Impact of NSCAT Winds on Tropical Cyclones in the ECMWF 4DVAR Assimilation System

Abstract

The impact of NASA Scatterometer (NSCAT) data on tropical cyclone forecasting on the European Centre for Medium-Range Weather Forecasts four-dimensional variational (4DVAR) data assimilation system is examined. Parallel runs with and without NSCAT data were conducted. The 4DVAR can use single-level data, such as scatterometer winds, to good advantage. The 4DVAR system uses data at appropriate times and has the potential to accurately resolve the ambiguity inherent in scatterometer data, by using a two-ambiguity cost function at each NSCAT location. Scatterometer data are shown to improve the depiction of the surface wind field in both tropical cyclones and extratropical lows, and can provide early detection of these features. Case studies of Hurricane Lili, and of Typhoons Yates and Zane (all in 1996), show major positive impacts of NSCAT data on forecasts of tropical cyclone intensity and position.