Mesoscale Alpine Climatology with VERA (VERACLIM)

The establishment of a 20 year Alpine climatology of intense weather phenomena is planned in the course of the proposed project. According to various authors, the frequency of severe weather episodes (heavy precipitation and wind storms) has increased considerably during this time period. The project is embedded in the scope of the international Mesoscale Alpine Programme (MAP), 1995-2004. The goals of MAP are the better understanding and prediction of severe weather events in the Alpine region and the climatological investigation of mesoscale phenomena with respect to global change. In contrast to most conventional climatologies, which are based on either large scale/ global fields of the Atmospheric state or on time series of single stations we intend to use high spatial and temporal resolution synoptic analyses of the Alpine atmosphere to study the frequency and eventually a change of certain mesoscale phenomena. These include pressure gradients accross the Alpine barriers in connection with gap flow and foehn, intense isallobaric features, connected with wind storms, shading and decoupling of air masses in valleys and basins, flow splitting points and mesovortices. The study will be carried out with the VERA (Vienna Enhanced Resolution Analysis) system which has been developed at our department during the last few years. It is unique in a sense, that it is based on synoptic observations, which are available in three hourly intervals. The spatial resolution (meso gamma) is below the resolution obtainable from station data solely, by making use of an effective downscaling procedure. The quality of downscaling is strongly influenced by the knowledge of very high resolution topographic data and short range special observation data fields, which have been collected during the field experiment of MAP (MAP-SOP, September to November 1999). It is a logical continuation of the research conducted so far by Austrian and international scientists within MAP and represents one important contribution within a bundle of research proposals.

The main goal of the VERACLIM project was to create an Alpine climatology in higher temporal and spatial resolution than those known from other, comparable investigations. One of the main tasks was to evaluate mesoscale phenomena such as thermal low- or thermal high pressure zones, strong pressure gradients over the Alps during Foehn-conditions, flows across or around the Alps, mean temperature distributions at any time of the day or of the year as well as the mean wind field in the Alpine region. Our evaluations were based on temperature-, pressure- and wind-analyses in a 3-hourly resolution for a 22-year period of time. The analysis scheme used has been developed at the Department of Meteorology and Geophysics of the University of Vienna. The input data were retrieved from ECMWF (European Centre for Medium Range Weather Forecasts). In the course of the project, the typical pressure distribution over the Alpine region in the presence of a thermal high- or thermal low pressure zone was investigated. As far as dynamically induced pressure features are concerned, it was found that Stau from the north occurs more than twice as often as Stau from the south. The mean temperature of the lowlands and valleys in the course of the year was determined for periods of 10 days. The corresponding set of mean charts was published in a climate atlas and it is expected to be well suited for comparing current meteorological conditions to the climatological mean of the period 1989-2001. In the framework of the investigation of thermally induced flows in the vicinity of the Alps ("Alpine Pumping"), intensity and temporal evolution of such flows were determined. Moreover, wind direction and wind speed for each of the 2752 grid points were investigated. The results of VERACLIM may not only serve as an important contribution to the investigation of Alpine climate but they could also find applications in economy and tourism.