A nonhydrostatic model based on a new approach

Summary ?The nonhydrostatic Meso model developed at NCEP (Janjic et al, 2001) is based on a new approach. Namely, a hydrostatic NWP model using mass based vertical coordinate has been extended to include the nonhydrostatic motions. In this way favorable features of the hydrostatic formulation have been preserved. This procedure did not require any linearization or approximation. The nonhydrostatic dynamics has been introduced through an add-on module. The nonhydrostatic module can be turned on and off, so that easy comparison can be made of hydrostatic and nonhydrostatic solutions. Here, the basic philosophy behind the discretization methods applied in the model, and not covered by Janjic et al (2001), is discussed, and the latest developments are reviewed. The forecast examples shown indicate that significant differences between hydrostatic and nonhydrostatic forecasts may develop even at relatively coarse resolution of 8 km. Possible future developments are considered. Received May 7, 2001; revised October 15, 2001