Parametrization and influence of subgridscale orography in general circulation and numerical weather prediction models
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Orography
Parametrization (atmospheric modeling)
Orographic lift
Wave drag
Envelope (radar)
Atmospheric Circulation
Effects of subgrid-scale orography are represented in most large-scale models of the atmosphere by means of parameterizing subgrid-scale orographic gravity wave drag and/or enhancing grid-scale orography, such as ‘envelope orography,’ with the use of subgrid-scale orographic variance. A new gravity wave parameterization scheme and an envelope orography have been implemented in the UCLA general circulation model. The impact of gravity wave drag and envelope orography on simulations using the tropospheric-stratospheric 15-layer version of the model are briefly discussed and compared. The gravity wave parameterization scheme and the envelope orography have a qualitatively similar and beneficial impact on ensemble means of simulated January climate. The midlatitude westerlies are weakened at all levels and the polar atmosphere is warmed in the Northern Hemisphere. A combination of the two produces the best results. Sensitivity experiments with the parameterization scheme indicate the importance of the selective enhancement of low-level drag. Although the overall impact of gravity wave drag on the mean fields is similar when using the standard version of orography or the envelope orography, the magnitudes of gravity wave drag are systematically different in simulations using the two representations of orography in the midlatitude Northern Hemisphere. The modification in the magnitudes of simulated meridional eddy momentum fluxes by gravity wave drag with the standard orography is as in earlier studies. This is, however, not the case with the envelope orography. Whereas the impact of gravity wave drag and the envelope orography on the mean fields is similar, it is not necessarily true in terms of the individual components of simulated angular momentum budget.
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Middle latitudes
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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.
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Abstract An evaluation of the performance of the subgrid orography parametrization scheme in the ECMWF Integrated Forecast System is made by examining the variation with resolution of the orographic torques (resolved and parametrized) in short‐range global forecasts. As the spectral resolution is increased from T95 to T511, the magnitudes of the resolved torques increase while those of the parametrized torques decrease. In general the changes in the total orographic torques are reasonably small, but an important exception is between 20 ° N and 50 ° N in winter. Here the total orographic torque decreases significantly with increasing resolution. Through examination of errors in the modelled angular momentum budgets, it is suggested that the problems lie primarily at the low resolution end, where the parametrized orographic torques seem to be excessive. Furthermore, recent work with the NCEP reanalysis data suggests that this result may not be unique to the ECMWF system. Copyright © 2004 Royal Meteorological Society
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Orography
Orographic lift
Deposition
East coast
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Orographic influence on the formation of clouds and its associated precipitation amount and distribution is dramatic. The influence of orography was well recognized very early in human history and documented in numerous meteorological and climatological literatures. When a moist airflow impinges on a mountain, the dynamical and cloud microphysical characteristics of the airflow are modified by orographic lifting and blocking which may modify and/or trigger cloud and precipitation systems in the vicinity of the mountain. Figure 11.1 shows the mean annual precipitation for the period 1971–1990 over Western Europe. Areas of heavy precipitation are concentrated on the Alpine mountains. Note that precipitation over the Alps is produced by weather systems coming from different directions, in particular, from the northern and southern sides.
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<p>Mountains are know to impact the atmospheric circulation on a variety of spatial scales and through a number of different processes. They exert a drag force on the atmosphere both locally through deflection of the flow and remotely through the generation of atmospheric gravity waves. The degree to which orographic drag parametrizations are able to capture the complex impacts on the circulation from realistic orography in high resolution simulations is examined here. We present results from COnstraing ORographic Drag Effects (COORDE), a project joint with the Working Group on Numerical Experimentation (WGNE) and Global Atmospheric System Studies (GASS). The aim of COORDE is to validate parametrized orographic drag in several operational models in order to determine both systematic and model dependent errors over complex terrain. To do this, we compare the effects of parametrized orographic drag on the circulation with those of the resolved orographic drag, deduced from km-scale resolution simulations which are able to resolve orographic low-level blocking and gravity-wave effects. We show that there is a large spread in the impact from parametrized orographic drag between the models but that the impact from resolved orography is much more robust. This is encouraging as it means that the km-scale simulations can be used to evaluate the caveats of the existing orographic drag parametrizations. Analysis of the parametrized drag tendencies and stresses shows that much of the spread in the parametrized orographic drag comes from differences in the partitioning of the drag into turbulent and flow blocking drag near the surface. What is more, much of the model error over complex terrain can be attributed to deficiencies in the parametrized orographic drag, particularly coming from the orographic gravity wave drag.</p>
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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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In recent years, the study of numerical weather prediction (NWP) in complex orographic areas has attracted a great deal of attention. Complex orography plays an important role in the occurrence and development of extreme rainfall events. In this study, the Yin–He Global Spectrum Model (YHGSM) was used, and the wave number truncation method was employed to decompose the orographic data to different resolutions. The obtained orographic data with different resolutions were used to simulate the extreme rainfall in Zhengzhou, Henan Province, China, to discuss the degree of influence and mechanism of the different orographic resolutions on the extreme rainfall. The results show that the simulation results of the YHGSM with high-resolution orography are better than those of the low-resolution orography in terms of the rainfall intensity and range. When the rainfall intensity is higher, the results of the low-resolution orography simulated the rainfall range of big heavy rainfall better. The orography mainly affected the rainfall by affecting the velocity of the updraft, but it had a limited influence on the maximum height that the updraft could reach. A strong updraft is one of the key factors leading to extreme rainfall in Henan Province. When the orographic resolution changes, the sensitivity of the vertical velocity of the updraft to the orographic resolution is the greatest, the sensitivity of the upper-air divergence and low-level vorticity to the orographic resolution is lower than that of the vertical velocity. In conclusion, the high-resolution orography is helpful in improving the model’s prediction of extreme rainfall, and when predicting extreme rainfall in complex orographic areas, forecasters may need to artificially increase rainfall based on model results.
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Complex orography is still a big challenge for all numerical weather prediction (NWP) models. Orography is an important factor that affects the NWP results. The orography in NWP mainly affects the main accuracy of the results through two aspects: orographic representation in models dynamics and orography-related parameterization schemes in the physical processes. To ensure the accuracy of NWP results, it is necessary to have a comprehensive understanding of the application of orographic data in NWP. This paper summarized the influence of orography on weather, the influence of orographic representation on prediction accuracy, and the parametrization of orography-related drag in NWP models. Finally, this paper elaborates the problems of the application of orographic data in NWP and looks forward to future directions in this field, hoping to improve the performance of NWP in complex orographic areas and provide a reference for better application in NWP.
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Orography strongly interacts with the atmospheric circulation, especially during frontal events, generating an enhanced spatial variability of the rainfall field. Regional models of extreme rainfall have to deal with these circumstances in order to provide good spatial estimation of the regionalized variable. We present a model for the regional estimation of the mean of the probability distribution of the annual daily rainfall maxima in a region (Campania, Southern Italy) with complex orography. In a recent work, we found that areas with enhanced variability of extreme rainfall, in the same region, correspond to a particular set of orographic objects, which had been classified through an automatic, GIS-based geomorphological procedure. Here, we propose an approach that considers the same orographic objects as building blocks for a regional model that is able to capture the amplification of extreme rainfall caused by orography. The regional model is then the product of a basic stationary random spatial process and an amplification factor, whose values are related to the topographic features of the orographic objects. This approach represents a step towards the improvement of the predictive ability of regional models of extreme rainfall within orographically complex areas.
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