CABL - Climatology of Alpine Boundary Layer Heights

The atmospheric boundary layer (ABL) is that part of the troposphere where the earth`s surface interacts with the large scale atmospheric flow. The ABL transfers energy and atmospheric constituents between the surface and higher atmospheric levels. It is essential to know the structure of the ABL for the simulation of air pollution dispersion as well as for correct local weather forecasts (e.g. of fog, minimum and maximum temperature, formation and erosion of cold air pools). Boundary layer behavior in homogenous and flat terrain has been investigated for decades and is usually described using several approximations. But changes in roughness, orographical variations and terrain-induced flows as valley winds and slope winds modify the boundary layer. Elevated heating surfaces as slopes and mountain tops influence the thermal structure. Due to these effects, the structure of the ABL is much more complex in the Alpine region and simple theoretical approximations do not hold under these conditions. No definitive methodology for ABL height estimation in complex terrain is available yet. The aim of the project CABL is to develop and test old and new methodologies to estimate ABL heights from measured vertical profiles of temperature, humidity and wind. This will be done using the extensive observational sounding network which was operated in the Alpine region during the three months field experiment of the Mesoscale Alpine Programme (MAP) between September and November 1999. Especially the boundary layer soundings conducted by ZAMG in the frame of the FWF Project "ROM Rhine Valley Ozone Study within MAP" give a valuable data-set for the evaluation of the methods. The complete data-set from radiosonde balloon soundings at 16 stations in the Alpine area between 1992 to 1999, which is also available in the MAP database, will be used to test the approaches under all weather conditions and for geographical positions all over the Alps. This calculation finally leads to an eight years long climatology of ABL heights in the Alpine region, which is evaluated statistically for trends of mixing heights and inversion frequencies.

Climate variations and growing settlements influence the atmospheric boundary layer. The temporal variability of the boundary layer depending on prevailing weather regimes has been evaluated based on radiosoundings from 21 stations in the alpine area under consideration of the location of the sites. Topography and land use have a direct impact on turbulence and stratification of the atmospheric boundary layer. Elevated heating surfaces on mountains, slope and valley winds and inhomogeneous surfaces change the mixing conditions and determine significantly pollution transport and diffusion in the alpine region. Mixing conditions in a large alpine valley are specially investigated based on extensive measurements which were conducted in the frame of the EU Program Mesoscale Alpine Program (among others within the FWF project P 13492-TEC). A program system for boundary layer height determination from measured vertical profiles of temperature, humidity and wind was developed, tested and documented. Different methods were tested and adapted concerning their applicability for mountainous terrain. Advantages and limits of some methods were demonstrated and different approaches for mechanical and convective mixing height determination were combined. Mean mixing properties change according to the frequency distribution of weather conditions. Radiosoundings near large cities probe the urban boundary layer during specific flow directions. Radiosonde data from 21 stations between 1991 and 1999 have been evaluated statistically in the project concerning these aspects. On one hand, the results are relevant for air quality assessment in the alpine region, for interpretation of pollution measurements as well as for air pollution modeling. On the other hand, conclusions can be drawn concerning the change of the alpine boundary layer due to climate change and regional development.