Nonhydrostatic simulations of orographic flows and some effects of time integration schemes

Flows on mountain regions show many pecularities and sub-grid scale characteristics. In high-resolution simulations over complex terrain, nonhydrostatic models have to be used, due to the importance of vertical accelerations. The variation of the vertical component of wind does not affect only dynamics, but the thermal and moisture distribution. The local character of nonhydrostatic dynamics is well evidenced in the alpine region and it is worthy shown by the mathematical characteristics of the pressure distribution equation. Furthermore, time integration schemes may have an impact on orographic waves. Many models make use of a semi-implicit time integration scheme, and some of them include a semi-Lagrangian treatment. Many runs, made by CEM multiscale model in the nonhydrostatic, semi-implicit and semi-Lagrangian versions, have been performed for the Alps. Due to the meteorological help given to a team involved with flight records by a motor sailplane in South America, daily simulations of orographic flows have been performed around the mountain regions of this area from November 2003 to January 2004. Particularly, results using different grid resolutions have been analysed for some real cases, on a domain covering northern Italy and nearby areas as well as the central and southern andine region for South America. A comparison, among the computations in different configurations, has been made. The differences between simulations, made by using the semi-implicit, semi-lagrangian time scheme or the semi-implicit one only, have been also explored. Some results will be presented, showing the influence of the different schemes and features on the representation of flow regimes in such a complex orography.