Inclusion of clouds and rain in atmospheric slant path calculations

The computation of aerosol absorption and scattering coefficients is normally a lengthy process and atmospheric slant-path calculations usually rely on selections from pre-calculated data. This paper describes an investigation in which the calculated coefficients are stored in a parameterized form. The objective was to encompass a wide variety of cloud and rain types in a data set of modest size. Atmospheric aerosols and rain are characterized by the mass density, size distribution n( r), and shape of the constituent water drops or ice crystals. Particle shapes may be irregular but, for the calculations reported here, are assumed to be spheres of equivalent volume of pure water or pure ice. For the drop-size distribution n( r), we adopt Deirmendjian's modified gamma function but replace its four parameters with two physically meaningful parameters r c (critical radius) and δ = αγ, which is proportional to the slope of n( r) around r c. We then calculate the absorption and scattering coefficients from Mie theory and express them as polynomials in temperature and frequency. The polynomial coefficients are stored for retrieval and interpolation during a slant-path calculation. The parameterization is quite general and contains the Marshall-Palmer relation for rain, the Laws and Parsons and inverse power distributions, etc. as special cases.