Results from the COORDE Project: A Model Comparison of Resolved and Parametrized Orographic Drag

Abstract:

The parametrization of orographic drag processes is a major source of circulation uncertainty in models. The COnstraining ORographic Drag Effects (COORDE) project makes a coordinated effort to narrow this uncertainty by bringing together the modelling community. The aims of the project are to: explore the variety of orographic drag parametrizations employed in current operational models; assess the resolution sensitivity of resolved and parametrized orographic drag across models; and validate the parametrized orographic drag in low resolution simulations using explicitly resolved orographic drag from high resolution simulations. Presented here are the results from 11 models spanning 8 major modelling centres, in which estimated resolved orographic drag from high resolution (km-scale) simulations and parametrized orographic drag from low resolution simulations, typical of those used for seasonal forecasting (~40 km) and climate projections (~100 km), are compared. In most models, at both seasonal and climate resolutions, the total (resolved plus parametrized) orographic gravity wave drag over land is shown to be underestimated by a considerable amount (up to 50%) over the Northern and Southern Hemisphere and by more than 60% over the Middle East region, with respect to the resolved gravity wave drag estimated from km-scale simulations. The km-scale simulations also provide evidence that the parametrized surface stress and the parametrized low-level orographic drag throughout the troposphere is overestimated in most models over the Middle East region, particularly at climate resolutions. Through this process-based evaluation, COORDE provides model developers new valuable information on the current representation of orographic drag at seasonal and climate resolutions, and the vertical partitioning of orographic low-level and gravity wave drag.