Recent studies have indicated that radiative scattering due to cloud droplets (those with diameters less than ~50 microns) is significant at frequencies greater than 20 GHZ. Most of the microwave radiative transfer studies of clouds with such small liquid droplets neglect the scattering component. As a result, the relationship between liquid water content and emitted radiation becomes linear and the problem is relatively easily solved. However, a rigorous radiative transfer method that includes both absorption and scattering must be used for improved accuracy. The inclusion of a scattering component in a radiative transfer study requires the specification of a droplet size distribution. Therefore it is also important to evaluate the sensitivity of emitted radiation due to changes in drop size distribution for a specified liquid water content.
In this paper, we solve for radiative transfer using both rigorous and approximate approaches. The approximate approach neglects scattering and uses emission or extinction components. It is shown that the down-welling radiation is 10 to 30% lower in the absence of scattering. Radiative transfer modeling computations are performed for a variety of wintertime aircraft icing conditions which have been documented in several field programs. The modeling study is used to retrieve water vapor amount, cloud liquid water content, droplet size and mean height of the cloud layer. The retrieval method is non-linear and is based on a neural network approach. Comparisons with actual measurements will be presented.
The results of this study have significant application for remote sensing of inflight icing hazards. Retrieval of liquid water content and droplet size will aid in quantifying the hazard to aviation, from either a ground-or airborne-based sensing system.
The 8th Conference on Aviation, Range, and Aerospace Meteorology