Retrieval of Cloud Microphysics from HIAPER Cloud Radar Measurements

This paper describes methodologies for estimating the precipitation amount and mean particle size using HIAPER Cloud Radar (HCR). The HCR is a pod-based millimeter wavelength radar. Its frequency of operation is 94 GHz (3 mm wavelength). The radar has been designed to fly on the NCAR Gulfstream V HIAPER aircraft or vertically pointing on the ground. Radar measurements of clouds and precipitation are, in general, proportional to one of the moments of the hydrometeor size distribution. For example, the radar reflectivity is proportional to the 6th moment and attenuation to the 3rd moment of the distribution in the case of Rayleigh scattering. Retrieval of liquid water content and mean particle size from millimeter wave radar is confounded by the presence of larger hydrometeors such as drizzle and ice crystals in the Mie-scattering size range (where the particle diameter is comparable to the radar wavelength). These larger hydrometeors will not affect the attenuation measurement at millimeter wavelength, but due to their larger sizes they will tend to dominate the reflectivity. Reflectivity and liquid water content (LWC) are not linearly related and the relation between them depends on additional independent parameters. Thus, LWC cannot be estimated accurately using reflectivity measurements alone.

Reflectivity measurements bounded by a surface reference technique, or coincident cloud radar and lidar measurements, have the potential for more accurately estimating the LWC and mean particle size. The capability of HCR is enhanced by coordination with High-Spectral Resolution Lidar (HSRL). The lidar, designed and built by the University of Wisconsin, provides unique measurements of both backscatter and extinction. The HCR measurements and coincident HCR and HSRL measurements are analyzed for their utility to estimate LWC and mean particle size. Retrievals of LWC and mean particle sizes from remote radar and lidar measurements over land and ocean surface will be presented.