The Circulation Response to Resolved Versus Parametrized Orographic Drag Over Complex Mountain Terrains
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Abstract The accuracy with which parametrizations of orographic blocking and orographic gravity wave drag (OGWD) are able to reproduce the explicitly resolved impacts on flow over complex terrain is investigated in two models: the Met Office Unified Model (MetUM) and the European Centre for Medium‐Range Weather Forecasts Integrated Forecasting System (ECMWF IFS). To this end, global and limited area short‐range forecast experiments across a range of horizontal resolutions, and their model errors relative to analyses, are assessed over two complex mountainous regions: the Himalayas and the Middle East. The impact of resolved orography on the circulation is deduced by taking the difference between high‐resolution experiments with a high (4 to 9 km) and low‐resolution (125 to 150 km) orography. This is then compared with the impact of parametrized orographic drag, deduced from global low‐resolution experiments with and without parametrized orographic drag. At resolutions ranging from tens to hundreds of kilometres, both the MetUM and ECMWF IFS exhibit too strong zonal winds relative to analyses in the lower stratosphere in the region of maximum resolved orographic gravity wave breaking, indicative of some deficiency in the parametrization of OGWD. Diagnosis of the parametrized physics and resolved dynamics tendencies across a range of OGWD parameter values reveal that this error is partly due to the manner in which the resolved dynamics interacts with the parametrized OGWD. This work introduces a method for quantifying the impacts of resolved versus parametrized orographic drag in models and highlights the importance of physics‐dynamics interactions.Keywords:
Orography
Orographic lift
Parametrization (atmospheric modeling)
Wave drag
Abstract The accuracy with which parametrizations of orographic blocking and orographic gravity wave drag (OGWD) are able to reproduce the explicitly resolved impacts on flow over complex terrain is investigated in two models: the Met Office Unified Model (MetUM) and the European Centre for Medium‐Range Weather Forecasts Integrated Forecasting System (ECMWF IFS). To this end, global and limited area short‐range forecast experiments across a range of horizontal resolutions, and their model errors relative to analyses, are assessed over two complex mountainous regions: the Himalayas and the Middle East. The impact of resolved orography on the circulation is deduced by taking the difference between high‐resolution experiments with a high (4 to 9 km) and low‐resolution (125 to 150 km) orography. This is then compared with the impact of parametrized orographic drag, deduced from global low‐resolution experiments with and without parametrized orographic drag. At resolutions ranging from tens to hundreds of kilometres, both the MetUM and ECMWF IFS exhibit too strong zonal winds relative to analyses in the lower stratosphere in the region of maximum resolved orographic gravity wave breaking, indicative of some deficiency in the parametrization of OGWD. Diagnosis of the parametrized physics and resolved dynamics tendencies across a range of OGWD parameter values reveal that this error is partly due to the manner in which the resolved dynamics interacts with the parametrized OGWD. This work introduces a method for quantifying the impacts of resolved versus parametrized orographic drag in models and highlights the importance of physics‐dynamics interactions.
Orography
Orographic lift
Parametrization (atmospheric modeling)
Wave drag
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Results are presented from extended annual-cycle integrations of the Meteorological Office 11-layer atmospheric general circulation model. an 8-year control integration produces a good simulation in the southern hemisphere in both summer and winter and also in the northern hemisphere in summer. In northern winter, however, there is excessive westerly near-surface flow polewards of about 40°N. It is argued that this indicates a lack of orographic forcing of the flow by the major mountain ranges. Two solutions which have been proposed for this problem are investigated. the first is envelope orography and the second is a parametrization of the effects of orographic gravity-wave drag. Each was included separately in 3 1/2 year integrations in parallel with the control. Both produce substantial changes in the model's climatology, but envelope orography does not improve the westerly problem over the European area and it also degrades the summer circulation. the gravity-wave drag scheme, however, gives results which in general are in very good agreement with the observations in both hemispheres and seasons. Comparison of the zonally averaged torques from the three experiments demonstrates that with envelope orography the northern hemisphere near-surface westerlies are reduced because of a dramatic increase in the mountain pressure torque. In the experiment with the gravity-wave scheme, however, the reduction is due to the gravity-wave drag and the mountain torque is similar in magnitude to that in the control. A study of the variability of the 500mb height fields on synoptic timescales demonstrates the improvement in the positions of the northern hemisphere storm tracks in this integration. Some remaining errors in the simulations are identified, particularly in the stratosphere. Finally, it is argued that despite these results it may be too early to come to a final conclusion as to the role of gravity-wave drag in the general circulation and also as to the relative importance of dynamical and radiative processes in the polar winter stratosphere.
Orography
Orographic lift
Parametrization (atmospheric modeling)
Wave drag
Westerlies
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Atmospheric Circulation
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Abstract A new anisotropic gravity‐wave‐drag parametrization‐scheme which represents high‐drag states modelled on hydraulic jump, flow blocking and internal‐wave‐reflection theory, including trapped lee‐waves, is presented. The scheme is shown to represent the breaking of waves over mountains better than previous schemes by comparison with mesoscale simulations of PYREX case studies. Extended simulations of the Meteorological Office Unified Model at climate resolution are presented, showing the impact of the new scheme and a combination of the scheme with a new orographic‐roughness parametrization. Results show a changed distribution of surface‐gravity‐wave surface‐stress, mostly due to mountain anisotropy, and a greater tendency for lower‐troposphere wavc‐drag. Inclusion of the orographic‐roughness parametrization halves the gravity‐wave stress, the combined effect of these schemes for climate integrations being a slight improvement to the tropospheric flow.
Parametrization (atmospheric modeling)
Wave drag
Orographic lift
Orography
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Orography
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Orographic lift
Wave drag
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Atmospheric Circulation
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High‐resolution simulations of flows over South Georgia (South Atlantic) are used to increase understanding of the likely influence of small isolated mountainous islands on the large‐scale flow and to ascertain the extent to which parametrization schemes can account for the missing drag in models where such islands are only partially resolved. Long‐duration (1 month) austral winter forecasts with a horizontal grid spacing of 1.5 km are used to quantify the mountain‐wave momentum fluxes generated by the island and the low‐level drag associated with flow‐blocking dynamics. The characteristics of the drag, such as the occurrence of high and low drag states, its dependence on wind direction and the spectral contributions to the mountain‐wave momentum flux, are considered. Flow splitting and low‐level wave breaking are shown to be responsible for wake regions that extend for hundreds of kilometres from the island. Regions of deceleration are also evident in the stratosphere, due to mountain‐wave dissipation. The extent to which an orographic drag parametrization scheme can reproduce the drag is investigated by comparison with coarse‐resolution (15 km grid spacing) simulations in which the orography is poorly resolved and a large proportion of the drag is parametrized. It is demonstrated that the total of the resolved plus parametrized drag in these simulations closely resembles that at high resolution, although it is underpredicted during instances of high drag. Simple modifications to the scheme, which enhance the drag when the low‐level flow is approximately normal to the major axis of the subgrid orography, are shown to rectify this. The study demonstrates that, at least for relatively simple isolated mountain ranges, the drag and mountain‐wave momentum fluxes can be predicted in a deterministic sense by a well‐tuned parametrization scheme of suitable complexity, although inclusion of stochastic effects might lead to yet further improvements.
Parametrization (atmospheric modeling)
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Abstract A simple gravity wave drag parametriiation over mountainous terrain is tested for its ability to reduce the systematic errors of medium‐range weather forecasts. Following Boer et al. (1984), this parametrization is a function of the low‐level wind speed and stability, the local Froude number, and the variance of the subgrid‐scale orographie features. A comparison study of ten 7‐day forecasts obtained with envelope orography, wave drag or standard orography, shows that wave drag is as effective as envelope orography in reducing the systematic errors. A further comparison where the combined effects of the wave drag and that of a complementary enhanced orography (that is one that includes only the subgrid‐scale elements not treated separately by wave drag) are taken into account shows this latter approach to be the most promising in reducing orographically‐related systematic errors.
Orography
Parametrization (atmospheric modeling)
Wave drag
Froude number
Envelope (radar)
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Citations (43)
Abstract Results are presented from extended annual‐cycle integrations of the Meteorological Office 11‐layer atmospheric general circulation model. an 8‐year control integration produces a good simulation in the southern hemisphere in both summer and winter and also in the northern hemisphere in summer. In northern winter, however, there is excessive westerly near‐surface flow polewards of about 40°N. It is argued that this indicates a lack of orographic forcing of the flow by the major mountain ranges. Two solutions which have been proposed for this problem are investigated. the first is envelope orography and the second is a parametrization of the effects of orographic gravity‐wave drag. Each was included separately in 3 1/2 year integrations in parallel with the control. Both produce substantial changes in the model's climatology, but envelope orography does not improve the westerly problem over the European area and it also degrades the summer circulation. the gravity‐wave drag scheme, however, gives results which in general are in very good agreement with the observations in both hemispheres and seasons. Comparison of the zonally averaged torques from the three experiments demonstrates that with envelope orography the northern hemisphere near‐surface westerlies are reduced because of a dramatic increase in the mountain pressure torque. In the experiment with the gravity‐wave scheme, however, the reduction is due to the gravity‐wave drag and the mountain torque is similar in magnitude to that in the control. A study of the variability of the 500mb height fields on synoptic timescales demonstrates the improvement in the positions of the northern hemisphere storm tracks in this integration. Some remaining errors in the simulations are identified, particularly in the stratosphere. Finally, it is argued that despite these results it may be too early to come to a final conclusion as to the role of gravity‐wave drag in the general circulation and also as to the relative importance of dynamical and radiative processes in the polar winter stratosphere.
Orography
Orographic lift
Parametrization (atmospheric modeling)
Wave drag
Westerlies
Forcing (mathematics)
Atmospheric Circulation
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Abstract The 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.
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Wave drag
Parametrization (atmospheric modeling)
Orography
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Various drag mechanisms are currently parametrized in numerical models of the atmosphere. For global models that include the middle atmosphere in particular, these mechanisms profoundly affect weather forecast as well as climate simulation.We have developed an extended-top version of the Navy Operational Global Atmospheric Prediction System (NOGAPS) to include the middle atmosphere by modifying some physical parametrizations and the vertical coordinate. We performed a series of ensemble simulations corresponding to January 2000 for investigating the response of the model to various drag mechanisms, such as mountain drag, orographic gravity wave drag, surface friction drag, and artificial model top drag. Based on the monthly mean fields obtained from the simulations, we first investigate the effect of gravity wave drag due to its direct impact through planetary wave activity as well as indirect impact through induced meridional circulation.We discuss the difficulties in partitioning between the mountain drag due to resolved orography and the gravity wave drag due to unresolved orography, first using conventional diagnostic measures. From analyses of the atmospheric angular momentum budget, we show that various model drag mechanisms when modified interact with one another by redistributing their drag while conserving the total amount. In particular, an overestimation of mountain drag is accompanied by an underestimation of gravity wave drag in the Northern Hemisphere mid-latitudes to conserve the total amount of drag in the model while likely breaking an optimal balance among the mechanisms. Under such a condition, the inclusion of a gravity wave drag parametrization — even if the drag amount itself is reasonable – does not necessarily improve the performance of the model. Diagnosis of this type of imbalance is not clear by conventional monthly mean fields of variables. In this paper, we argue that the budget of atmospheric angular momentum is a useful measure to diagnose impact of such changes in model physics with regard to the partition and balance among drag mechanisms. We also discuss the experimental results that led to the replacement of silhouette orography by mean orography in our model.
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Various drag mechanisms are currently parametrized in numerical models of the atmosphere. For global models that include the middle atmosphere in particular, these mechanisms profoundly affect weather forecast as well as climate simulation.We have developed an extended-top version of the Navy Operational Global Atmospheric Prediction System (NOGAPS) to include the middle atmosphere by modifying some physical parametrizations and the vertical coordinate. We performed a series of ensemble simulations corresponding to January 2000 for investigating the response of the model to various drag mechanisms, such as mountain drag, orographic gravity wave drag, surface friction drag, and artificial model top drag. Based on the monthly mean fields obtained from the simulations, we first investigate the effect of gravity wave drag due to its direct impact through planetary wave activity as well as indirect impact through induced meridional circulation.We discuss the difficulties in partitioning between the mountain drag due to resolved orography and the gravity wave drag due to unresolved orography, first using conventional diagnostic measures. From analyses of the atmospheric angular momentum budget, we show that various model drag mechanisms when modified interact with one another by redistributing their drag while conserving the total amount. In particular, an overestimation of mountain drag is accompanied by an underestimation of gravity wave drag in the Northern Hemisphere mid-latitudes to conserve the total amount of drag in the model while likely breaking an optimal balance among the mechanisms. Under such a condition, the inclusion of a gravity wave drag parametrization — even if the drag amount itself is reasonable – does not necessarily improve the performance of the model. Diagnosis of this type of imbalance is not clear by conventional monthly mean fields of variables. In this paper, we argue that the budget of atmospheric angular momentum is a useful measure to diagnose impact of such changes in model physics with regard to the partition and balance among drag mechanisms. We also discuss the experimental results that led to the replacement of silhouette orography by mean orography in our model.
Wave drag
Orography
Parametrization (atmospheric modeling)
Orographic lift
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