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Colleen A. Leary

Abstract

Profiles of temperature and humidity beneath precipitating anvil clouds in tropical convective systems suggest the presence of a mesoscale unsaturated downdraft there. In this paper, a one-dimensional, steady-state, hydrostatic model of a downdraft is used to simulate profiles of temperature and humidity beneath anvil clouds. With a realistic drop-size distribution, the model calculations produce profiles that closely resemble observed soundings beneath anvil clouds. Such calculated profiles can be useful in computing the mass, heat and moisture budgets of tropical convective systems.

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Colleen A. Leary

Abstract

The 5 September 1974 double cloud cluster formed in a large-scale wind field characterized by convergence of the trade winds along a line of confluence at the surface, a synoptic-scale wave in the mid-tropospheric easterlies and a difluent easterly flow near the tropopause. As the double cloud cluster matured, a closed center of cyclonic circulation formed in the surface wind field. After a time lag of several hours, the 700 mb flow developed a closed cyclonic circulation with a distinct double structure and the 200 mb flow was perturbed by a center of anticyclonic outflow. Both occurred over the region occupied by the dissipating cloud cluster. As the cloud cluster was dissipating, these wind perturbations on the scale of the cloud cluster weakened at all three levels. The wind perturbations followed a pattern similar to those produced by the 4–5 September 1974 squall-line system, suggesting that different types of cloud clusters have qualitatively similar feedbacks to their large-scale environment.

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Colleen A. Leary

Abstract

This paper describes the statistics of precipitation observed over the 42.5 × 104 km2 area covered by all four of the GATE digitized radars fair the period 2300 GMT 4 September 1974 to 2400 GMT 5 September 1974. The period coincided with the GATE double cloud cluster of 5 September, the latter stages in the life cycle of the squall line system of 4–5 September and the passage of the trough of an easterly wave in the midtropospheric flow. Hourly precipitation amounts over 4 km × 4 km data bins were stratified by time, data source, individual mesoscale precipitation feature (1 of 12), type of precipitation (convective or stratiform) and, if convective, cell top height. Each subtotal contained a total precipitation amount and the number of 4 km × 4 km bins which contributed to the total. From these subtotals a variety of precipitation statistics were compiled.

The precipitation statistics of the 5 September double cloud cluster confirm the importance of deep convection in producing observed tropical mesoscale precipitation patterns. They also underscore the importance of stratiform precipitation in non-squall cloud clusters as well as in squall line systems. The evolution of the total, convective and stratiform amounts of rainfall in the 5 September double cloud cluster is similar to that of the mesoscale precipitation features of which it is composed and also to the evolution of squall line systems consisting of a single intense mesoscale precipitation feature. The time lag in the development of stratiform precipitation suggests that storage of water condensed in convective cells after transfer to the anvil may be an important component of the water budget of a mesoscale precipitation feature. When precipitation from the 4–5 September squall line system and the 5 September double cloud cluster are combined to show the evolution of precipitation over the large mesoscale in and near the trough of the easterly wave, the total rainfall is nearly constant for over 24 hours. This result is consistent with the statistical quasi-equilibrium assumption Arakawa and Schubert used to provide a closed parameterization of cumulus convection for use in prognostic models of large-scale atmospheric motion.

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Colleen A. Leary
and
Edward N. Rappaport

Abstract

This paper describes the life cycle and precipitation structure of a Mesoscale Convective Complex (MCC) that passed through the data-collecting network of the Texas portion of the High Plains Cooperative Program (HIPLEX) on 8 June 1980. The MCC was the third in a sequence of five mesoscale convective systems that formed in association with a low-level frontal zone, short-wave perturbations in the 500 mb flow, outflow from previous convection, and upslope forcing. Quantitative radar data, together with surface, upper-air and satellite data, were used to determine the three-dimensional structure of the MCC. Isolated echoes formed over the Davis Mountains of far western Texas and merged as they moved eastward to form a mesoscale convective system with a lifetime of ∼24 h and a low-level precipitation pattern ∼500 km across. The leading edge of the low-level precipitation pattern was a north-south line of intense convective cells possessing the echo structure and low-level kinematic and thermodynamic characteristics typical of a squall line. Behind the leading edge was a well-defined minimum of reflectivity in the transition zone where convective cells dissipated. To the rear of the transition zone, an extensive region of precipitation was organized, during the system's mature stage, as a set of curved rainbands. Vertical cross sections through the rainbands show indications of weak cells in an otherwise stratiform radar reflectivity pattern possessing a distinctive radar bright band. A composite wind analysis shows a center of cyclonic inflow at 500 mb near the common center of curvature of the rainbands. The banded structure of precipitation behind the transition zone suggests an interaction between the mesoscale circulation of the MCC and the fine-wale substructure of its precipitation pattern. Anticyclonic outflow at 200 mb, together with inflow new 500 mb, suggests upward motion in the middle- and upper-level clouds in this region. Thermodynamic data behind the transition zone show the characteristic profile associated with a mesoscale unsaturated downdraft below a deep cloud based at middle levels.

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Robert A. Houze Jr.
and
Colleen A. Leary

Abstract

Convective fluxes of mass and beat in tropical easterly waves were computed by a slightly modified version of the method of Austin and Houze (1973). The input data for this method are radar and raingage observations of cloud heights and precipitation amounts. The computed fluxes were compared to convective fluxes inferred from budget studies of synoptic-scale rawinsonde data for the same easterly waves. Good agreement between the two methods was obtained for mass and heat fluxes in the middle troposphere. For the lower troposphere, our calculations showed smaller transports of man and heat than were indicated by large-scale budget studies. This result arises because many shallow convective clouds do not precipitate and thus axe not included in calculations based on precipitation data. When the effects of small clouds (tops below 6 km) were removed from the calculations based on large-scale budgets, excellent agreement was obtained in the lower as well as middle troposphere. This study shows that a particularly useful approach in future studies will be a combination of methods based on both precipitation and large-scale data.

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Colleen A. Leary
and
Robert A. Houze Jr.

Abstract

The existence of extensive precipitating anvil clouds in intense tropical convection suggests that vertical air motions associated with the anvil clouds play a significant role in the mass and heat budgets of these systems. This paper uses three different sets of assumptions about the water budget of an idealized mesoscale convective system to test the sensitivity of diagnostic calculations of vertical transports of mass and heat to the inclusion or exclusion of anvil clouds and their associated mesoscale vertical air motions. The properties of the mesoscale updraft and downdraft are evaluated using observations and the results of modeling studies. When a mesoscale updraft and downdraft are included in the diagnostic calculations, the profiles of vertical transports of mass and moist static energy are both qualitatively and quantitatively different from the results when mesoscale vertical air motions are excluded. Inclusion of mesoscale vertical motions in the diagnostic calculations leads to smaller upward mass transports below 4 km, larger upward mass sports above 4 km, less cooling below 4 km, and more cooling between 4.5 and 6.5 km than are obtained when mesoscale motions are not included in the calculations. These results imply that the effect of mesoscale vertical air motions on cloud mass flux and net beating profiles should be considered when parameterizing the effects of tropical convection on the larger scale environment.

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Colleen A. Leary
and
Robert A. Houze Jr.

Abstract

A large cloud cluster which occurred over the data network of the Global Atmospheric Research Program's Atlantic Tropical Experiment (GATE) on 5 September 1974 is examined. Data from four quantitative shipboard weather radars show that virtually all of the precipitation in the tropical cloud cluster was associated with six mesoscale precipitation features. A prototype for the structure and life cycle of these features is presented which is sufficiently general to describe all six precipitation features, one of which was a tropical squall-line system. These mesoscale features appear to he the primary entitles within which deep tropical convection occurs.

In their formative stage, mesoscale precipitation features consist of a line of isolated cumulonimbus cells oriented perpendicular to the low-level wind flow.

In the intensifying stage, the rain areas of the individual cells merge when new convective cells develop between and ahead of the existing cells, where the outflow from convective-scale downdrafts enhances low-level convergence. In the upper troposphere an overhang of cloud and precipitation particles extends downwind in the layer of outflow from deep convective updrafts.

The mature mesoscale precipitation feature possesses both a region of convective cells along its leading edge and a large area of horizontally uniform precipitation to the rear. The longevity and total rainfall in the area of horizontally uniform precipitation suggest that its maintenance may he due to organized mesoscale uplift in an anvil cloud extending from the 600–700 mb level to the upper troposphere. In the horizontally uniform rain area beneath the anvil cloud, aircraft observations show cold, dry, low-θw air consistent with the presence of a mesoscale unsaturated downdraft maintained by cooling due to the evaporation of failing rain. A pronounced radar bright band at the melting level is further evidence of cooling in this region.

In the dissipating stage, intense convective cells cease forming along the leading edge but the area of horizontally uniform precipitation persists for at least several hours longer.

Interactions among the six mesoscale precipitation features result in echo mergers that complicate the precipitation pattern of the cloud cluster, and give the cloud cluster of 5 September a distinctive double shape.

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Colleen A. Leary
and
Robert A. Houze Jr.

Abstract

Five cases of horizontally uniform precipitation associated with anvil clouds were investigated using weather radar, rawinsonde, satellite and raindrop size data collected during the Global Atmospheric Research Program's Atlantic Tropical Experiment (GATE).

The area of horizontally uniform precipitation was in each case characterized by rainfall rates of 1–10 mm h−1 in contrast to the 10–100 mm h−1 observed in convective cells. Concentrations of precipitation-sized ice particles above the melting layer and liquid water below the melting layer, together with observed particle spectra, suggest that aggregation occurs above the melting layer, and that riming occurs in sufficient amounts to produce graupel within the anvil cloud.

All five cases exhibited distinct radar bright bands in the melting layer. Cooling rates associated with the melting in this 1 km thick layer near the base of the anvil cloud were 1-7 K h−1. These cooling rates were comparable to the 0.2–6 K h−1 cooling rates due to evaporation of raindrops below the melting layer, suggesting that melting as well as evaporation plays a role in the initiation and maintenance of a mesoscale downdraft beneath the anvil cloud.

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Colleen A. Leary
and
Rory O. R. Y. Thompson

Abstract

A synoptic analysis of the only tropical depression in the BOMEX data set shows it to be a kink in the Intertropical Convergence Zone. The depression has a warm core through the troposphere to at least 250 mb, and strongest circulation in and just above the boundary layer. At 250 mb, an anticyclonic wind field lies above the depression. Divergence, vorticity, and relative humidity fields, as well as cloud patterns, corroborate a dynamical picture of the depression wherein air rising in a warm core is associated with convergence and cyclonic vorticity resulting from a kink in the ITCZ. The picture is compatible with the CISK mechanism.

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