A Theoretical Study of the Accuracy of Tomographic Retrieval of Cloud Liquid with an Airborne Radiometer

Abstract

Prior studies of the technique of inferring the Liquid water distribution in a cloud with microwave cloud tomography have concentrated on the use of ground-based instruments. An extension of the technique to the analysis of measurements at 31.65 GHz that can be made with an airborne radiometer is described. An improved retrieval method is presented and applied to simulated measurements to determine the accuracy attainable under various conditions. The result of the simulation study are summarized in a figure relating the average error in a retrieval to the size of the cloud, the maximum liquid water content, the resolution, and the number of measurement to be made with a sensitive radiometer. The error is of the order of 10% of the maximum liquid water content for nonprecipitating clouds with a maximum water content greater than 1 g m−3 for a spatial resolution of a few hundred meters. The question of the uniqueness of a retrieval and the likely spatial distribution of error are studied by analyzing the linear radiative transfer equation. Absorption and scattering coefficients for a range of liquid precipitation rates are computed with Mie theory. The results support a previous judgment that roughly 1 mm h−1 is the largest precipitation rate allowing the technique to remain useful.