Alpine radiosoundings feasible for mixing height Ddetermination?

Abstract

The depth of the mixing layer, i.e. of the domain, in which pollutants are dispersed by convection or mechanical turbulence, is an essential scaling parameter for the interpretation of air pollution measurements and for air pollution modeling. For flat terrain, several procedures are available to determine the mixing height from radiosonde profiles: In the convective boundary layer, the advanced parcel method and the evaluation of the gradient of the mixing ratio of water vapor are appropriate, in stable conditions approaches based on the Richardson number and on the low level wind profile. In stable and unstable cases, the detection of a critical inversion on top of the mixing layer renders a mixing height estimate. As direct observations of the mixing height (e.g. lidar measurements) are rare, the agreement of results from different approaches is a measure for the reliability of the mixing height estimate. The main task of this study is to investigate in how far the methods for mixing height determination can also be applied to radiosonde data in an Alpine valley. This question is investigated based on radiosonde profiles from five stations which were operated in the Alpine Rhine Valley from September to November 1999 during the Mesoscale Alpine Programme (MAP). Under weak pressure gradient conditions, the mixing height is well discernable from the soundings. The top of the convective boundary layer is horizontally flat. The mixing height in stable conditions is lower at the stations on the valley floor in the lower part than in the upper part of the valley. During foehn flow, mechanical mixing increases significantly and no mixing layer in the traditional sense is detectable.