Turbulent fluxes and thermal convection in a valley

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The proposed project is the 3rd year extension of a running project, which investigates the development of boundary layer convection in the Kinzig valley (center Black Forest) with the help of surface energy exchange measurements of the COPS experiment combined with large-eddy simulations (LES). Preliminary work was done setting up a quality controlled data base of surface turbulent flux measurements and checking the energy balance closure of all flux sites. In the Kinzig valley, relevant days with situations of free convection initiated near the ground were selected, which serve as a base for further LES studies. A LES model was adapted to the heterogeneous landscape of the Kinzig valley and idealized model runs with complex topography and heterogeneous forcing by heat fluxes at the lower boundary were carried out. Based on this preliminary work, the 3rd year extension will allow us to concentrate on the simulation of some selected free convection situations and test their sensitivity to surface inhomogeneities and orographic details and investigate their impact on the thermodynamic structure of the boundary layer. Moreover, the spatial and temporal scales of the convective events will be studied with ogive, spectral and wavelet analysis methods. An attempt to close the surface energy balance equation with the help of “virtual towers“ within the model domain will be made by considering flux contributions of secondary circulations, which develop over heterogeneous terrain and are not captured by stationary fixed-point turbulence measurements. This will lead to a correction of the flux measurements dependent on the flux contributions of the secondary circulations.