Incorporating orographic anisotropy and flow blocking effects in an orographic drag parameterization scheme

The orographic gravity-wave drag parameterization scheme of Kim and Arakawa is extended to include the 3-dimensional effects of orography and low-level subgrid-scale flow blocking. A new orographic statistics parameter is designed to account for the effect of orographic anisotropy. A blocked-layer drag formulation is derived basically following earlier studies, but with distinctly different methods to calculate key parameters. The orographic statistics required for the parameterization are also generalized for use in any global atmospheric models. The extended scheme is evaluated against mountain waves explicitly simulated with COAMPS®+ (Coupled Ocean/Atmosphere Mesoscale Prediction System) of NRL (Naval Research Laboratory). Mountain-wave simulations over Boulder, Colorado, USA, are used for representing realistic situations of different wave activity including severe downslope windstorms. The simulations are area-averaged and interpolated to the vertical grid of NOGAPS (Navy Operational Global Atmospheric Prediction System), and are used as the input to the extended Kim-Arakawa scheme. The scheme is calibrated by comparing the parameterized vertical distribution of the momentum fluxes with the counterpart obtained from the explicit mesoscale simulations. Validation results are presented to show that the calibrated scheme successfully represents the simulated magnitudes and vertical divergences of the momentum fluxes. A flow regime diagram is constructed utilizing a time series of the simulations to further evaluate the parameterization. Preliminary results of the experiments with the scheme implemented in NOGAPS are also presented.