Structure and Dynamics of a Tropical Squall–Line System

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  • 1 Department of Atmospheric Sciences, University of Washington, Seattle 98195
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Abstract

A tropical squall-line system which moved over the observational network of the Global Atmospheric Research Programme's Atlantic Tropical Experiment (GATE) was investigated using rawinsonde, weather radar, satellite, surface meteorological, acoustic sounder and cloud photographic data. Combining these data led to a detailed synthesis of the three-dimensional structure, dynamics and life cycle of the disturbance.

The squall-line system consisted of a squall line forming the leading edge of the system and a trailing anvil cloud region. The squall line was made up of discrete active centers of cumulonimbus convection, referred to as line elements (LE's). New LE's formed ahead of the squall line. Old LE's weekened toward the rear of the line and blended into the trailing anvil region as they dissipated. Each LE progressed through a period of rapid growth, with echo tops penetrating the tropopause to maximum heights of 16–17 km, then decreasing to heights of 13–14 km, which corresponds to the height of the anvil cloud with which the LE's merged at the end of their lifetimes.

The squall line was located along the leading edge of a mesoscale downdraft forming and spreading out in the middle and lower troposphere below the anvil cloud. Within the squall line, individual LE's contained smaller, convective-scale downdrafts which penetrated down to the sea surface. This cold, convective-scale downdraft air spread out at low levels providing lift for the sea on the leading side of the LE. The convective-scale downdraft air also spread out in a layer 200–400 mb deep toward the rear of the system. The top of this layer of cold air was bounded by a stable layer below which enhanced turbulent mixing occurred.

Precipitation failing from the trailing anvil cloud was stratiform and accounted for 40% of the total rain from the squall-line system. Thus, much of the anvil cloud is accounted for by the successive incorporation of weakened, but precipitation-laden old LE's from the back edge of the squall line and possibly by widespread upward air motion within the upper level anvil cloud.

Abstract

A tropical squall-line system which moved over the observational network of the Global Atmospheric Research Programme's Atlantic Tropical Experiment (GATE) was investigated using rawinsonde, weather radar, satellite, surface meteorological, acoustic sounder and cloud photographic data. Combining these data led to a detailed synthesis of the three-dimensional structure, dynamics and life cycle of the disturbance.

The squall-line system consisted of a squall line forming the leading edge of the system and a trailing anvil cloud region. The squall line was made up of discrete active centers of cumulonimbus convection, referred to as line elements (LE's). New LE's formed ahead of the squall line. Old LE's weekened toward the rear of the line and blended into the trailing anvil region as they dissipated. Each LE progressed through a period of rapid growth, with echo tops penetrating the tropopause to maximum heights of 16–17 km, then decreasing to heights of 13–14 km, which corresponds to the height of the anvil cloud with which the LE's merged at the end of their lifetimes.

The squall line was located along the leading edge of a mesoscale downdraft forming and spreading out in the middle and lower troposphere below the anvil cloud. Within the squall line, individual LE's contained smaller, convective-scale downdrafts which penetrated down to the sea surface. This cold, convective-scale downdraft air spread out at low levels providing lift for the sea on the leading side of the LE. The convective-scale downdraft air also spread out in a layer 200–400 mb deep toward the rear of the system. The top of this layer of cold air was bounded by a stable layer below which enhanced turbulent mixing occurred.

Precipitation failing from the trailing anvil cloud was stratiform and accounted for 40% of the total rain from the squall-line system. Thus, much of the anvil cloud is accounted for by the successive incorporation of weakened, but precipitation-laden old LE's from the back edge of the squall line and possibly by widespread upward air motion within the upper level anvil cloud.

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