ANALYSIS OF SEVERE CONVECTIVE STORMS OBSERVED BY RADAR-II

Ralph J. Donaldson Jr. Geophysics Research Directorate, Air Force Cambridge Research Center

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Abstract

The frequency of hail occurrence in New England thunderstorms during 1956 and 1957 was directly related to the maximum heights and maximum reflectivity factors of the thunderstorm echoes measured by CPS-9 radar. Hail frequencies in excess of 33 per cent occurred in storms with echo heights above 46,000 to 49,000 ft, with little dependence on reflectivity. Higher hail frequencies were found with even greater echo heights and were also strongly related to large values of radar reflectivity factor.

The over-all frequency of hail reported in all thunderstorms was 20.6 per cent, considerably larger than the value of somewhat less than five per cent computed for the region by Shands. The discrepancy is believed to reflect a difference in definition of thunderstorm area, and an observer bias in favor of the larger, hail-producing storms which was evident in the present study.

Storms that produced large tornadoes and destructive winds and hail were characterized by significantly greater than average echo heights but not by unusually high echo reflectivities at low altitude. Five storms that resulted in large tornadoes had maximum echo heights during their lifetime of 47,000 ft to 57,000 ft. In two tornado-bearing storms, limited evidence was obtained for extremely large values of radar reflectivity factor at high altitudes with a pronounced maximum around 20,000 ft.

Abstract

The frequency of hail occurrence in New England thunderstorms during 1956 and 1957 was directly related to the maximum heights and maximum reflectivity factors of the thunderstorm echoes measured by CPS-9 radar. Hail frequencies in excess of 33 per cent occurred in storms with echo heights above 46,000 to 49,000 ft, with little dependence on reflectivity. Higher hail frequencies were found with even greater echo heights and were also strongly related to large values of radar reflectivity factor.

The over-all frequency of hail reported in all thunderstorms was 20.6 per cent, considerably larger than the value of somewhat less than five per cent computed for the region by Shands. The discrepancy is believed to reflect a difference in definition of thunderstorm area, and an observer bias in favor of the larger, hail-producing storms which was evident in the present study.

Storms that produced large tornadoes and destructive winds and hail were characterized by significantly greater than average echo heights but not by unusually high echo reflectivities at low altitude. Five storms that resulted in large tornadoes had maximum echo heights during their lifetime of 47,000 ft to 57,000 ft. In two tornado-bearing storms, limited evidence was obtained for extremely large values of radar reflectivity factor at high altitudes with a pronounced maximum around 20,000 ft.

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