Differential Doppler Velocity: A Radar Parameter for Characterizing Hydrometeor Size Distributions

Observations of Doppler-resolved spectra of differential radar reflectivity provide estimates of particle shapes as a function of their terminal velocity, and they can be derived by having the antenna at a significant 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 authors call the "differential Doppler velocity" (DDV), is unaffected. Larger raindrops fall faster and are oblate, so values of DDV are positive. If a gamma function is used for the raindrop size spectrum, then the observed DDV andZ DRcorrespond to particular values of median drop diameterD₀ and the dispersion indexm. The scaling parameterN₀ is derived fromZ. Estimates ofmhave a mean value of 5 but vary substantially. An error in rainfall rate of up to ±15% results if the rainfall rate is computed fromZandZ DRalone, andmis assumed constant at 5. An overestimation of more than 30% occurs ifmis assumed to be 0. DDV values in stratiform ice are slightly negative. The values in ice are explicable in terms of a mixture of slowly falling oblate crystals and faster-falling spherical aggregates. In the bright band, DDV is consistent with the coexistence of oblate snowflakes and faster-falling raindrops.