Experimental study of five föhn events during the Mesoscale Alpine Programme: From synoptic scale to turbulence

Abstract

AbstractAircraft measurements made during the Mesoscale Alpine Programme are used to investigate the atmospheric turbulence inside the large Rhine Valley on the north side of the Alps during five föhn events induced by a southerly to south‐westerly flow. Prior to that goal, an analysis based on the observations of different instruments provides the mesoscale and large‐scale atmospheric conditions encountered during the aircraft flights. The temporal and spatial structure of the föhn event of 20 October 1999 is presented as an example. In this case, the three‐dimensional structure of the turbulence is investigated. An estimate of the different components of the turbulent kinetic energy budget is attempted. It shows that the source of the turbulence is mainly mechanical. In spite of the high inhomogeneity of the turbulence in space and between the cases, the study of five cases allows the description of some general features of the föhn events, such as the plume of turbulence extending downwards in the valley and the near homogeneity of the dissipative length‐scale. Copyright © 2003 Royal Meteorological Society.