Formulation and evaluation of a nonhydrostatic mesoscale vorticity model (TVM)

This paper describes the formulation and the application of the nonhydrosta tic anelastic vorticity model (TVM). This model is constructed using a meth od involving two horizontal streamfunctions and two horizontal vorticity co mponents. The evaluation of this formulation is performed by simulating var ious bidimensional hydrostatic and nonhydrostatic mountain wave cases. Resu lts are compared with analytical solutions and in particular with those dev eloped by Laprise and Peltier for nonlinear forcings. The anelastic formula tion is also validated with respect to the highly nonlinear 1972 Boulder wi ndstorm. TVM is shown to accurately reproduce these mountain wave test case s in both its incompressible and anelastic formulations. In the second part of this paper, the adequacy of the hydrostatic and anela stic assumptions in simulating thermally induced circulations is investigat ed and compared to previous works. For a moderate surface thermal forcing, typical geographical setups are used and show slight differences between hy drostatic and nonhydrostatic horizontal wind speeds. For vertical wind spee ds, differences are shown to be much larger and more sensitive to changes i n grid resolution. For more stringent thermal surface forcing, differences remain low for horizontal wind speeds bur increase considerably for the ver tical wind component. The comparison between anelastic and incompressible solutions For the same cases shows the adequacy of the incompressible assumption when circulations are forced by the surface and are characterized by a relatively shallow ve rtical extent. In such conditions, virtually no differences are observed be tween the two formulations.