Comparative analysis of albedo and surface energy balance of a grassland site and an adjacent Scots pine forest

Abstract

The climate conditions in the atmospheric boundary layer are influenced by the under- lying land-use type because of its impacts on surface energy balance. Furthermore, the perfor- mance of atmospheric models depends crucially on a realistic representation of surface processes, e.g. the partitioning of available energy into individual energy-balance components. The present study investigates the similarities and differences in the energy balance of grassland and forest. Data from the period March 1992 to September 1996 were gathered at 2 adjacent sites in the south- ern Upper Rhine Plain in southwest Germany: a Scots pine Pinus sylvestris L. forest site at Hartheim, and an adjacent grassland site at Bremgarten. The experimental sites constituted a seg- ment of the REKLIP (Regio-Klima-Projekt) network of energy-balance stations. Synoptic conditions were very similar for both land-use types owing to the short distance between the 2 sites. Differ- ences in available energy (AE) between both land-use types were mainly caused by their different albedo. On average, AE of the Scots pine forest was higher than that for grassland by 24%. Nor- malised by AE, the mean sensible heat flux H accounted for 38% of the AE for grassland and 47% of the AE for the Scots pine forest, while the mean latent heat flux LE reached 62% for grassland and only 53% for the Scots pine forest. Compared to the mean Bowen ratio β of 0.61 for grassland, the relatively high mean β value of 0.89 for the Scots pine forest resulted from the strong stomatal control of the pine trees due to the warm and dry site conditions. In the case of a sufficient water supply, forest AE was converted in equal proportions to LE and H, while for grassland, 60 and 40% of AE were converted to LE and H, respectively. With a low water supply, more AE was converted into H for both land-use types.