VAS Retrievals as a Source of Information for Convective Weather Forecasts: An Objective Assessment and Comparison with Other Sources of Upper-Air Observations

Abstract

Objective experiments have been carried out to determine which moisture and stability indices as derived from the VISSR Atmospheric Sounder (VAS) contain the greatest amount of predictive information with respect to thunderstorm and severe local storm events. In these experiments, stability and moisture parameters derived from 1700 UTC VAS retrievals were compared and correlated to storm observations made during the subsequent 2000?0000 UTC period. The amount of predictive information in these indices was also compared to that possessed by indices derived from VAS first-guess profiles, concurrently available rawinsonde measurements, and numerical model forecasts. The correlation in the form of the computed information ratio (Ic) was used as a measure of predictive power in these experiments. It was found that precipitable water and modified versions of the classic K index which included recent surface data had the highest values of Ic for general thunderstorm occurrence. The 50-kPa gradient wind speed (derived from VAS geopotential heights) and the temperature lapse rate in the 70?50 kPa layer were the best predictors for discriminating severe local storm cases from general thunderstorm cases. The 3-h and 6-h changes in VAS stability and moisture indices were poorly correlated to severe storm occurrence. Of all the variables examined, only the 3-h and 6-h change in surface blackbody temperature appeared to be even moderately correlated to severe storms. A series of probability forecast and verification experiments was carried out to determine if the incorporation of VAS observations might improve automated thunderstorm probability forecasts. It was found that the retrievals possessed more information with respect to thunderstorm occurrence than do their own first guess profiles, which were derived from a forecast of the 0000 UTC limited-area Fine Mesh (LFM) model. However, the VAS-derived stability at 1700 UTC appears to possess little or no additional information beyond that available from 1200 UTC rawinsonde measurements, and indices derived from 1200 UTC LFM forecasts valid near the observation period (2000?0000 UTC) have more information than the 1700 UTC VAS-based indices. Possible reasons for these findings, and their implications for future satellite operations and applied research are discussed.