Comparison of microwave radiative transfer calculations obtained with three different approximations of hydrometeor shape

The sensitivity of microwave radiative transfer through the atmosphere with respect to three different raindrop geometries (spheres, oblate spheroids and oblate raindrop shapes realistically approximated by a series of Chebyshev polynomials) is investigated with a one-dimensional numerical model. All rotational symmetric particles are aligned with their axis of rotation along the vertical. Single-scattering calculations are performed with an extended boundary condition T-matrix code provided by Mishchenko and the Discretized Mie Formalism. In contrast to the former code the latter allows to model the scattering by the Chebyshev-shaped raindrops. The change of brightness temperature when using one of the oblate particle shapes instead of the widely used spherical assumption reaches up to - 5 K for 85 GHz at upward directions. The calculated microwave signatures are most sensitive to the shape of liquid water drops at frequencies around 20 GHz, while the results at 37 and 85 GHz show larger impact by ice particles. Polarization differences exhibit large changes when using oblate particle shapes instead of spheres, leading to changes of + 6 K at upward and - 15 K at downward directions. The slight variation of geometry when using Chebyshev shapes gives additional changes of - 2 and + 4 K at the same directions of emerging radiation. Calculations with an ice particle layer above the rain layer prove that the scattering signature of the underlying rain layer will not be screened by the ice particle scattering. This result shows the importance of correct treatment of raindrop shape even in the presence of large ice particles. (C) 1999 Elsevier Science Ltd. All rights reserved.