Sensitivity of WRF Land Surface Schemes to Land Cover Classification over Complex Alpine Terrain
Authors: Simonet Gaspard , Lehner Manuela , Rotach Mathias W. 
Published: March 2022

&lt;p&gt;Presently, Limited Area or High-Resolution Mesoscale (HRM) models with grid spacing on the&lt;br&gt;order of 1 km are used for numerical weather forecasting. Mountainous terrain is, however,&lt;br&gt;characterized by large surface heterogeneity since steep topography, urban areas, and different land-&lt;br&gt;cover types co-exist on small spatial scales. Because of this surface heterogeneity, local small-scale&lt;br&gt;processes occur within the Mountain Boundary Layer (MoBL) that cannot be explicitly resolved&lt;br&gt;with a 1-km grid spacing and thus need to be parameterized by the Land Surface Model (LSM) and&lt;br&gt;the Planetary Boundary Layer (PBL) schemes. The large surface heterogeneity can be poorly&lt;br&gt;represented in the Land-Use Classification (LUC) and can further lead to errors within the model.&lt;br&gt;Correct land-use classification is, however, crucial to provide accurate surface characteristics (e.g.,&lt;br&gt;albedo, roughness length, thermal inertia, emissivity, and soil moisture availability) to correctly&lt;br&gt;calculate near-surface exchange processes in the LSM. A careful evaluation of the LUCs, the&lt;br&gt;associated surface characteristics, and their impact on the modeled land-atmosphere exchange&lt;br&gt;against observations is thus a key to a better understanding of the model&amp;#8217;s performance.&lt;br&gt;We will present Weather and Research Forecasting Model (WRF) simulations with a grid spacing&lt;br&gt;down to 1 km over the steep Alpine terrain of the Inn Valley, Austria. Focusing on convective&lt;br&gt;summer conditions, simulations are performed for individual undisturbed valley-wind days.&lt;br&gt;Various LSMs are tested with four LUCs, that is, the Corine Land Cover 2012 and the updated 2018&lt;br&gt;(CLC12 and CLC18) datasets and the WRF built-in MODIS and USGS datasets. Initial and&lt;br&gt;boundary conditions come from the ERA-5 reanalysis. The model simulations are evaluated against&lt;br&gt;high-quality observations from the i-Box measurement platform, which includes a temperature and&lt;br&gt;a humidity profiler and six eddy-covariance towers (including four full energy-balance stations),&lt;br&gt;which are located at various locations throughout the valley covering different surface&lt;br&gt;characteristics (e.g., slope aspect, slope angle, land cover, and elevation.) Automatic weather&lt;br&gt;stations in the Inn Valley and its surroundings increase the spatial coverage of observations&lt;br&gt;available for model evaluation.&lt;br&gt;Both standard meteorological variables (e.g., temperature, humidity, pressure, wind speed and&lt;br&gt;direction) and the full surface-energy balance (e.g., heat fluxes and radiation) will be evaluated&lt;br&gt;against observations for all the simulations to determine the impact of differences in LUC on&lt;br&gt;surface exchange in the LSMs. Because of the large spatial heterogeneity of the topography and the&lt;br&gt;land cover, an optimized grid-point selection is developed for evaluating the model against these&lt;br&gt;point measurements in addition to correcting for differences in elevation and height above ground&lt;br&gt;between the model and real topography. Surface fluxes integrated over the whole valley are further&lt;br&gt;analyzed to determine the impact of the LUC on the MoBL, such as the thermal structure and the&lt;br&gt;valley-wind circulation.&lt;/p&gt;