Extension of an orographic‐drag parametrization scheme to incorporate orographic anisotropy and flow blocking

Abstract

AbstractThe Kim–Arakawa orographic gravity‐wave drag parametrization scheme, which is a component of the US Navy's NOGAPS ALPHA (Navy Operational Global Atmospheric Prediction System, Advanced‐Level Physics and High Altitude), is extended to include the effects of orographic anisotropy and low‐level flow blocking. The algorithms to calculate the orographic statistics needed for the parametrization are also revised. 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, and are used as the input to the extended Kim–Arakawa scheme. The scheme is calibrated by comparing the parametrized vertical distribution of the momentum fluxes with the counterpart obtained from the explicit mesoscale simulations. Overall, 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 parametrization. The robustness of the orographic statistics, together with an approximate method to improve it, are also addressed. Copyright © 2005 Royal Meteorological Society.