Radiation balance over low-lying and mountainous areas in south-west Germany

Surface radiative fluxes play a major role in the energy exchange process b etween the atmosphere and earth surface and are thus very crucial to climat ic processes within the atmospheric boundary layer. Based on four years REK LIP (REgio-KLIma-Project) data set of measured radiative fluxes and additio nal supporting meteorological variables, the surface radiation regime for s elected lowland site (Bremgarten 212 m a.s.l.) and mountain sites (Geiersne st at 870 m a.s.l.; Feldberg 1489 m a.s.l.) in the southern Upper Rhine val ley region (south-west Germany) has been reported. Time series of radiative fluxes and surface albedo showed significant inter-site differences. Possi ble reasons for the observed differences have been made. Downward atmospher ic radiation Al at the study sites was parameterised in terms of air temper ature, vapour pressure and cloud amount, all of which strongly govern the v ariation of Ai. Effective terrestrial radiation amounted to about 50% of ab sorbed shortwave radiation at the study sites annually. During clear sky co nditions, global solar irradiance G(s) constituted about 76.0% of the incid ent extraterrestrial solar irradiance at Feldberg mountain site but only 68 .5% of that at Bremgarten lowland site. Annual cumulative of net radiative flux R-n amounted to 1722 MJm(-2) yr(-1) at the lowland site, while that at Geiersnest and Feldberg mountain sites constituted 84% and 73% respectivel y of the corresponding magnitude for the lowland site. In the same vein, an nual mean of radiation efficiency (defined here as R-n/G(s)) amounted to 0. 32 in Feldberg, 0.37 in Geiersnest and 0.41 in Bremgarten. Consequently the annual available energy, of which net radiative flux is representative, wa s smaller at the mountainous sites relative to the lowland site during the study period. Inter-annual Variability of net radiative flux, its constitue nt variables and derivatives at the study sites were generally below 10%, w ith longwave fluxes showing the lowest fluctuation. This renders the measur ed data quite suitable for modelling purposes. In winter, mean daily sums o f R-n showed a slow rise with cloud amount N at the lowland site but a shar p rise with N at Feldberg mountain site. In summer however; mean daily sums of R-n declined significantly with N as well as Linke turbidity factor at the study sites.