Studies on passive remote sensing of vapor, liquid, and ice water paths

Ground-based dual-channel radiometers can be used to monitor water vapor and cloud liquid water. Radiometers can provide measurements automatically and continuously. Westwater (1978) and Staelin (1966) investigated the microwave spectrum of the atmosphere and its sensitivities to atmospheric components and these investigations provided the basis for most radiometric retrieval methods. During their investigations, statistical tools were more or less utilized to deal with the variability of vapor and liquid components of atmospheric media both in time and space. Based on these findings, both simple physical and linear statistical algorithms were developed for ground-based radiometers to quantitatively retrieve water vapor and cloud liquid water information under non-precipitation conditions. Simple physical methods which oversimplify the real problems are less accurate than statistical ones, but statistical methods offer no insights into the physical processes and they provide little explanation as to the salient features of themselves. Most of the algorithms break down if the Mie scattering phenomenon is present. The authors´ objective for presenting a comprehensive physical model is two-fold. First, it outlines the radiative transfer processes observed by the ground-based radiometers; and thus provides physical insights into the dual-channel based statistical algorithms. Second, the retrieval method is enhanced to obtain an ice water path using 90 GHz- measurements.