Determination of surface heterogeneity from surface temperature maps

As there has been a lot of research on the turbulent fluxes over heterogeneous terrain it is important to determine the degree of heterogeneity of the underlying surface. Besides the heterogeneity scale based on the roughness length a heterogeneity scale considering the thermal properties of the surface can be derived. This thermal heterogeneity scale has been determined within this work in order to calculate a thermal blending height. Therefore, during the LITFASS-2003 field campaign maps of the surface temperature of a heterogeneous terrain have been generated. The experimental site was located in Lindenberg, south-east of Berlin and consisted of grassland, forest, agriculture and lakes. In order to measure the surface temperature innovative techniques such as a Tornado reconnaissance aircraft of the German airforce and the helicopter borne turbulence probe Helipod have been used. The Tornado aircraft, which was equipped with an high-resolution infrared camera, took snapshots of the surface temperature of the entire area. It only provided a relative temperature distribution as the infrared camera is optimized for maximum contrast. Hence, the grey-scale infrared images had to be calibrated with one-dimensional surface temperature information obtained by the Helipod. The resulting surface temperature maps achieved a spatial resolution of about 6 m and a relative accuracy of 2 K on an 20 x 20 km² area. Within the temperature maps the sizes of areas of similar temperatures are detected and divided into classes. The class with the highest frequency determines the averaged area size thus the heterogeneity scale of the experimental site. Using this heterogeneity scale a thermal blending height can be calculated. According to Bange et al. (2006) and Mahrt (2000) this blending height should be much more appropriate for this experiment than other definitions of the blending height which are derived from roughness length or the area-averaged heat flux.