All-weather wind measurements from intercalibrated active and passive microwave satellite sensors: The RSS Version 7 ocean suite

Accurate observations of high winds over the oceans are important for analyses of phenomena that range from local (tropical and extratropical storms) to global scales (air-sea exchanges, ocean circulation). However, the lack of in-situ observations of winds above 20 m/s poses severe limitations to the accuracy of satellite-retrieved high winds as insufficient data are available for training and validation of wind speed retrieval algorithms. An additional difficulty is the fact that many high wind observations are contaminated by rain. This leads to a strong degradation in the accuracy of wind speed measurements by active sensors (scatterometers) and a complete breakdown of the wind speed retrieval by passive sensors (radiometers).

A recent breakthrough in radiometer measurements of winds in rain has been achieved by Meissner and Wentz (IEEE TGRS 47(9), pp 3065 – 3983, 2009) who developed an all-weather wind speed and direction retrieval for the WindSat radiometer. It is a blended combination between the standard physical wind algorithm for non-raining atmospheres, whose accuracy is better than 1 m/s, and a new wind-through-rain algorithm. The latter one is mainly trained statistically for rainy conditions. Its key feature is the utilization of C-band and X-band frequencies, whose spectral signature makes it possible to find combinations between different channels in these bands that are sufficiently sensitive to wind speed but little sensitive to rain. A very similar technique has been employed successfully for many years in the Step Frequency Microwave Radiometer (SFMR), which is NOAA's primary airborne sensor for measuring tropical cyclone wind speeds. The WindSat all weather wind vector product is part of Remote Sensing Systems (RSS) new Version 7 ocean suite, which contains about 100 years of combined intercalibrated satellite observations from SSM/I, SSMIS, TMI, AMSR, WindSat and QuikSCAT. An extensive validation study using mainly wind fields from NOAA Hurricane Research Division (HRD) has shown that the RSS WindSat Version 7 wind vectors are accurate up to 40 m/s in both non-raining and raining conditions.

We have used the WindSat wind speeds for developing a revised geophysical model function for the QuikSCAT Ku-band scatterometer, which is the basis for retrieving scatterometer wind speeds and directions. The new QuikSCAT retrievals greatly improved at winds above 20 m/s. The QuikSCAT winds are part of the intercalibrated RSS Version 7 ocean suite.

In our presentation we give an overview of the physical basis of high wind measurements by passive and active microwave sensors. We discuss validation results for the RSS V7 WindSat and QuikSCAT wind speed and direction products that include a statistical assessment of the retrieval performance without and with rain and show results for selected tropical and extratropical storms.