisture content from reanalysis data, we examined the synoptic-dynamic evolution of the atmosphere and the movement of the cyclonic vortex that transported moisture to the affected region, resulting in substantial rainfall measurements exceeding 100 mm. The analysis also revealed that a vortex over the ocean slowed down the vortex near Senegal, prolonging the rainfall over a total period of 10 h. Additionally, this study presents a comprehensive comparative analysis of state-of-the-art satellite rainfall estimates, assessing their accuracy and reliability in capturing extreme rainfall events both spatially and at specific rainfall gauges situated in Dakar. This evaluation revealed that while satellite rainfall estimates are valuable, they tended to underestimate (up to 40%) the actual rainfall observed at the Dakar-Yoff station. Furthermore, extreme value analysis showed that there is a tendency to underestimate return levels for high-intensity events, with some cases showing underestimations by up to twice the actual values. Thus, this research advances our understanding of extreme rainfall events in West Africa and improves our knowledge of satellite-based rainfall estimates, contributing to future monitoring and preparedness. Furthermore, these findings highlight the importance of monitoring cyclonic systems associated with African Easterly Waves, contributing to a better understanding of extreme rainfall events in West Africa.

Revisiting the Seasonal Cycle of Rainfall over Central Africa

Abstract The intertropical convergence zone (ITCZ), with its twice-annual passage over central Africa, is considered as the main driver of the rainfall seasonality. In this ITCZ paradigm, high rainfall occurs over regions of large low-level convergence. But recently, this paradigm was challenged over central Africa. Here, we show that a shallow meridional overturning circulation—driven by surface conditions—plays a thermodynamical control on the rainfall seasonality over central Africa. Indeed, due to the local evaporative cooling effect, the foot of the ascending branch of Hadley cells occurs where the temperature is the warmest, indicating a thermal low. This distorts the southern Hadley cell by developing its bottom-heavy structure. As result, both shallow and deep Hadley cells coexist over central Africa year-round. The deep mode is associated with the poleward transport of atmospheric energy at upper levels. The shallow mode is characterized by a shallow meridional circulation, with its moisture transport vanishing and converging in the midtroposphere rather than at lower troposphere. This midtropospheric moisture convergence is also the dominant component that shapes the vertically integrated moisture flux convergence, with little contribution of African easterly jets. This convergence zone thus controls the precipitating convection. Its meridional migration highlights the interhemispheric rainfall contrast over central Africa and outlines the unimodal seasonality. On the other hand, forced by the Congo basin cell, the precipitable water regulates the deep convection from the vegetated surface of Congo basin, acting as a continental sea. This nonlinear mechanism separates the rainfall into three distinct regimes: the moisture-convergence-controlled regime, with convective rainfall exclusively occurring in the rainy season; the local evaporation-controlled regime with drizzle in the dry season; and the precipitable-water-controlled regime, with exponential rainfall increase in the dry season.

Unfavorable environmental conditions for tropical cyclone genesis over the western North Pacific during the Last Interglacial based on PMIP4 simulations

Investigating the variation in tropical cyclone (TC) activity over the western North Pacific (WNP) in past warm periods helps to better understand TC behaviors in a warming future. In this study, the authors analyze the changes in large-scale TC genesis factors and the associated mechanisms over the WNP during the Last Interglacial (LIG), based on multimodel outputs from the Paleoclimate Modelling Intercomparison Project Phase 4. The results show that potential intensity exhibits a general decrease over the WNP during the LIG in the storm season, dominated by a weakened thermodynamic disequilibrium. The moist entropy deficit shows an overall increase over the WNP, arising from the decreased mid-tropospheric moisture and weakened vertical temperature contrast. Vertical wind shear enhances over the central WNP but weakens over the southwestern WNP, which is induced by the changes in the meridional tropospheric temperature gradient and hence high-level zonal winds. The absolute vorticity shows a general decrease over the WNP, partially linked with the decreased SST over the western tropical Pacific. Based on a genesis potential index, the authors suggest a decrease in genesis potential over the WNP during the LIG, indicating unfavorable conditions for TC genesis. The results highlight the important role of Earth's orbit in regulating TC activity, which may shed light on TC behaviors in a warmer climate.

Dynamic comparative analysis of different types of precipitation caused by landfalling strong typhoons over South China

Based on multiple datasets, a comparative analysis has investigated the different rainfall distributions and the factors related to strong (weak) rainfall induced by landfalling strong typhoons moving northwest (SR-NWLFSTYs, WR-NWLFSTYs) which are defined from the tracks, the locations, and intensities of the rainfall center of LFSTYs based on a dynamic composite analysis method.The obviously stronger rainfall induced by SR-NWLFSTYs, with the intensive rainfall center presenting a cyclonic rotation in the lifecycle of typhoons, tends to be accompanied by the pronouncedly enhanced low-level jet and a warmer SST before and during landfall. Owing to the enhanced low-level jet, moisture convergence experiences an intensification, resulting in a warmer and more humid circulation. Sufficient water vapor and latent heat is favorable for the enhancement of the mid-troposphere warm core structure with longer maintenance which can prolong the impact period of typhoons, leading to continuous precipitation. Simultaneously, the coupling of low-level convergence and upper-level divergence provides favorable dynamic conditions, thus further promoting the upward transportation of moisture through Ekman pumping, which tends to increase the mid-troposphere moisture through moisture advection. The changes in the mid-troposphere moisture coincide well with the evolution of rainfall over the same region during the lifecycle of typhoons.Comparatively, the asymmetric rainfall features of WR-NWLFSTYs with intense rainfall center mainly placed in the northeast quarter of the TC center during the lifecycle of TCs is likely related to the relatively weaker low-level jet which remains almost constant during and after landfall. The weaker low-level flow transports less moisture into the cyclone, and prohibits the weakening of vertical wind shear after landfall, leading to reduced precipitation with an obvious asymmetric distribution. Concurrently, the configuration of the weakening upper-level divergence and low-level convergence after landfall has been unable to maintain the upward motion for precipitation, resulting in relatively weaker rainfall for WR-NWLFSTYs.

Meteorological and geographical factors associated with dry lightning in central and northern California

Lightning occurring with less than 2.5 mm of rainfall—typically referred to as ‘dry lightning’—is a major source of wildfire ignition in central and northern California. Despite being rare, dry lightning outbreaks have resulted in destructive fires in this region due to the intersection of dense, dry vegetation and a large population living adjacent to fire-prone lands. Since thunderstorms are much less common in this region relative to the interior West, the climatology and drivers of dry lightning have not been widely investigated in central and northern California. Using daily gridded lightning and precipitation observations (1987–2020) in combination with atmospheric reanalyses, we characterize the climatology of dry lightning and the associated meteorological conditions during the warm season (May–October) when wildfire risk is highest. Across the domain, nearly half (∼46%) of all cloud-to-ground lightning flashes occurred as dry lightning during the study period. We find that higher elevations (>2000 m) receive more dry lightning compared to lower elevations (<1000 m) with activity concentrated in July-August. Although local meteorological conditions show substantial spatial variation, we find regionwide enhancements in mid-tropospheric moisture and instability on dry lightning days relative to background climatology. Additionally, surface temperatures, lower-tropospheric dryness, and mid-tropospheric instability are increased across the region on dry versus wet lightning days. We also identify widespread dry lightning outbreaks in the historical record, quantify their seasonality and spatial extent, and analyze associated large-scale atmospheric patterns. Three of these four atmospheric patterns are characterized by different configurations of ridging over the continental interior and offshore troughing. Understanding the meteorology of dry lightning across this region can inform forecasting of possible wildfire ignitions and is relevant for assessing changes in dry lightning and wildfire risk in climate projections.

Close Linkage of the South China Sea Summer Monsoon Onset and Extreme Rainfall in May over Southeast Asia: Role of the Synoptic-Scale Systems

Abstract Based on multiple datasets and methods, this study reveals a close relation between the South China Sea summer monsoon (SCSSM) onset and the occurrence frequency of extreme rainfall over Southeast Asia (including the Indochina Peninsula, Hainan Island, and the Philippines) in May. Specifically, an early SCSSM onset tends to be accompanied by a higher chance of extreme rainfall over Southeast Asia, while a late SCSSM onset is likely to coincide with less heavy rainfall in May. It is suggested that the amplitude of tropical synoptic-scale systems may play an important role in the linkage between the SCSSM onset and the extreme rainfall over Southeast Asia. Accompanying an early SCSSM onset, anomalous cyclonic circulation appears over the SCS and Philippine Sea, which tends to increase the quantity of midtroposphere moisture via moisture advection and Ekman pumping. Besides, the barotropic energy conversion (from mean kinetic to eddy kinetic energy) associated with this anomalous cyclone will facilitate the development of the synoptic-scale systems. The vertical easterly wind shear accompanying an early SCSSM onset will confine the synoptic-scale wave train to the lower troposphere, which is also favorable for its growth. Once these synoptic-scale systems develop, the baroclinic process (from eddy available potential energy to eddy kinetic energy) will further promote their development. As such, the intensity of synoptic-scale systems is much stronger in early than late SCSSM onset years. The increased moisture and vigorous synoptic-scale systems further lead to an increase in the occurrence of extreme rainfall over Southeast Asia.

Deep Convection Initiation, Growth, and Environments in the Complex Terrain of Central Argentina during CACTI

Abstract This study characterizes the wide range of deep convective cloud life cycles and their relationships with ambient environments observed during the Cloud, Aerosol, and Complex Terrain Interactions (CACTI) field campaign near the Sierras de Córdoba (SDC) range in central Argentina. We develop a novel convective cell tracking database for the entire field campaign using C-band polarimetric radar observations. The cell tracking database includes timing, location, area, depth, merge/split information, microphysical properties, collocated satellite-retrieved cloud properties, and sounding-derived environmental conditions. Results show that the SDC exerts a strong control on convection initiation (CI) and growth. CI preferentially occurs east of the SDC ridge during the afternoon, and cells often undergo upscale growth through the evening as they travel eastward toward the plains. Larger and more intense cells tend to occur in more unstable and humid low-level environments, and surface-based cells are stronger than elevated cells. Midtropospheric relative humidity and vertical wind shear also jointly affect the size and depth of the cells. Rapid cell area growth rates exhibit dependence on both large environmental wind shear and low-level moisture. Evolution of convective cell macro- and microphysical properties are strongly influenced by convective available potential energy and low-level humidity, as well as the presence of other cells in their vicinity. This cell tracking database demonstrates a framework that ties measurements from various platforms centering around convective life cycles to facilitate process understanding of factors that control convective evolution. Significance Statement The purpose of this study is to develop a framework that ties coordinated radar, satellite, and radiosonde measurements around tracking convective storm life cycles to facilitate process understanding of atmospheric environments that control storm evolution. The processes coupling storm life cycles and local environments remain inadequately understood and are poorly represented in weather and climate models. Our results demonstrate the importance of atmospheric instability, low- and midtropospheric moisture, changes of wind with height, and interactions among nearby storms in affecting the formation and growth of convective storms. The storm database developed in this work enables future studies for comprehensive exploration of processes that lead to improved mechanistic understanding of storm evolution and their representations in models.

Moisture origin, transport pathways, and driving processes of intense wintertime moisture transport into the Arctic

Abstract. A substantial portion of the moisture transport into the Arctic occurs in episodic, high-amplitude events with strong impacts on the Arctic's climate system components such as sea ice. This study focuses on the origin of such moist-air intrusions during winter and examines the moisture sources, moisture transport pathways, and their linkage to the driving large-scale circulation patterns. For that purpose, 597 moist-air intrusions, defined as daily events of intense (exceeding the 90th anomaly percentile) zonal mean moisture transport into the polar cap (≥70∘ N), are identified. Kinematic backward trajectories combined with a Lagrangian moisture source diagnostic are then used to pinpoint the moisture sources and characterize the airstreams accomplishing the transport. The moisture source analyses show that the bulk of the moisture transported into the polar cap during these moist-air intrusions originates in the eastern North Atlantic with an uptake maximum poleward of 50∘ N. Trajectories further reveal an inverse relationship between moisture uptake latitude and the level at which moisture is injected into the polar cap, consistent with ascent of poleward-flowing air in a baroclinic atmosphere. Focusing on intrusions in the North Atlantic (424 intrusions), we find that lower tropospheric moisture transport is predominantly accomplished by two types of airstreams: (i) cold, polar air warmed and moistened by surface fluxes and (ii) air subsiding from the mid-troposphere into the boundary layer. Both airstreams contribute about 36 % each to the total transport. The former accounts for most of the moisture transport during intrusions associated with an anomalously high frequency of cyclones east of Greenland (218 intrusions), whereas the latter is more important in the presence of atmospheric blocking over Scandinavia and the Ural Mountains (145 events). Long-range moisture transport, accounting for 17 % of the total transport, dominates during intrusions with weak forcing by baroclinic weather systems (64 intrusions). Finally, mid-tropospheric moisture transport is invariably associated with (diabatically) ascending air and moisture origin in the central and western North Atlantic, including the Gulf Stream front, accounting for roughly 10 % of the total transport. In summary, our study shows that moist-air intrusions into the polar atmosphere result from a combination of airstreams with predominantly high-latitude or high-altitude origin, whose relative importance is determined by the underlying driving weather systems (i.e., cyclones and blocks).

Are elevated moist layers a blind spot for hyperspectral infrared sounders? A model study

Abstract. The ability of the hyperspectral satellite-based passive infrared (IR) instrument IASI to resolve elevated moist layers (EMLs) within the free troposphere is investigated. EMLs are strong moisture anomalies with significant impact on the radiative heating rate profile and typically coupled to freezing level detrainment from convective cells in the tropics. A previous case study by Stevens et al. (2017) indicated inherent deficiencies of passive satellite-based remote sensing instruments in resolving an EML. In this work, we first put the findings of Stevens et al. (2017) into the context of other retrieval case studies of EML-like structures, showing that such structures can in principle be retrieved, but retrievability depends on the retrieval method and the exact retrieval setup. To approach a first more systematic analysis of EML retrievability, we introduce our own basic optimal estimation (OEM) retrieval, which for the purpose of this study is based on forward-modelled (synthetic) clear-sky observations. By applying the OEM retrieval to the same EML case as Stevens et al. (2017), we find that a lack of independent temperature information can significantly deteriorate the humidity retrieval due to a strong temperature inversion at the EML top. However, we show that by employing a wider spectral range of the hyperspectral IR observation, this issue can be avoided and EMLs can generally be resolved. We introduce a new framework for the identification and characterization of moisture anomalies, a subset of which are EMLs, to specifically quantify the retrieval's ability to capture moisture anomalies. The new framework is applied to 1288 synthetic retrievals of tropical ocean short-range forecast model atmospheres, allowing for a direct statistical comparison of moisture anomalies between the retrieval and the reference dataset. With our basic OEM retrieval, we find that retrieved moisture anomalies are on average 17 % weaker and 15 % thicker than their true counterparts. We attribute this to the retrieval smoothing error and the fact that rather weak and narrow moisture anomalies are most frequently missed by the retrieval. Smoothing is found to also constrain the magnitude of local heating rate extremes associated with moisture anomalies, particularly for the strongest anomalies that are found in the lower to mid troposphere. In total, about 80 % of moisture anomalies in the reference dataset are found by the retrieval. Below 5 km altitude, this fraction is only of the order of 52 %. We conclude that the retrieval of lower- to mid-tropospheric moisture anomalies, in particular of EMLs, is possible when the anomaly is sufficiently strong and its thickness is at least of the order of about 1.5 km. This study sets the methodological basis for more comprehensively investigating EMLs based on real hyperspectral IR observations and their operational products in the future.

Interannual Variation in Stable Isotopes in Water Vapor Over the Northern Tibetan Plateau Linked to ENSO

AbstractThe contrasting responses of ice core δ18O records (δ18Oice) from the northern and southern Tibetan Plateau (TP) to the El Niño Southern Oscillation (ENSO) complicate δ18Oice‐based temperature reconstructions. These contrasting responses were explored using stable hydrogen isotopes in water vapor (δDv). We found that the interannual variation in δDv from the northern TP is influenced by midtropospheric moisture sources from the Indo‐Pacific Warm Pool, with enriched (depleted) δDv values in La Niña (El Niño) years due to enhanced (weakened) deep convection. In comparison, the interannual δDv variation in the southern TP is mainly influenced by low‐tropospheric moisture from the central Indian Continent, whose δDv is depleted (enriched) in La Niña (El Niño) years. These results demonstrate that moisture transported from different altitudes can lead to opposite isotopic signals of water vapor over the northern and southern TP and can be used to reconcile the contrasting responses of δ18Oice to ENSO.

Comparison of Convective Parameters Derived from ERA5 and MERRA-2 with Rawinsonde Data over Europe and North America

AbstractIn this study we compared 3.7 million rawinsonde observations from 232 stations over Europe and North America with proximal vertical profiles from ERA5 and MERRA-2 to examine how well reanalysis depicts observed convective parameters. Larger differences between soundings and reanalysis are found for thermodynamic theoretical parcel parameters, low-level lapse rates, and low-level wind shear. In contrast, reanalysis best represents temperature and moisture variables, midtropospheric lapse rates, and mean wind. Both reanalyses underestimate CAPE, low-level moisture, and wind shear, particularly when considering extreme values. Overestimation is observed for low-level lapse rates, midtropospheric moisture, and the level of free convection. Mixed-layer parcels have overall better accuracy when compared to most-unstable parcels, especially considering convective inhibition and lifted condensation level. Mean absolute error for both reanalyses has been steadily decreasing over the last 39 years for almost every analyzed variable. Compared to MERRA-2, ERA5 has higher correlations and lower mean absolute errors. MERRA-2 is typically drier and less unstable over central Europe and the Balkans, with the opposite pattern over western Russia. Both reanalyses underestimate CAPE and CIN over the Great Plains. Reanalyses are more reliable for lower elevation stations and struggle along boundaries such as coastal zones and mountains. Based on the results from this and prior studies we suggest that ERA5 is likely one of the most reliable available reanalyses for exploration of convective environments, mainly due to its improved resolution. For future studies we also recommend that computation of convective variables should use model levels that provide more accurate sampling of the boundary layer conditions compared to less numerous pressure levels.

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.

Simulation of Central Indian Ocean Mode in S2S Models

AbstractSimulation of monsoon intraseasonal oscillations (MISO) during the Indian summer monsoon (ISM) is a grand scientific challenge. The Subseasonal‐to‐Seasonal (S2S) prediction project provides a unique way to examine the key dynamics of MISO. Recently, a Central Indian Ocean (CIO) mode was proposed as an intrinsic climate mode over the Indian Ocean, and it has a close relation with MISO during the ISM. In this study, the simulations of the CIO mode events in S2S models are examined. The results confirm that a better rendition of the CIO mode in S2S models tends to result in a better simulation of northward propagating MISO and leads to stronger subseasonal rainfall during the ISM. The positive CIO mode cases are classified into well‐simulated and poorly simulated groups. It is shown that the barotropic energy conversion due to the meridional shear of background zonal winds ( ) enhances the kinetic energy at subseasonal timescales during the CIO mode in the well‐simulated group, which also agrees with observations. As a result, the enhanced meridional wind anomalies transport moisture from the tropics to the subtropics and reinforce midtropospheric moisture loading in the subtropics, which nourishes the northward propagating MISO. Therefore, a better simulation of and moisture transport by subseasonal meridional wind anomalies are required to improve the CIO mode simulation, which is expected to benefit the simulation and prediction of MISO during the ISM.

Interpreting and Stabilizing Machine-Learning Parametrizations of Convection

AbstractNeural networks are a promising technique for parameterizing subgrid-scale physics (e.g., moist atmospheric convection) in coarse-resolution climate models, but their lack of interpretability and reliability prevents widespread adoption. For instance, it is not fully understood why neural network parameterizations often cause dramatic instability when coupled to atmospheric fluid dynamics. This paper introduces tools for interpreting their behavior that are customized to the parameterization task. First, we assess the nonlinear sensitivity of a neural network to lower-tropospheric stability and the midtropospheric moisture, two widely studied controls of moist convection. Second, we couple the linearized response functions of these neural networks to simplified gravity wave dynamics, and analytically diagnose the corresponding phase speeds, growth rates, wavelengths, and spatial structures. To demonstrate their versatility, these techniques are tested on two sets of neural networks, one trained with a superparameterized version of the Community Atmosphere Model (SPCAM) and the second with a near-global cloud-resolving model (GCRM). Even though the SPCAM simulation has a warmer climate than the cloud-resolving model, both neural networks predict stronger heating/drying in moist and unstable environments, which is consistent with observations. Moreover, the spectral analysis can predict that instability occurs when GCMs are coupled to networks that support gravity waves that are unstable and have phase speeds larger than 5 m s−1. In contrast, standing unstable modes do not cause catastrophic instability. Using these tools, differences between the SPCAM-trained versus GCRM-trained neural networks are analyzed, and strategies to incrementally improve both of their coupled online performance unveiled.

The Interaction between Moist Convection and the Atmospheric Circulation in the Tropics

AbstractTheories of the interaction between moist convection and the atmospheric circulation in the tropics are reviewed. Two main schools of thought are highlighted: (i) one that emphasizes the lower-level control of convection through moisture convergence and variations in convective inhibition, and (ii) one that sees convection as an adjustment process in reaction to larger-scale instabilities, referred to as convective quasi-equilibrium theory. Conceptually the two views consider moist convection to have fundamentally different roles in the tropical circulation. In one case the presence of low-level inhibition and the conditional nature of the atmospheric instability allows for convective vertical motion and latent heating to drive and reinforce synoptic-scale disturbances and overturning circulations; in the other case, because low-level inhibition is not acknowledged to be a widespread controlling barrier, convection is believed to balance and dampen vertical instabilities at the rate they are created by larger-scale processes over the vertical extent of the atmosphere. More recently, investigations of the moisture dynamics surrounding organized convective structures have led to an emerging consensus on the theory of convection–circulation coupling in the tropics that acknowledges the important role of lower- to midtropospheric moisture variations, and the significance of moist convection and convective clouds for initiating and establishing circulations. However, the implementation of these new insights in numerical models lags behind. This is exemplified by the apparent inadequacy of climate models to correctly represent decadal variability in the tropical Pacific, a fact that potentially has implications for the confidence in climate change projections based on such models.

Assimilation of GOES-16 Radiances and Retrievals into the Warn-on-Forecast System

AbstractThe increasing maturity of the Warn-on-Forecast System (WoFS) coupled with the now operational GOES-16 satellite allows for the first time a comprehensive analysis of the relative impacts of assimilating GOES-16 all-sky 6.2-, 6.9-, and 7.3-μm channel radiances compared to other radar and satellite observations. The WoFS relies on cloud property retrievals such as cloud water path, which have been proven to increase forecast skill compared to only assimilating radar data and other conventional observations. The impacts of assimilating clear-sky radiances have also been explored and shown to provide useful information on midtropospheric moisture content in the near-storm environment. Assimilation of all-sky radiances adds a layer of complexity and is tested to determine its effectiveness across four events occurring in the spring and summer of 2019. Qualitative and object-based verification of severe weather and the near-storm environment are used to assess the impact of assimilating all-sky radiances compared to the current model configuration. We focus our study through the entire WoFS analysis and forecasting cycle (1900–0600 UTC, daily) so that the impacts throughout the evolution of convection from initiation to large upscale growth can be assessed. Overall, assimilating satellite data improves forecasts relative to radar-only assimilation experiments. The retrieval method with clear-sky radiances performs best overall, but assimilating all-sky radiances does have very positive impacts in certain conditions. In particular, all-sky radiance assimilation improved convective initiation forecast of severe storms in several instances. This work represents an initial attempt at assimilating all-sky radiances into the WoFS and additional research is ongoing to further improve forecast skill.

A Tripole Pattern of Summertime Rainfall and the Teleconnections Linking Northern China to the Indian Subcontinent

Abstract Because of the interactive margin between the East Asian summer monsoon and westerly circulation, summer rainfall in northern China (NC) exhibits high variability. By employing reanalysis data and geostationary satellite data from the Fengyun-2G (FY-2G) satellite and using the linear baroclinic model (LBM) and Hybrid Single-Particle Lagrangian Integrated Trajectory model, this study suggests a tripole pattern in summer rainfall over NC and the Indian subcontinent (IS) that is related to the Indian summer monsoon. The distributions of atmospheric circulation indicate three teleconnections: one is from the IS via the Indo-China Peninsula (ICP) and NC, enhancing the Pacific–Japan (PJ) pattern; another is from the IS via west-central Asia and NC, arousing a Eurasian wave pattern; and the third is an IS–TP–NC pattern via the Tibetan Plateau (TP). Those teleconnections modulate vorticity and atmospheric stability over NC. In addition, along with the circulation distribution related to those teleconnections, two pathways of moisture transport related to the IS rainfall are suggested, except for moisture transport via the Bay of Bengal: one is from the Indo-Pacific to NC due to enhancing cyclones over the Indo-Pacific and a PJ-like pattern; and another is from the IS to NC via the TP within the midtroposphere, which modulates midtroposphere moisture fluxes and atmospheric stability over NC. Both teleconnections and moisture transport result in anomalous rainfall over NC. This study reveals a new mechanism and pathway of the Indian summer monsoon impacting NC rainfall, possibly explaining the reason behind the high variability in NC rainfall.

Characteristics of high-frequency gravity waves generated during pre-monsoon season over a tropical location: A case study

Mesoscale convection weather events, associated with deep cumulonimbus clouds transport moisture, energy and momentum to upper troposphere and lower stratosphere (UTLS), affect the energetics and general circulation of the middle atmosphere. Therefore, it is necessary to understand the atmospheric dynamics during pre–monsoon periods (when strong mesoscale convective events occur) for improving the parameterization of model physics. The present study attempts to investigate the characteristics of high-frequency gravity waves (GWs) generated over Gadanki (13.5° N, 79.2° E), India during one such severe convection event in the pre–monsoon period. For this purpose, the Mesosphere Stratosphere Troposphere (MST) radar at Gadanki was operated continuously for the period of ∼10 h during 27–28 May 2015. A wide spectrum of high frequency GWs with different generation mechanisms are found during this convective event whose vertical propagation characteristics fulfill the non-hydrostatic GWs dispersion relation. The temperature and wind data of ERA (ECMWF Re-Analysis)-interim reanalysis and GPS radiosonde, launched at Gadanki, are also analyzed for ten days around the event day to determine the background atmospheric thermodynamical conditions. Strong up- and downdrafts are observed in the altitude range of ∼3.6–20 km (radar limited top height); presence of which even up to ∼20 km height is quite rare. The WRF (Weather Research and Forecasting) model simulated relative humidity on 27–28 May 2015 clearly depicts the mid-tropospheric moisture intrusion. Presence of moisture is also observed at the higher heights ∼17–18 km which could be due to strong low level convergence leading to high divergence value of deep cumulonimbus clouds in UTLS region. Furthermore, this is supported by the Doppler Weather Radar (DWR) and simulated reflectivity of WRF model reaching the tropopause height of ∼18 km. The altitude profiles of phase of some oscillations during convection period depict that the sources of these waves are near ∼20 km which is first of its kind outcome and it is corroborated with high resolution WRF model simulations.

Substantial Reductions in Cloud Cover and Moisture Transport by Dynamic Plant Responses

AbstractCumulus clouds make a significant contribution to the Earth's energy balance and hydrological cycle and are a major source of uncertainty in climate projections. Reducing uncertainty by expanding our understanding of the processes that drive cumulus convection is vital to the accurate identification of future global and regional climate impacts. Here we adopt an interdisciplinary approach that integrates interrelated scales from plant physiology to atmospheric turbulence. Our explicit simulations mimic the land‐atmosphere approach implemented in current numerical weather prediction, and global climate models enable us to conclude that neglecting local plant dynamic responses leads to misrepresentations in the cloud cover and midtropospheric moisture convection of up to 21% and 56%, respectively. Our approach offers insights into the key role played by the active vegetation on atmospheric convective mixing that has recently been identified as the source of half of the variance in global warming projections (i.e., equilibrium climate sensitivity).

Mesoscale Circulations and Organized Convection in African Easterly Waves

Abstract Global convection-permitting model simulations and remote sensing observations are used to investigate the interaction between organized convection, both moist and dry, and the atmospheric circulation in the case of an African easterly wave (AEW). The wave disturbance is associated with a quadrupole structure of divergence, with two convergence centers slightly ahead of the trough. Moisture transport from southeast of the trough to the area in front and lower midtropospheric moisture convergence precondition and organize convection. The main inflow into the squall-line cluster is from behind. The moisture-abundant inflow collides at the low level with monsoon air with high moist static energy and establishes a frontal line of updrafts at the leading edge of the propagating mesoscale convective system. A mantle of moisture surrounds the convective core. A potential vorticity budget analysis reveals that convective latent heating is driving the evolution of the wave but not in a quasi-steady way. The wave propagation includes a succession of convective bursts and subsequent dynamic adjustment processes. Dry convection associated with the Saharan air layer (SAL) and SAL intrusions into the wave trough together with vorticity advection can play a role in intensifying AEWs dynamically as they move from the West African coast across the Atlantic Ocean. Our analysis demonstrates that the synoptic-scale wave and convection are interlinked through mesoscale circulations on a continuum of scales. This implies that the relation between organized convection and the atmospheric circulation is intrinsically dynamic, which poses a particular challenge to subgrid convection parameterizations in numerical models.