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- Author or Editor: J. R. Holton x
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Abstract
Seasonal mean downward mass fluxes across the 100 mb level in the extratropics of both hemispheres are computed using the meridionally integrated residual vertical circulation as determined from the transformed Eulerian mean equations. The eddy momentum and heat flux data required for the calculation are taken from Oort's 15-year climatology. Upward mass flux from the troposphere to the stratosphere in the tropics is computed from mass continuity. The flux is a maximum during Northern Hemisphere winter and a minimum during Northern Hemisphere summer. The computed fluxes imply a 2.5 year turnover time for the global atmospheric layer above 100 mb.
Abstract
Seasonal mean downward mass fluxes across the 100 mb level in the extratropics of both hemispheres are computed using the meridionally integrated residual vertical circulation as determined from the transformed Eulerian mean equations. The eddy momentum and heat flux data required for the calculation are taken from Oort's 15-year climatology. Upward mass flux from the troposphere to the stratosphere in the tropics is computed from mass continuity. The flux is a maximum during Northern Hemisphere winter and a minimum during Northern Hemisphere summer. The computed fluxes imply a 2.5 year turnover time for the global atmospheric layer above 100 mb.
Abstract
A two-dimensional cloud-resolving model is used to examine the possible role of gravity waves generated by a simulated tropical squall line in forcing the quasi-biennial oscillation (QBO) of the zonal winds in the equatorial stratosphere. A simulation with constant background stratospheric winds is compared to simulations with background winds characteristic of the westerly and easterly QBO phases, respectively. In all three cases a broad spectrum of both eastward and westward propagating gravity waves is excited. In the constant background wind case the vertical momentum flux is nearly constant with height in the stratosphere, after correction for waves leaving the model domain. In the easterly and westerly shear cases, however, westward and eastward propagating waves, respectively, are strongly damped as they approach their critical levels, owing to the strongly scale-dependent vertical diffusion in the model. The profiles of zonal forcing induced by this wave damping are similar to profiles given by critical level absorption, but displaced slightly downward. The magnitude of the zonal forcing is of order 5 m s−1 day−1. It is estimated that if 2% of the area of the Tropics were occupied by storms of similar magnitude, mesoscale gravity waves could provide nearly 1/4 of the zonal forcing required for the QBO.
Abstract
A two-dimensional cloud-resolving model is used to examine the possible role of gravity waves generated by a simulated tropical squall line in forcing the quasi-biennial oscillation (QBO) of the zonal winds in the equatorial stratosphere. A simulation with constant background stratospheric winds is compared to simulations with background winds characteristic of the westerly and easterly QBO phases, respectively. In all three cases a broad spectrum of both eastward and westward propagating gravity waves is excited. In the constant background wind case the vertical momentum flux is nearly constant with height in the stratosphere, after correction for waves leaving the model domain. In the easterly and westerly shear cases, however, westward and eastward propagating waves, respectively, are strongly damped as they approach their critical levels, owing to the strongly scale-dependent vertical diffusion in the model. The profiles of zonal forcing induced by this wave damping are similar to profiles given by critical level absorption, but displaced slightly downward. The magnitude of the zonal forcing is of order 5 m s−1 day−1. It is estimated that if 2% of the area of the Tropics were occupied by storms of similar magnitude, mesoscale gravity waves could provide nearly 1/4 of the zonal forcing required for the QBO.
Abstract
Three years of zonally averaged N2O and CH4 data from the SAMS instrument on Nimbus 7 are utilized to investigate the annual and semiannual cycles in long-lived tracer mixing ratios. The annual and semiannual variations are shown to be approximately antisymmetric and symmetric about the equator, respectively. Using the first three components of the annual cycle to estimate the time tendency, the tracer continuity equation is solved diagnostically to obtain the effective transport velocity (i.e., the meridional circulation that can produce the observed seasonal variations in the tracer fields). The resulting circulation is qualitatively in agreement with the diabatic circulations computed by other workers. The present calculations, however, exhibit a stronger equinoctial subsidence in the equatorial upper stratosphere than deduced in other studies as required to produce a “double peak” tracer structure that has the amplitude and vertical extent that is observed.
Abstract
Three years of zonally averaged N2O and CH4 data from the SAMS instrument on Nimbus 7 are utilized to investigate the annual and semiannual cycles in long-lived tracer mixing ratios. The annual and semiannual variations are shown to be approximately antisymmetric and symmetric about the equator, respectively. Using the first three components of the annual cycle to estimate the time tendency, the tracer continuity equation is solved diagnostically to obtain the effective transport velocity (i.e., the meridional circulation that can produce the observed seasonal variations in the tracer fields). The resulting circulation is qualitatively in agreement with the diabatic circulations computed by other workers. The present calculations, however, exhibit a stronger equinoctial subsidence in the equatorial upper stratosphere than deduced in other studies as required to produce a “double peak” tracer structure that has the amplitude and vertical extent that is observed.
Abstract
Lindzen's parameterization for the drag and eddy diffusion produced by breaking internal gravity waves in the mesosphere and lower thermosphere is applied to a modified version of the β-plane channel model of Holton in which an isotropic source spectrum of waves is specified similar to that given in 1982 by Matsuno. The transmission for each wave component is influenced by Newtonian cooling and by eddy diffusion induced by the breaking of other wave components. In general the waves with smallest Doppler-shifted phase speeds break first and produce sufficient eddy diffusion to significantly raise the breaking heights for the higher speed components. Thus, the wave drag and diffusion is spread through a deep layer and the resulting mean wind profiles for both summer and winter solstice conditions are more realistic than those computed previously by Holton.
Abstract
Lindzen's parameterization for the drag and eddy diffusion produced by breaking internal gravity waves in the mesosphere and lower thermosphere is applied to a modified version of the β-plane channel model of Holton in which an isotropic source spectrum of waves is specified similar to that given in 1982 by Matsuno. The transmission for each wave component is influenced by Newtonian cooling and by eddy diffusion induced by the breaking of other wave components. In general the waves with smallest Doppler-shifted phase speeds break first and produce sufficient eddy diffusion to significantly raise the breaking heights for the higher speed components. Thus, the wave drag and diffusion is spread through a deep layer and the resulting mean wind profiles for both summer and winter solstice conditions are more realistic than those computed previously by Holton.
Abstract
A nonlinear model based on the shallow water equations is used to study cross-equatorial propagation of forced waves in the presence of a longitudinally varying time-mean basic-state zonal wind field. It is found that global-scale planetary waves are unable to propagate past a critical latitude where the mean zonal wind speed vanishes. However, if the longitudinally-asymmetric basic state includes a “duet” in which the zonal winds are westerly, waves of zonal scale less than the zonal scale of the westerly duct may propagate from one hemisphere to the other even though the zonally-symmetric mean zonal wind remains easterly in the equatorial region. The amplitude of the response in one hemisphere to forcing in the opposite hemisphere increases strongly with the magnitude of the westerlies in the equatorial duct. The existence and annual variations of a westerly duct region in the upper troposphere in the eastern Pacific appear to account for some features of the low-frequency variability in the Northern Hemisphere.
Abstract
A nonlinear model based on the shallow water equations is used to study cross-equatorial propagation of forced waves in the presence of a longitudinally varying time-mean basic-state zonal wind field. It is found that global-scale planetary waves are unable to propagate past a critical latitude where the mean zonal wind speed vanishes. However, if the longitudinally-asymmetric basic state includes a “duet” in which the zonal winds are westerly, waves of zonal scale less than the zonal scale of the westerly duct may propagate from one hemisphere to the other even though the zonally-symmetric mean zonal wind remains easterly in the equatorial region. The amplitude of the response in one hemisphere to forcing in the opposite hemisphere increases strongly with the magnitude of the westerlies in the equatorial duct. The existence and annual variations of a westerly duct region in the upper troposphere in the eastern Pacific appear to account for some features of the low-frequency variability in the Northern Hemisphere.
Abstract
The annual cycle of the zonally averaged circulation in the middle atmosphere (16–96 km) is simulated using; a severely truncated semi-spectral numerical model. The model includes only a single zonal harmonic wave component which interacts with the mean flow. The circulation is driven by diabatic heating and by a specified perturbation in the topography of the lower boundary, which is taken to be the 100 mb surface. Damping is included as Newtonian cooling and Rayleigh friction.
A comparison of the annual cycle simulated by this model with the results of an analogous two-dimensional model indicates that planetary waves have relatively little influence on the zonal mean temperature profiles and on the solstice mean zonal winds at high latitudes. The primary effects of the forced waves are in decelerating the mean winds in low latitudes in the winter hemisphere to produce a region of weak westerlies, and in generating “final warmings” at the spring equinoxes.
Computed mean zonal winds and wave amplitudes display significant hemispheric asymmetries. These differences are associated with the strong dependence of the eddy structure on the mean zonal wind profile.
The “residual” mean meridional circulation, computed by subtracting the portion of the Eulerian mean meridional flow which is exactly balanced by the eddy heat fluxes, is nearly identical at the solstices to the Eulerian mean circulation computed in the two-dimensional model. Except in low latitudes, the net mean flow forcing by the eddy fluxes is quite small at the solstices in agreement with the predictions of wave-mean flow non-acceleration theorems.
There are no major midwinter sudden warmings produced in the model. However, final warmings occur in the spring equinoctial seasons in both hemispheres. The rapid transient adjustments which occur during these final warmings make the equinoctial circulations in this model much different from the circulations computed for corresponding seasons in the two-dimensional model.
Abstract
The annual cycle of the zonally averaged circulation in the middle atmosphere (16–96 km) is simulated using; a severely truncated semi-spectral numerical model. The model includes only a single zonal harmonic wave component which interacts with the mean flow. The circulation is driven by diabatic heating and by a specified perturbation in the topography of the lower boundary, which is taken to be the 100 mb surface. Damping is included as Newtonian cooling and Rayleigh friction.
A comparison of the annual cycle simulated by this model with the results of an analogous two-dimensional model indicates that planetary waves have relatively little influence on the zonal mean temperature profiles and on the solstice mean zonal winds at high latitudes. The primary effects of the forced waves are in decelerating the mean winds in low latitudes in the winter hemisphere to produce a region of weak westerlies, and in generating “final warmings” at the spring equinoxes.
Computed mean zonal winds and wave amplitudes display significant hemispheric asymmetries. These differences are associated with the strong dependence of the eddy structure on the mean zonal wind profile.
The “residual” mean meridional circulation, computed by subtracting the portion of the Eulerian mean meridional flow which is exactly balanced by the eddy heat fluxes, is nearly identical at the solstices to the Eulerian mean circulation computed in the two-dimensional model. Except in low latitudes, the net mean flow forcing by the eddy fluxes is quite small at the solstices in agreement with the predictions of wave-mean flow non-acceleration theorems.
There are no major midwinter sudden warmings produced in the model. However, final warmings occur in the spring equinoctial seasons in both hemispheres. The rapid transient adjustments which occur during these final warmings make the equinoctial circulations in this model much different from the circulations computed for corresponding seasons in the two-dimensional model.
Abstract
The excitation and vertical propagation of gravity waves is simulated in a two-dimensional model of a mesoscale convective storm. It is shown that in a simulated squall line the gravity waves that are preferentially excited are those propagating opposite to the direction of motion of the storm. Solutions for cases with differing stratospheric mean zonal flow profiles are compared. It turns out that, in the absence of storm-relative mean winds in the stratosphere, the primary mode of excitation of gravity waves is by mechanical forcing owing to oscillatory updrafts. The stratospheric response consists of waves whose periods match the primary periods of the forcing. Owing to the tendency of the oscillating updrafts to propagate toward the rear of the storm, gravity wave propagation is limited primarily to the rearward direction, and there is a net downward momentum transport. When storm-relative mean winds are included in the model the waves excited by the oscillating updrafts are weaker, but a new class of waves, similar to topographic waves, appears in the stratosphere directly above the main updraft region.The cloud model results are compared with results from a dry model in which waves are excited by a specified compact momentum source designed to mimic the mechanical forcing caused by the regular development and rearward propagation of updraft cells. Results from this analog strongly support the notion that squall-line–generated gravity waves arise from mechanical forcing rather than thermal effects.
Abstract
The excitation and vertical propagation of gravity waves is simulated in a two-dimensional model of a mesoscale convective storm. It is shown that in a simulated squall line the gravity waves that are preferentially excited are those propagating opposite to the direction of motion of the storm. Solutions for cases with differing stratospheric mean zonal flow profiles are compared. It turns out that, in the absence of storm-relative mean winds in the stratosphere, the primary mode of excitation of gravity waves is by mechanical forcing owing to oscillatory updrafts. The stratospheric response consists of waves whose periods match the primary periods of the forcing. Owing to the tendency of the oscillating updrafts to propagate toward the rear of the storm, gravity wave propagation is limited primarily to the rearward direction, and there is a net downward momentum transport. When storm-relative mean winds are included in the model the waves excited by the oscillating updrafts are weaker, but a new class of waves, similar to topographic waves, appears in the stratosphere directly above the main updraft region.The cloud model results are compared with results from a dry model in which waves are excited by a specified compact momentum source designed to mimic the mechanical forcing caused by the regular development and rearward propagation of updraft cells. Results from this analog strongly support the notion that squall-line–generated gravity waves arise from mechanical forcing rather than thermal effects.
Abstract
High-frequency gravity waves generated by convective storms likely play an important role in the general circulation of the middle atmosphere. Yet little is known about waves from this source. This work utilizes a fully compressible, nonlinear, numerical, two-dimensional simulation of a midlatitude squall line to study vertically propagating waves generated by deep convection. The model includes a deep stratosphere layer with high enough resolution to characterize the wave motions at these altitudes. A spectral analysis of the stratospheric waves provides an understanding of the necessary characteristics of the spectrum for future studies of their effects on the middle atmosphere in realistic mean wind scenarios. The wave spectrum also displays specific characteristics that point to low physical mechanisms within the storm responsible for their forcing. Understanding these forcing mechanisms and the properties of the storm and atmosphere that control them are crucial first steps toward developing a parameterization of waves from this source. The simulation also provides a description of some observable signatures of convectively generated waves, which may promote observational verification of these results and help tie any such observations to their convective source.
Abstract
High-frequency gravity waves generated by convective storms likely play an important role in the general circulation of the middle atmosphere. Yet little is known about waves from this source. This work utilizes a fully compressible, nonlinear, numerical, two-dimensional simulation of a midlatitude squall line to study vertically propagating waves generated by deep convection. The model includes a deep stratosphere layer with high enough resolution to characterize the wave motions at these altitudes. A spectral analysis of the stratospheric waves provides an understanding of the necessary characteristics of the spectrum for future studies of their effects on the middle atmosphere in realistic mean wind scenarios. The wave spectrum also displays specific characteristics that point to low physical mechanisms within the storm responsible for their forcing. Understanding these forcing mechanisms and the properties of the storm and atmosphere that control them are crucial first steps toward developing a parameterization of waves from this source. The simulation also provides a description of some observable signatures of convectively generated waves, which may promote observational verification of these results and help tie any such observations to their convective source.
Abstract
No abstract available.
Abstract
No abstract available.
Abstract
Gravity waves generated by convective heating are widely believed to have vertical wavelengths approximately twice the depth of the heating. The frequency, horizontal, and vertical wavelengths of gravity waves are, however, mutually related through the gravity wave dispersion relationship. For forcing of a given frequency, waves of vertical wavelength of twice the depth of the heating will be efficiently excited only if the horizontal forcing projects significantly onto horizontal scales compatible with the vertical-to-horizontal wavenumber ratio given by the dispersion relationship. The preferred vertical wavelength depends on a nondimensional parameter relating the frequency, horizontal, and vertical scales of the forcing. For the high-frequency waves that dominate the momentum flux in the upper stratosphere and mesosphere, the maximum vertical flux of horizontal momentum generally occurs for waves with vertical wavelengths much greater than twice the depth of the heating.
Abstract
Gravity waves generated by convective heating are widely believed to have vertical wavelengths approximately twice the depth of the heating. The frequency, horizontal, and vertical wavelengths of gravity waves are, however, mutually related through the gravity wave dispersion relationship. For forcing of a given frequency, waves of vertical wavelength of twice the depth of the heating will be efficiently excited only if the horizontal forcing projects significantly onto horizontal scales compatible with the vertical-to-horizontal wavenumber ratio given by the dispersion relationship. The preferred vertical wavelength depends on a nondimensional parameter relating the frequency, horizontal, and vertical scales of the forcing. For the high-frequency waves that dominate the momentum flux in the upper stratosphere and mesosphere, the maximum vertical flux of horizontal momentum generally occurs for waves with vertical wavelengths much greater than twice the depth of the heating.
