Search Results
Abstract
A climatology of the middle atmosphere is determined from 11-yr integrations of the U.K. Meteorological Office Unified Model and compared with 18 years of satellite observations and 5 years of data assimilation fields. The model has an upper boundary at 0.1 mb, and above 20 mb uses Rayleigh friction as a substitute for gravity wave drag. Many of the results are, however, found to be relatively insensitive to enhancing the damping above 0.3 mb. As with most general circulation models, the polar night jet in both hemispheres is too strong and does not have the observed equatorward slope with height. The model suffers from the common “cold pole” problem and, apart from a local warm pool centered just below 100 mb in northern high latitudes in January, and another at about 30 mb at 70°S in July, has a cold bias throughout the stratosphere. At the level where polar stratospheric clouds occur, the temperature bias is about −4 K in the Northern Hemisphere and up to +6 K in the Southern Hemisphere. For the majority of the southern winters, local minimum temperatures in the lower stratosphere agree well with observations but in some years the behavior is more like the Northern Hemisphere with values rising rapidly in late winter. This feature of the simulation is also seen in the South Pole temperatures at 10 mb with midwinter warmings occurring in two of the years. At 10 mb, midwinter warming behavior at the North Pole is quite well reproduced, as is the annual cycle in extratropical circulation. In the Tropics, there is no quasi-biennial oscillation, and the semiannual oscillation in the upper stratosphere has a poorly simulated westerly phase, while the easterly phase lacks the observed seasonal asymmetry. Simulated stationary wave amplitudes in the upper stratosphere lack a strong hemispheric asymmetry and are overpredicted in both hemispheres despite having roughly the correct amplitudes at 100 mb. Interannual variability in the winter stratosphere is underestimated, and again there is evidence that the model does not produce the proper hemispheric asymmetries.
Abstract
A climatology of the middle atmosphere is determined from 11-yr integrations of the U.K. Meteorological Office Unified Model and compared with 18 years of satellite observations and 5 years of data assimilation fields. The model has an upper boundary at 0.1 mb, and above 20 mb uses Rayleigh friction as a substitute for gravity wave drag. Many of the results are, however, found to be relatively insensitive to enhancing the damping above 0.3 mb. As with most general circulation models, the polar night jet in both hemispheres is too strong and does not have the observed equatorward slope with height. The model suffers from the common “cold pole” problem and, apart from a local warm pool centered just below 100 mb in northern high latitudes in January, and another at about 30 mb at 70°S in July, has a cold bias throughout the stratosphere. At the level where polar stratospheric clouds occur, the temperature bias is about −4 K in the Northern Hemisphere and up to +6 K in the Southern Hemisphere. For the majority of the southern winters, local minimum temperatures in the lower stratosphere agree well with observations but in some years the behavior is more like the Northern Hemisphere with values rising rapidly in late winter. This feature of the simulation is also seen in the South Pole temperatures at 10 mb with midwinter warmings occurring in two of the years. At 10 mb, midwinter warming behavior at the North Pole is quite well reproduced, as is the annual cycle in extratropical circulation. In the Tropics, there is no quasi-biennial oscillation, and the semiannual oscillation in the upper stratosphere has a poorly simulated westerly phase, while the easterly phase lacks the observed seasonal asymmetry. Simulated stationary wave amplitudes in the upper stratosphere lack a strong hemispheric asymmetry and are overpredicted in both hemispheres despite having roughly the correct amplitudes at 100 mb. Interannual variability in the winter stratosphere is underestimated, and again there is evidence that the model does not produce the proper hemispheric asymmetries.
Abstract
Longitudinally asymmetric features of gravity wave propagation in a sudden warming are examined theoretically, using observed geostrophic wind fields in the stratosphere for three days of winter 1979. It is shown that the wind patterns accompanying a sudden warming act to reduce, but not eliminate, quasi-stationary gravity wave propagation to the mesosphere. The onset of large-amplitude planetary waves leads to the formation of propagating zones and forbidden zones for gravity waves of intermediate horizontal scale (50–200 km). Lateral ray movement and horizontal refraction are secondary but observable effects for these waves.
To the extent that these waves are excited isotropically in the troposphere, it is possible to evaluate the direction and magnitude of the average wavevector reaching the mesosphere as follows. Stationary waves with wavevector orthogonal to the local mean flow are selectively absorbed in the stratosphere, implying that for these waves the average wavevector transmitted to the mesosphere is antiparallel to the average of the mean flow orientation extrema in the underlying stratosphere.
Abstract
Longitudinally asymmetric features of gravity wave propagation in a sudden warming are examined theoretically, using observed geostrophic wind fields in the stratosphere for three days of winter 1979. It is shown that the wind patterns accompanying a sudden warming act to reduce, but not eliminate, quasi-stationary gravity wave propagation to the mesosphere. The onset of large-amplitude planetary waves leads to the formation of propagating zones and forbidden zones for gravity waves of intermediate horizontal scale (50–200 km). Lateral ray movement and horizontal refraction are secondary but observable effects for these waves.
To the extent that these waves are excited isotropically in the troposphere, it is possible to evaluate the direction and magnitude of the average wavevector reaching the mesosphere as follows. Stationary waves with wavevector orthogonal to the local mean flow are selectively absorbed in the stratosphere, implying that for these waves the average wavevector transmitted to the mesosphere is antiparallel to the average of the mean flow orientation extrema in the underlying stratosphere.
Abstract
Data retrieved from the LIMS (Limb lnfrared Monitor of the Stratosphere) experiment are used 10 calculate daily isentropic distributions of Ertel's potential vorticity, ozone, water vapor and nitric acid at the 850 K level in the Northern Hemisphere stratosphere for the period 25 October 1978 through 2 April 1979. Systematic redistributions of the quasi-conservative tracers are investigated by following the evolutions of the horizontal projection of the areas enclosed by isopleths of tracer on the isentropic surface. If the horizontal velocity is nondivergent on an isentropic surface, the areas change in response to nonconservative processes and /or irreversible mixing to unresolvable scales and so provide a diagnostic for quantifying the net cited of these two processes. The effects of the seasonal variation of the solar heating on the areas are identified from the evolutions of the hemispheric means and, for the potential vorticity, from a comparison with an annual Mile integration of a zonally symmetric, general circulation model. Superimposed on the seasonal trends are changes in areas on shorter time scales, and the LIMS potential vorticity, ozone and water vapor distributions each show the distinctive “surf-zone, main–,vortex structure” described by McIntyre and Palmer. As winter progresses the main vortex decreases in size while the surf zone expands. The evidence of the observations, combined with estimates of the strength of the radiative processes acting on the potential vorticity field, indicates fairly convincingly that irreversible mixing is an important mechanism involved in the formation of the surf-zone, main-vortex structure, and the subsequent erosion in size of the vortex. In addition, there is evidence of strong diabatic cross-isentropic transport of air parcels in the surf zone acting to restore the large-scale gradients destroyed by the mixing. The only LIMS measured constituent for which mixing was not always the dominant mechanism of redistribution was nitric acid, and it is speculated that the effects of dynamically induced changes to the effective sources and sinks of nitric acid on the 850 K surface are overshadowing other processes, at least in late January and February. Implications to tracer transport studies are examined by using the isentropic potential vorticity field as a basis for calculating low resolution approximations to the Lagrangian-mean tracer mixing ratios. The results demonstrate the feasibility of the approach to the longer-species but indicate a need for further research to distinguish between dynamical and radiactive/photochemical effects.
Abstract
Data retrieved from the LIMS (Limb lnfrared Monitor of the Stratosphere) experiment are used 10 calculate daily isentropic distributions of Ertel's potential vorticity, ozone, water vapor and nitric acid at the 850 K level in the Northern Hemisphere stratosphere for the period 25 October 1978 through 2 April 1979. Systematic redistributions of the quasi-conservative tracers are investigated by following the evolutions of the horizontal projection of the areas enclosed by isopleths of tracer on the isentropic surface. If the horizontal velocity is nondivergent on an isentropic surface, the areas change in response to nonconservative processes and /or irreversible mixing to unresolvable scales and so provide a diagnostic for quantifying the net cited of these two processes. The effects of the seasonal variation of the solar heating on the areas are identified from the evolutions of the hemispheric means and, for the potential vorticity, from a comparison with an annual Mile integration of a zonally symmetric, general circulation model. Superimposed on the seasonal trends are changes in areas on shorter time scales, and the LIMS potential vorticity, ozone and water vapor distributions each show the distinctive “surf-zone, main–,vortex structure” described by McIntyre and Palmer. As winter progresses the main vortex decreases in size while the surf zone expands. The evidence of the observations, combined with estimates of the strength of the radiative processes acting on the potential vorticity field, indicates fairly convincingly that irreversible mixing is an important mechanism involved in the formation of the surf-zone, main-vortex structure, and the subsequent erosion in size of the vortex. In addition, there is evidence of strong diabatic cross-isentropic transport of air parcels in the surf zone acting to restore the large-scale gradients destroyed by the mixing. The only LIMS measured constituent for which mixing was not always the dominant mechanism of redistribution was nitric acid, and it is speculated that the effects of dynamically induced changes to the effective sources and sinks of nitric acid on the 850 K surface are overshadowing other processes, at least in late January and February. Implications to tracer transport studies are examined by using the isentropic potential vorticity field as a basis for calculating low resolution approximations to the Lagrangian-mean tracer mixing ratios. The results demonstrate the feasibility of the approach to the longer-species but indicate a need for further research to distinguish between dynamical and radiactive/photochemical effects.
Abstract
This paper investigates the vertical filtering of parameterized gravity wave pseudomomentum flux in the troposphere–stratosphere version of the Met Office Unified Model. Gravity wave forcing is parameterized using the Warner and McIntyre spectral gravity wave parameterization. The same amount of isotropic pseudomomentum flux per unit mass is launched from the planetary boundary layer at each grid point. The parameterization models the azimuthally dependent Doppler shifting and breaking of the gravity wave spectrum as it is filtered by the background atmosphere. The result is an anisotropic distribution of pseudomomentum flux among azimuthal sectors that varies greatly with altitude and location. This gives an idealized global climatology of nonorographic gravity waves. The filtering effect of the atmosphere in this climatology is diagnosed using the “zonal anisotropy.”
Results show areas where observational measurements could be targeted to find the most prominent features in the gravity wave field. Such areas include, for example, the summer stratosphere where zonal anisotropy is very large and where there is a significant localization in latitude and longitude of patches of high zonal anisotropy. Comparisons are also made with recent observational estimates of gravity wave fluxes and test whether wind filtering of a homogeneous, azimuthally isotropic source is enough to reproduce observed features of the gravity wave field.
Abstract
This paper investigates the vertical filtering of parameterized gravity wave pseudomomentum flux in the troposphere–stratosphere version of the Met Office Unified Model. Gravity wave forcing is parameterized using the Warner and McIntyre spectral gravity wave parameterization. The same amount of isotropic pseudomomentum flux per unit mass is launched from the planetary boundary layer at each grid point. The parameterization models the azimuthally dependent Doppler shifting and breaking of the gravity wave spectrum as it is filtered by the background atmosphere. The result is an anisotropic distribution of pseudomomentum flux among azimuthal sectors that varies greatly with altitude and location. This gives an idealized global climatology of nonorographic gravity waves. The filtering effect of the atmosphere in this climatology is diagnosed using the “zonal anisotropy.”
Results show areas where observational measurements could be targeted to find the most prominent features in the gravity wave field. Such areas include, for example, the summer stratosphere where zonal anisotropy is very large and where there is a significant localization in latitude and longitude of patches of high zonal anisotropy. Comparisons are also made with recent observational estimates of gravity wave fluxes and test whether wind filtering of a homogeneous, azimuthally isotropic source is enough to reproduce observed features of the gravity wave field.
Abstract
Transformed Eulerian mean (TEM) equations and Eliassen–Palm (EP) flux diagnostics are presented for the general nonhydrostatic, fully compressible, deep atmosphere formulation of the primitive equations in spherical geometric coordinates. The TEM equations are applied to a general circulation model (GCM) based on these general primitive equations. It is demonstrated that a naive application in this model of the widely used approximations to the EP diagnostics, valid for the hydrostatic primitive equations using log-pressure as a vertical coordinate and presented, for example, by Andrews et al. in 1987 can lead to misleading features in these diagnostics. These features can be of the same order of magnitude as the diagnostics themselves throughout the winter stratosphere. Similar conclusions are found to hold for “downward control” calculations. The reasons are traced to the change of vertical coordinate from geometric height to log-pressure. Implications for the modeling community, including comparison of model output with that from reanalysis products available only on pressure surfaces, are discussed.
Abstract
Transformed Eulerian mean (TEM) equations and Eliassen–Palm (EP) flux diagnostics are presented for the general nonhydrostatic, fully compressible, deep atmosphere formulation of the primitive equations in spherical geometric coordinates. The TEM equations are applied to a general circulation model (GCM) based on these general primitive equations. It is demonstrated that a naive application in this model of the widely used approximations to the EP diagnostics, valid for the hydrostatic primitive equations using log-pressure as a vertical coordinate and presented, for example, by Andrews et al. in 1987 can lead to misleading features in these diagnostics. These features can be of the same order of magnitude as the diagnostics themselves throughout the winter stratosphere. Similar conclusions are found to hold for “downward control” calculations. The reasons are traced to the change of vertical coordinate from geometric height to log-pressure. Implications for the modeling community, including comparison of model output with that from reanalysis products available only on pressure surfaces, are discussed.
Abstract
With extreme variability of the Arctic polar vortex being a key link for stratosphere–troposphere influences, its evolution into the twenty-first century is important for projections of changing surface climate in response to greenhouse gases. Variability of the stratospheric vortex is examined using a state-of-the-art climate model and a suite of specifically developed vortex diagnostics. The model has a fully coupled ocean and a fully resolved stratosphere. Analysis of the standard stratospheric zonal mean wind diagnostic shows no significant increase over the twenty-first century in the number of major sudden stratospheric warmings (SSWs) from its historical value of 0.7 events per decade, although the monthly distribution of SSWs does vary, with events becoming more evenly dispersed throughout the winter. However, further analyses using geometric-based vortex diagnostics show that the vortex mean state becomes weaker, and the vortex centroid is climatologically more equatorward by up to 2.5°, especially during early winter. The results using these diagnostics not only characterize the vortex structure and evolution but also emphasize the need for vortex-centric diagnostics over zonally averaged measures. Finally, vortex variability is subdivided into wave-1 (displaced) and -2 (split) components, and it is implied that vortex displacement events increase in frequency under climate change, whereas little change is observed in splitting events.
Abstract
With extreme variability of the Arctic polar vortex being a key link for stratosphere–troposphere influences, its evolution into the twenty-first century is important for projections of changing surface climate in response to greenhouse gases. Variability of the stratospheric vortex is examined using a state-of-the-art climate model and a suite of specifically developed vortex diagnostics. The model has a fully coupled ocean and a fully resolved stratosphere. Analysis of the standard stratospheric zonal mean wind diagnostic shows no significant increase over the twenty-first century in the number of major sudden stratospheric warmings (SSWs) from its historical value of 0.7 events per decade, although the monthly distribution of SSWs does vary, with events becoming more evenly dispersed throughout the winter. However, further analyses using geometric-based vortex diagnostics show that the vortex mean state becomes weaker, and the vortex centroid is climatologically more equatorward by up to 2.5°, especially during early winter. The results using these diagnostics not only characterize the vortex structure and evolution but also emphasize the need for vortex-centric diagnostics over zonally averaged measures. Finally, vortex variability is subdivided into wave-1 (displaced) and -2 (split) components, and it is implied that vortex displacement events increase in frequency under climate change, whereas little change is observed in splitting events.
Abstract
Stratospheric variability is examined in a vertically extended version of the Met Office global climate model. Equatorial variability includes the simulation of an internally generated quasi-biennial oscillation (QBO) and semiannual oscillation (SAO). Polar variability includes an examination of the frequency of sudden stratospheric warmings (SSW) and annular mode variability. Results from two different horizontal resolutions are also compared. Changes in gravity wave filtering at the higher resolution result in a slightly longer QBO that extends deeper into the lower stratosphere. At the higher resolution there is also a reduction in the occurrence rate of sudden stratospheric warmings, in better agreement with observations. This is linked with reduced levels of resolved waves entering the high-latitude stratosphere. Covariability of the tropical and extratropical stratosphere is seen, linking the phase of the QBO with disturbed NH winters, although this linkage is sporadic, in agreement with observations. Finally, tropospheric persistence time scales and seasonal variability for the northern and southern annular modes are significantly improved at the higher resolution, consistent with findings from other studies.
Abstract
Stratospheric variability is examined in a vertically extended version of the Met Office global climate model. Equatorial variability includes the simulation of an internally generated quasi-biennial oscillation (QBO) and semiannual oscillation (SAO). Polar variability includes an examination of the frequency of sudden stratospheric warmings (SSW) and annular mode variability. Results from two different horizontal resolutions are also compared. Changes in gravity wave filtering at the higher resolution result in a slightly longer QBO that extends deeper into the lower stratosphere. At the higher resolution there is also a reduction in the occurrence rate of sudden stratospheric warmings, in better agreement with observations. This is linked with reduced levels of resolved waves entering the high-latitude stratosphere. Covariability of the tropical and extratropical stratosphere is seen, linking the phase of the QBO with disturbed NH winters, although this linkage is sporadic, in agreement with observations. Finally, tropospheric persistence time scales and seasonal variability for the northern and southern annular modes are significantly improved at the higher resolution, consistent with findings from other studies.
Abstract
Analysis of a high-resolution, convection-permitting simulation of the tropical Indian Ocean has revealed empirical relationships between precipitation and gravity wave vertical momentum flux on grid scales typical of earth system models. Hence, the authors take a rough functional form, whereby the wave flux source spectrum has an amplitude proportional to the square root of total precipitation, to represent gravity wave source strengths in the Met Office global model’s spectral nonorographic scheme. Key advantages of the new source are simplicity and responsiveness to changes in convection processes without dependence upon model-specific details of their representation. Thus, the new source scheme is potentially a straightforward adaptation for a class of spectral gravity wave schemes widely used for current state-of-the-art earth system models. Against an invariant source, the new parameterized source generates launch-level flux amplitudes with greater spatial and temporal variability, producing probability density functions for absolute momentum flux over the ocean that have extended tails of large-amplitude, low-occurrence events. Such distributions appear more realistic in comparison with reported balloon observations. Source intermittency at the launch level affects mean fluxes at higher levels in two ways: directly, as a result of upward propagation of the new source variation, and indirectly, through changes in filtering characteristics that arise from intermittency. Initial assessment of the new scheme in the Met Office global model indicates an improved representation of the quasi-biennial oscillation and sensitivity that offers potential for further impact in the future.
Abstract
Analysis of a high-resolution, convection-permitting simulation of the tropical Indian Ocean has revealed empirical relationships between precipitation and gravity wave vertical momentum flux on grid scales typical of earth system models. Hence, the authors take a rough functional form, whereby the wave flux source spectrum has an amplitude proportional to the square root of total precipitation, to represent gravity wave source strengths in the Met Office global model’s spectral nonorographic scheme. Key advantages of the new source are simplicity and responsiveness to changes in convection processes without dependence upon model-specific details of their representation. Thus, the new source scheme is potentially a straightforward adaptation for a class of spectral gravity wave schemes widely used for current state-of-the-art earth system models. Against an invariant source, the new parameterized source generates launch-level flux amplitudes with greater spatial and temporal variability, producing probability density functions for absolute momentum flux over the ocean that have extended tails of large-amplitude, low-occurrence events. Such distributions appear more realistic in comparison with reported balloon observations. Source intermittency at the launch level affects mean fluxes at higher levels in two ways: directly, as a result of upward propagation of the new source variation, and indirectly, through changes in filtering characteristics that arise from intermittency. Initial assessment of the new scheme in the Met Office global model indicates an improved representation of the quasi-biennial oscillation and sensitivity that offers potential for further impact in the future.
