The Nature of Turbulent Kinetic Energy in a Deep and Narrow Valley under Convective (?) Conditions

This contribution investigates the nature of turbulent kinetic energy (TKE) in a steep and narrow Alpine valley under fair-weather summertime conditions. The Riviera Valley in southern Switzerland has been chosen for a detailed case study, in which the evaluation of aircraft data (obtained from the MAP-Riviera field campaign) is combined with the application of high-resolution (350 m) large-eddy simulations using the model ARPS. The simulations verify what has already been observed on the basis of measurement data: TKE profiles scale surprisingly well if the convective velocity scale w∗ is obtained from the sun-exposed eastern slope rather than from the surface directly underneath the profiles considered. ARPS is then used to evaluate the TKE-budget equation, showing that, despite sunny conditions, wind shear is the dominant production mechanism. Therefore, the surface heat fluxes (and thus w∗) on the eastern slope do not determine the TKE evolution directly but rather, as we believe, indirectly via the interaction of thermally-driven crossvalley and along-valley flow. Excellent correlations between w ∗ and the up-valley wind speed solidify this hypothesis.