Simple Altitude Adjustments to Raindrop Velocities for Doppler Radar Analysis

Abstract

The altitude factor for adjusting raindrop velocities from sea level depends primarily on air density and drop size. An adjustment factor is formulated as the air density ratio raised to a power m that varies linearly with drop diameter from 0.4 at 1 mm to 0.5 at 5 mm: m(D) = 0.375 + 0.025D (mm). The improved velocity adjustment can be directly incorporated into the standard equation for calculating the size distribution from the Doppler spectrum. An integral mean value (m?) is provided to adjust sea level values of rainfall rate and mean (and mean square) Doppler velocities. The recommended value of m? varies from 0.41 to 0.46 depending on application. The simplest adjustment is obtained using 0.42 for rainfall rate and 0.45 for mean Doppler velocity.