Impacts of orography on large-scale atmospheric circulation

Sandu, Irina; Niekerk, Annelize van; Shepherd, Theodore G.; Vosper, Simon; Zadra, Ayrton; Bacmeister, Julio T.; Beljaars, Anton; Brown, Andrew R.; Dörnbrack, Andreas; McFarlane, N. A.; Pithan, Felix; Svensson, Gunilla

doi:10.1038/s41612-019-0065-9

Cited by 78 publications

(84 citation statements)

References 76 publications

(78 reference statements)

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“…Recent study by Sandu et al. () suggests that parameterization schemes on topographic processes (e.g., turbulent form drag, low‐level flow blocking, and mountain waves) may also impact on the atmospheric circulation. Optimizing the parameterization schemes could be helpful for further improving the RWS simulation.…”

Section: Summary and Discussionmentioning

confidence: 99%

“…Han et al, 2016;Van Weverberg et al, 2012) have shown that parameterization schemes on convection, subgrid-scale cloud microphysical process of the convective clouds and resultant precipitation all can affect the convergence/divergence in models. Recent study by Sandu et al (2019) suggests that parameterization schemes on topographic processes (e.g., turbulent form drag, low-level flow blocking, and mountain waves) may also impact on the atmospheric circulation. Optimizing the parameterization schemes could be helpful for further improving the RWS simulation.…”

Section: Summary and Discussionmentioning

confidence: 99%

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Winter and Summer Rossby Wave Sources in the CMIP5 Models

Nie

Zhang

Yang

et al. 2019

Earth and Space Science

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The Rossby wave source (RWS) in the upper troposphere plays an important role in the tropical‐extratropical teleconnections. Using the daily outputs from the Phase 5 of the Coupled Model Intercomparison Project (CMIP5) models, the overall model performances in simulating the climatological Rossby wave sources in both winter and summer are evaluated. The ensemble mean of the CMIP5 models can simulate the large‐scale geographical distributions of the RWS reasonably close to the observations, with the simulations of RWS in general better in the Southern Hemisphere. For the Northern Hemisphere, most models overestimate the subtropical RWS but underestimate the midlatitude RWS in both winter and summer. Many models even fail to simulate the seasonal source‐sink shift of RWS in East Asia. Greatest intermodel differences are shown in East Asia, western North America in both seasons, and in the subtropical belt in winter hemisphere. Possible reasons for the model biases in RWS are further investigated. In the Northern Hemisphere, our analysis shows that model performance in simulating the local divergence, which might relate to the overly smoothed topography in Asia and western North America in the model, is most responsible for the biases of the RWS simulations. In the Southern Hemisphere, the bias in subtropical divergence pattern and tropical convection all contribute to the intermodel divergence of RWS simulation.

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Section: Summary and Discussionmentioning

confidence: 99%

Section: Summary and Discussionmentioning

confidence: 99%

Winter and Summer Rossby Wave Sources in the CMIP5 Models

Nie

Zhang

Yang

et al. 2019

Earth and Space Science

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“…The drag imparted on the atmosphere by orography plays an important role in the momentum budget and, correspondingly, the atmospheric circulation. The representation of orographic drag in numerical models therefore influences the reliability of predictions across all time‐scales, from days (Sandu et al, ) to climate time‐scales (Pithan et al, ; van Niekerk et al, ). However, models used for numerical weather prediction (NWP), seasonal forecasting and climate projections typically only resolve a portion of the orographic scales, resulting in the need for orographic drag parametrisation schemes.…”

Section: Introductionmentioning

confidence: 99%

What can we learn about orographic drag parametrisation from high‐resolution models? A case study over the Rocky Mountains

Niekerk

Elvidge

Sandu

et al. 2020

Quart J Royal Meteoro Soc

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Comprehensive high‐resolution numerical weather prediction models provide a virtual laboratory for modelling the atmospheric flow over complex mountain ranges. In this study, global and regional simulations with horizontal grid spacing ranging from 2 to 32 km, focused over the northern Rocky Mountains, are used to assess the orographic blocking and gravity wave drag parametrisations employed in the Met Office Unified Model (UM) and the European Centre for Medium‐Range Weather Forecasts Integrated Forecasting System (IFS). The total, resolved and parametrised drag components in coarse‐resolution simulations are compared with those in high‐resolution simulations, in which the orographic drag processes are better resolved. The total surface stresses and gravity wave momentum fluxes in the free atmosphere of the global 16 km UM and IFS simulations are shown to compare well with 2 km regional simulations in terms of variability and mean. While the total gravity wave momentum flux is somewhat underestimated by both global models, its vertical distribution is well captured. The “seamlessness” of the parametrisation scheme is then assessed by comparing the total orographic stress – and its components – across several horizontal resolutions of the UM. The surface stress remains relatively constant across resolutions, such that the reduction in resolved orographic stress at coarser resolutions is compensated for by an almost equivalent increase in parametrised orographic stress. However, the parametrised orographic gravity wave momentum flux in the free atmosphere remains almost constant with resolution, failing to compensate for the lack of resolved flux at coarse resolutions. This leads to an underestimation of the total gravity wave drag at coarser resolutions. Further analysis suggests that this underestimation is due to the monochromatic wave assumption made by the gravity wave drag parametrisation scheme.

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“…As a result of the multiscale nature of orography and its impacts, orographic processes are only resolved in part by numerical models; processes occurring at scales smaller than the typical resolution of global climate (O(100 km)) or NWP models (O(10 km)) need to be parametrized. To date, the representation of unresolved orographic processes in numerical models remains a major challenge (Sandu et al, ).…”

Section: Introductionmentioning

confidence: 99%

“…Ideally, if the orographic drag parametrizations would be perfect, they should account for unresolved orographic effects at all resolutions and the skill gained by increasing the orographic resolution should be negligible. In practice, however, parametrizations of orographic effects are known to be uncertain and poorly constrained (Sandu et al, , ; Zadra, ). They also behave inconsistently across resolutions, in the sense that they do not accurately account for the handover between resolved and parametrized orographic drag as the resolution is varied (Brown, ; van Niekerk et al, ; Vosper, ).…”

Section: Introductionmentioning

confidence: 99%

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

Kanehama

Sandu

Beljaars

et al. 2019

J Adv Model Earth Syst

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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.

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Impacts of orography on large-scale atmospheric circulation

Cited by 78 publications

References 76 publications

Winter and Summer Rossby Wave Sources in the CMIP5 Models

Winter and Summer Rossby Wave Sources in the CMIP5 Models

What can we learn about orographic drag parametrisation from high‐resolution models? A case study over the Rocky Mountains

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

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