Ambient Air Motion and Raindrop Size Distributions Retrieved from UHF Vertical Incident Profiler Observations

The raindrop size distribution is a fundamental quantity used to describe the characteristics of rain. Vertically pointing Doppler radar profilers are well suited to retrieve the raindrop size distributions because of their operating frequency and data collection methodology. Doppler radar profilers operating at UHF are sensitive to both Bragg scattering from the radio refractive index of turbulence and Rayleigh scattering from distributed targets. During light precipitation, both scattering processes are resolved in the Doppler velocity spectra. During moderate to heavy precipitation, the ambient air motion is not resolved in the Doppler velocity spectra. The Sans Air Motion (SAM) Model is introduced in this study and uses only the Rayleigh scattering portion of the Doppler velocity spectrum to estimate the ambient vertical air motion, the air motion turbulence, and the raindrop size distribution.

The SAM Model is applied to 915 MHz profiler observations made in central Florida deployed in support of the TRMM Ground Validation Program. There is good agreement between the near surface SAM Model retrieved rainrate and mass weighted mean diameter with simultaneous surface disdrometer observations. The SAM Model is applied to the profile of Doppler velocity spectra to yield estimates the rainrate, mass weighted mean diameter, and ambient vertical air motion from 300 meters to just under the melting level at 4 km.