Evaluation of a New Operational Technique for Producing Clear Radiances

To produce atmospheric temperature profiles from measurements of infrared data, it is necessary to obtain infrared radiances for clear areas. Clear radiances are obtained either by identifying spots that are completely clear or by extracting clear values from areas that are partly overcast. Until 10 June 1980, clear radiances were obtained using an algorithm described by Smith and Woolf (1976). At this time, the algorithm was replaced by a new algorithm using current techniques. The new algorithm produced more accurate results which resulted in improved consistency between retrievals from the clear and partly cloudy areas. In addition, the new algorithm produced many partly cloudy retrievals in areas where the former method had produced less accurate retrievals using only microwave channels. During the study, it was discovered that the crucial assumption of a single layer or even a two- or three-layer cloud is seldom satisfied. The new algorithm uses various tests to identify pairs of spots with the same cloud heights. These tests are an important feature of the new algorithm. The clear radiance procedure is described and the results of the evaluation are discussed.