Diploma Thesis
Besonderheiten der vertikalen Windverteilung am Waldstein
Stephanie Dix (Schier) (02/2008-05/2009)
Support: Thomas Foken, Johannes Lüers
Teilbetreuung durch Prof. Meixner (MPI für Chemie, Mainz)
Main aim of this thesis was the investigation and characterization of local wind phenomena in the atmospheric boundary layer (ABL) at the Waldstein/Weidenbrunnen FLUXNET site (DE-Bay). The data were collected during two intensive observation periods (IOP) within the EGER project (ExchanGE processes in mountainous Regions). The ABL was profiled with a SODAR-RASS system and a miniSODAR. Additionally the data of a 2D Sonic in 32 m a.g.l. and the data from the Windprofiler (WPR) at the Oschenberg site near Bayreuth controlled by the German Weather Service (DWD) were used. In comparison to each other the different instruments are in good agreement. So they can be used together in future experiments. This is of particular interest for the SODARRASS in combination with the miniSODAR, because the miniSODAR provides much better vertical resolution of the 3D wind vector in the first 200 m of the ABL. The WPR located at 25 km distance represents the general atmospheric flow of the Waldstein site and can be used for model input. Varying wind directions with low turbulence and wind speed are observed at times of sunrise and sunset. The standard deviation of the wind direction increases with decreasing wind speed, particularly for wind speeds less than 2 m s-1. This relationship is more intensive in labile ABLs. Sometimes a stream from the south in 35 – 50 m a.g.l. is found in a general flow pattern from westerly directions by the miniSODAR. At midday secondary circulations due to convection over a big clear cut in the south of the site are possible. More investigations and modelling are necessary in this respect. The general approaching flow from westerly directions over the whole profile and a known secondary maximum for the wind from the southeast over the canopy is confirmed. At night times and during the morning hours the profile of the wind vector often shows a strong turn of the wind direction with increasing height. The flows above the canopy come from the east to the south-east directions while the general geostrophic wind approaches from south-westerly directions. The topography and their influence on the wind field in a stable ABL are the most likely reasons. This phenomenon is observed to be independent of the general weather situation. During IOP1 in 8 of 19 nights a low-level jets (LLJ) with duration times of several hours were observed. In IOP2 in 11 of 39 nights LLJ-activities were found. Maximum horizontal wind speed (vh_max) was in the range of 8.2 to 13.0 m s-1. The height of vh_max varied between 100 and 400 m a.g.l. For some simple jet parameters a positive relation between jet maximum and maximum wind speed is found. Although the existence of a LLJ seems to be independent of the general weather situation, but the direction is shown to be not. Most of the LLJ events in IOP1 were characterized by an approaching flow from south-westerly directions. In IOP2 they were found from south-easterly directions. Also a higher wind shear in more stable nights is shown