Comparing Ku-band NSCAT scatterometer and ERS-2 altimeter winds

Differences between sea surface wind speed estimated from the measurements of two different Ku-band satellite-borne sensors are investigated. The first sensor is the altimeter radar on board the European Remote Sensing Satellite, ERS-2, of the European Space Agency. The second one is the NASA scatterometer, NSCAT, on board the Japanese satellite ADvanced Earth Observing System (ADEOS). Because of the different physical processes involved in the measurements of the two sensors, differences are expected between the two retrieved surface wind speed data. The analyzed data set was extracted from the ERS-2 acid NSCAT collocated data base set up, within ADEOS validation activities, by the French Centre ERS d´Archivage et de Traitement (CERSAT), from the Institut Francais de Recherche pour l´Exploitation de la Mer. The dataset covers almost the whole life of NSCAT, from September, 1996, to June, 1997, and consists in more than 800000 pairs of collocated altimeter and scatterometer measurements. On average, wind speed estimates were found to be in agreement over the medium 4-9 ms^-1 wind speed range. At low speed, NSCAT wind speed is overestimated relative to the altimeter one, and inversely, at high wind speed, over 10-17 ms^-1 range, NSCAT is underestimated. The impact of significant wave height and incidence angle on difference between wind speed retrievals from the two sensors was then investigated. Some systematic behaviors were observed as a function of these two parameters and discussed in view of other available analysis and backscatter modeling development