Characteristics Of Mesoscale Convective Systems Research Articles

Climatological Tracking and Lifecycle Characteristics of Mesoscale Convective Systems in Northwestern South America

AbstractMesoscale convective systems (MCSs) are crucial in shaping large‐scale tropical circulation and the hydrological cycle, particularly in Northwestern South America (NwSA), a region marked by complex terrain and significant MCS activity. Understanding MCSs in NwSA is vital due to their impact on precipitation patterns and potential for severe weather events. To enhance this understanding, the ATRACKCS algorithm was developed for tracking convective systems, utilizing precipitation and brightness temperature data sets. This research focuses on documenting the spatiotemporal variability of MCS occurrence, life cycle, and movement. Notably, MCS hotspots were identified to the west of the major orographic features in the region, with maximum occurrences at night, contrasting with the region's typical afternoon peak in land convection. MCS movement is also heavily influenced by topography, with higher velocities on the eastern (windward) side of the Andes compared to velocities on the western (leeward) side. MCSs generally move westward, driven by easterly winds, but this pattern is not consistent throughout the year or region. Northward movement is predominant to the west of the Andes, while southward movement is observed to the east. These seasonal and regional movement variations are linked to factors such as the intertropical convergence zone position, moisture availability, topography, and low‐level jets. This research underscores the complexity of MCSs in NwSA and emphasizes the need for detailed studies on the atmospheric environment shaping these systems. Additionally, it provides a robust 21‐year MCS database for NwSA and an advanced tracking tool for research in various geographic contexts and impact areas.

Characteristics of mesoscale convective systems and related precipitation in the three‐river‐source region of China

AbstractMesoscale convective systems (MCSs) are important air water sources to the Three‐river‐source (TRS) region known as the “Chinese water tower.” Using hourly equivalent blackbody temperature (TBB) data from geostationary satellites of Chinese Fengyun‐2 series during the warm season (May–August) in 2005–2020 and an objective algorithm, MCSs in the TRS are divided into meso‐α (MαCS), meso‐β (MβCS), and meso‐γ (MγCS), and MαCS and MβCS are subdivided into larger meso‐α (LMαCS), smaller meso‐α (SMαCS), larger meso‐β (LMβCS), and smaller meso‐β (SMβCS). Results show that a high‐frequency zone of MCSs in the TRS distributes along the source of the rivers. Most MCSs, except LMαCS, develop and dissipate in situ. The interannual variation in MCS frequency exhibits a decreasing trend, especially after 2013, mainly due to the decrease in MCSs in the source region of the Yellow–Lancang River. The occurrence of MCSs peaks in August, but MCSs are most likely to produce precipitation in July and usually generate between 1600–2200 h LST (UTC + 8). The precipitation caused by MCSs to the total precipitation (precipitation ratio, PR) accounts for about 40%; MCS PR is closely related to, and increases with, the horizontal scale of the MCS, with MαCS PR being the highest, exceeding 67%. The contribution of MCSs to precipitation is mainly reflected in weak precipitation, smaller than 10.0 mm/h. Most of the maximum precipitation of MCSs appears after MCSs reach their prime, with the maximum lag by MαCS up to 2 h.

Impact of the Madden–Julian oscillation and equatorial waves on tracked mesoscale convective systems over southeast Asia

AbstractSoutheast Asia is a region dominated by high‐impact weather, but numerical weather prediction here is a challenge owing to the complex orography and interactions between small‐ and large‐scale phenomena. Localised mesoscale convective systems (MCSs) can produce intense precipitation. Here, we track MCSs over a 5‐year period in Himawari satellite data, characterise the distribution of MCSs in the region, and investigate how they are modulated by the Madden–Julian oscillation (MJO) and equatorial waves. Between 10°S and 10°N in southeast Asia, MCSs account for 45–70% of the precipitation during boreal extended winter (November–April). Over most of the region, the fractional MCS contribution to rainfall is higher than average on days with extreme rainfall (>55%). Long‐lived (>12 hr) MCSs contribute disproportionately, providing 85% of the rainfall despite comprising only 34% of all MCSs. Variability in MCS rainfall accounts for >50% of the total rainfall variability during an MJO cycle, mostly due to larger numbers of MCSs in convectively active MJO phases. Variations in MCS size and mean rain rate due to shifts in the stratiform proportion provide compensating effects. In the west of the region, a shift to faster moving MCSs in active MJO phases and slower moving MCSs in inactive phases resulted in fast‐moving MCSs having the greatest impact on the MJO‐associated variability. Variability is larger in the west than in the east. Equatorial Kelvin waves modulate MCS rainfall, with MCSs accounting for 20–50% of local rainfall anomalies. This variability is again enhanced in the west. By contrast, rainfall anomalies due to westward‐propagating mixed Rossby–gravity waves and Rossby‐1 waves are dominated by tropical‐cyclone‐related rainfall. Skill at local scales may be extracted from forecasts of subseasonal drivers such as the MJO and Kelvin waves, by understanding how these modulate the number and characteristics of MCSs.

Summertime Near‐Surface Temperature Biases Over the Central United States in Convection‐Permitting Simulations

AbstractConvection‐Permitting Model (CPM) simulations of the Central United States climate for the summer of 2011 are studied to understand the causes of warm biases in 2‐m air temperature (T2m) and related underestimates of precipitation including that from mesoscale convective systems (MCSs). Based on 10 CPM simulations and 9 coarser‐resolution model simulations, we quantify contributions from evaporative fraction (EF) and radiation to the T2m bias with both types of models overestimating T2m largely because they underestimate EF. The performance of CPMs in capturing MCS characteristics (frequency, rainfall, propagation) varies. The pre‐summer precipitation bias has large correlation with mean summertime T2m bias but the relationship between summertime MCS mean rainfall bias and T2m bias is non‐monotonic. Analysis of lifting condensation level deficit and convective available potential energy suggests that models with T2m warm biases and low EF have too dry and stable boundary layers, inhibiting the formation of clouds, precipitation and MCSs. Among the CPMs with differing model formulations (e.g., transpiration, infiltration, cloud macrophysics and microphysics), evidence suggests that altering the land‐surface model is more effective than altering the atmospheric model in reducing T2m biases. These results demonstrate that land‐atmosphere interactions play a very important role in determining the summertime climate of the Central United States.

Review of the History of Mesoscale Convective System Forecasts on Aviation

Airports are important national resources, and Aviation Weather Services are critical to the aviation industry's success. According to the National Transportation Safety Board's (NTSB) analysis of weather-related circumstances that influence near- surface aircraft operations, wind and turbulence caused 1381 accidents, visibility, ceiling height (hc), and precipitation-related accidents occurred 485 times, and aircraft icing caused 150 accidents between 2003 and 2007. Mesoscale convective systems (MCSs) arise when cumulonimbus clouds merge into a single entity that can span hundreds of miles and continue for hours, posing a higher threat to aviation due to its size and duration. The mesoscale downdraft of a squall-line MCS's stratiform area sometimes merges with the convective downdrafts in the leading line of convection, and these mergers can produce strong effects, with the gust front surging forward and triggering new convection in the form of a “bow echo," according to Doppler radar. Bow echo events are of particular concern to forecasters because they are typically associated with strong, damaging surface winds. Because MCSs are still a major socioeconomic issue, it's critical to construct climate models that incorporate them, whether through cloud-resolving modeling or parameterization. MCS characteristics are influenced by the increasingly contaminated aerosol environment in most parts of the world, and as the Earth warms, MCS patterns will certainly change.

Mesoscale convective systems in the third pole region: Characteristics, mechanisms and impact on precipitation

The climate system of the Third Pole region, including the (TP) and its surroundings, is highly sensitive to global warming. Mesoscale convective systems (MCSs) are understood to be a vital component of this climate system. Driven by the monsoon circulation, surface heating, and large-scale and local moisture supply, they frequently occur during summer and mostly over the central and eastern TP as well as in the downstream regions. Further, MCSs have been highlighted as important contributors to total precipitation as they are efficient rain producers affecting water availability (seasonal precipitation) and potential flood risk (extreme precipitation) in the densely populated downstream regions. The availability of multi-decadal satellite observations and high-resolution climate model datasets has made it possible to study the role of MCSs in the under-observed TP water balance. However, the usage of different methods for MCS identification and the different focuses on specific subregions currently hamper a systematic and consistent assessment of the role played by MCSs and their impact on precipitation over the TP headwaters and its downstream regions. Here, we review observational and model studies of MCSs in the TP region within a common framework to elucidate their main characteristics, underlying mechanisms, and impact on seasonal and extreme precipitation. We also identify major knowledge gaps and provide suggestions on how these can be addressed using recently published high-resolution model datasets. Three important identified knowledge gaps are 1) the feedback of MCSs to other components of the TP climate system, 2) the impact of the changing climate on future MCS characteristics, and 3) the basin-scale assessment of flood and drought risks associated with changes in MCS frequency and intensity. A particularly promising tool to address these knowledge gaps are convection-permitting climate simulations. Therefore, the systematic evaluation of existing historical convection-permitting climate simulations over the TP is an urgent requirement for reliable future climate change assessments.

Climatic characteristics of mesoscale convective systems in the warm season in North China

In this study, a total of 339 mesoscale convective systems (MCSs) are obtained in North China using the temperature of brightness blackbody (TBB) data from the FY-2E in the warm season from 2010 to 2018. The number of meso-α-scale convective systems (MαCSs) is much more than that of meso-β-scale convective systems (MβCSs). The number of mesoscale elongated convective systems (MECSs) is more than that of mesoscale circular convective systems (MCCSs). Most MCSs occur in July and August, which have the widest influence range, the longest duration, and the strongest convection. The MαCS develops slowly and weakens rapidly. The diurnal variation of MαCSs presents a bimodal distribution, most of MαCSs form in the afternoon, while some of MαCSs form in the evening. The MCSs activities in the warm season of North China are concentrated in two belts, namely, the east–west-oriented belt along Henan Province, Shandong Province and the Yellow Sea, and the south–north-oriented belt along central-western Shandong, Tianjin City, the west of Bohai Sea and the northeast of Hebei Province. MCSs mainly move eastward, and only some MECSs move southwestward and northwestward. The easterly and northerly moving MCSs are mainly affected by the steering flow, while the southerly moving MCSs are mainly affected by storm propagation. The MCSs of North China mainly form in the high temperature, high humidity and high energy area, with favorable dynamic conditions, such as middle-level trough or vortex, low-level shear line, surface inverted trough or surface convergence line, and the terrain. Meanwhile, the MCS pregnant environment is often accompanied by low-level jet and relatively strong vertical wind shear.

Climatologies of Mesoscale Convective Systems over China Observed by Spaceborne Radars

Abstract This study investigates the characteristics of mesoscale convective systems (MCSs) as a function of MCS organizational mode over China, using long-term precipitation radar observations from the Tropical Rainfall Measuring Mission (TRMM) and Global Precipitation Measurement Mission (GPM). The spaceborne radar-based MCS climatology shows maximum population over low-elevation regions, marked decrease over the foothills, and minimum frequency over the Tibetan Plateau. Linear and nonlinear MCSs account for 17% and 83% of the MCSs over China, respectively. Linear MCSs have much stronger convective intensity and heavier precipitation than nonlinear MCSs, as indicated by TRMM convective proxies. Interestingly, though broad-stratiform MCSs have the weakest convection, they produce the heaviest (maximum) rain rate and the largest amount of heavy rainfall among nonlinear MCSs. Among various types of linear MCSs, bow echoes (BEs) and no-stratiform (NS) systems exhibit the strongest convective intensity, embedded lines exhibit the weakest, and convective lines with trailing/leading stratiform in between. BEs and NSs share the most vertically extended structure, strongest microwave ice scattering, and highest lightning flash rates, but NS systems have a much lower surface rain rate likely due to a drier environment. Vertical radar reflectivity profiles suggest that both ice-based and warm-rain processes play an important role in the precipitation processes of linear MCSs over China, including the most intense BE storms. In short, this study helps to better understand the convective organization, precipitation structure, and ensemble microphysical properties of MCSs over China, and potentially provides guidelines for evaluating high-resolution model simulations and satellite rainfall retrievals for monsoonal MCSs.

The characteristics of mesoscale convective systems generated over the Yunnan–Guizhou Plateau during the warm seasons

AbstractThe characteristics of mesoscale convective systems (MCSs) over the Yunnan–Guizhou Plateau (YGP) during the warm seasons (April–August) are investigated using an automatic tracking algorithm based on long‐term (2000–2018) hourly geostationary satellites data of temperature of black body. A total of 1,845 MCSs generated over the YGP are identified and further classified into the eastward moving type (EMT; ~13.1%) and noneastward moving/dissipating type (NDT; ~86.9%). The two types of MCSs exhibit varying characteristics. The EMTs are mainly active in the eastern flank of the YGP with a longer mean lifespan (~16.5 hr), while the NDTs occur anywhere over the YGP with a preference for the central YGP with a shorter mean lifespan (~7.6 hr). The MCSs are observed most frequently during June–July, while the ratio of EMTs reaches the highest in April due to the strongest steering flows. The abundant moisture supply plays a vital role in the generation and development of MCSs in June, whereas the dynamic forcing and high CAPE are favourable to MCSs in July. In terms of the diurnal cycle, the NDTs are generally initiated in the afternoon, reach mature in the late afternoon, and dissipate at night. By contrast, the mature stage of EMTs shows double diurnal peaks in the late afternoon and early morning. The MCSs are usually generated in an instable environment accompanied by strong vertical wind shear and intense low‐level water vapour flux over the YGP, and the MCSs tend to vacate the YGP with strong mid‐level westerlies and instable environment in the downstream regions. Compared to MCSs in Tibetan Plateau, the ratio of the EMTs is higher with longer lifespans though fewer MCSs are generated over the YGP.

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.

Mesoscale Convective Systems in a Superparameterized E3SM Simulation at High Resolution

AbstractAccurately representing mesoscale convective systems (MCSs) is crucial to simulating the energy and water cycles in global climate models. Using a novel MCS identification and tracking algorithm applied to observations and model simulations, we evaluate how well the Energy Exascale Earth System Model (E3SM) simulates MCSs over the central US. Simulations performed by E3SM at 25 km grid spacing with and without superparameterization (SP) are compared and evaluated against observations using multiple metrics highlighting important MCS characteristics. Compared to E3SM, the superparameterized model (SP‐E3SM) better simulates MCS number and MCS precipitation amount, diurnal cycle, propagation, and the probability distribution of precipitation rate in both spring and summer. The improvement from SP is partly contributed by improvement in simulating the large‐scale environments, featuring enhanced atmospheric low‐level moisture and larger moisture transport to the central US relative to E3SM. However, SP‐E3SM still underestimates MCS precipitation amount, particularly in summer. This underestimation is closely related to the negative bias in MCS precipitation intensity, although the drier environments simulated during summer also contributes to underestimation of MCS frequency. Without SP, the larger bias in MCS precipitation amount is closely related to the negative bias in MCS frequency, while the substantial dry bias in the large‐scale environments also contributes to underestimation of MCS intensity. Our results suggest that SP improves MCS simulation by improving modeling of the large‐scale environments and convection initiation, which are both major limiting factors in E3SM even at 25 km grid spacing where deep convection is represented by a cumulus parameterization.

Statistical Characteristics of Mesoscale Convective Systems Initiated over the Tibetan Plateau in Summer by Fengyun Satellite and Precipitation Estimates

In order to investigate the key characteristics of mesoscale convective systems (MCSs) initiated over the Tibetan Plateau (TP) in recent years and the main differences in circulation and environmental factors between different types of MCSs, an automatic MCS identification and tracking method was applied based on the data from China’s Fengyun satellite and precipitation estimates. In total, 8820 MCSs were found to have been initiated over the TP during the summers from 2013 to 2019, and a total of 9.3% of them were able to move eastward out of the TP (EO). The number of MCSs showed a monthly variation, with a maximum in July and a minimum in June, while most EOs occurred in June. Compared with other types of MCSs, EOs usually had a lower cloud-top temperature, a greater rainfall intensity, a longer life duration, more rapid development, larger areas of rainfall and convective clouds, longer tracks and a wider influence range, indicating that EOs are more vigorous than the other types of MCSs. The movement of MCSs is mainly due to the mid- to high-level dynamic conditions, and moisture is an essential factor in their development and maintenance.

Characteristics of mesoscale convective systems during the warm season over the Tibetan Plateau based on FY‐2 satellite datasets

AbstractMesoscale convective systems (MCSs) often cause heavy precipitation events during the warm season over the Tibetan Plateau (TP). By using the 1‐hr gridded data sets with the resolution of 0.1° × 0.1° from the geostationary satellites of Fengyun‐2 series (FY‐2C/2E), the characteristics of MCSs over the TP during the warm season (May–August) from 2005 to 2012 are analysed. Based on objective criteria, MCSs over the TP can be divided into six categories, namely mesoscale convection complexes (MCCs), persistent elongated convection systems (PECSs), meso‐β circular convection systems (MβCCSs), meso‐β elongated convection systems (MβECSs), smaller meso‐β circular convection systems (SMβCCSs), as well as smaller meso‐β elongated convection systems (SMβECSs). The results show that there are 1,465 MCSs occurring over the TP during the warm seasons of 2005–2012, and most MCSs occur in a banded area around 30°N with two high‐frequency centres. The frequency of MCSs reaches a maximum in July, followed by August and June, while the lowest frequency occurs in May. The high‐frequency centre is located in the northern part of the TP in May, and moves southward to the central and southern part of the TP from June to August. 82.3% of the MCSs over the TP belong to the elongated categories, out of which MβECSs account for the most. MCSs over the TP have significant diurnal variations, which usually initiate at around 1700 LST (LST, LST = UTC + 8 hr), then develop and reach maturity at 2000 LST, then weaken and dissipate at 2200 LST. The life cycles of MCSs are characterized by slow growth and rapid weakening. The larger the scale of MCSs over the TP is, the longer the duration will be.

Создание картографической базы данных и веб-сервиса «Конвективные опасные метеорологические явления на территории Центрального федерального округа»

Hazardous convective weather events (HCWE), such as heavy rainfall, large hail, squalls and tornadoes, are one of the substantial sources of natural emergencies in Russia. The territory of the Central Federal District (CFD) is characterized by the highest population density in Russia. On the one hand, this leads to increased risks associated with HCWE, but on the other hand, it provides the possibilities for collecting the most detailed information on them (including the events missed by the observation network and reported based on damage assessment). In this study, we consider the structure and information content of the GIS database of HCWE for the territory of the CFD. The main advantage of the developed database comparing with existing analogues is its structure, which includes information on both the events themselves and their consequences, and the conditions of their occurrence. This includes, in particular, the characteristics of meso-scale convective systems (convective storms) based on the images from meteorological satellites and diagnostic variables characterizing the atmospheric environments according to the data from ERA-5 and CFS reanalysis systems. Also, the developed database is associated with previously published databases on tornadoes in Northern Eurasia and large-scale windthrow events in European Russia. At present, we compiled the data on more than 2,000 cases of HCWE in the CFD for the period 2001–2020, most of which were reported based on damage assessment. The open-source PostgreSQL DBMS is used to manage and edit the database. Open access to the database on the Internet is implemented through an online web map service available at http://convective-storms.psu.ru/.

Large-Scale Environmental Influences on Tropical Cyclone Formation Processes and Development Time

AbstractThe development of tropical cloud clusters (TCCs) to tropical cyclones (TCs) is the process of TC formation. This study identifies five main environmental transitions for the development of TCCs to TCs in the western North Pacific by using a cluster analysis method. Of these, three transitions indicate TCCs that develop in monsoon environments and two in easterly environments. Their numbers, distributions, and interannual variability differ. On average, the development time, defined as the period from the TCC forming to it developing into a TC, for TCCs that develop in easterly environments is shorter than that in monsoon environments. For the development of TCC to TC in easterly environments, TCCs have fewer embedded mesoscale convective systems (MCSs), which are located closer to the TCC center. Moreover, there is a stronger inward short-term (less than 10 days) angular momentum flux (AMF) at middle levels (800–500 hPa) before TCC formation. Conversely, in monsoon environments, TCCs have more MCSs, which are located farther from the TCC center. A stronger inward short-term AMF at low levels (1000–850 hPa) is observed before TCC formation and develops upward during the development of TCC to TC. The characteristics of MCS and AMF are significantly correlated with the development time of TCC to TC. In summary, large-scale easterly and monsoon environments cause TCCs to have different MCS and AMF characteristics, leading to higher efficiency for TCCs developing into TCs in easterly environments compared to monsoon environments.

The formation, character and changing nature of mesoscale convective systems

Mesoscale convective systems (MCSs) describe organized groupings of thunderstorms in the tropics and mid-latitudes that span thousands of square kilometres. While recognized for over a century, the advent of satellite and radar observations, as well as atmospheric-model simulations, has brought about their increased understanding. In this Review, we synthesize current knowledge on MCS formation, climatological characteristics, hazardous weather, predictive capacity and projected changes with anthropogenic warming. Driven by typical deep moist convective processes (moisture, lift and instability) and vertical wind shear, MCS formation occurs preferentially in locations where these ingredients are present and can be maintained by large-scale ascent and the cold pools that they produce. MCSs also generate hazardous weather, including extreme rainfall, flooding, derechos and, sometimes, tornadoes and hail, all of which have substantial economic and societal impacts. Given that MCSs also produce a large fraction of warm-season rainfall, there is critical need for both short-term forecasts and long-term projections, presently challenged by inadequate model resolution. Yet, with continually improving modelling capabilities, as well as greater theoretical basis, it is suggested that MCSs might increase in frequency and intensity under a warming climate. Further modelling progress, in turn, offers improved understanding of MCS characteristics, from their life cycle through to impacts. Mesoscale convective systems are an important source of precipitation in many tropical and mid-latitude regions, but can also produce hazardous weather, such as extreme rain, derechos and tornadoes. This Review discusses the formation of mesoscale convective systems, their hazardous weather, predictive capabilities and projected changes with anthropogenic warming.

Observed Warm‐Season Characteristics of MCS and Non‐MCS Rainfall and Their Recent Changes in the Central United States

AbstractWarm‐season rainfall characteristics in the central United States are investigated as they play important roles in ecohydrology and agricultural productivity. Using rainfall observations, we compare the April–August mesoscale convective systems (MCS) and non‐MCS rainfall characteristics and examine their linear trends between 1997 and 2018. MCS rainfall is found to be approximately seven times more intense than non‐MCS rainfall but it occurs less frequently in time and space. MCS rainfall peaks in nocturnal hours, with synchronized timing of rainfall intensity, area, and occurrence, while non‐MCS rainfall peaks in late‐afternoon hours, mostly attributed to the timing of peak rainfall area. MCS rainfall has increased in the last 22 years due to an increase in frequency and a longer duration per MCS. In contrast, non‐MCS rainfall has decreased mainly due to a reduction in rainfall area, leading to fewer total wet days and increased dry intervals between events.

Characteristics of Deep Cloud Systems under Weak and Strong Synoptic Forcing during the Indian Summer Monsoon Season

AbstractSynoptic-scale weather systems are often responsible for initiating mesoscale convective systems (MCSs). Here, we explore how synoptic forcing influences MCS characteristics, such as the maximum size, lifespan, cloud-top height, propagation speed, and triggering over the Indian region. We used 30-min interval infrared (IR) data of the Indian Kalpana-1 geostationary satellite. Cloud systems (CSs) in this data are identified and tracked using an object tracking algorithm. ERA-Interim 850-hPa vorticity is taken as a proxy for the synoptic forcing. The probability of CSs being larger, longer lived, and deeper is more in the presence of a synoptic-scale vorticity field; however, the influence of synoptic forcing is not evident on the westward propagation of CSs over land. There exists a linear relationship between maximum size, lifespan, and average cloud-top height of CSs regardless of the nature of synoptic forcing. Formation of CSs peaks around 1500 LST over land, which is independent of synoptic forcing. Over the north Bay of Bengal, CSs formation is predominantly nocturnal when synoptic forcing is strong, whereas, 0300 and 1200 LST are the preferred times when synoptic forcing is weak. Long-lived CSs are preferentially triggered in the western flank of the 850-hPa vorticity gradient field of a monsoon low pressure system. Once triggered, CSs propagate westward and ahead of the synoptic system and dissipate around midnight. Formation of new CSs on the next day occurs in the afternoon hours in the wake of previous day’s CSs and where vorticity gradient is also present. Formation and westward propagations of CSs on successive days move the synoptic envelope westward.

Sistemas Convectivos de Mesosescala Associados a Eventos Extremos de Precipitação Sobre o Semiárido do Nordeste do Brasil

Este trabalho apresenta um estudo das caracteristicas fisicas e morfologicas dos Sistemas Convectivos de Mesoescala (SCM) que ocorreram entre os anos de 2010 e 2011 sobre a regiao Semiarida do Nordeste do Brasil em situacoes de eventos de precipitacao fraca (EPF) e intensa (EPI). Para tanto, foram utilizadas imagens do satelite Geostationary Operational Environmental Satellite (GOES-12) no canal infravermelho, obtidas a cada 30 minutos, dados esses utilizados pelo Forecasting and Tracking the Evolution of Cloud Clusters (10,7 μm) para identificar, rastrear e determinar as caracteristicas dos SCM. Alem disso, foram utilizados dados de precipitacao diaria, fornecidos pela Agencia Nacional de Aguas (ANA) e pelo Instituto Nacional de Meteorologia (INMET) para identificacao dos eventos extremos, por meio da tecnica dos quantis. Os resultados mostraram que os EPI foram mais frequentes que EPF, principalmente no mes de marco. Ambos SCM relacionados a EPF e EPI, apresentaram, em sua maioria, formato do tipo irregular. As temperaturas medias do topo da nuvem foram semelhantes (-52oC), porem as temperaturas minimas do topo dos SCM tiveram maiores ocorrencias nos meses mais quentes, com valor de -79oC. Os SCM relacionados aos EPI apresentaram maiores tamanhos. A partir desses resultados evidencia-se que os SCM atuantes sobre o Semiarido do Nordeste do Brasil possuem desenvolvimento vertical intenso, possivelmente favorecido pela presenca de um ou mais sistemas atmosfericos atuando simultaneamente.

Characteristics of mesoscale convective systems in central East China and their reliance on atmospheric circulation patterns

ABSTRACTBased on mosaics of composite radar reflectivity for a 4‐year period from July 2007 to June 2011, a total of 354 mesoscale convective systems (MCSs) over central East China were identified and their characteristics were investigated. Using the objective classification method of obliquely rotated principal analysis in T mode, the atmospheric circulation patterns over central East China were classified into nine typical types based on the geopotential height fields at 850 hPa. It was found that 62.2% of daytime MCSs and 67.7% of nocturnal MCSs occurred in meridional atmospheric circulations, which were associated with the Western North Pacific Subtropical High (WNPSH) to the east and a low‐pressure system to the west. The relationship between atmospheric circulation types and characteristics of MCSs was also investigated. Composite analyses revealed two different atmospheric circulation patterns at 500 hPa in meridional atmospheric circulations, the pre‐short‐trough pattern and the cold vortex pattern. The former featured relatively stronger vertical wind shear, warm advection, and convergence at the exit region of low‐level jets (LLJs), while the latter was associated with relatively stronger frontal zones and frontogenesis process. Numbers of MCSs decreased significantly from 2008 to 2010, which might be attributed to changes in the location of WNPSH, the trough to the west and LLJs between the two systems.