Evaluation of the SSMI rain analyses for selective storms in the ERICA project
Cataldo, Edmund F.
Wash, Carlyle H.
Nuss, Wendell A.
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Evaluation of the SSMI HAC precipitation algorithm is presented. SSMI rain rate data from five passes during ERICA IOP 2 and 3 were compared to all available ship observations, dropwinsonde soundings and coastal radar. Four different techniques were applied to the seven SSMI channels to analyze rain rate. They are: SSMI HAC algorithm, the Tb(19H) GHz channel with a threshold of 160 deg K, the Tb(37H) GHz channel with a threshold of 190 K, and the Tb(37V- 37H) image with a threshold of less than a 30 deg K difference. For the two IOP 2 passes the Spencer et al (1989) Polarized Correction Temperature (PCT) algorithm using the two 85 GHz channels were also studied. There is considerable uncertainty in the interpretation of the SSMI HAC rain rate algorithm. Specifically large areas of out-of-limit values are present in the vicinity of mid-latitude winter cyclones. Study of the SSMI HAC rain rate has indicated the out-of-limit areas occur when the rain flag is triggered, but the calculated rain rate from the HAC algorithm is less than zero. From this study it is obvious that the four channel SSMI HAC regression algorithm, in its current form, can not satisfactorily analyze the precipitation. Further study is needed to determine if a regression equation can be used to estimate precipitation areas, particularly those with light precipitation. Treating the out-of-limits values as light precipitation would dramatically improve the quality of the SSM I HAC analysis. However, if a regression equation can not be used to estimate precipitation, using the Tb(37H) channel for a better overall analysis of light precipitation and showers and the Tb(19H) channel for a better analysis of the moderate to heavy precipitation is a viable solution.
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