A comparative study of near concurrent DMSP-SSM / I and Geosat-altimeter measurements of surface winds over the Indian oceanic region

Abstract. In this paper, colocated, near simultaneous measurements of ocean surface wind speed near the Indian subcontinent from the Special Sensor Microwave/Imager (SSM/I) and Radar Altimeter onboard, respectively, DMSP and Geosat satellites are analysed. It is found that for about 84 per cent of cases, difference of two winds is within 3 m s 1. However, when the colocated winds are averaged for entire period and over the grid boxes of 2.5 2.5 , the correlation between the two winds appears to be very high. It is found that the absolute difference of the two winds has no apparent relation with the amount of water vapour and the amount of cloud liquid water present in the atmosphere. Further, the variations of absolute difference of two winds with SSM/I winds, altimeter winds and significant wave height have indicated that altimeter measurements of high wind speed are likely to be more inaccurate compared to SSM/I measurements. The variation of bias and standard deviation of the difference of the two winds as measured in each 1 m s 1 bin of altimeter wind indicated that the SSM/I wind and the Altimeter wind are in good agreement within 12 m s 1 of altimeter wind, and thus in this range any one can be used in the absence of another.