It is generally accepted that increased horizontal resolution improves the representation of atmospheric circulation in global weather and climate models. Understanding which processes contribute toward this improvement can help to focus future model development efforts. In this study, a set of 10-day global weather forecasts, performed with different atmospheric and orographic resolutions ranging from 180 to 9 km, are used to examine the impacts of resolving increasingly smaller orographic scales on the forecast skill of the Northern Hemisphere winter circulation. These experiments aim to answer two main questions: What is the relative contribution from increases in atmospheric versus orographic resolution to the overall improvement in the Northern Hemisphere winter medium-range forecast skill obtained when increasing the horizontal resolution? and How do different orographic scales affect different scales of the atmospheric flow? For experiments in which the subgrid-scale orography parametrizations are turned off, increases in orographic resolution are responsible for almost all of the increase in skill within the troposphere. In the stratosphere, higher atmospheric resolution also contributes to skill improvements, likely due to a better representation of gravity wave propagation and breaking. All scales of orography considered here are found to be important for the obtained changes in the circulation and appear to rapidly affect all considered scales of the flow. In experiments in which the subgrid-scale orography parametrizations are turned on, the benefits of increasing the horizontal resolution decrease, but do not entirely disappear, suggesting that these parametrizations are not perfect substitutes for the unresolved orography. Plain Language SummaryThe skill of global weather forecasts has dramatically improved over the past decades. This is in part due to the fact that the resolution of global numerical weather prediction models has increased over time, from hundreds of kilometers to approximately 10 km. Here we demonstrated that during winter in the Northern Hemisphere, weather forecasts improve when the horizontal resolution is increased across this resolution range mostly because the impacts of orography on the atmospheric circulation are better resolved. In the troposphere, the increases in forecast skill obtained when increasing the model resolution are largely due to increases in orographic resolution, and little forecast skill can be gained by increasing the atmospheric resolution alone. We also showed that even approximately 10-km scales of orography can affect the largest scales of the atmospheric flow. Finally, we demonstrated that the parametrizations used in models to mimic effects of orographic features with scales smaller than the model grid box do not perfectly capture these unresolved effects and need to be improved.

• A multi-model 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

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