Oceanic Mesoscale Convective Systems Research Articles

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

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

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

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

Long-Lived Mesoscale Convective Systems over Eastern South Africa

AbstractA climatology of large, long-lived mesoscale convective systems (MCSs) over eastern South Africa for the extended austral summer (September–April) from 1985 to 2008 is presented. On average, 63 MCSs occur here in summer, but with considerable interannual variability in frequency. The systems mainly occur between November and March, with a December peak. This seasonal cycle in MCS activity is shown to coincide with favorable CAPE and vertical shear profiles across the domain. Most systems tend to occur along the eastern escarpment and adjacent warm waters of the northern Agulhas Current with a nocturnal life cycle. Typically, initiation begins in the early afternoon, MCS status is reached midafternoon, maximum extent early in the night, and termination around midnight or shortly thereafter. It is found that most MCSs initiate over land, but systems that initiate over the ocean tend to last longer than those that develop over land. The results also show that there are differences in the seasonal cycle between continental and oceanic MCSs, with oceanic systems containing two intraseasonal peaks (December and April). There is a relatively strong positive relationship between the southern annular mode (SAM) and early summer MCS frequency. For the late summer, the frequency of MCSs appears related to the strength of the Mascarene high and Mozambique Channel trough, which modulate the inflow of moisture into eastern South Africa and the stability of the lower atmosphere over the region.

A Global High‐Resolution Mesoscale Convective System Database Using Satellite‐Derived Cloud Tops, Surface Precipitation, and Tracking

AbstractA new methodology is developed to construct a global (60°S–60°N) long‐term (2000–2019) high‐resolution (∼10‐km h) mesoscale convective system (MCS) database by tracking MCS jointly using geostationary satellite infrared brightness temperature (Tb) and precipitation feature (PF) characteristics from the Integrated Multi‐satellitE Retrievals for GPM precipitation data sets. Independent validation shows that the satellite‐based MCS data set is able to reproduce important MCS statistics derived from ground‐based radar network observations in the United States and China. We show that by carefully considering key PF characteristics in addition to Tb signatures, the new method significantly improves upon previous Tb‐only methods in detecting MCSs in the midlatitudes for all seasons. Results show that MCSs account for over 50% of annual total rainfall across most of the tropical belt and in selected regions of the midlatitudes, with a strong seasonality over many regions of the globe. The tracking database allows Lagrangian aspects such as MCS lifetime and translational speed and direction to be analyzed. The longest‐lived MCSs preferentially occur over the subtropical oceans. The land MCSs have higher cloud‐tops associated with more intense convection, and oceanic MCSs have much higher rainfall production. While MCSs are observed in many regions of the globe, there are fundamental differences in their dynamic and thermodynamic structures that warrant a better understanding of processes that control their evolution. This global database provides significant opportunities for observational and modeling studies of MCSs, their characteristics, and roles in regional and global water and energy cycles, as well as their hydrologic and other impacts.

Evaluating the Detection of Mesoscale Outflow Boundaries Using Scatterometer Winds at Different Spatial Resolutions

Outflow boundaries induced by cold-pools are a key characteristic of convective systems related to microphysical and kinematic processes during the mature stage of their lifecycle. Over the ocean, such kinematic processes are associated with low-level wind modulations that are captured by scatterometers. This study investigates the ability of the Advanced Scatterometer (ASCAT) wind retrievals to detect the outflow boundary associated with an oceanic mesoscale convective system (MCS). Leveraging a new technique to identify cold pools that is based on features that enclose elevated magnitude of the gradient of the wind, termed as ‘Gradient Feature’ (GF), wind retrievals at 50-, 25- and 7-km spatial resolution were utilized to explore how the characteristics of the outflow boundary vary with resolution. Ground-based radar retrievals were also implemented to assess and correct, when possible, the performance of the ASCAT retrievals. The magnitude of the gradient of the wind for the coarser resolution was an order of magnitude smaller (10−4 s−1) than the finer ones (10−3 s−1). An increase in the magnitude of the gradient wind field associated with the outflow boundary was captured by all resolutions and a respective feature was identified by the GF method. The location of the features relative to the distance from the front edge of the MCS decreased with resolution, indicating the importance of the high resolution ASCAT product to capture their extent, as well as additional smaller scale features. The effect of the background wind field on the selection of the final wind field during the ambiguity removal process for the high-resolution product is also discussed.

A 17 year climatology of the macrophysical properties of convection in Darwin

Abstract. The validation of convective processes in global climate models (GCMs) could benefit from the use of large datasets that provide long-term climatologies of the spatial statistics of convection. To that regard, echo top heights (ETHs), convective areas, and frequencies of mesoscale convective systems (MCSs) from 17 years of data from a C-band polarization (CPOL) radar are analyzed in varying phases of the Madden–Julian Oscillation (MJO) and northern Australian monsoon in order to provide ample validation statistics for GCM validation. The ETHs calculated using velocity texture and reflectivity provide similar results, showing that the ETHs are insensitive to various techniques that can be used. Retrieved ETHs are correlated with those from cloud top heights retrieved by Multifunctional Transport Satellites (MTSATs), showing that the ETHs capture the relative variability in cloud top heights over seasonal scales. Bimodal distributions of ETH, likely attributable to the cumulus congestus clouds and mature stages of convection, are more commonly observed when the active phase of the MJO is over Australia due to greater mid-level moisture during the active phase of the MJO. The presence of a convectively stable layer at around 5 km altitude over Darwin inhibiting convection past this level can explain the position of the modes at around 2–4 km and 7–9 km. Larger cells were observed during break conditions compared to monsoon conditions, but only during the inactive phase of the MJO. The spatial distributions show that Hector, a deep convective system that occurs almost daily during the wet season over the Tiwi Islands, and sea-breeze convergence lines are likely more common in break conditions. Oceanic MCSs are more common during the night over Darwin. Convective areas were generally smaller and MCSs more frequent during active monsoon conditions. In general, the MJO is a greater control on the ETHs in the deep convective mode observed over Darwin, with higher distributions of ETH when the MJO is active over Darwin.

A long-term tropical mesoscale convective systems dataset based on a novel objective automatic tracking algorithm

Mesoscale convective systems (MCSs) are important components of tropical weather systems and the climate system. Long-term data of MCS are of great significance in weather and climate research. Using long-term (1985–2008) global satellite infrared (IR) data, we developed a novel objective automatic tracking algorithm, which combines a Kalman filter (KF) with the conventional area-overlapping method, to generate a comprehensive MCS dataset. The new algorithm can effectively track small and fast-moving MCSs and thus obtain more realistic and complete tracking results than previous studies. A few examples are provided to illustrate the potential application of the dataset with a focus on the diurnal variations of MCSs over land and ocean regions. We find that the MCSs occurring over land tend to initiate in the afternoon with greater intensity, but the oceanic MCSs are more likely to initiate in the early morning with weaker intensity. A double peak in the maximum spatial coverage is noted over the western Pacific, especially over the southwestern Pacific during the austral summer. Oceanic MCSs also persist for approximately 1 h longer than their continental counterparts.

Comparison of observed and simulated spatial patterns of ice microphysical processes in tropical oceanic mesoscale convective systems

AbstractTo equitably compare the spatial pattern of ice microphysical processes produced by three microphysical parameterizations with each other, observations, and theory, simulations of tropical oceanic mesoscale convective systems (MCSs) in the Weather Research and Forecasting (WRF) model were forced to develop the same mesoscale circulations as observations by assimilating radial velocity data from a Doppler radar. The same general layering of microphysical processes was found in observations and simulations with deposition anywhere above the 0°C level, aggregation at and above the 0°C level, melting at and below the 0°C level, and riming near the 0°C level. Thus, this study is consistent with the layered ice microphysical pattern portrayed in previous conceptual models and indicated by dual‐polarization radar data. Spatial variability of riming in the simulations suggests that riming in the midlevel inflow is related to convective‐scale vertical velocity perturbations. Finally, this study sheds light on limitations of current generally available bulk microphysical parameterizations. In each parameterization, the layers in which aggregation and riming took place were generally too thick and the frequency of riming was generally too high compared to the observations and theory. Additionally, none of the parameterizations produced similar details in every microphysical spatial pattern. Discrepancies in the patterns of microphysical processes between parameterizations likely factor into creating substantial differences in model reflectivity patterns. It is concluded that improved parameterizations of ice‐phase microphysics will be essential to obtain reliable, consistent model simulations of tropical oceanic MCSs.

Tropical Cyclone Mekkhala’s (2008) Formation over the South China Sea: Mesoscale, Synoptic-Scale, and Large-Scale Contributions

Abstract Tropical cyclone formation close to the coastline of the Asian continent presents a significant threat to heavily populated coastal countries. A case study of Tropical Storm Mekkhala (2008) that developed off the coast of Vietnam is presented using the high-resolution analyses of the European Centre for Medium-Range Weather Forecasts/Year of Tropical Convection and multiple satellite observations. The authors have analyzed contributions to the formation from large-scale intraseasonal variability, synoptic perturbations, and mesoscale convective systems (MCSs). Within a large-scale westerly wind burst (WWB) associated with the Madden–Julian oscillation (MJO), synoptic perturbations generated by two preceding tropical cyclones initiated the pre-Mekkhala low-level vortex over the Philippine Sea. Typhoon Hagupit produced a synoptic-scale wave train that contributed to the development of Jangmi, but likely suppressed the Mekkhala formation. The low-level vortex of the pre-Mekkhala disturbance was then initiated in a confluent zone between northeasterlies in advance of Typhoon Jangmi and the WWB. A key contribution to the development of Mekkhala was from diurnally varying MCSs that were invigorated in the WWB. The oceanic MCSs, which typically develop off the west coast of the Philippines in the morning and dissipate in the afternoon, were prolonged beyond the regular diurnal cycle. A combination with the MCSs developing downstream of the Philippines led to the critical structure change of the oceanic convective cluster, which implies the critical role of mesoscale processes. Therefore, the diurnally varying mesoscale convective processes over both the ocean and land are shown to have an essential role in the formation of Mekkhala in conjunction with large-scale MJO and the synoptic-scale TC influences.

Precipitation hydrometeor type relative to the mesoscale airflow in mature oceanic deep convection of the Madden‐Julian Oscillation

AbstractComposite analysis of mature near‐equatorial oceanic mesoscale convective systems (MCSs) during the active stage of the Madden‐Julian Oscillation (MJO) shows where different hydrometeor types occur relative to convective updraft and stratiform midlevel inflow layers. The National Center for Atmospheric Research (NCAR) S‐PolKa radar observed these MCSs during the Dynamics of the Madden‐Julian Oscillation/Atmospheric Radiation Measurement‐MJO Investigation Experiment (DYNAMO/AMIE). NCAR's particle identification algorithm (PID) is applied to S‐PolKa's polarimetric data to identify the dominant hydrometeor type in each radar sample volume. Combining S‐PolKa's Doppler velocity data with the PID demonstrates that hydrometeors have a systematic relationship to the airflow within mature MCSs. In the convective region, moderate rain occurs within the updraft core; the heaviest rain occurs just downwind of the core; wet aggregates occur immediately below the melting layer; narrow zones containing graupel/rimed aggregates occur just downstream of the updraft core at midlevels; dry aggregates dominate above the melting level; and smaller ice particles occur along the edges of the convective zone. In the stratiform region, rain intensity decreases toward the anvil; melting aggregates occur in horizontally extensive but vertically thin regions at the melting layer; intermittent pockets of graupel/rimed aggregates occur atop the melting layer; dry aggregates and small ice particles occur sequentially above the melting level; and horizontally oriented ice crystals occur between −10°C and −20°C in turbulent air above the descending midlevel inflow, suggesting enhanced depositional growth of dendrites. The organization of hydrometeors within the midlevel inflow layer is insensitive to the presence or absence of a leading convective line.

Microphysical properties and high ice water content in continental and oceanic mesoscale convective systems and potential implications for commercial aircraft at flight altitude

Abstract. Two complementary case studies are conducted to analyse convective system properties in the region where strong cloud-top lidar backscatter anomalies are observed as reported by Platt et al. (2011). These anomalies were reported for the first time using in situ microphysical measurements in an isolated continental convective cloud over Germany during the CIRCLE2 experiment (Gayet et al., 2012). In this case, in situ observations quasi-collocated with CALIPSO (Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observation), CloudSat and Meteosat-9/SEVIRI observations confirm that regions of backscatter anomalies represent the most active and dense convective cloud parts with likely the strongest core updrafts and unusually high values of the particle concentration, extinction and ice water content (IWC), with the occurrence of small ice crystal sizes. Similar spaceborne observations of a maritime mesoscale cloud system (MCS) located off the Brazilian coast between 0° and 3° N latitude on 20 June 2008 are then analysed. Near cloud-top backscatter anomalies are evidenced in a region which corresponds to the coldest temperatures with maximum cloud top altitudes derived from collocated CALIPSO/IIR and Meteosat-9/SEVIRI infrared brightness temperatures. The interpretation of CALIOP (Cloud Aerosol Lidar with Orthogonal Polarization) data highlights significant differences in microphysical properties from those observed in the continental isolated convective cloud. Indeed, SEVIRI (Spinning Enhanced Visible and InfraRed Imager) retrievals in the visible spectrum confirm much smaller ice particles near the top of the isolated continental convective cloud, i.e. effective radius (Reff) ~ 15 μm as opposed to 22–27 μm in the whole MCS area. Cloud profiling observations at 94 GHz from CloudSat are then used to describe the properties of the most active cloud regions at and below cloud top. The cloud ice-water content and effective radius retrieved with the CloudSat 2B-IWC and DARDAR (raDAR/liDAR) inversion techniques, show that at usual cruise altitudes of commercial aircraft (FL 350 or ~ 10 700 m level), high IWC (i.e. up to 2 to 4 g m−3) could be identified according to specific IWC–Z (Z being the reflectivity factor) relationships. These values correspond to a maximum reflectivity factor of +18 dBZ (at 94 GHz). Near-top cloud properties also indicate signatures of microphysical characteristics according to the cloud-stage evolution as revealed by SEVIRI images to identify the development of new cells within the MCS cluster. It is argued that the availability of real-time information (on the kilometre-scale) about cloud top IR brightness temperature decreases with respect to the cloud environment would help identify MCS cloud areas with potentially high ice water content and small particle sizes against which onboard meteorological radars may not be able to provide timely warning.

Reducing the Biases in Simulated Radar Reflectivities from a Bulk Microphysics Scheme: Tropical Convective Systems

Abstract A well-known bias common to many bulk microphysics schemes currently being used in cloud-resolving models is the tendency to produce excessively large reflectivity values (e.g., 40 dBZ) in the middle and upper troposphere in simulated convective systems. The Rutledge and Hobbs–based bulk microphysics scheme in the Goddard Cumulus Ensemble model is modified to reduce this bias and improve realistic aspects. Modifications include lowering the efficiencies for snow/graupel riming and snow accreting cloud ice; converting less rimed snow to graupel; allowing snow/graupel sublimation; adding rime splintering, immersion freezing, and contact nucleation; replacing the Fletcher formulation for activated ice nuclei with that of Meyers et al.; allowing for ice supersaturation in the saturation adjustment; accounting for ambient RH in the growth of cloud ice to snow; and adding/accounting for cloud ice fall speeds. In addition, size-mapping schemes for snow/graupel were added as functions of temperature and mixing ratio, lowering particle sizes at colder temperatures but allowing larger particles near the melting level and at higher mixing ratios. The modifications were applied to a weakly organized continental case and an oceanic mesoscale convective system (MCS). Strong echoes in the middle and upper troposphere were reduced in both cases. Peak reflectivities agreed well with radar for the weaker land case but, despite improvement, remained too high for the MCS. Reflectivity distributions versus height were much improved versus radar for the less organized land case but not for the MCS despite fewer excessively strong echoes aloft due to a bias toward weaker echoes at storm top.

Characteristics of Tropical Convection over the Ocean near Kwajalein

Abstract Radar observations have been analyzed to determine characteristics of convection over the oceanic region around Kwajalein in the tropical western Pacific. Generally, the echo areas, heights, and durations exhibited lognormal distributions. Heights were greater under conditions of higher midtropospheric humidity and correlated with echo area, with a wide spread of values. Mergers and splits often truncated echo lifetimes. The most surprising result was the distribution of orientation angles of echo lines, which statistically verify the shear-parallel and shear-normal modes of convective line organization seen elsewhere over tropical oceans. The two modes typically coexisted within the area of radar coverage, indicating a potential difficulty in predicting line orientation in terms of large-scale variables. The largest members of the echo population were mesoscale convective systems (MCSs), which had large stratiform components. They occurred in environments of increased humidity and higher midlevel buoyancy associated with westward-propagating synoptic-scale disturbances. MCSs moved with the easterlies in low to midlevels and exhibited the jump updraft, overturning updraft, and subsiding midlevel inflow characteristic of tropical oceanic MCSs. The Kwajalein Experiment (KWAJEX) radar echo population resembles that of the eastern tropical Atlantic in terms of the shapes of the distributions of area, height, and lifetime, the prevalence of echo mergers and splits, and the tendency to form shear-parallel and shear-normal lines. These characteristics appear to be endemic to oceanic convective populations. Large-scale conditions appear to modulate these basic population characteristics rather than qualitatively alter them.

The oceanic mesoscale convective system and associated mesovortex observed 12 December 1992 during TOGA-COARE

This study documents the precipitation and kinematic structure of a mature/mature-to-decaying, westward propagating, near-equatorial oceanic mesoscale convective system (MCS) observed by airborne Doppler radars during TOGA-COARE, the Tropical Ocean/Global Atmosphere Coupled Ocean-Atmosphere Response Experiment. This system occurred on 12 December 1992 during the convectively active phase of an intraseasonal oscillation, and was followed, at its dissipating stage, by a redevelopment of convection that led to the MCS observed on 13 December 1992. These two successive MCSs were associated with a two-day atmospheric disturbance. Radar-deduced airflows at two time periods reveal many similarities with other tropical oceanic cloud clusters. In particular, a marked rear inflow jet associated with a midlevel cyclonic mesovortex was observed within the rear of the stratiform region. The mesovortex was better defined both in size and depth during the mature-to-decaying stage and was located more inside the system within a region of convergence, due to the westward propagation of the rear inflow. Countergradient transports of momentum normal to the mean orientation of the system at midlevels, and downgradient transports at lower and higher levels helped to intensify this rear inflow. These transports were mostly accomplished by eddy structures. Cyclonic vorticity was concentrated at the rear of the MCS and peaked at the centre of the observed closed wind circulation. As previously observed, stretching of the pre-existing vertical vorticity was the dominant dynamical mechanism which helped to amplify the mesovortex at mid-to-upper levels, while tilting of the horizontal vorticity into the vertical was a lesser mechanism and had an opposite effect. In most respects, advective processes were negatively correlated with the stretching and tilting mechanisms, but not sufficiently to be balanced. The resulting tendency was an increase of cyclonic vorticity at low-to-mid levels and a net decrease above.

The oceanic mesoscale convective system and associated mesovortex observed 12 December 1992 during TOGA-COARE

The oceanic mesoscale convective system and associated mesovortex observed 12 December 1992 during TOGA-COARE

Effect of Three-Dimensional Structure on the Stormwide Horizontal Accelerations and Momentum Budget of a Simulated Squall Line

Abstract Past studies of the effects of mesoscale convective systems (MCSs) on the environmental flow have been limited by data coverage and resolution. In the current study the MCS-scale (stormwide) horizontal accelerations and momentum budget associated with an oceanic MCS are analyzed using output from a high-resolution three-dimensional numerical model integrated over a large domain. The simulation is based on an observed MCS that occurred on 22 February 1993 during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment. An important aspect of both the observed and simulated MCS is its evolution from a quasi-two-dimensional to an asymmetric three-dimensional morphology, which was demonstrated in companion studies to result from the finite length of the MCS interacting with environmental vertical shear that varies in direction with height. Herein, the authors focus on the effects of the three-dimensional structure on MCS-scale horizontal accelerations. The horizontal accelera...

The Intensification of the Low-Level Jet during the Development of Mesoscale Convective Systems on a Mei-Yu Front

Abstract During the period of 21–25 June 1991, a mei-yu front, observed by the post–Taiwan Area Mesoscale Experiment, produced heavy precipitation along the western side of the Central Mountain Range of Taiwan. Several oceanic mesoscale convective systems were also generated in an area extending from Taiwan to Hong Kong. Numerical experiments using the Penn State–NCAR MM5 mesoscale model were used to understand the intensification of the low-level jet (LLJ). These processes include thermal wind adjustment and convective, inertial, and conditional symmetric instabilities. Three particular circulations are important in the development of the mei-yu front. First, there is a northward branch of the circulation that develops across the upper-level jet and is mainly caused by the thermal wind adjustment as air parcels enter an upper-level jet streak. The upper-level divergence associated with this branch of the circulation triggers convection. Second, the southward branch of the circulation, with its rising mot...

The oceanic mesoscale convective system observed with airborne Doppler radars on 9 February 1993 during TOGA-COARE: Structure, evolution aand budgets

The oceanic mesoscale convective system observed with airborne Doppler radars on 9 February 1993 during TOGA-COARE: Structure, evolution aand budgets

The oceanic mesoscale convective system observed with airborne Doppler radars on 9 February 1993 during TOGA–COARE: Structure, evolution and budgets

AbstractData, collected on 9 February 1993 during the Tropical Ocean and Global Atmosphere Programme—Coupled Ocean‐Atmosphere Response Experiment (TOGA‐COARE) with the Doppler radars on board the two National Oceanic and Atmospheric Administration WP‐3D and the National Center for Atmospheric Research Electra aircraft, are analysed to study the structure and evolution of an equatorial oceanic mesoscale convective system (MCS). Observations lasted more than five hours, a period during which the cloud‐cluster characteristics changed from convective to stratiform. This mesoscale system was approximately aligned along the tropospheric wind shear, but it was nearly perpendicular to the low‐level shear.Series of three‐dimensional fields of reflectivity, air‐velocity components, retrieved pressure and temperature perturbations show similarities with previously observed convective systems over the tropical oceans. In particular, although the cloud top reached 18 km altitude, relatively weak vertical motions and temperature perturbations resulted from a moderate convective instability. In the low levels, a cold front‐to‐rear flow intensified and deepened as the overall structure evolved from convective to stratiform. The vertical profiles of mean horizontal divergence evolved from low‐level convergence in the cross‐line direction for the earliest analyses to mid‐level convergence from all directions during the later stratiform phase. Vertical momentum fluxes were important and led to an increase in the cross‐line wind shear and a decrease in the along‐line one.Analysis of radiosounding data from the nearest stations of the TOGA‐COARE array shows that the period during which airborne Doppler observations were conducted was characterized by relatively strong large‐scale ascent and ‘apparent’ sources of heat, moisture and momentum. These ‘diagnosed’ values and the mean radarderived ones agree fairly well. This shows that the early convective and late stratiform phases of the MCS are equally important for the ‘apparent’ sources of heat and momentum.

Passive Microwave Observations of the Stratiform Regions of Two Tropical Oceanic Mesoscale Convective Systems

Abstract High-resolution passive microwave observations within the stratiform regions of two different tropical oceanic mesoscale convective systems are investigated in detail. The observations were obtained from the Advanced Microwave Precipitation Radiometer (AMPR) during the Tropical Ocean and Global Atmosphere Coupled Ocean-Atmosphere Response Experiment. The AMPR records data at 10.7, 19.35, 37.1, and 85.5 GHz. Data collected during one ER-2 leg over the stratiform regions of each system are examined in detail. (Each leg is well coordinated with the low-flying WP-3s.) The passive microwave observations of the two stratiform regions, with similar surface rain rates, are found to differ significantly from one another. The average 85.5-GHz brightness temperature was 30 K colder on 22 February compared with 20 February. This greater ice scattering on 22 February is found to be consistent with the simultaneous radar reflectivity profile, which shows that upper-level reflectivities are greater through a de...