Near surface winds in the vicinity of SST gradients as observed by QuikSCAT

Forecasting near surface winds over the western North Atlantic waters is a significant challenge for NOAA Ocean Prediction Center (OPC) forecasters. These waters are favored for extratropical cyclone development, strong frontal passages, and tropical cyclone activity. They also contain the complex sea surface temperature (SST) gradients of the Gulf Stream and continental shelf waters. Using QuikSCAT near real time winds, OPC forecasters have observed significant differences in wind speeds across moderate to strong sea surface temperature gradients near the Gulf Stream and the Shelf-Break Front. Typically higher wind speeds have been observed over warmer waters with weaker winds over cooler waters. This most often occurs when a significant component of the low-level wind is flowing from warmer sea surface to cooler waters, primarily in southerly flow. Often the magnitude of the QuikSCAT derived wind speeds over the cooler waters is half of what is observed over the warmer waters. This difference in wind speed is most likely due to differences in the static stability of the marine atmospheric boundary layer. NCEP numerical model low-level winds compare well globally with QuikSCAT derived winds. However, some of the most significant differences between numerical model winds and QuikSCAT winds are observed across these waters. This adds to the difficulty faced by forecasters. OPC forecasters now have a variety of tools available to them to aid in forecasting near surface winds across the offshore waters of the East Coast of the United States. These tools include model sounding data, low-level stability indices, and a variety of numerical model wind fields. Forecasters can view (in real time) the difference in magnitude between QuikSCAT 25 km NRT winds and the NCEP 10 meter (GFS and NAM) and 9950 sigma winds (GFS). Forecasters can also view the 30-day bias between numerical model wind speeds and QuikSCAT for 4 categories of low-level stability. To improve wind forecasts this bias is applied to NCEP NAM and GFS forecast winds and can be used as the basis for forecasts. Several examples of significant wind speed differences across SST gradients of the western North Atlantic will be shown. Examples of forecast tools available to OPC forecasters will also be discussed.