Medium-range forecasting with a hybrid-isentropic global circulation model

Attempts to simulate atmospheric motion with a numerical model using potential temperature as vertical coordinate date back to the mid-1960s, with D.Johnson, J.Dutton and A.Eliassen being the main protagonists. One of the latest contributions to this line of work is a global model employing a "hybrid" vertical coordinate that is terrain-following near the surface but quickly transitions to potential temperature aloft. The model, developed at NOAA-ESRL, is hydrostatic and uses an icosahedral horizontal mesh to achieve near-uniform global grid resolution. Daily real-time forecasts at 15-30km grid resolution, based on initial fields supplied by NCEP-GFS, are presently being evaluated and compared to operational forecasts by a number of weather centrals.

Having proven its ability to deliver competitive medium-range forecasts, the model (called FIM -- see http://fim.noaa.gov) is presently being tested for possible use in intraseasonal to interannual prediction work. The interactive ocean model needed on these time scales also uses an icosahedral grid (to simplify coupling) and a vertical grid analogous to the hybrid FIM grid. As such, the ocean model is yet another manifestation of the "isentropic viewpoint" championed by Johnson.