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

Abstract

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.

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

“…The stratosphere is now well known to be important for medium‐range weather prediction, seasonal forecasting and climate projections (Butler et al., 2019), and orographic gravity waves play a significant role in its accurate representation (Alexander et al., 2010). Better resolved orography also leads to a better representation of the low‐level flow across mountains and valleys (Vannière et al., 2019) and, more generally, of the large‐scale circulation throughout the atmosphere (Kanehama et al., 2019).…”

“…The atmospheric circulation is much better represented in these high‐resolution simulations than with coarser grid spacing (tens to hundreds of km). Several studies have shown that the improvement in circulation, particularly over the Northern Hemisphere (NH) during winter, comes predominantly from the fact that the orography is better resolved (e.g., Berckmans et al., 2013; Kanehama et al., 2019). At these resolutions, important orographic effects become resolved, and there is a lesser need to parametrize them.…”

Orographic resolution driving the improvements associated with horizontal resolution increase in the Northern Hemisphere winter mid-latitudes

“…To more easily disentangle the impact of the resolved orography on the winter Northern Hemisphere circulation, both the TOFD and SSO schemes are turned off. This follows what was done in Kanehama et al (2019), in order to avoid interactions between resolved and unresolved orographic processes. As expected, turning off the two parametrizations leads to a considerable deterioration of the mean climate, causing an increase in the jet speed at both low and upper levels.…”

“…In this study, one of these aspects is explored: to what extent the increased fidelity of the higher horizontal resolution climate simulations is due to the fact that more orographic effects become explicitly resolved. Following Kanehama et al (2019), who explored this question for weather timescales, hereafter the term "atmospheric resolution' will be used to refer to the grid spacing at which the atmosphere and land surface are discretized. Similarly, the term "orographic resolution" will be used to 2 https://doi.org/10.5194/wcd-2021-51 Preprint.…”

“…This allows to create a clear distinction between the model grid spacing and the level of details of the orography. Kanehama et al (2019) demonstrated that (in the absence of orographic drag parametrizations) increases in orographic resolution are responsible for most of the improvement in the Northern Hemisphere winter medium-range forecast skill obtained when increasing the horizontal resolution of a numerical weather prediction system. Previous evidence also suggests that at climate timescales the orographic resolution may be responsible for a large part of the benefits of the increase in the horizontal resolution (Jung et al, 2012;Berckmans et al, 2013).…”