Diploma Thesis

Characterisation of the Atmospheric Boundary-Layer in a Complex Terrain using SODAR-RASS

Jens-Christopher Mayer (06/2004-02/2005)

Support: Thomas Foken, Christoph Thomas

This work investigates wind- and temperature profiles at a complex meteorological measurement site in a mountain range. In complex terrain, the wind field is strongly affected by topography, and thus the profiles strongly depend on wind direction. The knowledge of these dependencies is important for the interpretation of measurements performed at this site.
The wind- and temperature profiles were obtained during the complex exchange experiment WALDATEM-2003 from May to July 2003, using a SODAR system METEK DSDPA.90/64 with a 1.29 GHz RASS extension. The experiment was conducted at the Waldstein Weidenbrunnen measurement site in the Fichtelgebirge mountains, NE Bavaria, Germany. The site is located at the NE flank of the mountains on a saddle, 765 m above sea level. Additional measurements for a comparison between the SODAR-RASS system and measurements from a meteorological tower were carried out during the ECHO 2003 campaign at the Research Centre Jülich, Germany from July to August 2003. The evaluated wind- and temperature profiles have a height resolution of 20 m and a maximum height of 1 km above ground level.
In this study the occurrence of wind shear layers under different wind directions is presented. Further investigations focus on the development of the mixed layer and the occurrence of low-level jets at a complex site, results being compared experiments in flat terrain or urban regions. For the Waldstein Weidenbrunnen site, the wind direction’s frequency distribution indicated three main wind directions, defining three main sectors: North, Southeast and West. Wind profiles within the North sector were characterised by a counter clockwise turn with in creasing height and frequently occurring low-level jets, wind profiles within the Southeast sector showed a clockwise turn of the wind by up to 90 ° with increasing height and a secondary wind maximum in the bottom 200 m. Profiles within the West sector showed an exponential increase of the wind speed and a clockwise turn of the wind by 30 ° with increasing height. An evaluation of wind profiles under different conditions of atmospheric stability revealed a significant influence of the topography especially for the Southeast sector. Three low-level jet events were observed during the experiment. Their duration ranged from one to ten hours, with peak wind speeds between 9 ms-1 and 12 ms-1, all three occurred for wind directions from the north sector. The development of the mixed layer was derived after different criteria for several days, results were compared to model calculations. Elevating rates for the mixed layer height were found to vary between 140 mh-1 and 400 mh-1.

last modified 2005-04-11