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COnstraining ORographic Drag Effects (COORDE): a model comparison of resolved and parametrized orographic drag


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 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. Eleven models from 8 major modelling centres are used to estimate resolved orographic drag from high resolution (km‐scale) simulations and parametrized orographic drag from low resolution simulations, typically used for seasonal forecasting (~40 km) and climate projections (~100 km). 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.

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