Spatio-temporal Structure of the Boundary Layer under the Impact of Mountain Waves

During the Hydrological cycle in the Mediterranean Experiment (HyMeX) in autumn 2012 intensive measurements were conducted in the Tavignano Valley, which extends from the centre to the coast of the island of Corsica. On the investigated day, the atmospheric boundary layer (ABL) in the valley showed a distinctive spatio-temporal variability, which resulted from the interaction and superposition of mesoscale dynamically- and thermally-driven processes and dry convection. Based on the observations, not all of the observed ABL characteristics could be explained and hypotheses on the involved processes were formulated in a previous study. To close the observational gaps and to test the hypotheses, high-resolution simulations with the COSMO (Consortium for Small-scale Modeling) model were now performed. The model was able to reproduce the main ABL characteristics and could hence be used to address the processes affecting the ABL. The main features were: in the upper part of the valley, the stable nocturnal ABL was eroded from top and bottom alike by shear-generated turbulent mixing in the vicinity of a mountain wave and buoyancy- and shear-driven surface-based turbulent mixing, leading to a very abrupt increase of the daytime ABL depth. ... mehrIn the lower part of the valley, the ABL remained rather shallow and was dominated by a superimposed thermally-driven sea breeze and upvalley wind. In the afternoon, the formerly deep ABL in the upper part of the valley rapidly decreased when the combined sea breeze and upvalley wind moved up the valley. While the ABL depth was rather horizontally homogeneous in the lower part of the valley and near the coast, it showed a considerable variability in the valley's upper part on scales of a few kilometres due to the varying dominance of the different processes. The local ABL depth also varied considerably in time depending on which influence dominated, i.e. of surface heating, mountain wave or sea breeze and upvalley wind. As the simulated sea breeze strongly depended on the sea-surface temperature, the results were sensitive to the chosen value in the model.