The impact of forest cover on the modeled valley atmosphere

The spatial resolution and accuracy of land-cover datasets used in numerical models can have a significant impact on the modeled mountain boundary layer. The land-surface cover influences the surface-energy budget through, for example, the effect of albedo on net shortwave radiation and roughness length on the turbulent exchange between the surface and the atmosphere. Local heating and cooling of the near-surface valley atmosphere are thus equally affected by the land-surface cover, which in turn influences the development of thermally driven slope and valley winds. In addition, the roughness length impacts near-surface turbulent momentum transport and flow fields, which may be of particular importance for shallow slope winds.

We have performed a series of WRF simulations for the Inn Valley, Austria, using three different land-use datasets and three idealized land-cover distributions. The two standard WRF land-use datasets MODIS and USGS strongly overestimate the amount of forest cover in the valley compared to the newer and better resolved CORINE Land Cover (CLC18) dataset. To further analyze the impact of this overestimation in forest cover, semi-idealized simulations are performed with a prescribed amount of forest cover across the model domain. The presentation will show the impact of the land cover on the local surface-energy budget and near-surface atmosphere as well as on the bulk valley atmosphere. Differences in the local sensible heat flux averaged over the surface of the valley are linked to total heating of the valley and the resulting valley-wind circulation.