8.2 Evaluation of the WRF hybrid vertical coordinate for an air turbulence simulation

Park, Sang-Hun, Yonsei University, Korea, Joseph Klemp, National Center for Atmospheric Reserarch, and Jung-Hoon Kim, Colorado State University/Cooperative Institute for Research in the Atmosphere and National Oceanic and Atmospheric Administration/Aviation Weather Center

Complex topography provides important mesoscale forcing in atmospheric simulations. However, with terrain following vertical coordinates, artificial circulations or waves may be generated that look like they are forced by small-scale terrain influences. In this study, we investigate role of a vertical hybrid coordinate in air turbulence over mountainous terrain in WRF simulations and evaluate the impact of this new feature in the WRF model numerics. These simulations demonstrate that with the basic pressure-based terrain following coordinate, significant artificial disturbances may arise in the lower stratosphere that appear to be caused by numerical error incurred in computing the horizontal advection along the terrain-following coordinate surfaces in the presence of strong horizontal flow. These upper-level flow distortions do not appear with the hybrid coordinate since terrain influences are effectively removed form the coordinate surfaces at upper levels. We will discuss potential issues associated with small-scale topography and terrain-following coordinates using this air-turbulence case study and illustrate comparative behavior using horizontal kinetic energy spectra.