Towards a more “scale‐aware” orographic gravity wave drag parametrization: Description and initial testing

Abstract

AbstractMany current orographic gravity wave drag parametrizations employed in numerical weather prediction and climate models assume idealised elliptical subgrid orography. As a result, they do not fully capture the spectrum of subgrid orographic scales and do not exhibit the desired behaviour when the model grid length is varied. This article describes the motivation, formulation, and testing of a new orographic gravity wave drag parametrization in the Met Office Unified Model that is more “scale‐aware”. The scheme circumvents the need to make assumptions about the shape and distribution of the subgrid orography, moving away from elliptical mountains, through the use of Fourier transforms. The reduction in gravity wave surface stress when orographic flow blocking is present is also accounted for. The result is a new scheme that represents subgrid mountain scales more faithfully and consistently, rather than assuming a single mountain scale for each grid box. Tests using both regional and global simulations, ranging in grid spacing from 2–130 km, demonstrate that the scale‐aware scheme provides a more constant total, resolved plus parametrized, orographic gravity wave momentum flux, when the grid length is varied over most regions of the globe. Global five‐day forecasts also show a significant reduction in the too‐strong zonal wind bias within the Northern and Southern Hemisphere upper troposphere to lower stratosphere at grid spacings of 130 and 40 km when the scale‐aware scheme is employed. Some discussion on future developments for the scheme is also presented, noting that improvements to the orographic flow‐blocking drag component are required before the scheme can be implemented operationally.