Abstract : 2E.12
"Seegrubenwolke" a thermal cloud in the area of Innsbruck, Austria

Peter Rafelsberger
Institute of Meteorology and Geophysics, University of Innsbruck

We investigate the occurrence of a specific convective cloud and the associated meteorological conditions. This is based on time-lapse photography capturing the sun exposed slopes north of Innsbruck in 30 seconds intervals during the period Feb2002 to Aug2004. There, "Seegrubenwolke" is featuring a convectively induced cumulus humilis or cumulus mediocris, which is outstandingly constrained by local topography. While this cloud is a well known phenomenon amongst local glider pilots it did not yet receive meteorologically oriented interest. We approach the topic in terms of basic statistics and case studies. The meteorological interpretation is based on synoptic data as well as slope and radiosonde temperature profiles.

The basic evaluation procedure comprises geometrical rectification of the records and a visual selection of the relevant frames. In this context, the occurrence of the cloud is subjectively categorized according to criteria like position, size, shape and surface conditions along the slope. Thus, a multi-year phenology of the cloud is established, which reveals that the cloud can occur throughout February until November. Notably, the cloud was observed with snow covered slopes, too. On a daily basis, the appearance of the cloud is clearly associated to synoptic conditions with weak upper air winds, above normal solar insolation, high air temperatures and a well developed valley wind system along the Inntal. Case studies indicate that the diurnal development is related to topographically modified solar insolation at the slope and to the influence of up valley winds during afternoon. Moreover, cloud base height is related to air temperature and humidity in the valley. Inspection of the diurnal evolution of the vertical profiles of air temperature derived from radiosoundings and slope stations exemplifies the break up of nocturnal inversions and the development of a well mixed slope boundary layer.