COnstraining ORographic Drag Effects (COORDE): A Model Comparison of Resolved and Parametrized Orographic Drag

• Published in: Journal of advances in modeling earth systems Vol. 12; no. 11
• Main Authors: Niekerk, Annelize, Sandu, Irina, Zadra, Ayrton, Bazile, Eric, Kanehama, Takafumi, Köhler, Martin, Koo, Myung‐Seo, Choi, Hyun‐Joo, Kuroki, Yukihiro, Toy, Michael D., Vosper, Simon B., Yudin, Valery
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 01-11-2020; American Geophysical Union (AGU)
• Subjects: atmospheric modelling; boundary layer meteorology; Climate; Climate change; Drag; Drag effects; Forecasting; Gravity wave drag; Gravity waves; Modelling; numerical weather prediction; orographic drag; Resolution; Seasonal forecasting; Simulation; Stratosphere; stratospheric dynamics; Troposphere; Uncertainty; Wave drag; Wind stress
• ISSN: 1942-2466; 1942-2466
• DOI: 10.1029/2020MS002160

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 modeling 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 to validate the parametrized orographic drag in low‐resolution simulations using explicitly resolved orographic drag from high‐resolution simulations. Eleven models from eight major modeling centers 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 are 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.
Plain Language Summary
Numerical models used for seasonal, climate, or ensemble atmospheric predictions typically cannot resolve mountains with horizontal scales less than a few tens to hundreds of km, which are known to affect the accuracy of these models. As a result, their impacts on the winds, known as drag, are accounted for using simplified theory and approximations, which contain many uncertainties. This work sets out to investigate these uncertainties by comparing these approximations of the drag across several atmospheric models to the drag directly modeled in high‐resolution simulations, in which more of the effects of mountains on the atmospheric flow are explicitly resolved. Results show that, in many of the models considered, the approximated mountain drag is underestimated in the upper part of the atmosphere but overestimated in the lower part of the atmosphere.
Key Points
A multimodel comparison of resolved and parametrized orographic drag is used to evaluate the representation of orographic drag processes
Models show an underestimation in total orographic gravity wave drag in the stratosphere at climate and seasonal prediction resolutions
Some models show an overestimation in their parametrized orographic drag within the troposphere at climate resolutions