Diploma Thesis
Das Windprofil in den untersten 100 m der Atmosphäre unter besonderer Berücksichtigung der Stabilität
Mathias Göckede (02/2000-08/2000)
Support: Thomas Foken
This study presents a method for the extrapolation of wind speeds measured near the ground. The method is based on routine meteorological data and can describe wind profiles at heights above the top of the atmospheric surface layer. By the use of this procedure it is possible to improve the prognosis of wind speeds at great heights above the ground level, for example for the purpose of wind energy applications.
A new parameterization scheme was developed in order to determine the stability of a given atmospheric stratification,. The scheme is based on formulas propounded by BUSINGER et al. (1971) and further develops an approach by HOLTSLAG & VAN ULDEN (1983). The method makes it possible to compute the dimensionless height in the surface layer z without temperature gradient input. Instead the neccessary sensible heat flux value is determined by use of radiation measurements. In addition to this a procedure was developed to parameterize the radiation budget without cloud observation data. In addition, several approaches to the extrapolation of surface based wind speed measurements were compared. These had to meet the requirements to work with routine meteorological input data and not to be restricted to the atmospheric surface layer. The best results could be obtained with the power-law formulas presented by IRWIN (1978) and SEDEFIAN (1980), respectively. In the last phase of the development of the new prognosis method certain algorithms were established that make possible the use of a footprint-model (SCHMID, 1997) for the determination of roughness-lengths. These formulas, in combination with the stability-parameterizations scheme, were executed in a FORTRAN-routine named FOOTSTAB. The FOOTSTAB-program is able to compute individual roughness-lengths for each measurement height of a given mast. The results vary according to the actual meteorological situation as a consequence of the changing position and dimensions of the calculated source area. With the use of these roughness values it is possible to determine a wind profile without any data bias from a heterogenous surrounding area.
The combined application of the FOOTSTAB roughness lengths and the power-law formulas by IRWIN (1978) and SEDEFIAN (1980), respectively, improves the prognosis of wind speeds at great heights above ground level. The neccessary input data is comprised of one measurement each of wind speed, air temperature and vapor pressure, and in addition to this downwelling shortwave radiation, atmospheric pressure and wind direction. Further information about the surrounding area of the position is also neccessary, and may be obtained from topographical maps. The method was tested on two different meteorological datasets. This analysis showed special advantages in comparison with traditional extrapolation methods in cases of heterogeneous aerodynamical properties of the surrounding area. In the structured environment of the Meteorological Observatory Lindenberg of the German Weather Service, where many of areas can be assigned to roughness class 3 according to TROEN & PETERSEN (1989), a clear improvement over the compared procedures was observed. The use of the new method in the shore region of the Baltic Sea, which is more homogeneous, with predominating roughness classes 1 and 2 according to TROEN & PETERSEN (1989), improved the results only slightly.