On the proper use of temperature screen-level measurements in weather forecasting models over mountains

Abstract. Near surface air temperature, considered to be 2 m above the ground, is a key meteorological parameter with a wealth of uses for mankind. However, its accurate estimation in mountain regions is impeded by persistent limits inherent to atmospheric modelling over complex terrain. In the present study, we analyze the role of structural inhomogeneities of the valleys and mountains observational network in France, to highlight their contribution to the misrepresentation of near-surface air temperature over mountain regions in the numerical weather prediction (NWP) system Arome-France. We scrutinized the disparity in height above ground of the temperature measurements, the inhomogeneous geographical distribution of stations that are preferentially located in valleys, and the relief mismatch between station location and model grid points. The consequences of these inhomogeneities are analyzed for model evaluation and throughout the data assimilation process. In France, high altitude stations usually measure temperature at about 7 m over the snow-free ground, and on average one meter lower when the ground is snow-covered in winter. We show this height difference with respect to standard stations measuring at 2 m, should be considered when evaluating the model performances and in assimilation. We show that due to the current 3DVar assimilation system, the assimilation of valley stations affects the near-surface temperature analysis at all altitudes in the mountains. The altitude mismatch between observation points and model grid points does not play an important rôle, probably in part due to its relatively marginal occurrence in an NWP system with 1.3 km grid spacing. In summary, this study describes new methods for comparing models with mountain observation data, both in terms of assimilation and performance assessment.