Relationship between snow parameters and microwave satellite measurements: Theory compared with Advanced Microwave Sounding Unit observations from 23 to 150 GHz

Abstract

A theoretical model is used to examine the relationship between snow parameters and satellite‐based microwave radiometer measurements between 23 and 150 GHz. The model is based on the two‐stream radiative transfer equation whose optical parameters are obtained from dense media theory. Model simulations are used to show how the grain size, density, snow depth, and an ice crust affect the emissivity and brightness temperatures measured by the Advanced Microwave Sounding Unit (AMSU). The AMSU channels at 23, 31, 89, and 150 GHz are shown to be more sensitive to grain size variations than the other snow parameters. For ice particle diameters less than 1 mm the emissivity and brightness temperatures are found to decrease continuously up to 150 GHz. However, for larger particles, the emissivity and brightness temperature results in a flatter frequency response beyond 89 GHz as the wavelength approaches the particle size. Model results are tested using AMSU measurements for snow events over the central United States during a 2‐week period in the late fall. In agreement with model simulations, the brightness temperature is shown to decrease continuously up to 150 GHz for the small grain size associated with new snow, whereas the brightness temperature no longer decreases beyond 89 GHz as the snow ages and larger grains form owing to snow metamorphosis. Most importantly, the difference between the 89 and 150 GHz AMSU measurements is shown to provide a unique signature for identifying and separating new from aged snow. Consequently, it is now possible to identify four different snow types (wet, new, aged, and stratified snow) from AMSU.