Topo Wind Intercomparison


It is widely acknowledged that the Weather Research and Forecasting (WRF) model has a high surface wind speed bias, especially over plains and valleys (e.g., Bernardet et al. 2005; Roux et al. 2009; Mass and Ovens 2010, 2011). In recent versions of WRF, two new surface drag parameterization options, both associated with the Yonsei University (YSU) planetary boundary layer (PBL) scheme, have been developed. The Developmental Testbed Center (DTC) has performed testing and evaluation of three WRF model configurations with the Advanced Research WRF (ARW) core (Skamarock et al. 2008).

The baseline configuration utilized the ARW High-Resolution Window (HIRESW) forecast system physics suite run operationally at the National Centers for Environmental Prediction (NCEP). The two comparative configurations tested the effects of the surface drag parameterization scheme namelist option, topo_wind, which aims to correct the high wind bias seen in WRF. One configuration was run with topo_wind=1 (TWIND1; Jimenez and Dudhia 2011), which is based on the concept of a momentum sink term and makes use of the standard deviation of the subgrid-scale orography as well as the Laplacian of the topographic field. The second configuration was run with topo_wind=2 (TWIND2; Mass and Ovens 2012), which determines the subgrid terrain variance and makes the surface drag, or roughness, used in the model dependent on it; also included is additional consideration for stability and wind speed. The baseline configuration had topo_wind=0 (turned off, default). These runs were cold start cases initialized every 36 hours and run out to 48 hours for one full year. All three configurations were run over a 15-km and 5-km 2-way nested (feedback=1) domain, with a near-North American outer domain and an inner domain covering the CONUS.