Application of AVHRR data to a one-dimensional variational retrieval scheme for cloudy TOVS data

The use of the Advanced Very High Resolution Radiometer (AVHRR) data in a o ne-dimensional variational scheme is examined to retrieve cloud parameters and atmospheric profiles. The variational scheme used TIROS Operational Ver tical Sounder radiance data for retrieval. The AVHRR data were used in the partly cloudy and cloudy cases to provide initial guesses for cloud paramet ers in the iterative scheme, to detect the presence of cirrus clouds, and t o determine the sea surface temperature used in retrieval. Sensitivity test s showed that the error in the initial guesses of cloud parameters has subs tantial impact on the accuracy of the retrieved fields; this sensitivity in creases with increased cloudiness. Cloud parameters deduced from AVHRR data are nearly optimal, in terms of maximizing the efficiency of convergence, as the initial guesses for the retrieval scheme. In the absence of cirrus c loud, a retrieval procedure incorporating AVHRR initial guesses produced te mperature and humidity profiles for partly cloudy cases that are about as a ccurate as those for clear cases. In both cases the maximum improvement mad e in the retrieval procedure over background error was about 0.2 K in the t emperature profile, and 0.05 tin logarithm of mixing ratio) in the humidity profile. For partly cloudy cases, best retrieval results were obtained for a low cloud top, or a middle cloud top bur with small cloud fraction. Cirr us cloud remains a problem, as its presence generally degrades the quality of retrieval.