ANALYSIS OF SEVERE CONVECTIVE STORMS OBSERVED BY RADAR

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  • 1 Geophysics Research Directorate, Air Force Cambridge Research Center, Air Research and Development Command
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Abstract

A survey of thunderstorms observed during 1956 by CPS-9 radar from Blue Hill (Milton, Massachusetts) revealed a high frequency of hail occurrence (as reported by a regional network of cooperative observers) in storms in which the radar echo attained high altitudes and large values of radar reflectivity factor. Hail frequencies are related to these criteria, and to the single criterion of maximum echo height, since a correlation was noted between echo height and radar reflectivity factor in all convective storms, Most of the observed damaging windstorms and tornadoes were associated with parent echoes extending above 40,000 ft and the most severe tornadoes occurred along or above the curve of 50 per cent hail frequency and had maximum echo heights above 47,000 ft.

In one tornado situation, the echo heights of several neighboring storms increased and decreased nearly in phase with one another, and the wind damage and hail at the ground occurred during the periods of rapid increase of echo height. This suggests the possibility of control of a group of storms by a meso-scale system. The median of a number of observations of the variation with height of radar reflectivity factor shows a steady decrease with height in a thunderstorm. The decrease is slow up to about 20,000 ft but more rapid at higher altitudes. Just before the appearance of the tornado, however, the radar reflectivity factor in the tornado storm increased substantially with height up to 20,000 ft.

Abstract

A survey of thunderstorms observed during 1956 by CPS-9 radar from Blue Hill (Milton, Massachusetts) revealed a high frequency of hail occurrence (as reported by a regional network of cooperative observers) in storms in which the radar echo attained high altitudes and large values of radar reflectivity factor. Hail frequencies are related to these criteria, and to the single criterion of maximum echo height, since a correlation was noted between echo height and radar reflectivity factor in all convective storms, Most of the observed damaging windstorms and tornadoes were associated with parent echoes extending above 40,000 ft and the most severe tornadoes occurred along or above the curve of 50 per cent hail frequency and had maximum echo heights above 47,000 ft.

In one tornado situation, the echo heights of several neighboring storms increased and decreased nearly in phase with one another, and the wind damage and hail at the ground occurred during the periods of rapid increase of echo height. This suggests the possibility of control of a group of storms by a meso-scale system. The median of a number of observations of the variation with height of radar reflectivity factor shows a steady decrease with height in a thunderstorm. The decrease is slow up to about 20,000 ft but more rapid at higher altitudes. Just before the appearance of the tornado, however, the radar reflectivity factor in the tornado storm increased substantially with height up to 20,000 ft.

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