Updates of Upper-Level Turbulence Forecasts by Reducing Unphysical Topography with Hybrid Vertical Coordinate in WRF Model

On 2 November 2015, unrealistically large areas of light-or-stronger turbulence were predicted by the WRF-RAP (Weather Research and Forecast Rapid Refresh)-based operational turbulence forecast system over the western U.S. mountainous regions, which were not supported by available observations. First these areas are reduced by applying additional terrain averaging, which damps out the unphysical components of small-scale (~2Δx) energy aloft induced by unfiltered topography in the initialization of the RAP model. Second, these areas are also mitigated by using sigma-pressure hybrid vertical coordinate in the WRF model. A control simulation with the same design of the WRF-RAP model shows that the large-scale atmospheric conditions are well simulated but predict strong turbulence over the western mountainous region. Four experiments with different levels of additional terrain smoothing with or without hybrid vertical coordination are applied in the initialization of the model integrations, which significantly reduce spurious mountain-wave like features, leading to better turbulence forecasts more consistent with the observed data.