Assimilation of WindSat winds into the Navy Operational Global Atmospheric Prediction System

The operational assimilation of WindSat winds into the Navy Operational Global Atmospheric Prediction System (NOGAPS) at Fleet Numerical Meteorology and Oceanography Center (FNMOC) was initiated in December 2006. The polar-orbiting WindSat instrument is a multifrequency polarimetric microwave radiometer developed by the Naval Research Laboratory (NRL). WindSat wind observations are generated at FNMOC from the raw radiances using software developed by NRL. Prior to assimilation, these wind observations are processed in a manner similar to that developed for QuikSCAT. Up to four aliases are available at each location; currently the alias whose direction is closest to the NOGAPS background surface wind direction is chosen. After quality control is performed (e.g., rejecting duplicate and flagged observations), superobs are generated for ocean areas roughly 160 km square. Data assimilation experiments were conducted using observations from November 2006 to evaluate the impact of the WindSat observations. These experiments were performed using the current operational configuration of NOGAPS consisting of the NRL Atmospheric Variational Data Assimilation System (NAVDAS) with the assimilation of all available conventional and satellite observations and a T239L30 global spectral model. It was found that the addition of the WindSat winds led to an improvement in performance at longer forecast ranges, especially in the Southern Hemisphere.