Stratiform Precipitation Areas Research Articles

Springtime mesoscale convective systems over South China: a historic radar image-based analysis of climatological features, interannual variability, and potential connections with surface aerosol

We objectively analyzed historic radar reflectivity images and diagnosed mature mesoscale convective systems (MCSs) in South China during the spring season (March to May) of 2009–2019. Our goal was to understand the climatological features of mature MCSs, their interannual variations, and potential connections with surface aerosol pollution. Springtime MCSs over South China were most frequently observed in the central and east-coastal parts of Guangdong Province. The mean monthly half-hourly counts of MCSs over South China in March, April, and May were 103 ± 83, 274 ± 298, and 337 ± 225, respectively, with considerable variability from year to year. Approximately 89% of springtime MCSs over South China had a linear or quasi-linear structure, with convective precipitation covering on average 34% of the total precipitating area of each individual MCS, anmied 63% of MCSs consisted of a stratiform precipitation area trailing the convective precipitation. In March, MCSs occurred most frequently mid-day; in April and May, MCSs were most frequent around midnight. From 2013 to 2019, the MCS occurrences in April were significantly lower during years with more aerosol pollution days. This finding potentially supported our previous model study’s finding that elevated anthropogenic aerosol levels may suppress April MCS occurrences in South China via aerosol-cloud-radiation interactions. Further research is required to better understand the intricate relationship between aerosol abundance and MCS activities in this region.

Analysis of Convective and Stratiform Precipitation Characteristics in Xinjiang, China Based on GPM Dual‐Frequency Precipitation Radar

The spatial distribution of precipitation in Xinjiang, China, is extremely heterogeneous due to its complex subsurface, and microphysical studies of cloud precipitation over Xinjiang are severely constrained by observational means. The Global Precipitation Measurement (GPM) dual‐frequency precipitation radar (DPR) provides improved three‐dimensional observation data of strong and weak precipitation and performs effective observations in alpine mountain regions. Therefore, this study used GPM/DPR Level‐3 data from January 2015 to December 2022 to analyze the convective and stratiform precipitation characteristics over Xinjiang. The results showed that (1) the range of maximum precipitation reflectivity factors (PRF_max) near the ground and at different heights were larger in summer and maximum PRF_max values appeared in areas along the Altai, Tianshan, and Kunlun Mountains and plains on the respective mountain slopes. In summer, the PRF_max of stratiform (convective) precipitation nearsurface ranged from 24 to 34 dBZ (28–40 dBZ). (2) The two types of precipitation exhibited the highest storm top heights in summer, with convective precipitation having higher values than stratiform precipitation and mountainous areas having higher values than the surrounding basins due to the effects of topographic uplift. (3) The value domains of the PRF, particle mass‐weighted diameter (Dm), and particle number concentration for stratiform (convective) precipitation were more concentrated (wider), that is, PRF: 14–30 dBZ (12–40 dBZ), Dm: 0.9–1.4 mm (0.7–2.1 mm), and the particle concentration: 29–35 (24–39). The height of the center of the probability density of each variable was higher in the three major mountain ranges than in the two major basins owing to the influence of topography. PRF increased and then decreased with height. Convective raindrops were in an active state of high concentration and exhibited a large scale in the lower layers, whereas the particle spectral parameters of stratiform precipitation showed a more concentrated and symmetric distribution and insignificant changes with height.

Cloud‐Radiation Feedback Facilitates Secondary Eyewall Formation of Tropical Cyclones

AbstractRecent studies show that the formation of outer rainbands (ORBs) is essence to secondary eyewall formation (SEF) of tropical cyclones (TCs). This study reveals that cloud‐radiation feedback (CRF) facilitates SEF of TCs by promoting the formation of ORBs and their stratiform sectors. In detail, CRF engenders an upper‐level anomalous radiative warming in the outer‐core region, which induces a secondary circulation that accelerates the release of microphysical heating above the freezing level. The enhanced upper‐level latent heating further promotes the outer‐core convection and the top‐down development of the ORBs. Moreover, CRF promotes a broader stratiform precipitation area and stronger stratiform heating profiles of the ORBs, which forces convective updrafts at the radially inward edge of the stratiform precipitation area and eventually leads to SEF. On the contrary, when the anomalous radiative warming is removed, the organization of ORBs and the development of stratiform precipitation are suppressed due to the absence of the radiation‐induced upper‐level secondary circulation, thus inhibiting SEF.

Heavy Winter Precipitation Events with Extratropical Cyclone Diagnosed by GPM Products and Trajectory Analysis

Heavy precipitation events were identified during the cold seasons of 2014–2019 using two-day accumulated precipitation data at 137 stations of the Japan Meteorological Agency. The mechanisms for producing heavy precipitation regarding the structure of an occluding extratropical cyclone were analyzed using the products of the Dual-frequency Precipitation Radar (DPR) onboard the Global Precipitation Measurement (GPM) core satellite and trajectory analysis on the European Centre for Medium-range Weather Forecasts atmospheric reanalysis data. Upper-ranked events with heavy precipitation were predominantly caused by extratropical cyclones and were in mature stages. In the top 50 ranked events, three south-coast cyclones were nominated, and the relationships between developing the mesoscale precipitation system and airstreams were intensively diagnosed. Hourly precipitation changes at stations that recorded heavy precipitation were primarily affected by a combination of the warm conveyor belt (WCB), cold conveyor belt (CCB), and dry intrusion (DI). Wide-ranging stratiform precipitation east of the cyclone center was composed of low-level WCB over the CCB and upper WCB, and convective clouds around the cyclone center were associated with the upper DI over the WCB that provided an extreme precipitation rate at the surface, including the formation of a band-shaped precipitation system. Convective cloud activities also contributed to moist air advection over the stationary stratiform precipitation areas recognized as the upper WCB. DPR products also identified deep stratiform precipitation in the cloud-head area behind the cyclone center with mid-level (near-surface) latent heat release (absorption) with increased potential vorticity along the CCB, making feedback intensification of the cyclone possible.

Cloud‐Resolving Model Intercomparison of an MC3E Squall Line Case: Part II. Stratiform Precipitation Properties

AbstractIn this second part of a cloud microphysics scheme intercomparison study, we focus on biases and variabilities of stratiform precipitation properties for a midlatitude squall line event simulated with a cloud‐resolving model implemented with eight cloud microphysics schemes. Most of the microphysics schemes underestimate total stratiform precipitation, mainly due to underestimation of stratiform precipitation area. All schemes underestimate the frequency of moderate stratiform rain rates (2–6 mm/hr), which may result from low‐biased ice number and mass concentrations for 0.2–2‐mm diameter particles in the stratiform ice region. Most simulations overestimate ice water content (IWC) at altitudes above 7 km for temperatures colder than −20 °C but produce a decrease of IWC approaching the melting level, which is opposite to the trend shown by in situ observations. This leads to general underestimations of stratiform IWC below 5‐km altitude and rainwater content above 1‐km altitude for a given rain rate. Stratiform precipitation area positively correlates with the convective condensate detrainment flux but is modulated by hydrometeor type, size, and fall speed. Stratiform precipitation area also changes by up to 17%–25% through alterations of the lateral boundary condition forcing frequency. Stratiform precipitation, rain rate, and area across the simulations vary by a factor of 1.5. This large variability is primarily a result of variability in the stratiform downward ice mass flux, which is highly correlated with convective condensate horizontal detrainment strength. The variability of simulated local microphysical processes in the stratiform region plays a secondary role in explaining variability in simulated stratiform rainfall properties.

The Role of Vertically Propagating Gravity Waves Forced by Melting-Induced Cooling in the Formation and Evolution of Wide Cold-Frontal Rainbands

Abstract Realistic mesoscale model simulations using the Weather Research and Forecasting (WRF) Model and idealized dry simulations were used to study the mechanisms responsible for the formation and evolution of wide cold-frontal rainbands (WCFRs) associated with a wintertime cyclone that moved onto the Washington coast. The WRF simulation reproduced observed characteristics of three successively formed WCFRs, including their spacing and movement as well as the timing of the formation of two WCFRs behind the first. Sensitivity experiments showed that melting-induced cooling in the stratiform precipitation area behind the surface cold front was essential for the formation of the first and second WCFRs, whereas the third WCFR was formed by the release of potential instability within an ascent forced by upper-level frontogenesis. Enhanced frontal updrafts responsible for the first and second WCFRs were created by a superposition of a broad updraft caused by frontal dynamics and upward-propagating gravity waves generated by the melting-induced cooling. The dry simulations forced by specified cooling revealed specific mechanisms for the wave generation and the evolution of the first and second WCFRs. The gravity waves were generated at the intersection of the low-level frontal zone and the melting layer, where strong vertical shear of the cross-front wind and upshear-sloped cooling by melting cooperatively enhanced the wave generation. The formation of the second WCFR behind the first and subsequent dissipation of these WCFRs was attributed to the evolution of a wave pattern associated with the evolution of cross-front flow above the frontal zone.

Latent heating characteristics of the MJO computed from TRMM Observations

AbstractThe Tropical Rainfall Measurement Mission's (TRMM) Spectral Latent Heating algorithm shows the contributions of different forms of convection to the latent heating profiles of the Madden‐Julian Oscillation over the central Indian and West Pacific Oceans. In both oceanic regions, storms containing broad stratiform regions produce increased upper level heating during active Madden‐Julian Oscillation (MJO) phases. The largest differences between the central Indian and West Pacific Ocean heating are associated with heating produced by convective elements. Examination of the most extreme forms of convection shows that mesoscale organized convection often produces at least as much latent heat as young vigorous deep convection. Heating from nonextreme (often midlevel‐topped) convection is an important component of the MJO heating in both regions in all stages of the MJO. Over the central Indian Ocean the heating profile changes from having a maximum at 2 km due to nonextreme convection to a profile during the active stage that has two maxima: one at 3 km due to nonextreme convection and 6 km owing to numerous mature mesoscale storms with broad stratiform precipitation components. Over the West Pacific, the maxima at 3 and 6 km are present in all MJO stages, but the magnitude of the 6 km maximum sharply increases in the active MJO stage due to an increase in the number of storms with broad stratiform precipitation areas.

The cloud population and onset of the Madden‐Julian Oscillation over the Indian Ocean during DYNAMO‐AMIE

AbstractVariability of the cloud population in the central equatorial Indian Ocean was observed in the context of the Madden‐Julian Oscillation (MJO) during the Dynamics of the Madden‐Julian Oscillation (DYNAMO) and Atmospheric Radiation Measurement Madden‐Julian Investigation Experiment (AMIE) field campaigns. Radar observations from the polarimetric S‐band radar on Addu Atoll in the Maldives characterize the types of convective and stratiform radar echoes and the heights their 20 dBZ contours reach. To gain insight into the relationship between clouds and humidification of the troposphere leading up to and during an active MJO event, the work relates variability of the observed precipitation structure to that of tropospheric humidity and upper level zonal wind. The variability in stratiform precipitation areas dominates variability in the nature of precipitating convection associated with the MJO. Areal coverage of precipitating radar echo, convective echo top height, and tropospheric humidity above 850 hPa rapidly increase over ~3–7 days near MJO onset. This rate of increase is substantially faster than the 10–20 days needed for buildup of moisture prior to MJO onset as hypothesized by the “discharge‐recharge” hypothesis. Convective echoes become more common during the days prior to MJO onset, and the increased convection occurs before low‐tropospheric moistening. The upper troposphere rapidly moistens as the first widespread stratiform region passes over an area. Thus, clouds likely play a role in tropospheric humidification. Whether increased low‐tropospheric humidity causes vertical growth of convection has not yet been determined.

Classifying convective and stratiform rain using multispectral infrared Meteosat Second Generation satellite data

This paper investigates the potential for developing schemes that classify convective and stratiform precipitation areas using the high infrared spectral resolution of the Meteosat Second Generation—Spinning Enhanced Visible and Infrared Imager (MSG-SEVIRI). Two different classification schemes were proposed that use the brightness temperature (BT) Τ10.8 along with the brightness temperature differences (BTDs) Τ10.8–Τ12.1, Τ8.7–Τ10.8, and Τ6.2–Τ10.8 as spectral parameters, which provide information about cloud parameters. The first is a common multispectral thresholding scheme used to partition the space of the spectral cloud parameters and the second is an algorithm based on the probability of convective rain (PCR) for each pixel of the satellite data. Both schemes were calibrated using as a reference convective\stratiform rain classification fields derived from 87 stations in Greece for six rainy days with high convective activity. As a result, one single infrared technique (TB10) and two multidimensional techniques (BTDall and PCR) were constructed and evaluated against an independent sample of rain gauge data for four daily convective precipitation events. It was found that the introduction of BTDs as additional information to a technique works in improving the discrimination of convective from stratiform rainy pixels compared to the single infrared technique BT10. During the training phase, BTDall performed slightly better than BT10 while PCR technique outperformed both threshold techniques. All techniques clearly overestimate the convective rain occurrences detected by the rain gauge network. When evaluating against the independent dataset, both threshold techniques exhibited the same performance with that of the dependent dataset whereas the PCR technique showed a notable skill degradation. As a result, BTDall performed best followed at a short distance by PCR and BT10. These findings showed that it is possible to apply a convective/stratiform rain classification algorithm based on the enhanced infrared spectral resolution of MSG-SEVIRI, for nowcasting or climate purposes, despite the highly variable nature of convective precipitation.

Anvil clouds of tropical mesoscale convective systems in monsoon regions

AbstractThe anvil clouds of tropical mesoscale convective systems (MCSs) in West Africa, the Maritime Continent and the Bay of Bengal have been examined with TRMM and CloudSat satellite data and ARM ground‐based radar observations. The anvils spreading out from the precipitating cores of MCSs are subdivided into thick, medium and thin portions. The thick portions of anvils show distinct differences from one climatological regime to another. In their upper portions, the thick anvils of West Africa MCSs have a broad, flat histogram of reflectivity, and a maximum of reflectivity in their lower portions. The reflectivity histogram of the Bay of Bengal thick anvils has a sharply peaked distribution of reflectivity at all altitudes with modal values that increase monotonically downward. The reflectivity histogram of the Maritime Continent thick anvils is intermediate between that of the West Africa and Bay of Bengal anvils, consistent with the fact this region comprises a mix of land and ocean influences. It is suggested that the difference between the statistics of the continental and oceanic anvils is related to some combination of two factors: (1) the West African anvils tend to be closely tied to the convective regions of MCSs while the oceanic anvils are more likely to be extending outward from large stratiform precipitation areas of MCSs, and (2) the West African MCSs result from greater buoyancy, so that the convective cells are more likely to produce graupel particles and detrain them into anvils. Copyright © 2009 Royal Meteorological Society

First results on a process-oriented rain area classification technique using Meteosat Second Generation SEVIRI nighttime data

Abstract. A new technique for process-oriented rain area classification using Meteosat Second Generation SEVIRI nighttime data is introduced. It is based on a combination of the Advective Convective Technique (ACT) which focuses on precipitation areas connected to convective processes and the Rain Area Delineation Scheme during Nighttime (RADS-N) a new technique for the improved detection of stratiform precipitation areas (e.g. in connection with mid-latitude frontal systems). The ACT which uses positive brightness temperature differences between the water vapour (WV) and the infrared (IR) channels (ΔTWV-IR) for the detection of convective clouds and connected precipitating clouds has been transferred from Meteosat First Generation (MFG) Metesoat Visible and Infra-Red Imager radiometer (MVIRI) to Meteosat Second Generation (MSG) Spinning Enhanced Visible and InfraRed Imager (SEVIRI). RADS-N is based on the new conceptual model that precipitating cloud areas are characterised by a large cloud water path (cwp) and the presence of ice particles in the upper part of the cloud. The technique considers information about both parameters inherent in the channel differences ΔT3.9-10.8, ΔT3.9-7.3, ΔT8.7-10.8, and ΔT10.8-12.1, to detect potentially precipitating cloud areas. All four channel differences are used to gain implicit knowledge about the cwp. ΔT8.7-10.8 and ΔT10.8-12.1 are additionally considered to gain information about the cloud phase. First results of a comparison study between the classified rain areas and corresponding ground based radar data for precipitation events in connection with a cold front occlusion show encouraging performance of the new proposed process-oriented rain area classification scheme.

From Hot Towers to TRMM: Joanne Simpson and Advances in Tropical Convection Research

Joanne Simpson began contributing to advances in tropical convection about half a century ago. The hot tower hypothesis jointly put forth by Joanne Simpson and Herbert Riehl postulated that deep convective clouds populating the “equatorial trough zone” were responsible for transporting heat from the boundary layer to the upper troposphere. This hypothesis was the beginning of a 50-year quest to describe and understand near-equatorial deep convection. Tropical field experiments in the 1970s [Global Atmospheric Research Program Atlantic Tropical Experiment (GATE) and the Monsoon Experiment (MONEX)] in which Joanne participated documented the mesoscale structure of the convective systems, in particular the deep, stratiform, dynamically active mesoscale clouds that are connected with the hot towers. In the 1980s these new data led to better understanding of how tropical mesoscale convective systems vertically transport heat and momentum. The role of the mesoscale stratiform circulation in this transport was quantified. Tropical field work in the 1990s [especially the Coupled Ocean-Atmosphere Response Experiment (COARE), in which Joanne again participated] showed the importance of a still larger scale of convective organization, the “supercluster.” This larger scale of organization has a middle-level inflow circulation that appears to be an important transporter of momentum. The mesoscale and supercluster scale of organization in tropical convective systems are associated with the stratiform components of the cloud systems. Joint analysis of satellite and radar data from COARE show a complex, possibly chaotic relationship between cloud-top temperature and the size of a stratiform precipitation area. The Tropical Rainfall Measuring Mission (TRMM) satellite, for which Joanne served as project scientist for nearly a decade, is now providing a global census of mesoscale and supercluster-scale organization of tropical convection. The TRMM dataset should therefore provide some closure to the question of the nature of deep convection in the equatorial trough zone.

A Texture-Polarization Method for Estimating Convective–Stratiform Precipitation Area Coverage from Passive Microwave Radiometer Data

Observational and modeling studies have described the relationships between convective/stratiform rain proportion and the vertical distributions of vertical motion, latent heating, and moistening in mesoscale convective systems. Therefore, remote sensing techniques which can quantify the relative areal proportion of convective and stratiform, rainfall can provide useful information regarding the dynamic and thermodynamic processes in these systems. In the present study, two methods for deducing the convective/stratiform areal extent of precipitation from satellite passive microwave radiometer measurements are combined to yield an improved method. If sufficient microwave scattering by ice-phase precipitating hydrometeors is detected, the method relies mainly on the degree of polarization in oblique-view, 85.5 GHz radiances to estimate the area fraction of convective rain within the radiometer footprint. In situations where ice scattering is minimal, the method draws mostly on texture information in radiometer imagery at lower microwave frequencies to estimate the convective area fraction. Based upon observations of ten convective systems over ocean and nine systems over land, instantaneous 0.5 degree resolution estimates of convective area fraction from the Tropical Rainfall Measuring Mission Microwave Imager (TRMM TMI) are compared to nearly coincident estimates from the TRMM Precipitation Radar (TRMM PR). The TMI convective area fraction estimates are slightly low-biased with respect to the PR, with TMI-PR correlations of 0.78 and 0.84 over ocean and land backgrounds, respectively. TMI monthly-average convective area percentages in the tropics and subtropics from February 1998 exhibit the greatest values along the ITCZ and in continental regions of the summer (southern) hemisphere. Although convective area percentages. from the TMI are systematically lower than those from the PR, monthly rain patterns derived from the TMI and PR rain algorithms are very similar. TMI rain depths are significantly higher than corresponding rain depths from the PR in the ITCZ, but are similar in magnitude elsewhere.

Wake Low Severe Wind Events in the Mississippi River Valley: A Case Study of Two Contrasting Events

Abstract Severe winds, in excess of 50 kt (25 m s−1), brought widespread damage across the mid–Mississippi Valley on the mornings of 11 April 1995 and 28 April 1996. These severe winds were unusual and unexpected as they trailed behind stratiform precipitation areas where severe weather is not normally expected. The wake low on 11 April 1995 appeared to be the result of strong adiabatic warming due to dry air subsidence while the wake low on 28 April 1996 appeared to be associated with a gravity wave. Because wake lows are short-lived mesoscale phenomena, they are generally not well understood by the operational forecasting community and have been nearly impossible for forecasters to predict. With the modernization of the National Weather Service, it may become feasible to anticipate and track these mesoscale phenomena with Doppler radar, surface observations, and finer-resolution models. With an evaluation of some of the differences and similarities in the generation and maintenance of severe wake lows, ...

Separation of Convective and Stratiform Precipitation Using Microwave Brightness Temperature

Abstract This paper presents a new scheme that classifies convective and stratiform (C/S) precipitation areas over oceans using microwave brightness temperature. In this scheme, data are first screened to eliminate nonraining pixels. For raining pixels, C/S indices are computed from brightness temperatures and their variability for emission (19 and 37 GHz) and scattering (85 GHz). Since lower-resolution satellite data generally contain mixtures of convective and stratiform precipitation, a probability matching method is employed to relate the C/S index to a convective fraction of precipitation area. The scheme has been applied on synthetic data generated from a dynamical cloud model and radiative transfer computations to simulate the frequencies and resolutions of the Tropical Rainfall Measuring Mission (TRMM) Microwave (TMI) Imager as well as the Special Sensor Microwave/Imager (SSM/I). The results from simulated TMI data during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Exper...

Stratiform Precipitation in Regions of Convection: A Meteorological Paradox?

It was once generally thought that stratiform precipitation was something occurring primarily, if not exclusively, in middle latitudes—in baroclinic cyclones and fronts. Early radar observations in the Tropics, however, showed large radar echoes composed of convective rain alongside stratiform precipitation, with the stratiform echoes covering great areas and accounting for a large portion of the tropical rainfall. These observations seemed paradoxical, since stratiform precipitation should not have been occurring in the Tropics, where baroclinic cyclones do not occur. Instead it was falling from convection-generated clouds, generally thought to be too violent to be compatible with the layered, gently settling behavior of stratiform precipitation. In meteorology, convection is a dynamic concept; specifically, it is the rapid, efficient, vigorous overturning of the atmosphere required to neutralize an unstable vertical distribution of moist static energy. Most clouds in the Tropics are convection-generated cumulonimbus. These cumulonimbus clouds contain an evolving pattern of newer and older precipitation. The young portions of the cumulonimbus are too violent to produce stratiform precipitation. In young, vigorous convective regions of the cumulonimbus, precipitation particles increase their mass by collection of cloud water, and the particles fall out in heavy showers, which appear on radar as vertically oriented convective “cells.” In regions of older convection, however, the vertical air motions are generally weaker, and the precipitation particles drift downward, with the particles increasing their mass by vapor diffusion. In these regions the radar echoes are stratiform, and typically these echoes occur adjacent to regions of younger convective showers. Thus, the stratiform and convective precipitation both occur within the same complex of convection-generated cumulonimbus cloud. The feedbacks of the apparent heat source and moisture sink of tropical cumulonimbus convection to the large-scale dynamics of the atmosphere are distinctly separable by precipitation region. The part of the atmospheric response deriving from the areas of young, vigorous convective cells is two layered, with air converging into the active convection at low levels and diverging aloft. The older, weaker intermediary and stratiform precipitation areas induce a three-layered response, in which environmental air converges into the weak precipitation area at midlevels and diverges from it at lower and upper levels. If global precipitation data, such as that to be provided by the Tropical Rainfall Measuring Mission, are to be used to validate the heating patterns predicted by climate and general circulation models, algorithms must be applied to the precipitation data that will identify the two principal modes of heating, by separating the convective component of the precipitation from the remainder.

The Mesoscale Structure of Severe Precipitation Systems in Switzerland

Abstract The structures of severe mesoscale precipitation systems (MPS) in Switzerland have been classified by analyzing radar images obtained over a 5-yr period. Severe MPSs were defined to be those producing most of the damage on days on which at least 5 (out of 2400) communities reported water and/or at least 20 reported hail damage. Of 94 MPSs selected, 82 had radar reflectivity of 47 dBZ or greater and were referred to as mesoscale convective systems (MCS). The 12 remaining MPSs consisted of less intense, long-lasting, and widespread frontal or orographic rainfall. Subclasses of MCSs were defined according to their internal arrangements of cell complexes (CC). A CC was defined as an echo contour of 40 dBZ surrounding echo maxima of at least 47 dBZ. Four general categories of organization were found: isolated CC, a group of CCs, and a broken or continuous line of CCs. All categories can be purely convective at the mature stage, or the CCs may be juxtaposed with a stratiform precipitation area, usually...

Diabatic Divergence Profiles in Western Pacific Mesoscale Convective Systems

Abstract Heating in the atmosphere can be expressed as diabatic divergence δd, which is nearly equal to the actual horizontal divergence δ in tropical convection. High-quality δ profile measurements from airborne Doppler radar “purls” in ten mesoscale convective systems (MCS) observed during TOGA-COARE are examined, and the mean profile is compared with rawinsonde array measurements. Young convective features have strong near-surface convergence, while older cells with better-developed downdrafts and stratiform precipitation areas have their peak convergence aloft. In the mean, then, surface flow is only weakly convergent or oven divergent, so that the main convergence into MCSs is deep and peaked aloft, with a sharp “melting convergence” at 0°C. Divergence prevails above ∼10 km altitude but was undersampled by the radar. Unusual but well-sampled observations in the purl dataset include: a persistent, widespread δ profile feature in one well-sampled MCS (a cyclone rainband); oscillatory “reverberations” c...

Melting and freezing in a mesoscale convective system

AbstractMicrophysical and thermodynamic fields retrieved from dual‐Doppler synthesized winds are used to examine the distribution of melting and freezing rates within a midlatitude squall line with trailing stratiform precipitation. Previously, the effect of the heating/cooling processes associated with the release/consumption of the latent heat of fusion have emphasized the melting layer in the stratiform precipitation region, where the layer is typically marked by a radar bright band. The retrieved fields reveal the melting and freezing in the convective as well as stratiform region. Two‐dimensional retrieved fields indicate that the strong melting effect is not confined to the stratiform region, but extends across the entire breadth of the storm. The peak rates of cooling in the stratiform precipitation region are −2 to −3 K h−1, in general agreement with previous studies. In contrast, cooling rates in the convective precipitation region reach −14 K h−1. The heating by freezing is concentrated in a vertical column associated with the convective updraughts, with peak heating rates of 8 K h−1 just above the 0 °C level. For the mature stage of the storm, the convective region is found to account for 56% of the total cooling by melting, and 87% of the total heating by freezing within the squall line.A one‐dimensional retrieval model is applied to mean vertical‐velocity profiles to retrieve area‐mean profiles of the melting and freezing rates. In the convective region, average cooling rates of −10 K h−1 are found. The strong cooling by melting is expected to exert a strong influence on the development of convective downdraughts, the cold pool, and the gust front. The heating by freezing occurs through a deep layer, with a peak average heating rate of about 4 K h−1 just above the 0 °C level. In the stratiform precipitation region, a melting rate of about −2 K h−1 produced a near 0 °C isothermal layer approximately 500 m deep, similar to isothermal layers observed in stratiform precipitation areas associated with fronts.

Synthetic Dual-Doppler Analysis of Mesoscale Convective Systems

The synthetic dual-Doppler (SDD) analysis technique is applied to the 6–7 May 1985 mesoscale convective system (MCS) that occurred during the Oklahoma-Kansas Preliminary Regional Experiment-STORM Central. This system had a cyclonic mesoscale circulation in its stratiform precipitation region. The SDD analyses are compared to corresponding actual dual-Doppler analyses. The sensitivity of the former to various parameters is discussed. The SDD analysis technique is also applied to an MCS that passed by the NEXRAD (Next Generation Weather Radar) facility in Norman, Oklahoma, on 13 June 1989. Analyses of both the leading line of convective cells and the trailing stratiform precipitation area we presented; the salient features were similar to those found in dual-Doppler analyses of other systems. There was a mesoscale cyclonic circulation present at low and midlevels in the stratiform precipitation area. Vertical wind profiles obtained from the SDD analysis are compared to those obtained front velocity-azimuth display (VAD) analyses and from a conventional sounding. Vertical profiles of divergence and vertical velocity that were determined from VAD analyses indicated upward motion ahead of the convective line, and sinking motion and rising motion in the lower and upper troposphere, respectively, in the stratiform precipitation area behind the leading line. Rising motion and sinking motion in the lower and upper troposphere, respectively, were indicated in the vicinity of the mesoscale cyclonic circulation.