ON THE PROBLEM OF MEASURING INTERANNUAL WIND-SPEED VARIATIONS USING SSMI DATA

The first Special Sensor Microwave Imager (SSMI) was launched on the D efence Meteorological Satellite Program (DMSP) F8 spacecraft in July 1 987, and wind speed was no longer retrieved after December 1991. A sec ond SSMI was launched on DMSP F10 in December 1990. Interpretation of the 1987-1993 (or longer) SSMI wind speed time series is dependent upo n the space and time characteristics of the differences between F8 and F10 SSMI measurements. The 10-degrees-zonal averaged monthly mean F8- F10 wind speed difference was negative (positive) for wind speeds less (greater) than 7.9 m s-1, reaching -0.43 (0.32) m s-1 at 5 (10) m s-1 . Between 60-degrees-S and 60-degrees-N the 10-degrees-zonal averaged monthly mean F8-F10 wind speed bias was greater than +/- 0.5 m s-1 on several occasions. From 60-degrees-S-60-degrees-N the 1991 average val ue of the monthly mean root-mean-square difference between daily F8 an d F10 wind speeds in 10-degrees-longitudinal bands was 2.0 m s-1. In t he 60-degrees-S-60-degrees-N region, about 50% of the daily F8 and F10 wind speed differences was caused by measurement non-simultaneity and about 50% of the difference was attributed to other factors, such as instrument noise and the different azimuthal orientations of each SSMI . Caution is advised in the interpretation of large-scale monthly mean analyses obtained from a combined F8 and F10 wind speed time series.