ASSESSMENT OF THE QUALITY OF TOVS RETRIEVALS OBTAINED WITH THE 3I ALGORITHM FOR ANTARCTIC CONDITIONS

The quality of layer-mean temperatures and humidities retrieved with t he improved initialization inversion (31) (version 2) algorithm from m easurements of the TIROS operational vertical sounder (TOVS) radiomete r has been studied for two summer periods in 1987 and 1990 in the Wedd ell Sea region (Antarctica) by means of collocations with radiosoundin gs, European Centre for Medium-Range Weather Forecasts (ECMWF) analyse s and special sensor microwave / imager (SSM/I) retrievals. Appropriat e corrections of the atmospheric transmission functions have been used for the two periods. A significant influence on the accuracy of the r etrievals has been observed. Collocation statistics with radiosounding s for layer-mean temperatures based on 160 cases show very satisfying results. The standard deviation is very close to the deviations expect ed from collocation errors alone, but an overall negative bias of the retrieved temperatures is observed, Considerably smaller biases in cle ar conditions suggest that the frequent cloudy situations are one reas on. The atmospheric stability index derived from the retrievals corres ponds well to the observations. However, surface inversions and invers ions capping the boundary layer (being characteristics of the polar en vironment) cannot be resolved by the TOVS. Layer-mean humidities and i ntegrated water vapor content are calculated by the 31 algorithm with a good absolute accuracy and errors are comparable to those occurring at lower latitudes. But problems have been found in the ability to cor rectly retrieve varying water vapor co?tents in different situations w hich is caused by weak signals in the very dry Antarctic conditions. C omparisons of the total water vapor content to results based on SSM/I measurements show a good agreement between the two different instrumen ts and retrieval methods. Comparisons of temperature and humidity retr ievals to ECMWF analyses show a better correspondence between retrieva ls and analyses for 1990, related probably to model and assimilation s ystem changes. The ECMWF analyses show deviations from radiosoundings comparable to those of the retrievals emphasizing the value of TOVS re trievals as an important source of information in this data-sparse reg ion.