Sensitivity of WRF Land Surface Schemes to Land Cover Classification over Complex Alpine Terrain

Abstract

<p>Presently, Limited Area or High-Resolution Mesoscale (HRM) models with grid spacing on the<br>order of 1 km are used for numerical weather forecasting. Mountainous terrain is, however,<br>characterized by large surface heterogeneity since steep topography, urban areas, and different land-<br>cover types co-exist on small spatial scales. Because of this surface heterogeneity, local small-scale<br>processes occur within the Mountain Boundary Layer (MoBL) that cannot be explicitly resolved<br>with a 1-km grid spacing and thus need to be parameterized by the Land Surface Model (LSM) and<br>the Planetary Boundary Layer (PBL) schemes. The large surface heterogeneity can be poorly<br>represented in the Land-Use Classification (LUC) and can further lead to errors within the model.<br>Correct land-use classification is, however, crucial to provide accurate surface characteristics (e.g.,<br>albedo, roughness length, thermal inertia, emissivity, and soil moisture availability) to correctly<br>calculate near-surface exchange processes in the LSM. A careful evaluation of the LUCs, the<br>associated surface characteristics, and their impact on the modeled land-atmosphere exchange<br>against observations is thus a key to a better understanding of the model’s performance.<br>We will present Weather and Research Forecasting Model (WRF) simulations with a grid spacing<br>down to 1 km over the steep Alpine terrain of the Inn Valley, Austria. Focusing on convective<br>summer conditions, simulations are performed for individual undisturbed valley-wind days.<br>Various LSMs are tested with four LUCs, that is, the Corine Land Cover 2012 and the updated 2018<br>(CLC12 and CLC18) datasets and the WRF built-in MODIS and USGS datasets. Initial and<br>boundary conditions come from the ERA-5 reanalysis. The model simulations are evaluated against<br>high-quality observations from the i-Box measurement platform, which includes a temperature and<br>a humidity profiler and six eddy-covariance towers (including four full energy-balance stations),<br>which are located at various locations throughout the valley covering different surface<br>characteristics (e.g., slope aspect, slope angle, land cover, and elevation.) Automatic weather<br>stations in the Inn Valley and its surroundings increase the spatial coverage of observations<br>available for model evaluation.<br>Both standard meteorological variables (e.g., temperature, humidity, pressure, wind speed and<br>direction) and the full surface-energy balance (e.g., heat fluxes and radiation) will be evaluated<br>against observations for all the simulations to determine the impact of differences in LUC on<br>surface exchange in the LSMs. Because of the large spatial heterogeneity of the topography and the<br>land cover, an optimized grid-point selection is developed for evaluating the model against these<br>point measurements in addition to correcting for differences in elevation and height above ground<br>between the model and real topography. Surface fluxes integrated over the whole valley are further<br>analyzed to determine the impact of the LUC on the MoBL, such as the thermal structure and the<br>valley-wind circulation.</p>