Modelling the Wind Climate in Mountain Valleys Using the MIUU Mesoscale Model

Abstract: High average wind speeds have been measured in many valleys. The reasons for these high winds are channelling effects of two physical origins, forced channelling and pressure driven channelling. In this study, the MIUU-model, a three-dimensional mesoscale model with a higher order closure, has been used to investigate these effects in idealised valleys. The simulations have been performed in both two- and three-dimensions with a multitude of conditions. The valley dimensions, as depth, width and length, have been changed in order to investigate the flow within the valley. The dependence of strength and direction of the geostrophic wind have also been examined. Most of the simulations are performed for wintertime conditions at high latitudes with low roughness. In addition, simulations representing summertime conditions are made to check the influence of the diurnal variation of incoming radiation and thermal stability. In the two-dimensional simulations the mean wind speed along the valley is found to be independent of the magnitude of the geostrophic wind (at least up to 10 ms-1) if the large-scale flow is perpendicular to the valley. In this case, only the pressure driven channelling effect is of importance. When the geostrophic wind is higher than10 ms-1, the valley winds are coupled to the ambient flow and gravity waves are affecting the results. The mean and maximum wind speeds are found to be a function of the valley depth. In a valley with a water body (a lake) at the bottom, the simulations gave higher within valley winds for lower water temperatures, which increases the stability. In the three-dimensional simulations, it is shown that the maximum valley winds are almost independent of the valley length. However, the mean valley wind speed is more affected. Furthermore, it was found that close to the area where the valley widens - towards the region with lower synoptic pressure - a wind maximum is found in all simulations. A simulation of the Torneträsk area was made to investigate the flow in a real topography. With geostrophic wind perpendicular to the valley, high wind speeds occur at low levels in accordance with measurements. For the conditions used, channelling effects dominated the within valley flow.