Evolution Of Mesoscale Convective System Research Articles

Environment of severe storm formations over West Africa on the 26‐28 June 2018

AbstractUnderstanding the environmental evolution of mesoscale convective systems (MCSs) is critical for forecasting weather in West Africa. This study investigated the thermodynamic and synoptic environments of MCSs over West Africa on 26 (storm 1) and 28 (storm 2) June 2018. Primary datasets used to assess the diurnal evolution of the storms were obtained from ERA5. The results showed a trapped gravity wave, enhanced by a well‐established African Easterly Jet and monsoon trough, was responsible for the initiation of storm 1. Both storms also initiated in the presence of several moist lower (925–850 hPa) to mid‐tropospheric (600 hPa) cyclonic and anticyclonic vortices, controlling inland moisture advection. The lower troposphere was moistened through moisture advection by the West African westerly jet for storm 1 and the nocturnal low‐level jet prior to initiation for storm 2. For both storms, the evolution of outgoing longwave radiation showed a consistent atmosphere of deep afternoon convection. Boundary layer height increased significantly during storm evolution to support the increasing ascent of warm air. Vegetation cover differences may have also likely aided the evolution of storm 2. The passage of gravity waves from decaying storms can aid forecasters to nowcast likely regions of afternoon convection with high accuracy. Under the GCRF African Science for Weather Information and Forecasting Techniques (SWIFT), these findings are crucial in fulfilling the project's aims of improving weather forecasting capability and communication over West Africa.

Temporal and Spatial Evolution of Precipitation under the Summer Sprite Parent Mesoscale Convective Systems in Japan

Transient luminous events (TLEs) are electrical discharges in the upper atmosphere caused by vigorous thunderstorms. Six sprites, which are part of TLEs, were observed on 22 July 2013 from Mt. Fuji (3776 m above sea level), Japan. All the six sprites were associated with intense positive cloud-to-ground strikes (+CGs), whose causative positive charges can reside in the stratiform region. Consequently, we assumed that the main sprites causative charges could generate an in situ charging mechanism, accompanied by precipitation growth in the extensive stratiform region. Thus, we supposed that there can be a relationship between the time sequence of surface precipitation intensity and the sprite emissions. In this study, we conclude that time sequences and horizontal evolution of Mesoscale Convective Systems (MCSs) precipitation are associated with sprites. As the result, prior to sprites 1–5, the areal amount of strong precipitation (≥8 mm/h) increased considerably, and only a small increase occurred during sprite 6. Analyzing the time sequence of the percentage of strong and weak precipitation with respect to the total precipitation, it was found that sprites 1–6 occurred within 20 min after the local peaks with respect to strong precipitation compared to total precipitation. In particular, sprites 2–5 occurred very close to local peaks. The rise time to the first peak of the strong precipitation rate associated with the first sprite was 80 min, while the rise time to the last peak associated with sprite 6 was 30 min. The temporal differences until the peaks suggest that the charging speeds, or mechanisms, related to precipitation differ between sprites 1–5 and sprite 6 in parent MCSs.

Influence of the interaction between different low‐ and mid‐level wind couplings and orography on the evolution of mesoscale convective systems in northwest China: A case study

A mesoscale convective systems (MCSs) event occurring in Northwest China is investigated to discuss the influences of interaction between different low- and mid-level winds couplings and orography on the evolution of MCSs, using high-resolution reanalysis data. The formation of a low-level anticyclonic circulation related to the plateau high caused the sequential occurrence of reverse and perpendicular low- and mid-level winds couplings and the stagnation and expansion of a high humidity zone originating from the mountains. In the case of reverse low- and mid-level winds coupling, mid-level winds were constantly lifted by low-level winds ascending the mountains, resulting in strong updrafts and relevant inertia-gravity waves occurring continuously in the upstream direction of mid-level winds. The unstable stratification generated by the high humidity zone prevented the inertia-gravity waves from propagating into the updrafts, leading to the maintenance and connection of the updrafts. Meanwhile, a strong downdraft occurred and maintained on the leeward side of the updrafts due to the coupling of the inertia-gravity waves associated with the uplift of mid-level winds with the inertia-gravity wave triggered by low-level winds ascending the mountains. Consequently, MCSs developed as they propagated into the updrafts, and perished abruptly when they reached the leeward-side downdraft. In the case of perpendicular low- and mid-level winds coupling, a broad and strong low-level updraft band formed and stayed on the windward side of mountain ridge due to terrain-induced uplift. Significant mid-level updraft and associated inertia-gravity wave were triggered on the side of the low-level updraft band blown by mid-level winds. The unstable stratification generated by the high humidity zone hindered the propagation of gravity wave into the low-level updraft band, leading to the maintenance of strength and continuity of the low-level updraft band. Consequently, MCSs maintained during their entire propagation along the low-level updraft band. This article is protected by copyright. All rights reserved.

The Impacts of Hydrometeors and Water Vapor in Stratiform Region on the Evolutions of MCSs and Embedded Convective Cells in a Pre‐Summer Rainfall Event Over South China

AbstractBoth the stratiform and convective regions play important roles in the development of mesoscale convective systems (MCSs). In this study, the impacts of hydrometeors and water vapor in stratiform region on the evolution of MCSs during a heavy rainfall event over South China are investigated by the Weather Research and Forecasting (WRF) model with Thompson microphysics. A series of sensitivity experiments including the reductions of water vapor to 95%, 80% (water vapor sensitivity experiments), and the reductions of hydrometeor mixing ratios to 10% (hydrometeor sensitivity experiments) in stratiform region are conducted, respectively. Results show that the control run reasonably reproduces the spatial distribution and temporal evolution of convective rainbands. The hydrometeor flux convergence partly indicates the changes of convection organization and movement direction. The intensities of precipitation and MCSs are significantly weakened by decreasing moisture advection to convective region in water vapor sensitivity experiments, but are a little weaker in hydrometeor sensitivity experiments. The enhanced water vapor advection and reduced sedimentation and evaporation of rain water in hydrometeor sensitivity experiments are responsible for the restoration of stratiform cloud. The stratiform region with more hydrometeors is generally more conducive to the development of convection than less one when they are at the same evolution stage. However, it is found that stronger convective cell can also exist in the stratiform region with less hydrometeors that is generated by the old convection cloud and at its dissipating stage, owing to the more transfer of water vapor and hydrometeors from stratiform to convective regions.

Impact of Convectively Generated Low-Frequency Gravity Waves on Evolution of Mesoscale Convective Systems

AbstractIdealized numerical simulations of mesoscale convective systems (MCSs) over a range of instabilities and shears were conducted to examine low-frequency gravity waves generated during initial and mature stages of convection. In all simulations, at initial updraft development a first-order wave was generated by heating extending through the depth of the troposphere. Additional first-order wave modes were generated each time the convective updraft reintensified. Each of these waves stabilized the environment in advance of the system. As precipitation descended below cloud base, and as a stratiform precipitation region developed, second-order wave modes were generated by cooling extending from the midlevels to the surface. These waves destabilized the environment ahead of the system but weakened the 0–5 km shear. Third-order wave modes could be generated by midlevel cooling caused by rear inflow intensification; these wave modes cooled the midlevels destabilizing the environment. The developing stage of each MCS was characterized by a cyclical process: developing updraft, generation of n = 1 wave, increase in precipitation, generation of n = 2 wave, and subsequent environmental destabilization reinvigorating the updraft. After rearward expansion of the stratiform region, the MCSs entered their mature stage and the method of updraft reinvigoration shifted to absorbing discrete convective cells produced in advance of each system. Higher-order wave modes destabilized the environment, making it more favorable to development of these cells and maintenance of the MCS. As initial simulation shear or instability increased, the transition from cyclical wave/updraft development to discrete cell/updraft development occurred more quickly.

Sensitivity Analysis of Ensemble Simulations on a Torrential Rainfall Case over South China Using Multiple PBL and SL Parameterizations

A good representation of the interaction between the planetary boundary layer (PBL) and the surface layer (SL) in numerical models is of great importance for the prediction of the initiation and development of convection. This study examined an ensemble that consists of the available suites of PBL and SL parameterizations based on a torrential rainfall event over south China. The sensitivity of the simulations was investigated against objective measurements using multiple PBL and SL parameterization schemes. The main causes of the bias from different parameterization schemes were further analysed by comparing the good and bad ensemble members. The results showed that good members tended to underestimate the rainfall amount but presented a decent evolution of mesoscale convective systems that were responsible for the torrential rainfall. Using the total energy mass flux (TEMF) scheme, the bad members overestimated the amount and spatial coverage of rainfall. The failure of the bad member was due to a spurious convection initiation (CI) resulting from the overestimated high-<i>θ</i>e elevated air. The spurious CI developed and expanded rapidly, causing intensive and extensive rainfall over south China. Consistent with previous studies, the TEMF scheme tends to produce a warmer and moister PBL environment. The detailed sensitivity analysis of this case may provide reference for the operational forecast of rainfall over south China using multiple PBL and SL parameterizations.

A Study of Cloud Microphysical Processes Associated With Torrential Rainfall Event Over Beijing

AbstractThe evolution of mesoscale convective systems (MCSs) leading to a heavy rainstorm event that occurred in Beijing on 21 July 2012 was simulated using the Weather Research and Forecasting model. Observational analyses indicated that this event can be divided into an earlier‐occurring warm‐sector precipitation (WSP) and a later‐occurring cold‐frontal precipitation (CFP). Owing to the considerable differences in their ambient weather conditions, the features and evolution of the cloud microphysics were different. Diagnoses of the mass‐ and heat‐hydrometeor budgets showed that the major differences in rainwater source were that the graupel melting (PGMLT) and the collection of snow by rain (PRACS_s2r) had similar magnitudes in the WSP, and PGMLT was larger than PRACS_s2r in the CFP, while the accretion growth of cloud droplets (PRA) was always the largest in both phases. The main cooling effect in the WSP was due to the evaporation of rainwater (PRE) and cloud water, while it was PRE and PGMLT for the CFP. The mechanisms of how microphysical processes influenced the precipitation were explored. It was found that the strong PRA in the WSP was conducive to the formation of a supercooled water level and evoked a seeding effect. However, graupel processes were crucial for the CFP. The strong sublimation processes of graupel and snow associated with the collection of droplets by graupel caused more latent heat release and drove airflow to reach a higher convection height. Moreover, the stronger PGMLT cooled the air in the MCS and reduced the effect of cloud droplet accretion growth.

Structure and Evolution of Mesoscale Convective Systems: Sensitivity to Cloud Microphysics in Convection‐Permitting Simulations Over the United States

AbstractRegional climate simulations over the continental United States were conducted for the 2011 warm season using the Weather Research and Forecasting model at convection‐permitting resolution (4 km) with two commonly used microphysics parameterizations (Thompson and Morrison). Sensitivities of the simulated mesoscale convective system (MCS) properties and feedbacks to large‐scale environments are systematically examined against high‐resolution geostationary satellite and 3‐D mosaic radar observations. MCS precipitation including precipitation amount, diurnal cycle, and distribution of hourly precipitation intensity are reasonably captured by the two simulations despite significant differences in their simulated MCS properties. In general, the Thompson simulation produces better agreement with observations for MCS upper level cloud shield and precipitation area, convective feature horizontal and vertical extents, and partitioning between convective and stratiform precipitation. More importantly, Thompson simulates more stratiform rainfall, which agrees better with observations and results in top‐heavier heating profiles from robust MCSs compared to Morrison. A stronger dynamical feedback to the large‐scale environment is therefore seen in Thompson, wherein an enhanced mesoscale vortex behind the MCS strengthens the synoptic‐scale trough and promotes advection of cool and dry air into the rear of the MCS region. The latter prolongs the MCS lifetimes in the Thompson relative to the Morrison simulations. Hence, different treatment of cloud microphysics not only alters MCS convective‐scale dynamics but also has significant impacts on their macrophysical properties such as lifetime and precipitation. As long‐lived MCSs produced 2–3 times the amount of rainfall compared to short‐lived ones, cloud microphysics parameterizations have profound impact in simulating extreme precipitation and the hydrologic cycle.

Impact of different parameterization schemes on simulation of mesoscale convective system over south-east India

Main objective of the present paper is to examine the role of various parameterization schemes in simulating the evolution of mesoscale convective system (MCS) occurred over south-east India. Using the Weather Research and Forecasting (WRF) model, numerical experiments are conducted by considering various planetary boundary layer, microphysics, and cumulus parameterization schemes. Performances of different schemes are evaluated by examining boundary layer, reflectivity, and precipitation features of MCS using ground-based and satellite observations. Among various physical parameterization schemes, Mellor–Yamada–Janjic (MYJ) boundary layer scheme is able to produce deep boundary layer height by simulating warm temperatures necessary for storm initiation; Thompson (THM) microphysics scheme is capable to simulate the reflectivity by reasonable distribution of different hydrometeors during various stages of system; Betts–Miller–Janjic (BMJ) cumulus scheme is able to capture the precipitation by proper representation of convective instability associated with MCS. Present analysis suggests that MYJ, a local turbulent kinetic energy boundary layer scheme, which accounts strong vertical mixing; THM, a six-class hybrid moment microphysics scheme, which considers number concentration along with mixing ratio of rain hydrometeors; and BMJ, a closure cumulus scheme, which adjusts thermodynamic profiles based on climatological profiles might have contributed for better performance of respective model simulations. Numerical simulation carried out using the above combination of schemes is able to capture storm initiation, propagation, surface variations, thermodynamic structure, and precipitation features reasonably well. This study clearly demonstrates that the simulation of MCS characteristics is highly sensitive to the choice of parameterization schemes.

Forecasting mesoscale convective systems in the Urals using the WRF model and remote sensing data

The results are presented ofmodeling the formation and evolution ofmesoscale convective systems (MCS) accompanied by severe weather events over the territory of the Western Urals by the WRF-ARW numerical model of the atmosphere. Twenty-three cases of mesoscale convective complexes and mesoscale squall lines are considered for 2002-2015. The Terra/Aqua MODIS data, the data of weather radars installed in Perm and Izhevsk, and the data from the Roshydromet observation network were used to verify the model forecasts. It is demonstrated that the parameters of MCS intensity are simulated by the model with high reliability; however, the quality of the forecast of the spatial position of MCS is unsatisfactory in most cases. It is revealed that the model grid spacing strongly affects the forecast skill scores. In some cases the model successfully simulates the formation and evolution of MCS accompanied by severe weather events and can be used for their short-range forecast with the time accuracy of ±(1-2) hours.

Formation and evolution of mesoscale convective systems that brought the heavy rainfall over Seoul on September 21, 2010

An investigation has been carried out using observational data and a numerical model to explain the formation and development of heavy precipitation systems on September 21, 2010. These systems were responsible for heavy rainfall over the middle Korean peninsula, with a maximum 24-h rainfall amount greater than 290 mm in the Seoul metropolitan area. Both observational analysis and a numerical simulation indicate that an important starting condition for this heavy rainfall event is the presence of a pressure trough over the Shandong peninsula and the Yellow Sea. Convective cells formed in the early morning over this trough area, grew into larger systems as they moved eastward, and induced the formation of a meso low over the Yellow Sea around 0000 UTC on September 21, 2010. A stationary front with significant vertical circulation developed in response to the deformation of flow associated with the meso low. In the meantime, multicell-type convective systems continuously developed and moved along the front. These storms developed further and produced heavy rainfall over the middle Korean peninsula, which includes the Seoul metropolitan area. According to observations, the band structure appeared to change after 0700 UTC as a narrow convection band developed over the sea, upstream of the existing band of multicell storms. Numerical simulation showed a similar transition. However, it failed to reproduce the stationary behavior of the observed band.

TRMM-retrieved cloud structure and evolution of MCSs over the northern South China Sea and impacts of CAPE and vertical wind shear

Cloud structure and evolution of Mesoscale Convective Systems (MCSs) retrieved from the Tropical Rainfall Measuring Mission Microwave Imager (TRMM TMI) and Precipitation Radar (PR) were investigated and compared with some pioneer studies based on soundings and models over the northern South China Sea (SCS). The impacts of Convective Available Potential Energy (CAPE) and environmental vertical wind shear on MCSs were also explored.

Observational and numerical analysis of the characteristics of MCS development associated with trigger effect of isolated islands

In order to clarify the characteristics of Mesoscale Convective System (MCS) development and understand the impact of the trigger effect of isolated islands, observational and numerical analysis of the heavy rainfall were carried out over the southwestern part of the Korean Peninsula on July 14, 2004. Satellite based remote sensing data and numerical model MM5 with observational data adjustment were used in this study. The MCS development, in this case, was accompanied not by directional wind shear, but by speed shear which was strongly associated with development of the updraft cloud. An inversion layer at a 750 hPa level is one of the fundamental factors in increasing instability. Effective separation of the upper and lower level atmospheric structure due to an inversion layer at a 750 hPa level creates a suitable condition to develop a MCS. According to numerical analysis it has been found that isolated islands located off the southwestern part of the Korean Peninsula are not a principal factor in causing the heavy rainfall due to the evolution of MCS in this case. Transferable topographic forcing of the downwind side can often induce the variation of MCS intensity, while associated with the precipitation amount over the lee side of the isolated islands at a mature stage of MCS development.

Comment on “Variations of tropical upper tropospheric clouds with sea surface temperature and implications for radiative effects” by H. Su et al.

Using TRMM VIRS data, we attempt to replicate the analysis made by Su et al. (2008) to quantify the effect of methodological choices on the magnitude of the observed correlations between upper‐level cloud cover and SST. Using brightness temperature thresholds to identify upper‐level cloud, we recover a relatively small change in the normalized area of cirrus clouds with SST (∼−6%/K compared to ∼−2%/K found by Su et al. (2008)). We discuss the effect of several methodological choices on the magnitude of the signal, namely, the classification of cloudy regions into convective updrafts and anvil, the use of cloud weighted SST, and the truncation and sampling error of the orbital satellite data with respect to the evolution of mesoscale convective systems. Accounting for some of these methodological differences could resolve the discrepancy between the weak signal documented by Su et al. (2008) and the stronger signal documented originally by Lindzen et al. (2001) and others, including the results reported in this comment.

A nudging scheme to assimilate satellite brightness temperature in a meteorological model: Impact on representation of African mesoscale convective systems

AbstractThe study presents the implementation of a nudging procedure in a mesoscale meteorological model, and its evaluation in improving the representation of mesoscale convective systems in the region of West Africa. The generation and development of convection are related to processes with different spatial scales, making it often difficult for meteorological models to reproduce their dynamics.The assimilation approach is based on the continuous assimilation of METEOSAT infrared brightness temperatures within the model in order to trigger convection, where observations show the presence of large convective systems. The nudging also inhibits convection, when the model reproduces unrealistic convective precipitation and coherently modifies the dynamical fields.Model sensitivity to the nudging scheme has been tested to evaluate the role of the forcing term. The nudging improves the geographical distribution and time evolution of mesoscale convective systems reproduced by the model; the impact of assimilation is positive up to 13 h after the end of the nudging period. It is also shown that the nudging improves the simulated amount and spatial distribution of precipitation.Finally, the assimilation is performed throughout the entire simulation, showing how the proposed approach can be used to improve meteorological analyses in terms of convective activity and rainfall. Copyright © 2010 Royal Meteorological Society

Evolution of Mesoscale Convective Systems and its Relationship with the Madden-Julian Oscillation in the Indo-Pacific Region

In this study we investigated the influence of sea surface temperature (SST) and Madden-Julian Oscillation (MJO) on the organization, strength, and distribution of Mesoscale Convective Systems (MCSs) over the Indo-Pacific region for understanding El Nino Southern Oscillation (ENSO). The study is based on the Tropical Rainfall Measuring Mission’s (TRMM) precipitation features dataset during boreal winters of 2001-02 to 2006-07. The results of time series of hot spot analyses show that MCSs are predominantly clustered in the Maritime Continent and West Pacific Regions, and are preferentially observed over ocean compared to land. The largest quantity of MCSs were observed to the south of the equator in January 2002, and the distribution of MCSs as a function of SST for these months peaked between 29-30° Celsius. MJO explains 11% to nearly 30% of the characteristics of MCSs in the Indo-Pacific. The percentage of MCSs over the ocean in January is larger when the MJO is in the active phase than the passive phase from the Indian Ocean to the West Pacific Region. The warm waters under the MCSs may be related to the coupled MJO process that convectively generate and maintain the storms. The effect of the MJO is strongest when it is located within the Maritime Continent region. The MJO analysis indicates that MCSs are more likely accompanied by westerly winds during the eastward propagation of the MJO and the westerly wind bursts have been linked to the onset of El Nino in the Pacific.

A Study of the Evolution of Mesoscale Convective Systems Using WSR-88D Data

While mesoscale convective systems (MCSs) are major contributors to severe weather and precipitation in the central United States, details of their evolution are still not fully understood. In particular, recent observational and modeling studies have indicated that many MCSs tend to exhibit an asymmetric precipitation pattern during the mature-to-later stages of their life cycles. However, to what extent and how this evolution occurs in all cases is still a matter of conjecture. The recently established WSR-88D network has afforded the opportunity to investigate the evolution of MCSs in greater detail than has been possible in the past. As a preliminary effort in this direction, 13 MCSs having leading line-trailing stratiform structures that occurred over the lower Midwest during May and June 1995 have been studied. It is found that indeed there does appear to be an evolution of such MCSs toward asymmetry later in their life cycles; however, the nature of the evolution is complex. The trailing stratiform region develops an asymmetric pattern (stratiform precipitation located toward the northern edge of the MCS) well before the system reaches maturity. The convective line also evolves to an asymmetric pattern (the most intense convective cells concentrated along the southern end of the line), but the appearance of asymmetry occurs several hours later than it does in the stratiform component. The limited sample size, however, precludes definitive conclusions about the significance of this timing difference. Contributions to asymmetry by hydrometeor advection in the along-line component of the flow aloft have also been inferred from sounding data. Results indicate that the system-relative, along-line flow in the pre-MCS environment is comparable to and often greater than the cross-line flow. This evidence suggests that both along-line and cross-line, system-relative flows, along with other processes (e.g., Coriolis effects), may contribute to an evolution toward asymmetry.

The Role of Environmental Shear and Thermodynamic Conditions in Determining the Structure and Evolution of Mesoscale Convective Systems during TOGA COARE

Abstract A collection of case studies is used to elucidate the influence of environmental soundings on the structure and evolution of the convection in the mesoscale convective systems sampled by the turboprop aircraft in the Tropical Ocean Global Atmosphere (TOGA) Coupled Ocean–Atmosphere Response Experiment (COARE). The soundings were constructed primarily from aircraft data below 5–6 km and primarily from radiosonde data at higher altitudes. The well-documented role of the vertical shear of the horizontal wind in determining the mesoscale structure of tropical convection is confirmed and extended. As noted by earlier investigators, nearly all convective bands occurring in environments with appreciable shear below a low-level wind maximum are oriented nearly normal to the shear beneath the wind maximum and propagate in the direction of the low-level shear at a speed close to the wind maximum; when there is appreciable shear at middle levels (800–400 mb), convective bands form parallel to the shear. With...

Lagrangian approach for deriving cloud characteristics from satellite observations and its implications to cloud parameterization

A Lagrangian view is adopted for establishing the spatiotemporal cloud statistics and the scale dependent radiative properties using satellite data. Individual clouds are identified using a newly developed scheme. We sort all clouds by cloud type, cloud area, and number of clouds in each area bin, as well as their radiative properties. For seven different cloud types our analyses provide radiative properties, such as albedo and cloud top temperature, as a function of the cloud spatial scale. All clouds are marked by local time, and large clouds are tracked over time. These analyses provide diurnal variability, lifetimes, and evolution of cloud systems as a function of their spatial scales. These scale dependent cloud properties can be objectively used in guiding the development and evaluation of cloud parameterization in global climate models (GCMs). Particularly, we show how our Lagrangian approach can be used to establish the relative importance of resolvable and fully parameterized clouds to the total cloudy area and to the total amount of reflected visible irradiance. Focus in this 1 month satellite study is on the convective‐stratiform cloud systems over the western and central tropical Pacific Ocean, including the so‐called warm pool. We adopt the hourly Japanese geostationary satellite (GMS) window channel radiances in the visible and IR window region for cloud classification and characterization. To study the radiative contributions of different clouds in the area, we computed the bidirectional model (BDM) for the Visible and Infrared Spin Scan Radiometer instrument aboard GMS, which we show to agree well with the BDM of the Earth Radiation Budget instrument aboard the Nimbus 7 satellite. An iterative two‐stage cloud detection scheme was developed to identify individual clouds. Furthermore, a tracking algorithm was developed to study the time evolution of mesoscale convective systems (MCS). It operates on area and orientationally equivalent ellipsoidal representations of these MCS. We show that the temporal statistics of these convective anvil clouds show good agreement with those reported in the literature. Our data indicate that for the convective‐stratiform systems in the tropical Pacific, 95% of the radiatively important clouds (containing a core with an effective brightness temperature &lt;219 K) are of scales resolvable by a GCM of about 50 km × 50 km. On the other hand, a GCM of 250 km × 250 km will only be able to resolve 50% of the radiatively important clouds. This, however, does not mean that the processes responsible for the formation and maintenance of these systems are also resolvable. The low clouds that are unattached to convective‐stratiform systems are mostly unresolvable by available GCMs.

Environment and Evolution of a Cold-Frontal Mesoscale Convective System

Data obtained from the June 26-27, 1985 period of the Kansas-Oklahoma PRE-STORM field phase are employed to describe the evolution of a mesoscale convective system (MCS) responsible for a heavy rainfall event over a large portion of Oklahoma and Kansas. In the case examined, the eastward advancement of deep convection was aided by the formation of a series of nearly parallel rainbands before the main precipitation area. These rainbands, which developed in a field of boundary layer cloud streets, without the assistance of gust-front convergence, redefined the leading edge of the MCS and became the locus of the most intense convection in the precipitation system.