Impact of Mountain Waves on the Spatio-Temporal Development of the Boundary-Layer over Corsica

During the HyMeX field campaign in autumn 2012 intensive measurements were conducted in a valley, which extends from the centre to the coast of the island of Corsica. On the investigated day, the atmospheric boundary layer (ABL) showed a distinctive spatio-temporal variability, which resulted from the superposition of dynamically- and thermally-driven processes and 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 model were performed.

The model was able to produce the main ABL characteristics and could hence be used to address the processes affecting the ABL depth. The main features are: in the upper part of the valley, the ABL depth increased very abruptly in the morning when shear-generated turbulent mixing in the vicinity of a mountain wave and buoyancy driven surface based turbulent mixing alike eroded the stable nocturnal ABL from top and bottom. In the lower part of the valley, the ABL remained rather shallow and was dominated by superimposed thermally-driven sea breeze and valley wind. In the afternoon, the formerly deep ABL in the upper part rapidly decreased when the combined sea breeze and valley 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 small-scale variability on scales of few kilometres due to the varying dominance of the different processes. The ABL depth at a specific site in the valley also varied considerably in time depending on which process dominated, i.e. the surface heating, mountain wave or the sea breeze and valley wind. As the simulated sea breeze strongly depended on the sea-surface temperature, the results were very sensitive to the chosen value in the model.