[1] Mesoscale convective systems (MCSs) have regions of both convective and stratiform precipitation, and they develop mesoscale circulations as they mature. The upward motion takes the form of a deep-layer ascent drawn into the MCS in response to the latent heating and cooling in the convective region. The ascending layer overturns as it rises but overall retains a coherent layer structure. A middle level layer of inflow enters the stratiform region of the MCS from a direction determined by the large-scale flow and descends in response to diabatic cooling at middle-to-low levels. A middle level mesoscale convective vortex (MCV) develops in the stratiform region, prolongs the MCS, and may contribute to tropical cyclone development. The propagation of an MCS may have a discrete component but may further be influenced by waves and disturbances generated both in response to the MCS and external to the MCS. Waves of a larger scale may affect the propagation velocity by phase locking with the MCS in a cooperative mode. The horizontal scale of an MCS may be limited either by a balance between the formation rate of convective precipitation and dissipation of stratiform precipitation or by the Rossby radius of the MCV. The vertical redistribution of momentum by an MCS depends on the size of the stratiform region, while the net vertical profile of heating of the large-scale environment depends on the amount of stratiform rain. Regional variability of the stratiform rain from MCSs affects the large-scale circulation’s response to

A new two-moment cloud microphysics scheme predicting the mixing ratios and number concentrations of five species (i.e., cloud droplets, cloud ice, snow, rain, and graupel) has been implemented into the Weather Research and Forecasting model (WRF). This scheme is used to investigate the formation and evolution of trailing stratiform precipitation in an idealized two-dimensional squall line. Results are compared to those using a one-moment version of the scheme that predicts only the mixing ratios of the species, and diagnoses the number concentrations from the specified size distribution intercept parameter and predicted mixing ratio. The overall structure of the storm is similar using either the one- or two-moment schemes, although there are notable differences. The two-moment (2-M) scheme produces a widespread region of trailing stratiform precipitation within several hours of the storm formation. In contrast, there is negligible trailing stratiform precipitation using the one-moment (1-M) scheme. The primary reason for this difference are reduced rain evaporation rates in 2-M compared to 1-M in the trailing stratiform region, leading directly to greater rain mixing ratios and surface rainfall rates. Second, increased rain evaporation rates in 2-M com-pared to 1-M in the convective region at midlevels result in weaker convective updraft cells and increased midlevel detrainment and flux of positively buoyant air from the convective into the stratiform region. This

The kinematic, microphysical, and electrical structures of two mesoscale convective systems (MCSs) observed during the 1991 Cooperative Oklahoma Profiler Studies (COPS91) experiment are analyzed. Profiles of the vertical electric field structure and charge density were obtained from a series of balloon-borne electric field meter (EFM) flights into each MCS. Contrasting electric field structures were found in the stratiform regions of these MCSs. In both systems, the EFM data indicate that the MCS charge structure was characterized by horizontally extensive regions of charge and charge density magnitudes on the order of what is typically observed in convective cores (�5 nCm�3). However, the vertical electric field profiles were each related to unique MCS precipitation and kinematic structures, with a five-layer charge profile (at T � 0�C) associated with the ‘‘symmetric’’ MCS and a simpler three-layer charge profile (at T � 0�C) associated with the ‘‘asymmetric’ ’ MCSs. The observational analysis identified several kinematic, thermodynamic, and microphysical differences between the two systems that offer at least some explanation for the observed electrical structures. First, ice particles detrained from the convective line of the symmetric MCS had much shorter ‘‘residence times’ ’ in the unfavorable growth/charging region associated with the transition zone downdraft compared to the asymmetric case. Second, upon entering the trailing stratiform region, ice particles in the symmetric system were immersed

An algorithm, the spectral latent heating (SLH) algorithm, has been developed to estimate latent heating profiles for the Tropical Rainfall Measuring Mission precipitation radar with a cloud-resolving model (CRM). Heating-profile lookup tables for the three rain types—convective, shallow stratiform, and anvil rain (deep stratiform with a melting level)—were produced with numerical simulations of tropical cloud systems in the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment. For convective and shallow stratiform regions, the lookup table refers to the precipitation-top height (PTH). For the anvil region, on the other hand, the lookup table refers to the precipitation rate at the melting level instead of PTH. A consistency check of the SLH algorithm was also done with the CRM-simulated outputs. The first advantage of this algorithm is that differences of heating profiles between the shallow convective stage and the deep convective stage can be retrieved. This is a result of the utilization of observed information, not only on precipitation type and intensity, but also on the precipitation depth. The second advantage is that heating profiles in the decaying stage with no surface rain can also be retrieved. This comes from utilization of the precipitation rate at the melting level for anvil regions. 1.

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The evolution of a small, vigorous mesoscale convective system (MCS) over northern Alabama is described using Doppler radar, GOES satellite, surface mesonet, lightning, and sounding data. The MCS formed near noon in a relatively unstable environment having weak synoptic forcing and weak shear. The initiation of separate lines and clusters of deep convection occurred in regions exhibiting cumulus cloud streets, horizontal variations in stratocumulus cloud cover, and variations in inferred soil moisture. MCS growth via merger of storms within clusters and lines, and among the clusters, was accomplished largely through intersection of storm-scale and mesoscale outflow boundaries. The MCS maximum anvil area (;60 000 km2 at 220 K) and lifetime (8 h) were about 50 % that of the typical Great Plains mesoscale convective complex (MCC). Despite its smaller size, this MCS displayed many aspects that typify the mostly nocturnal Great Plains MCS. The precipitation output was highly variable due to the transient nature of the intense convective elements, many of which produced microbursts. The radar measurements documented the formation of a stratiform region along the trailing side of an intense convective line. This stratiform region formed as decaying convective cores coalesced, rather than through advection of precipitation particles directly from the convective region. Combined GOES IR imagery and radar reflectivity analyses within the stratiform region show a sinking anvil cloud top

This paper describes calculations of the spatial and temporal variation of the radiation budget of a tropical mesoscale convective system (MCS). A combination of cloud model simula-tions, radiation model simulations, and analyses of observations obtained during the Equatorial Mesoscale Experiment (EMEX), the Stratosphere-Troposphere Exchange Program (STEP), and the Australian Monsoon Experiment (AMEX) are used to obtain these heating rates. The two-dimensional version of the Colorado State University regional atmospheric modeling system is used to simulate a tropical MCS that occurred during EMEX mission 9 on February 2, 1987. The simulation is shown to broadly agree with the observations reported in a related paper. The spatial radiative heating distributions derived from a two-stream radiative transfer model corre-sponding to the mature stage of the simulated cloud system indicate that significant horizontal inhomogeneities exist. According to the model results the effects of the MCS are to (1) increase in the infrared emission to the surface and to decrease in the net infrared energy loss from the atmosphere relative to the clear sky emission and (2) change the transmission of solar flux to the surface, the shortwave albedo of the atmosphere, and the solar absorption in the atmosphere. The results show how the MCS significantly reduces the solar flux to the surface relative to the clear

This paper is the first in a two part series in which the interactions between a growing mesoscale convective system (MCS) and its surrounding environment are investigated. The system studied here developed in north-eastern Colorado on 19 July 1993 and propagated into Kansas as a long-lived nocturnal MCS. High-resolution dual-Doppler and surface mesonet data collected from this system are discussed in Part I, while the results of a numerical simulation are discussed in Part II. The observations show that organized mesoscale surface pressure and flow features appeared very early in the lifetime of this system, long before the development of any trailing stratiform precipitation. Most of the stratiform anvil advected ahead of the convective line in the strong upper-tropospheric westerlies. In accordance with this, most of the mid- and upper-tropospheric storm-relative flow behind the line remained westerly, or rear-to-front. Despite the westerlies, the strongest flow perturbations with respect to the ambient winds developed to the rear of the line. The structure of these perturbations was similar to the upper-tropospheric front-to-rear and midtropospheric rear-to-front flows typically found in more mature leading-line/trailing-stratiform systems. The presence of these perturbations on the upwind side of the convective line indicates that gravity wave propagation was primarily responsible for their development. 1.

A global climatology of the altitude of the freezing level (08C isotherm) is computed using 20 yr of 6-hourly output from the National Centers for Environmental Prediction (NCEP) reanalysis system. Mean statistics dis-cussed include monthly means and climatological monthly means. Variance statistics include the standard de-viation of the 6-hourly values with the month and the standard deviation of the monthly means. In the Tropics, freezing levels are highest (;5000 m) and both intramonth and interannual variability is lowest. Freezing levels are lower and variability is higher in the subtropics and midlatitudes. In 1998 there are unusually high freezing levels in the eastern Pacific Ocean relative to the 20-yr climatology, consistent with elevated sea surface tem-peratures associated with the 1997–98 El Niño. Freezing levels return to near-climatological values during the last half of 1998. The individual monthly means for 1998 and the 20-yr climatology are compared with monthly means of the altitude of the bright band (melting layer) retrieved from Tropical Rainfall Measuring Mission (TRMM) precipitation radar data. Differences between TRMM and NCEP typically range from about 2300 to 2900 m. Differences are somewhat larger over landmasses and in zonal bands centered on 6208 latitude. 1.

The difficult problem of parameterizing tropical convection in large-scale models of the atmosphere led to the Global Atmospheric Research Program's Atlantic Tropical Experiment (GATE), whose goal was to improve basic understanding of tropical convection and its role in the global atmospheric circula-tion. A dense network of instrumented ships equipped with upper air sounding equipment and quan-titative weather radars were located over the Atlantic Ocean, in the intertropical convergence zone (ITCZ), just west of equatorial Africa. The ship network was supplemented by a fleet of research aircraft and a geosynchronous meteorological satellite. The data obtained show that the deep convection in the ITCZ was concentrated in two types of 'cloud clusters, ' rapidly moving squall clusters, and slowly mov-ing nonsquail clusters. The clusters were characterized by large mid-to-upper level cloud shields, or 'anvil clouds, ' that emanated from penetrative cumulonimbus convection. Accompanying the deep cumulo-nimbus in each cluster was a log normal spectrum of smaller convective features ranging from moderate cumulonimbus down to tiny nonprecipitating cumulus. The large cumulonimbus were typically grouped within a cluster into one or more mesoscale precipitation features (or MPF's), which were apparently triggered in mesoscale regions of intensified low-level convergence. As an MPF matured it developed a region of stratiform precipitation adjacent to its active deep convective cells. The stratiform precipitation