THEORETICAL ASPECTS OF MODELING BACKSCATTERING BY CIRRUS ICE PARTICLES AT MILLIMETER WAVELENGTHS

This research attempts to understand how nonspherical ice particles ba ckscatter electromagnetic radiation at millimetric wavelengths. The di screte dipole approximation (DDA) is employed to examine backscatterin g by single particles to (i) explore the limits of the Rayleigh approx imation and (ii) test the use and validity of spheroidal models to mod el semirealistic cirrus particles. It is shown that spheroids are reas onable models of cirrus ice particles at wavelengths of 3 and 8 mm. Fu rthermore, with careful consideration of optical size it is possible t o exploit the Rayleigh approximation for spheroids under many circumst ances. The sensitivity of backscattered radiation to variations in mic rophysical properties is examined, based on DDA calculations for ensem bles of ice particles. The most important factor in the ice crystal si ze distribution is found to be the median diameter (D-m) of the ice cr ystal volume distribution. In particular, for values of D-m typical of cirrus, the contribution of crystals whose major dimension is D less than or equal to 100 mu m is masked by the signal of larger crystals. Simulations of ice water content-effective radar reflectivity factor r elations (JWC-Z(l)) are also presented. Comparison with available empi rical relations indicates a functional dependence of the IWC on D-m (i .e., the relative number of large crystals) and also suggests upper an d lower bounds on D-m. It is demonstrated that the effective radar ref lectivity cannot be used in an unambiguous way to determine the IWC. T he difference between reflectivities at 3.16 and 8.66 mm are found to be insignificant. Implications for the remote sensing of ice clouds at millimeter wavelengths are discussed.