Preliminary Tests of First Guess at Appropriate Time (FGAT) with WRF 3DVAR and WRF Model

A First Guess at Appropriate Time (FGAT) procedure is implemented in the WRF 3DVAR system, which allows the use of background information closest to the observation time. In the traditional 3DVAR scheme, every observation is assumed to be valid at the analysis time and is used to compute the innovation. Except for the calculation of innovation vector, the remaining steps of minimization are exactly the same as the standard 3DVAR procedure. Experiments with and without FGAT over a 10-day period (38 cases) indicate positive impacts of FGAT in terms of initial cost function, innovation vector, analysis, and forecast errors. FGAT is most effective for the analysis of observations from moving platforms such as PILOT, Aircraft, Satellite data, and ship. Surface observations with high temporal resolution also benefit from the use of FGAT.