MILLIMETER-WAVE SCATTERING AND PROPAGATION IN RAIN - A COMPUTATIONAL STUDY AT 94 AND 140 GHZ FOR OBLATE SPHEROIDAL AND SPHERICAL RAINDROPS

Electromagnetic scattering from raindrops at 94 and 140 GHz frequencies is investigated. The differences in the scattering properties of spherical and oblate spheroidal model raindrops are illustrated for side and vertical incidence cases. Various backscattering and propagation parameters are evaluated using the Marshall-Palmer, Joss thunderstorm, and Joss drizzle drop size distributions. The Doppler spectrum at vertical incidence is substantially affected by the model raindrop shape. The radar reflectivity eta-H and the specific attenuation A(H), where the subscript H denotes horizontal polarization, are not significantly affected. For side incidence, the ratio of the reflectivities at horizontal and vertical polarizations is negligibly small, less than 0.12 dB, and almost independent of the rainfall rate R. On the other hand, specific differential attenuation (DELTA-A) and phase shift (DELTA-phi) show considerable variation with the rainfall rate. Power-law relationships of the form X = aR(b) are derived for X = A(H), DELTA-A, and eta-H. Linear relationships between DELTA-phi and R are also generated for the three drop size distribution models.