The Influence of the Vertical Coordinate on Simulations of a PV Streamer Crossing the Alps

Abstract

In this note the impact of the vertical coordinate system on upper-tropospheric and lower-stratospheric structures is studied using the fifth-generation Pennsylvania State University?NCAR Mesoscale Model (MM5). Two different simulations are compared. One uses the original sigma coordinate, and the second one uses a modified coordinate system having smoother coordinate surfaces in the free atmosphere. Fields of atmospheric variables, such as wind and temperature, show only a weak signal related to the vertical coordinate system. However, diagnostic quantities involving horizontal and vertical derivatives react very sensitively to the vertical coordinate. The results indicate that, in the presence of steep topography, a meaningful computation of the potential vorticity (PV) field in the tropopause region is possible with the modified coordinate system only. This is mainly because disturbances in the horizontal wind field that appear at first sight unimportant induce large errors in the relative vorticity field with the original coordinate system. In addition, the simulation with the original coordinate indicates a spurious moisture transport across the tropopause above the orography. On the other hand, the impact of the coordinate system on the structure and the amplitude of orographic gravity waves turns out to be quite small.