Numerical Simulations of the Foehn in the Rhine Valley on 24 October 1999 (MAP IOP 10)

Abstract

This paper presents numerical simulations of the MAP IOP 10 foehn case (24 October 1999) for the lower Alpine Rhine Valley, which was one of the two target areas selected for foehn observations. The simulations have been performed with a modified version of the fifth-generation Pennsylvania State University?NCAR Mesoscale Model MM5. Most importantly, the horizontal diffusion of temperature and the mixing ratios of water vapor and cloud water is computed truly horizontally rather than along the model surfaces, as is the case in the original MM5. In the Rhine Valley, the foehn case under consideration was characterized by a complex and rapidly changing flow field. In the early morning, the foehn was restricted to the upper parts of the valley while a cold air pool was present in the lower parts and around the exit toward Lake Constance. During the following hours, the cold air pool was gradually eroded, and the foehn reached Lake Constance around noon. Only 3 h later, a shallow cold front passing from the west terminated the foehn at Lake Constance and in the adjacent parts of the Rhine Valley. The cold front then gradually moved up the Rhine Valley and lifted the warm foehn air off the ground. With the modified model version, the observed flow evolution could be reproduced essentially correctly. Discrepancies between model results and observations are largely restricted to small errors in the time of foehn breakthrough and in the time of the cold front passage. In agreement with observations, the simulations show a pronounced downvalley increase of the surface potential temperature. The highest potential temperatures as well as the highest surface wind speeds are attained around Vaduz (Liechtenstein). The high wind speeds appear to be related to a complex three-dimensional gravity wave field excited over the adjacent mountain ridges. The downvalley increase of the surface potential temperature can be partly traced back to turbulent vertical mixing of stably stratified air. In addition, advection of potentially warm air from an adjacent mountain range is found in the region around Vaduz. Sensitivity experiments with the original diffusion scheme show much larger discrepancies between model results and observations. Because the temperature diffusion along the coordinate surfaces tends to dissipate the initial cold air pool, the foehn breakthrough toward Lake Constance is simulated several hours too early. Reducing the horizontal resolution by a factor of 3 while retaining the modified diffusion scheme has a less detrimental effect on the model results than computing the diffusion along the coordinate surfaces.