Turbulent Fluxes and thermal convection in a valley
Zusammenfassung der Projektergebnisse
The project concentrated on the investigation of thermal convection in a valley starting near the ground. For this purpose, energy exchange measurements must be undertaken to represent the link between the atmosphere and the underlying soil-vegetation system. This was realized by the energy balance and turbulence network installed during the COPS (Convective and Orographically-induced Precipitation Study) field campaign 2007. The working group of the University of Bayreuth coordinated the network and was responsible for a uniform quality control concept applied to all participating turbulence measurement stations and for the upload process on the data base. A comprehensive and valuable data set was collected at the World Data Center for Climate (WDCC) in Hamburg, which is provided for further investigation within the COPS scientific community, e.g. for the forcing and validation of applied mesoscale models. Furthermore, the early-morning boundary layer evolution in a low mountain valley was studied in this project in combination with large-eddy simulations (LES). The numerical model was successfully adapted to the complex conditions of the Kinzig valley. The valley atmosphere in these situations is characterized by a shrinking stable valley core and vanishing wind speeds of 1-2 hours during the reversal period of the valley wind system from down-valley winds which prevail at night to up-valley winds in the daytime. Contrary to the expectations, the energy balance in these situations was found to be closed. Free convective conditions were detected in the height of the eddy-covariance (EC) measurements, which implicates that large-scale eddies of the mixed layer reach down to the ground. This could be confirmed by spectral analysis of the eddy scales. As these large-scale eddies transport part of the fluxes, the energy balance is closed during these situations. With the onset of the upvalley wind, these large-scale structures are organized above the EC tower, which means part of the fluxes is not captured any more by the measurements and the energy balance becomes unclosed. The application of the LES showed that the vertical transport in the valley is strongly modified by the valley wind. During the low wind speed period in the morning, the vertical transport is strongly enhanced compared to the up-valley wind period afterwards. This finding draws our attention to the importance of these low wind speed periods for the effective release of surface layer air masses into higher regions of the boundary layer. Thus, it was demonstrated that these early-morning situations are of great relevance for the preconditioning of the boundary layer for a possible further development of convection in the course of the day. Moreover, this study showed that during the early-morning low wind speed period coherent convective structures always evolve at the same place within the valley. It was demonstrated by means of LES that the surrounding orography forces the turbulence in the valley to form these convective structures at predictable, specific locations. As a consequence, there also exist preferential locations in the valley which are mainly responsible for the vertical transport into higher regions of the boundary layer. These strong updrafts were shown to penetrate through the entire depth of the inversion core in the valley. From this finding the classical theory of the shrinking of the inversion core has to be refined. Besides anabatic winds, leading to a subsequent subsidence of the inversion core together with a developing shallow mixed layer at the bottom of the valley, strong updrafts are splitting the inversion core, thus increasing the area for further entrainment processes.
