A NUMERICAL SENSITIVITY STUDY ON THE BACKSCATTERING AT 35.8 GHZ FROM PRECIPITATION-SIZED HYDROMETEORS

In this paper we study the behavior of backscattering quantities that are of primary importance for the interpretation of millimeter-wavelen gth multiparameter radar observations (i.e., the backscattering cross sections at horizontal and vertical polarization, the differential ref lectivity, and the linear and circular depolarization ratios) for rota tionally symmetric nonspherical particles having a conical-elongated f orm that can be continuously varied. The scattering calculations at 35 .8 GHz have been carried out using a computer code based on the Extend ed Boundary Condition Method for liquid water and soft-ice hydrometeor s in the equal-volume sphere size ranges 0.05 less than or equal to r( ev) less than or equal to 0.4 cm and 0.1 less than or equal to r(ev) l ess than or equal to 1.0 cm, respectively. The calculated backscatteri ng quantities are examined for the effects of four factors: particle s ize, conicity, elongation, and orientation with respect to the inciden t field (i.e., radar elevation and particle canting). Results of this sensitivity study show that the effects of nonsphericity tend to incre ase as size increases; for hydrometeors smaller than r(ev) approximate to 0.4-0.5 cm, however, oscillations dominate the behavior of the con sidered backscattering quantities with size. Below such sizes, particl e elongation and radar elevation are by far the main factors affecting the backscattering. As size increases, however, particle conicity bec omes more and more important; at the largest sizes it is comparable to particle elongation. Our results also show that depending on particle shape, even limited variations in radar elevation or in particle orie ntation due to hydrometeor canting may significantly affect millimeter radar measurements of precipitating clouds. In any case, generally th ere is a considerable (often, large) sensitivity to particle parameter s; in our view this augurs well for the existence of backscattering '' signatures'' that could be used to characterize the observed hydromete ors. On the other hand, it also implies that interpretation of millime ter radar measurements should be based on a statistical approach, rath er than on a deterministic one, due to the variability of hydrometeor shape and orientation in nature and to their imperfect a priori inform ation.