Mesoscale Convective Research Articles

A Dual Regime of Mesoscale Convective Systems in the East Asian Monsoon Annual Cycle

AbstractEastern China is perennially hit by mesoscale convective systems (MCSs), yet it remains an open question as to whether and how their general properties change throughout the annual cycle. By leveraging high‐resolution satellite observations and a hybrid tracking algorithm, we present a 20‐year (2001–2020) climatology of MCSs over eastern China during eight subseasonal‐to‐seasonal (S2S) monsoon stages. MCSs contribute up to 50% of the stage‐total precipitation and over 60% of the rainband‐zone precipitation during the Pre‐Meiyu and Meiyu episodes, and their contributions remain non‐trivial during the transition seasons. We discover two contrasting regimes of MCSs that alternate immediately upon the Pre‐Meiyu and the Fall onsets. The wintertime regime features fast‐propagating shallow systems initiated at night in a strongly sheared and dry environment. The summertime regime, instead, features an outbreak of MCSs in the south, where the systems often initiate in the afternoon and propagate 2–3 times slower than the cold‐season ones. Based on multivariate clustering analysis, the nighttime MCSs that dictate the winter stages often initiate over a deep nocturnal boundary layer with moist rear inflows upgliding above it. In contrast, the afternoon‐initiated MCSs under the summertime regime feature a pronounced convective available potential energy (CAPE) "tongue" and upslope flows in their southeast quadrant. The southwesterly moist ascending flows are likely to shallow the convective inhibition at the tip of the CAPE tongue and facilitate the intense updrafts of the MCSs. Findings here may offer insights into S2S weather prediction and facilitate agricultural and water management throughout the year.

Lidar Observations and Data Assimilation of Low-Level Moist Inflows Causing Severe Local Rainfall Associated with a Mesoscale Convective System

Abstract We conducted an observational survey using a ground-based water vapor Raman lidar (RL) during the warm season in Japan to investigate the water vapor structure of low-level inflows that contribute to the formation of a mesoscale convective system (MCS). After the passage of a warm front, low-level moisture convergence contributed to the initiation and development of numerous convective clouds that composed the MCS. The RL observations showed that the vertical profiles of the water vapor mixing ratio (WVMR) associated with low-level inflows into the MCS exceeded 20 g kg−1 below 500 m above sea level, which is comparable to WVMRs in previous reports associated with MCSs in Japan and the United States. We conducted two assimilation experiments using a four-dimensional variational data assimilation system: one is to assimilate operational observational data (CNTL), and the other is to assimilate WVMR vertical profiles and operational observational data (TEST). A comparison between TEST and CNTL showed that data assimilation of the WVMR vertical profiles not only modified the moisture field but also the wind field. It appears that the modifications observed in horizontal wind are related to the modification of the WVMR in the analysis fields. These WVMR and wind modifications improved the reproduction of the frontal surface and forecasting of 6-h precipitation amount slightly. Data assimilation of vertical profiles of the WVMR has positive and negative impacts on the WVMR and horizontal wind, respectively, implying that the vertical profiles of both the horizontal wind and the WVMR might better estimate initial conditions and forecasts. Significance Statement Low-level moisture inflows are one of the key parameters involved in the formation of mesoscale convective systems (MCSs). Therefore, data assimilation of low-level moisture profiles is one of the prospective methods for better forecasting heavy precipitation associated with MCSs. However, few direct observations of the low-level moisture structure associated with MCSs and data assimilation experiments have been undertaken to date. We observed the vertical profiles of moisture associated with an MCS in Japan using a ground-based water vapor Raman lidar and show the existence of a relatively moist low-level inflow into the MCS. The data assimilation of low-level moisture has positive and negative impacts on moisture and horizontal wind, respectively, and improves slightly 6-h precipitation forecasts.

Anomalous convective transport of the tokamak edge plasma, caused by the inhomogeneous ion cyclotron parametric turbulence

In this paper, we develop the kinetic and hydrodynamic theories of the convective mesoscale flows driven by the spatially inhomogeneous electrostatic ion cyclotron parametric microturbulence in the pedestal plasma with a sheared poloidal flow. The developed kinetic theory predicts the generation of the sheared poloidal convective flow and of the radial compressed flow with radial flow velocity gradient. The developed hydrodynamic theory of the convective flows reveals the radial compressed convective flow as the dominant factor in the formation of the steep pedestal density profile with density gradient exponentially growing with time. This gradient density growth is limited by the formation of the radial oscillating with time ion outflow of pedestal plasma to the scrape-off layer.

IDENTIFICATION OF TROPICAL SQUALL LINE USING INFRARED CHANNEL HIMAWARI-8 SATELLITE IMAGERY (CASE STUDY OF 6-7 DECEMBER 2020 IN THE INDIAN OCEAN)

<p>Tropical squall line is a linear type of Mesoscale Convective Systems (MCS) phenomenon. On December 6-7, 2020, the Infrared (IR1) Himawari-8 satellite image in the Indian Ocean of Indonesian region, shows a cloud line identified as the tropical squall line. This study aims to identify the characteristics of the tropical squall line phenomenon that occurs in the Indian Ocean south of West Java using Himawari-8 Infrared (IR1) satellite imagery. Satellite image data is processed using an algorithm adapted to the MCC Maddox 1980 criteria. Furthermore, an objective analysis is carried out on the data based on the criteria from previous studies. The result shows that the tropical squall occurred for 19 hours with the initial type of tropical squall formation as intersecting convective band. In the mature stage, the trailing stratiform region and convective line develops an asymmetric pattern and shows a vortex (Mesoscale Convective Vortices) that forms inside the stratiform region. The result of rainfall distribution using the GSMaP model shows a category of heavy rain with rainfall in tropical squall areas exceeding 10 mm per hour.</p>

Polarimetric Size Sorting Signatures in the Convective Regions of Mesoscale Convective Systems in PECAN: Implications on Kinematics, Thermodynamics, and Precipitation Pathways

An object-based technique was utilized to identify hydrometeor size-sorting signatures at lower levels in the convective regions of 10 mesoscale convective systems (MCSs) during the 2015 Plains Elevated Convection at Night (PECAN) field campaign. Composite statistical analysis indicates that the magnitudes of size-sorting signatures (the separation distances between rain diameter maxima and concentration maxima) are nonlinearly correlated to the echo-top height, rain mass beneath the melting level, and precipitation rates at higher percentiles. To explore this correlation, the weather forecasting and research model was used to simulate the 20 June 2015 MCS during PECAN. Statistical analysis of the model outputs indicates more active riming growth and quicker graupel fallout at warmer temperatures near areas with larger separation distances. While updraft intensity above the melting level was also positively correlated to separation distances, this correlation was only statistically significant within certain temperature ranges. Additional analyses reveal that the higher intense precipitation potential near signatures with large separation distances could be attributed to precipitation production from the melted graupel. Finally, spatial correspondence between graupel distribution at the melting level and rain distribution at the lowest model level illustrates the critical role of graupel sedimentation and sorting in creating size-sorting signatures in MCSs during the PECAN field experiment.

A Comparison of Mesoscale Pressure Features Observed with Smartphones and Conventional Observations

Abstract To examine the utility of smartphone pressure observations (SPOs), a climatology of mesoscale pressure features was developed to evaluate whether SPOs could better resolve mesoscale phenomena than existing surface pressure networks (MADIS). A comparison between MADIS and smartphone pressure analyses was performed by tracking and characterizing bandpass-filtered, mesoscale pressure features. Over the year 2018, nearly 3000 pressure features were tracked across the central and eastern United States. Pressure features identified by smartphone observations lasted, on average, 25 min longer, traveled 25 km farther, and exhibited larger amplitudes than features observed by MADIS. An examination of smartphone pressure features tracks by season and location found that almost all pressure features propagated eastward. With over 87% of observed pressure features associated with convection, the climatology of surface pressure features largely reflects the geographic and seasonal variation of mesoscale convection. Phase relationships between pressure features and other surface variables were consistent with those expected for mesohighs and wake lows. These results suggest that SPOs could enhance convective analyses and forecasts compared to existing surface networks like MADIS by better resolving mesoscale structures and features, such as wake lows and mesohighs. Significance Statement While smartphone pressure networks provide unprecedented observation coverage and density, it was unclear whether they can add value to existing surface pressure networks. This study addresses this question by developing a yearlong record of mesoscale pressure features over the eastern and central United States. Analysis of this record revealed that smartphone analyses better resolved mesoscale pressure features, especially across the central United States where existing surface pressure networks are sparser. Nearly all observed pressure features were observed near precipitation, with five in six associated with convection. Relationships between mesoscale pressure features and other surface state variables were consistent with those expected for mesohighs and wake lows.

Impact assessment of the West African monsoon on convective precipitations over the far north region of Cameroon

Squall lines observed over the sahelo-sudanese regions are responsible for most of the annual rainfall recorded in these regions. They originate from the development of convective processes embedded in the Mesoscale Convective Systems (MCS). The impact assessment of the West African Monsoon (WAM) on the development of squall lines in the sahelo-sudanese regions, particularly the far north region of Cameroon is studied. In this study, the Tropical Rainfall Measuring Mission data (TRMM 3A12 and 3B43) and ERA5 reanalysis data from 1998 to 2008 have been used. The technique used is based on the use of the Climate Data Operator (CDO) software and the NCAR command language (NCL). Analysis of these data shows that the seasonal distribution of winds from the saharan heat-low and the Saint Helena high is the dominant atmospheric dynamics associated with the disturbances behind the development of squall lines. This distribution makes it possible to specifically draw up the evolution of the InterTropical Discontinuity (ITD) on the African continent, and thus, highlight the contribution of the WAM on precipitation in the far north region of Cameroon. This study is performed to contribute to the prediction of hydro-meteorological phenomena to improve disaster risks management associated with floods and landslides in this region, the most vulnerable in Cameroon.

An Observational Analysis of a Persistent Extreme Precipitation Event in the Post-Flood Season over a Tropical Island in China

Featuring unique tropical land–sea contrast and mesoscale terrain, Hainan Island in China is endowed with active mesoscale convections of special regional characteristics. Persistent extreme precipitation events (PEPs) during the post-flood season, triggered by multi-scale interactions among mid-latitude and tropical weather systems, exhibit notable mesoscale features, long duration and high rainfall rates with low forecasting performance. This study is motivated by a failure to forecast a PEP in two stages with distinct characteristics and predictabilities, in the post-flood season over Hainan Island on 16–18 October 2020. Based on multiple sources of remote sensing and high-resolution rain gauge records, detailed observational analyses were conducted using a flow decomposition method. Water vapor divergence (WVD) and its three components were used to investigate the spatial distribution and temporal evolution of two stages with distinct characteristics and predictabilities during this PEP. Decomposed moisture components can be used to determine how and to what extent large- and sub-synoptic scale moisture convergence contributes to PEPs in the tropics, under similar synoptic backgrounds. Joint applications of multiple sources of remote sensing data and flow decomposed WVD are proposed to further assist predicting PEPs in terms of rainfall location and evolution.

A Climatology of Mesoscale Convective Systems in Northwest Mexico during the North American Monsoon

Mesoscale Convective Systems (MCS) may vary greatly with respect to their morphology, propagation mechanism, intensity, and under which synoptic-scale conditions as a function of topographic complexity. In this study, we develop a long-term climatology of MCS during the North American Monsoon focusing on MCS morphology, lifecycle, and intensity as well as possible propagation mechanisms. We employ an MCS tracking and classification technique based on 23 years (1995 to 2017) of GOES IR satellite data. MCS intensity is also gauged with 7 years (2011 to 2017) of Vaisala GLD360 lightning data and, finally, monthly and interannual variability in synoptic conditions are examined with ERA5 reanalysis data. Our results based on 1594 identified MCS reveal that 98% are morphologically classified as Persistent Elongated Convective Systems. During the 23 summers (June through September) observed, the number of MCS varied considerably, averaging 70 MCS with minimum of 41 and maximum of 94. MCS typically have an average duration of around 8 h ± with a 2 h standard deviation. Propagation speeds, estimated with Hovmöller diagrams in addition to MCS centroid initial and final position, vary slightly depending on the trajectory. A notable result suggests that MCS propagation speeds are more consistent density currents or cold pools and not gravity waves nor steering-level winds. The results of this study could also provide a dataset for examining larger-scale controls on MCS frequency in addition to assesing convective parameterization and convective-resolving models in regions of complex topography.

Improving Forecast of Severe Oceanic Mesoscale Convective Systems Using FY-4A Lightning Data Assimilation with WRF-FDDA

The Fengyun-4A (FY-4A) geostationary satellite carries the Lightning Mapping Imager that measures total lightning rate of convective systems from space at high spatial and temporal resolutions. In this study, the performance of FY-4A lightning data assimilation (LDA) on the forecast of non-typhoon oceanic mesoscale convective systems (MCSs) is investigated by using an LDA method implemented in the Weather Research and Forecasting-Four Dimensional Data Assimilation (WRF-FDDA). With the LDA scheme, three-dimensional graupel mixing ratio fields retrieved from the FY-4A lightning data and the corresponding latent heating rates are assimilated into the Weather Research and Forecasting model via nudging terms. Two oceanic MCS cases over the South China Sea were selected to perform the study. The subjective evaluation results demonstrate that most of the oceanic convective cells missed by the control experiments are recovered in the analysis period by assimilating FY-4A lightning data, due to the promoted updrafts by latent-heat nudging, the more accurate and faster simulations of the cold pools, and the associated gust-fronts at the observed lightning locations. The cold pools and gust-fronts generated during the analysis period helped to maintain the development of the MCSs, and reduced the morphology and displacement errors of the simulations in the short-term forecast periods. The quantitative evaluation indicates that the most effective periods of the LDA for simulation enhancement were at the analysis time and the nowcasting (0–2 h forecast) periods.

Mesoscale Convective Systems Simulated by a High‐Resolution Global Nonhydrostatic Model Over the United States and China

AbstractMesoscale convective systems (MCSs) contribute a large fraction of warm‐season precipitation and generate hazardous weather with substantial socioeconomic impacts. Uncertainties in convection parameterizations in climate models limit our understanding of MCS characteristics and reliability of future projection. We examine MCSs simulated by the global 14 km Nonhydrostatic ICosahedral Atmospheric Model (NICAM) without cumulus parameterization against satellite observation from Global Precipitation Measurement during 2001–2008. We focus on MCSs over the central United States and eastern China where MCSs are prevalent from March to August. A process‐oriented tracking method incorporating both cloud and precipitation criteria is used to identify and track MCSs. About 140/100 MCSs initiate in the central United States/eastern China per warm season and most of them initiate in the east of high mountains and in coastal regions. The frequency distribution of MCS lifetime is well captured by NICAM. However, the simulated MCSs have stronger precipitation, smaller precipitation area, and larger cold cloud system than that observed in both regions, which may be caused by weak entrainment as it is not well resolved at 14 km resolution. The simulated MCS number is also underestimated in summer. By examining the climatological and MCS large‐scale environments, the significant underestimation of MCS number in summer over the central United States may be attributed to the large climatological dry bias in the atmosphere. For China, mean moisture in summer is well simulated but deficiency in capturing the dynamic condition related to the coastal topography for triggering convection may have contributed to underestimation of MCS even in a sufficiently moist environment.

Precipitation‐Moisture Coupling Over Tropical Oceans: Sequential Roles of Shallow, Deep, and Mesoscale Convective Systems

AbstractPrecipitation over tropical oceans rapidly increases when the environmental column saturation fraction (CSF) increases past a critical value of ∼0.7. Past studies suggested that increased stratiform rainfall greatly contributes to the rapid rainfall enhancement. In this study, the sequential roles of non‐deep convection, deep convection, and mesoscale convective system (MCS) in precipitation‐moisture interactions are examined using 19 years of satellite observations. When CSF is below ∼0.5, non‐deep convection dominates total rainfall, and predominantly contributes to moistening of the environment. Between the CSF range of 0.5–0.7, transition to deep convective rainfall begins. Meanwhile, MCS contribution to total rain rapidly increases, and the environment is further moistened. MCS becomes the major rainfall type above the critical CSF value (∼0.7), with the rapid increase of total rain mostly explained by the rapid increase in MCS rain area. Rainfall reduction at high CSF values is jointly contributed by MCS and non‐deep convection.

Initiation and Evolution of Long-Lived Eastward-Propagating Mesoscale Convective Systems over the Second-Step Terrain along Yangtze-Huaihe River Valley

Initiation and Evolution of Long-Lived Eastward-Propagating Mesoscale Convective Systems over the Second-Step Terrain along Yangtze-Huaihe River Valley

A Numerical Simulation of the Development Process of a Mesoscale Convection Complex Causing Severe Rainstorm in the Yangtze River Delta Region behind a Northward Moving Typhoon

In recent years, due to the influence of global warming, extreme weather events occur frequently, such as the continuous heavy precipitation, regional high temperature, super typhoon, etc. Tropical cyclones make frequent landfall, heavy rains and flood disasters caused by landfall typhoons have a huge impact, and typhoon rainstorms are often closely related to mesoscale and small-scale system activities. The application 2020 NCEP (National Centers for Environmental Prediction) final operational global analysis data and WRF (Weather Research and Forecasting model, version 3.9) mesoscale numerical prediction model successfully simulates the evolution characteristics of the mesoscale convective complex (MCC) that caused an extreme rainstorm in the Yangtze River delta region behind a northwards typhoon in this article. The results show that a meso-β-scale vortex existed in the mid- to upper troposphere in the region where the MCC occurred; accompanied by the occurrence of the meso-β-scale vortex, the convective cloud clusters developed violently, and its shape is a typical vortex structure. The simulation-sensitive experiment shows that the development of the meso-β-scale cyclonic vortex is the main reason for the enhancement of MCC. The occurrence and development of the MCC is manifested as a vertical positive vorticity column and a strong vertical ascending motion region in the dynamic field. In the development and maturity stage of the MCC, the vorticity and vertical rising velocity in the MCC area are significantly greater than those in the weakened typhoon circulation, which shows significant mesoscale convective system characteristics. The diagnostic analysis of the vorticity equation shows that the positive vorticity advection caused by the meso-β-scale cyclonic vortex in the mid- to upper troposphere plays important roles in the development of the MCC. Enhanced low-level convergence enhances vertical ascending motion. The convective latent heat release also plays an important role on the development of the MCC, changes the atmospheric instability by heating, enhances the upward movement, and delivers positive vorticity to the upper level, making the convection develop higher, forming a positive feedback mechanism between low-level convergence and high-level divergence. The simulation-sensitive experiment also shows that the meso-β-scale cyclonic vortex formation in this process is related to convective latent heat release.

Climatological Characteristics of Mesoscale Convective Systems in Northeast Brazil

Resumo As características meteorológicas de uma determinada região são de grande importância na manutenção das atividades humanas, uma vez que afetam a sociedade tanto de maneira positiva quanto negativa. Assim, o objetivo do estudo foi analisar as principais características dos Sistemas Convectivos de Mesoescala (SCMs) no Nordeste do Brasil (NEB). Utilizando o ForTraCC, um algoritmo para rastreamento de aglomerados convectivos, as seguintes características foram analisadas: ciclo de vida, distribuição espacial, período de maior frequência e o tamanho. Os resultados mostram que anualmente ocorrem, em média, 321 (±137) SCMs no NEB. O mês de março é o que apresenta a maior frequência na ocorrência de SCMs, enquanto agosto a menor. O quadrimestre fevereiro-março-abril-maio (FMAM) é o período com maior ocorrência de SCMs. No NEB, os SCMs se formam com uma frequência maior durante o dia, além de possuir um tempo médio de duração de 7 h (±5 h) e atingirem o estágio de maduro após 3 ou 4 h de sua gênese. Na fase de maturação, o tamanho médio é de 50.020 km2 (±86.453 km2). A densidade de gênese dos SCMs no NEB é maior sobre o continente do que sobre o oceano Atlântico Tropical e ocorre de forma desigual no território.

Characteristics of Regions with High-Density Initiation of Flashes in Mesoscale Convective Systems

To investigate the characteristics of regions exhibiting multiple lightning initiations within a finite volume and a short time, the lightning location data obtained from the convective regions of 14 mesoscale convective systems were analyzed in combination with data from radar. In total, 415 out of 5996 radar grids (1 km × 1 km × 0.5 km) were found to initiate more than one flash within 6 min. Only 49 grids showed an initiation density of more than two flashes within 6 min. The grids with high flash initiation densities were found to have a similar distribution to those with one lightning initiation within 6 min, in terms of altitude and reflectivity relative to altitude. They also showed similar trends in their frequency evolution. The grids with higher initiation densities seemed to be more concentrated in the altitude range of 9–13 km. However, only one was found to form at a lower altitude near the melting level when lightning initiation clearly declined. Moreover, the spatial relationship of this lower higher-initiation density grid to the reflectivity core was different to that in the main altitude range. In this paper, the possible dynamic and electrical mechanisms of the formation of this lower higher-initiation density grid are discussed.

Simulation and Analysis of Mesoscale Convective Systems (MCSs) Leading to a Severe Flood Over Iran

Simulation and Analysis of Mesoscale Convective Systems (MCSs) Leading to a Severe Flood Over Iran

A climatology of open and closed mesoscale cellular convection over the Southern Ocean derived from Himawari-8 observations

Abstract. Marine atmospheric boundary layer clouds cover vast areas of the Southern Ocean (SO), where they are commonly organized into mesoscale cellular convection (MCC). Using 3 years of Himawari-8 geostationary satellite observations, open and closed MCC structures are identified using a hybrid convolutional neural network. The results of the climatology show that open MCC clouds are roughly uniformly distributed over the SO storm track across midlatitudes, while closed MCC clouds are most predominant in the southeast Indian Ocean, with a second maximum along the storm track. The ocean polar front, derived from ECMWF-ERA5 sea surface temperature gradients, is found to be aligned with the southern boundaries for both MCC types. Along the storm track, both closed and open MCCs are commonly located in post-frontal, cold air masses. The hourly classification of closed MCC reveals a pronounced daily cycle, with a peak occurring late night/early morning. Seasonally, the diurnal cycle of closed MCC is most intense during the summer months (December–February; DJF). Conversely, almost no diurnal cycle is evident for open MCC.

Rain, Wind, and Dust Connections in the Sahel

AbstractThe Sahel is a dust source region where dust emission could be drastically modified in the future due to climatic and land use changes. Based on observations of meteorological parameters and dust concentration for about 1,000 rain events, we investigated the processes leading to dust emission during the rainy season when Mesoscale Convective Systems (MCSs) regularly cross the Sahel. We show that the highest wind speed is strongly linked to the MCS cold pool intensity, which is characterized by a drop in surface temperature. This is observed during the premonsoon period (∼May to June) when the midtroposphere is still sufficiently dry to allow intense evaporation of raindrops. Because this coincides with the time of the year that the surface is the least protected by the vegetative residue, the premonsoon wind speed leads to the highest observed dust concentration in our record. Most of the highest wind speed occur before or just at the beginning of a rainy event allowing a large part of the dust raised to be transported ahead the rain limiting dust removal by wet scavenging. Finally, we show that the number of 5‐min dust concentration higher than 5,000 μg m−3 is almost only occurring during the rainy season. These results suggest that until the dust models fail to correctly resolve MCS, it will be difficulty to obtain reliable estimates of dust emission from the Sahel for the present or future scenarios.

Crucial Role of Mesoscale Convective Systems in the Vertical Mass, Water, and Energy Transports of the South Asian Summer Monsoon

AbstractConvective vertical transport is critical in the monsoonal overturning, but the relative roles of different convective systems are not well understood. This study used a cloud classification and tracking technique to decompose a convection-permitting simulation of the South Asian summer monsoon (SASM) into subregimes of mesoscale convective systems (MCSs), non-MCS deep convection (non-MCS), congestus, and shallow convection/clear sky. Isentropic analysis is adopted to quantify the contributions of different convective systems to the total SASM vertical mass, water, and energy transports. The results underscore the crucial roles of MCSs in the SASM vertical transports. Compared to non-MCSs, the total mass and energy transports by MCSs are at least 1.5 times stronger throughout the troposphere, with a larger contributing fraction from convective updrafts compared to upward motion in stratiform regions. Occurrence frequency of non-MCSs is around 40 times higher than that of MCSs. However, per instantaneous convection features, the vertical transports and net moist static energy (MSE) exported by MCSs are about 70–100 and 58 times stronger than that of non-MCSs. While these differences are dominantly contributed by differences in the per-feature MCS and non-MCS area coverage, MCSs also show stronger transport intensities than non-MCSs over both ocean and land. Oceanic MCSs and non-MCSs show more obvious top-heavy structures than their inland counterparts, which are closely related to the widespread stratiform over ocean. Compared to the monsoon break phase, MCSs occur more frequently (~1.6 times) but their vertical transport intensity slightly weakens (by ~10%) during the active phases. These results are useful for understanding the SASM and advancing the energetic framework.