SUMMARYRe ned diagnostic and prognostic equations for the depth of the stably strati ed barotropic Ekman boundary later (SBL) are derived employing a recently developed non-local formulation for the eddy viscosity. In wellstudied cases of the thoroughly neutral SBL, the nocturnal atmospheric SBL and the oceanic SBL dominantly affected by the static stability in the thermocline, the proposed diagnostic equation reduces to the RossbyMontgomery, Zilitinkevich and Pollard-Rhines-Thompson equations, respectively. In its general form it is applicable to a range of regimes including long-lived atmospheric SBLs affected by the near-surface buoyancy ux and the static stability in the free atmosphere. Both diagnostic and prognostic SBL depth equations are validated against recent data from atmospheric measurements.
Measurements of the net radiation balance of two forested sites on the opposite slopes of a valley in south-western Germany, made over 3 years, are presented in this study. Radiation sensors were mounted horizontally on two measurement towers above two beech stands. The direct part of the measured short-wave incoming radiation was adjusted according to the slope's angle to convert horizontally measured radiation data into surface-parallel radiation fluxes. During periods when contemporaneous measurements of slope-parallel and horizontal radiation fluxes were available, the calculation of surface-parallel radiation fluxes from the horizontally recorded net radiation components were compared with measured values. The net radiative fluxes parallel to the slopes were calculated for a period of 36 months and analysed. Results show that the different aspects of both sites cause significant differences of the net radiation balance. In June, when the elevation of the sun is highest, incoming solar radiation K downward arrow received on the NE-slope was 9% lower than K downward arrow received on the SW-slope. During the winter months, the differences were much greater and incoming solar radiation to the NE-slope was 50% of that to the SW-slope. Due to the differing solar irradiance, net radiation fluxes were significantly higher on the SW-slope than on the NE-slope. For long-wave radiation only small differences between both slopes could be found. Since radiative fluxes determine the energy balance and hence the microclimate and water balance of a forest stand, these differences in the net radiation balance between the slopes are important for the vegetation.
The climate conditions in the atmospheric boundary layer are influenced by the underlying land-use type because of its impacts on surface energy balance. Furthermore, the performance 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 southern 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 segment 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. Differences 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%. Normalised 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.
Impacts of climate change on the behaviour of vegetation in the Rhine valley Following results of regional climatic models, the regional impact of climate change in the southern Rhine valley in Central Europe will result in hotter and dryer summer seasons. Because this region is already the warmest and dryest area in Germany, it seems to be favourite to perform interdisciplinary research on climate change impacts. Which modifications in plant water supply will result in which yield changes and in which carbon stock changes in relation to the sense of the protocol of Kyoto? The international working group defined these questions to be a research task in finding answers. Test plants are wheat (France), pine (Germany) and corn (Switzerland). We have different water use efficiencies (C3-and C4-plants). With new research methods, jointly applying the individual specific contributions of the four partners it will be possible to explain the relations between the atmospheric carbon supply and the biomass production and its variations in time and related to the weather. The project consists in simultaneous measurements at the three sites combined with isotopic analyses in the laboratory and joint data analyses. Micrometeorological flux and storage term estimates of CO 2 are completed with physiological transpiration measurements by means of 13 C and 18 O tracers and porometric measurements. Then budget calculations are required to estimate the storage capacity of the ecosystems under investigation. This will enable the three states involved in the project to present more precise capacity calculations in carbon stock of the vegetation and carbon exchange with the atmosphere.
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