Abstract
AbstractThe 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.
Highlights
Eleven models from eight major modeling centers are used to estimate resolved orographic drag from high‐resolution simulations and parametrized orographic drag from low‐resolution simulations, typically used for seasonal forecasting (∼40 km) and climate projections (∼100 km). At both seasonal and climate resolutions, the total orographic gravity wave drag over land is shown to be underestimated by a considerable amount 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
Building on previous Working Group for Numerical Experimentation (WGNE) drag model comparisons (Elvidge et al, 2019; Zadra et al, 2013) and on two recent studies which use km‐scale simulations to evaluate parametrized orographic drag, the COnstraining ORographic Drag Effects (COORDE) project further investigates the uncertainties in the representation of orographic drag processes at resolutions typical of seasonal forecasting and climate projections
The low‐level drag schemes, which account for nonhydrostatic gravity waves and orographic flow blocking, were found to vary much more in their formulation than those that represent vertically propagating gravity waves
Summary
Short to medium range (up to 10 days) regional and global numerical weather predictions (NWP) are being performed using fine horizontal grid spacings of ∼1–10 km. This is done by applying the approaches and techniques of van Niekerk et al (2018) and Vosper et al (2019) to a wider number of models from several operational modeling centers In these previous studies, high‐resolution (km‐scale) simulations that explicitly resolve orographic low‐level blocking and gravity wave effects to a large extent were used to evaluate the representation of orographic drag and its impacts on circulation in two operational models, that is, the Met Office Unified Model and the Integrated Forecasting System of the European Centre for Medium‐Range Weather Forecasts (ECMWF). COORDE uses coordinated simulations performed with 11 models spanning eight major modeling centers Simulations are performed both at low resolutions, in which drag processes need to be parametrized (i.e., resolutions typically used for climate projections and seasonal forecasting) and at high (km‐scale) resolutions, in which low‐level flow blocking and gravity waves are largely resolved so that any remaining parameterized effect is significantly smaller than at low resolution.
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