Equation for the microwave backscatter cross section of aggregate snowflakes using the self-similar Rayleigh-Gans approximation

Abstract

In this paper an equation is derived for the mean backscatter cross section of an ensemble of snowflakes at centimeter and millimeter wavelengths. It uses the Rayleigh-Gans approximation, which has previously been found to be applicable at these wavelengths due to the low density of snow aggregates. Although the internal structure of an individual snowflake is random and unpredictable, the authors find from simulations of the aggregation process that their structure is "self-similar" and can be described by a power law. This enables an analytic expression to be derived for the backscatter cross section of an ensemble of particles as a function of their maximum dimension in the direction of propagation of the radiation, the volume of ice they contain, a variable describing their mean shape, and two variables describing the shape of the power spectrum. The exponent of the power law is found to be -5/3. In the case of 1-cm snowflakes observed by a 3.2-mmwavelength radar, the backscatter is 40-100 times larger than that of a homogeneous ice-air spheroid with the same mass, size, and aspect ratio. © 2014 American Meteorological Society.