Evaluation of ASCAT high wind retrievals at NOAA Ocean Prediction Center

The Advanced SCATterometer (ASCAT) was launched abroad the European MetOp-A satellite in October 2006. Utilizing a fan-beam antenna system, ASCAT collects the C-band microwave backscatter at multiple azimuth angles from the wind roughed ocean surface, and infers the oceanic wind vector (magnitude and direction) by inverting a Geophysical Model Function (GMF) that relates the backscatter measurements to the near surface wind vector. The ASCAT Ocean Surface Vector Wind (OSVW) data are processed and distributed in near real-time by NOAA / NESDIS, and have been available to the operational weather forecasting community since the summer of 2007.

The NOAA Ocean Prediction Center (OPC) is an integral component of the National Centers for Environmental Prediction (NCEP) within the National Weather Service (NWS), and has marine forecast and wind warning responsibilities over the North Atlantic and North Pacific extra-tropical high seas, including the offshore waters of the continental United States. The remotely sensed OSVW data from QuikSCAT and ASCAT are used on a daily basis by OPC marine forecasters, and have proven to provide extremely useful information over an otherwise data sparse area of responsibility.

A previous assessment study of ASCAT retrievals, conducted at OPC, showed that ASCAT can reliably retrieve low to moderate surface wind speeds in all weather conditions. However, for higher wind speeds, ASCAT retrievals were found to have an increasingly low wind speed bias compared to QuikSCAT and Global Forecast System (GFS) winds. The low wind speed bias limited the utility of ASCAT winds in support of OPC's high wind warnings functions, especially, the most dangerous Hurricane Force (HF) category.

In an effort to improve the performance of the ASCAT high wind retrievals, NOAA / NESDIS has developed a new GMF by exploiting additional sensitivity of ASCAT backscatter measurements to collocated high wind events observed by QuikSCAT. The new GMF has been recently implemented in the ASCAT wind processing algorithm, and new ASCAT OSVW retrievals are being produced and distributed to operational weather centers.

In this paper, we present an evaluation of the new ASCAT retrievals in support of OPC's analysis and warning operations, with emphasis on the utility of the ASCAT OSVW data in detecting HF extratropical cyclones, and estimating the appropriate wind warning category during the past two cold hurricane seasons of Oct'07 ~ May'08 and Oct'08 ~ May'09. Hundreds of ASCAT passes are examined, and compared to wind retrievals from QuikSCAT, and the output of Numerical Weather Prediction models, and where available, to conventional buoy / ship observations. Results indicate that ASCAT low to moderate surface wind speeds remain nearly unchanged, and in very good agreement with independent wind observations. For higher wind speeds, ASCAT retrievals show a smaller low wind bias, and are in closer agreement to QuikSCAT measurements. The reduced high wind speed bias significantly improves the ASCAT capability in detecting extratropical cyclones that reach HF conditions.