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THUNDERSTORM STRUCTURE AND CIRCULATION

Horace R. ByersU. S. Weather Bureau

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Roscoe R. Braham JrU. S. Weather Bureau

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Abstract

Data from Thunderstorm Project observations in Florida, derived mainly from airplane traverses through thunderstorms, are analyzed in order to obtain a description of thunderstorm structure and circulation. Thunderstorms are normally found to consist of several more-or-less independent convective systems or “cells”. Each cell goes through a life cycle represented by three fairly distinct stages-the cumulus stage, the mature stage, and the dissipating stage.

In the cumulus stage the cell is formed from an updraft of air which, as in the other stages, “entrains” air from the environment. In this stage no rain has yet reached the ground. In the mature stage, rain is occurring and a large part of the cell consists of a downdraft which characterizes the rain area. The updraft continues in a portion of the cell in the low and intermediate levels and in all parts of the top levels. In the dissipating stage, downdrafts are present throughout, although weak upward motion is apparent in the upper parts.

The thunderstorm of 9 July 1946 is taken as typical of the structure and vertical currents that have been described. Radar photographs and data from the surface micronetwork are used in substantiation of the airplane findings. Strong horizontal divergence at the surface in the downdraft and rain areas and convergence under the updrafts are shown.

From the abundant upper-air wind and temperature-humidity soundings made with balloons in and around the thunderstorms, data from the various sources are combined to study the thermodynamics involved in the circulation. The entrainment of air into the cells has an important effect which by actual measurement and introduction of reasonable values of entrainment satisfactorily accounts for the observed air currents. With entrainment, the updraft lapse rate approaches that of the environment. It is then demonstrated that it is possible for falling rain to “trigger” a downdraft of cold air that reaches and spreads out over the surface of the earth as the cold core of the rain area.

Abstract

Data from Thunderstorm Project observations in Florida, derived mainly from airplane traverses through thunderstorms, are analyzed in order to obtain a description of thunderstorm structure and circulation. Thunderstorms are normally found to consist of several more-or-less independent convective systems or “cells”. Each cell goes through a life cycle represented by three fairly distinct stages-the cumulus stage, the mature stage, and the dissipating stage.

In the cumulus stage the cell is formed from an updraft of air which, as in the other stages, “entrains” air from the environment. In this stage no rain has yet reached the ground. In the mature stage, rain is occurring and a large part of the cell consists of a downdraft which characterizes the rain area. The updraft continues in a portion of the cell in the low and intermediate levels and in all parts of the top levels. In the dissipating stage, downdrafts are present throughout, although weak upward motion is apparent in the upper parts.

The thunderstorm of 9 July 1946 is taken as typical of the structure and vertical currents that have been described. Radar photographs and data from the surface micronetwork are used in substantiation of the airplane findings. Strong horizontal divergence at the surface in the downdraft and rain areas and convergence under the updrafts are shown.

From the abundant upper-air wind and temperature-humidity soundings made with balloons in and around the thunderstorms, data from the various sources are combined to study the thermodynamics involved in the circulation. The entrainment of air into the cells has an important effect which by actual measurement and introduction of reasonable values of entrainment satisfactorily accounts for the observed air currents. With entrainment, the updraft lapse rate approaches that of the environment. It is then demonstrated that it is possible for falling rain to “trigger” a downdraft of cold air that reaches and spreads out over the surface of the earth as the cold core of the rain area.

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