Doppler Radar Observations of Drop-Size Distributions in a Thunderstorm

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  • 1 Dept. of the Geophysical Sciences, The University of Chicago
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Abstract

Data obtained in a thunderstorm with a vertically pointing Doppler radar are analyzed to find the size distribution of raindrops at heights below the 0C level. Drop-size distributions were computed using the updraft UR deduced by Rogers' method and two other updrafts, namely, UR − 1 and UR + 1 m sec−1. It is found that the drop-size data at all the heights may be well represented by the exponential equation, N (D) = N0 exp(− ΛD), in which N (DD is the concentration of drops in the diameter range D to D + ΔD, N0 = 0.07 R0.37 [cm−4], and Λ = 38R−0.14 [cm−1], R being the rainfall rate (mm hr−1). For R ≳ 3 mm hr−1, the distribution is steeper and N0 is greater as compared to the Marshall-Palmer distribution. For radar reflectivity factors Z in the range 1–105 mm6 m−3, the relationship between the mean Doppler velocity and Z for the distribution agrees with that given by Rogers to within 1 m sec−1. The following equations have been found between the water content M, median volume diameter D0, radar reflectivity factor Z, and the rainfall rate R. M = 0.052 R0.94 [gm m−3], D0 = 0.13 R0.14 [mm] Z = 300R1.35 [mm6 m−3].

Abstract

Data obtained in a thunderstorm with a vertically pointing Doppler radar are analyzed to find the size distribution of raindrops at heights below the 0C level. Drop-size distributions were computed using the updraft UR deduced by Rogers' method and two other updrafts, namely, UR − 1 and UR + 1 m sec−1. It is found that the drop-size data at all the heights may be well represented by the exponential equation, N (D) = N0 exp(− ΛD), in which N (DD is the concentration of drops in the diameter range D to D + ΔD, N0 = 0.07 R0.37 [cm−4], and Λ = 38R−0.14 [cm−1], R being the rainfall rate (mm hr−1). For R ≳ 3 mm hr−1, the distribution is steeper and N0 is greater as compared to the Marshall-Palmer distribution. For radar reflectivity factors Z in the range 1–105 mm6 m−3, the relationship between the mean Doppler velocity and Z for the distribution agrees with that given by Rogers to within 1 m sec−1. The following equations have been found between the water content M, median volume diameter D0, radar reflectivity factor Z, and the rainfall rate R. M = 0.052 R0.94 [gm m−3], D0 = 0.13 R0.14 [mm] Z = 300R1.35 [mm6 m−3].

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