DIFFERENTIAL DOPPLER VELOCITY - A RADAR PARAMETER FOR CHARACTERIZING HYDROMETEOR SIZE DISTRIBUTIONS

Observations of Doppler-resolved spectra of differential radar reflect ivity provide estimates of particle shapes as a function of their term inal velocity, and they can be derived by having the antenna at a sign ificant elevation angle. Turbulence tends to smear out the details of the actual spectra observed, but the difference in the mean values of velocity using horizontal and vertical polarizations, which the author s call the ''differential Doppler velocity'' (DDV), is unaffected. Lar ger raindrops fall faster and are oblate, so values of DDV are positiv e. If a gamma function is used for the raindrop size spectrum, then th e observed DDV and Z(DR) correspond to particular values of median dro p diameter D-0 and the dispersion index m. The scaling parameter N-0 i s derived from Z. Estimates of m have a mean value of 5 but vary subst antially. An error in rainfall rate of up to +/-15% results if the rai nfall rate is computed from Z and Z(DR) alone, and m is assumed consta nt at 5. An overestimation of more than 30% occurs if m is assumed to be 0. DDV values in stratiform ice are slightly negative. The values i n ice are explicable in terms of a mixture of slowly falling oblate cr ystals and faster-falling spherical aggregates. In the bright band, DD V is consistent with the coexistence of oblate snowflakes and faster-f alling raindrops.