Evaluation of the Weather Research and Forecasting model in the Durance Valley complex terrain during the KASCADE field campaign for simulations of the Durance down-valley wind

In the winter of 2012-2013, the KASCADE observational campaign was conducted in the Durance valley in southeast France to characterize the wind and thermodynamic structure of the (stable) planetary boundary layer (PBL). The study area is characterized by numerous hills and valleys in between the French Alps and the Mediterranean Sea. Data were collected with two micro-meteorological towers, a SODAR, a tethered balloon and radiosoundings. We use this extensive dataset to evaluate the boundary-layer representation in the WRF model for these challenging and critical conditions. Also the model performance for mountain-to-plain circulation and the Durance down-valley wind is evaluated.
The model underestimates the diurnal temperature range (DTR) and both longwave radiation components, though evapotranspiration is overestimated. For a typical case,the model physics is thoroughly evaluated for five PBL parameterization schemes and two land-surface schemes. The model results appear to be robust with respect to PBL parameterization, while the sophisticated Noah land-surface model represents the extremes in skin temperature better than a more simple thermal diffusion scheme. Sensitivity tests were performed regarding land-surface-atmosphere coupling, initial soil moisture content and radiation parameterization. Relatively strong surface coupling and low soil moisture content results in a larger sensible heat flux, a deeper PBL and a larger DTR. However, the larger sensible heat flux is not supported by the observations. The sensitivity study underlines the importance of initial soil moisture values. The work resulted in an optimal configuration, which has been used to simulate 23 intensive observational periods of KASCADE. These simulations generalized the afore-mentioned discrepancies.
Despite the model deficiencies, the WRF model simulates the Durance down-valley wind to a reasonable extent. Generally, the onset and cessation timing – typically within 6 hours after sunset and sunrise, respectively – , and the depth of the wind are well simulated.