High-resolution large-eddy simulations of the Riviera Valley: assessment of the flow structure and the heat and moisture budgets

This paper focuses on the description of the three-dimensional flow structure and on the evaluation of the heat and moisture budgets in a typical medium-sized and steep Alpine valley under convective conditions. In our approach, we combine the evaluation of data from the MAP-Riviera field campaign (carried out during summer/fall 1999 in the Riviera Valley in southern Switzerland) with the application of a high-resolution large-eddy simulation code, the Advanced Regional Prediction System (ARPS). The simulations are run at a horizontal grid-resolution of 150 m (with a minimum vertical resolution of 20 m). In a companion paper, the setup of the model for the Riviera Valley is described, and its performance is quantitatively assessed by comparisons with data from surface stations and radio soundings. Aircraft measurements from the MAP-Riviera field campaign reveal a very pronounced valley-wind system, including a strong secondary circulation in the southern valley entrance region which is probably due to sharp curvature in the topography. The model turns out to reproduce these features very well. This allows for a quantitative assessment of the energetic processes which determine the thermal structure of the valley atmosphere. Surprising in this context is the strong dominance of advection terms both in the heat and in the moisture budget. The heating of the valley atmosphere appears to be mainly due to warm air advection from above – a consequence of the cross-valley circulation pattern. Turbulent heat flux divergence, on the other hand, seems to have a comparatively small effect on the thermal structure of the valley atmosphere. It is restricted to a relatively shallow “mixed” layer (about 300 m deep) at the valley floor. Analogous results apply for the moisture budget. These findings lead to a general discussion of the length scales which dominate the structure of such a valley atmosphere under fair weather conditions.