Abstract
We evaluate the near-surface representation of thermally driven winds in the Swiss Alps in a numerical weather prediction model at km-scale resolution. In addition, the influence of grid resolution (2.2 km and 1.1 km), topography filtering, and land surface datasets on the accuracy of the simulated valley winds is investigated. The simulations are evaluated against a comprehensive set of surface observations for an 18-day fair-weather summer period in July 2006. The episode is characterized by strong diurnal wind systems and the formation of shallow convection over the mountains, which transitions to precipitating convection in some areas. The near-surface winds (10 m above ground level) follow a typical diurnal pattern with strong daytime up-valley flow and weaker nighttime down-valley flow. At a 2.2 km resolution the valley winds are poorly simulated for most stations, while at a 1.1 km resolution the diurnal cycle of the valley winds is well represented in most large (e.g., Rhein valley at Chur and Rhone valley at Visp) and medium-sized valleys (e.g., Linth valley at Glarus). In the smaller valleys (e.g., Maggia valley at Cevio), the amplitude of the valley wind is still significantly underestimated, even at a 1.1 km resolution. Detailed sensitivity experiments show that the use of high-resolution land surface datasets, for both the soil characteristics as well as for the land cover, and reduced filtering of the topography are essential to achieve good performance at a 1.1 km resolution.
Highlights
Diurnal mountain winds are frequently observed over mountainous terrain, in particular under fair-weather conditions
We evaluate the skill of a numerical weather prediction model with a configuration close to the operational setup at MeteoSwiss, in reproducing the diurnal valley winds in the Swiss Alps
We investigated the impact of horizontal grid spacing, topography filtering, and the choice of land surface datasets on the accuracy of the simulated valley winds
Summary
Diurnal mountain winds are frequently observed over mountainous terrain, in particular under fair-weather conditions. In the deep valleys of the Swiss Alps, these thermally driven flows can occur throughout the year and they are a key characteristic of the local weather and climate (e.g., [1]). Apart from their direct impact on the near-surface weather conditions, they influence the formation of clouds and local thunderstorms and the distribution of radiation and precipitation over complex terrain (e.g., [2,3]). The accurate simulation of these diurnal wind systems is important for the prediction of local weather in mountainous regions and adjacent areas. It can be regarded as a necessary but not Atmosphere 2018, 9, 196; doi:10.3390/atmos9050196 www.mdpi.com/journal/atmosphere
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