Which Orographic Scales Matter Most for Medium-Range Forecast Skill in the Northern Hemisphere Winter?

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.