Multiple atmospheric layering and mixing-layer height in the Inn valley observed by remote sensing

Automatic mixing layer height monitoring was performed by continuous sodar and ceilometer measurements in the Inn valley east of Innsbruck, Austria during a winter measuring campaign on air and noise pollution. The ceilometer, an eye-safe commercial lidar originally designed to detect cloud base heights and vertical visibility for aviation safety purposes, was operated for about two months; the sodar was operated for more than four months. Special software for this ceilometer provides routine retrievals of multiple aerosol layers and mixing layer height from the optical backscatter data. An existing retrieval method for the mixing layer height from sodar data has also been enhanced in order to detect multiple atmospheric layering. Particular emphasis is given to the detection of thermally stable layers and inversions within the lower valley atmosphere and their temporal development. Such layers influence significantly the diurnal variations of air pollution and traffic noise impact. A comparison is performed with parallel mixing layer height retrievals from the sodar and the ceilometer. In clear and cold winter nights sometimes several layers one above the other can be detected with both instruments. These multiple layers form due to an interaction between the mountain wind and the down-slope winds. In the absence of low clouds and precipitation ceilometers can estimate the mixing-layerheight fairly well. Ceilometer and sodar partly complement each other.