Raindrop Size Distributions and the Radar Bright Band

Abstract

The relationship between raindrop size distribution, measured with a disdrometer, and a radar parameter of the melting-layer bright band is investigated. The data, obtained in July 1993 in Switzerland, cover 120 h of precipitation. A good correlation (about −0.7) between the strength of the bright band (ΔZe) and the intercept and slope parameters of the Marshall-Palmer drop size distribution (N0, ∇) is observed for horizontally uniform precipitation of moderate intensity (one-third of total rainfall). Steep spectrum with many small drops and no large drops are associated with small values for ΔZe, whereas flat spectra with relatively few small drops and with large drops exhibit a well-defined bright band with a large ΔZe. Considering ΔZe allows a significantly better derivation of the rainfall rate from radar measurements than is possible with standard Z-R relationships. The rms errors of the 10-min averages of rainfall intensity can be reduced by 20%–40%.