Vertical Evolution of Raindrop Size Distribution due to Breakup and Coalescence Observed with Vertically Pointing Radars

Vertically pointing radars can observe the vertical structure of precipitation as precipitation advects over the radar site. By operating two vertically pointing Doppler radars side-by-side and transmitting at 3- and 35-GHz, the mean Doppler velocities of these two radars will be different due to the Rayleigh scatter and non-Rayleigh scattering of backscattered energy from the same distribution of hydrometeors. This differential Doppler velocity (DDV) is dependent on the shape of the raindrop size distribution (DSD) and is independent of reflectivity and signal attenuation through the rain. Using DDV, Doppler velocities, and measured reflectivities, the DSD is estimated from about 700 m above the surface to just below the melting layer. The DSD is described using three physical parameters to represent the DSD scale and shape: normalized number concentration N_w, mass spectrum mean diameter D_m, and mass spectrum effective variance n_m (mass spectrum variance / mean diameter squared).

A persistent vertical pattern in DSD parameters was observed during a stratiform rain event over the DOE ARM Central Facility in Northern Oklahoma during the MC3E (Mid-latitude Continental Convective Clouds Experiment, April-May 2011) field campaign. While the reflectivity was nearly uniform with height, the normalized number concentration (N_w) and mean diameter (D_m) had opposite vertical structures with N_w decreasing and D_m increasing from the melting layer down to the surface. Interestingly, the mass spectrum effective variance (n_m) decreased as the raindrops fall indicating that the DSD was evolving into a narrower effective mass spectrum with a loss of small and/or large raindrops. This vertical structure of DSD parameters suggests breakup, coalescence, and evaporation were occurring in the vertical column.