Untersuchungen zur räumlich verteilten Modellierung des fühlbaren Wärmestroms über Schnee in einem komplexen Gelände

Abstract

Measurements as well as lot of General Circulation Models (GCM) simulations indicate an increase in air temperature especially for polar regions. In order to improve the quality of GCM predictions for these regions it is necessary to achieve a good parameterization of the energy exchange between atmosphere and snow, because the high latitudes are snow covered for as many as nine months. During snowmelt period sensible heat flux plays an important role for energy input into the snow especially in periods of warm air advection, which are frequently accompanied by inversions. During the snowmelt periods in 1995 and 1998, Eddy-Covariance and profile measurements of wind speed and air temperature were carried out in the Karkevagge, a valley in the Abisko Mountains in Swedish Lapland. Sensible heat flux was calculated by the Profile- and Bulk-method and compared to the directly-measured sensible heat flux. The Profile- and the Bulk-method gave reliable results as long as all data were measured within the surface layer and radiation errors for air-temperature data were small. The Bulk-method yielded better results than the profile method in periods with high values of upwelling shortwave radiation. All measurements conducted outside the surface layer, which was frequently shallow in 1998, were unusable for the Profile-method. Similarly, the Eddy-Covariance-method produced unreliable results if the instrument was operating outside the surface layer. In such a case, however, the Bulk-method produced at least a minimal estimate for the sensible heat flux. None of the methods yielded reasonable results for all meteorological conditions. These aspects should be noted when seeking new methods to improve the calculation of sensible heat flux not only in GCMs. The area distributions of air temperature and wind speed, which are not simply related to topographic parameters, were analyzed for the area of Karkevagge, because sensible heat flux depends on the vertical temperature gradient and the turbulence in the surface layer. Especially wind speed shows a high variability in complex terrain. Area distribution of wind speed was calculated with two simulation models for a strong wind event and compared with measurements of two stations. MetPhoMod gave better results than the MSFD-Model compared to the measured data. Despite the very complex topography and the restrictions of the models the overall results found with both models are encouraging.