The role of the mean flow and gravity wave forcing in the observed seasonal variability of the migra...
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The role of the mean flow and gravity wave forcing in the observed seasonal variability of the migrating diurnal tide. David A. Ortland NorthWest Research Associates Charles McLandress University of Toronto. Main questions:.

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David a ortland northwest research associates charles mclandress university of toronto

The role of the mean flow and gravity wave forcing in the observed seasonal variability of the migrating diurnal tide.

David A. Ortland

NorthWest Research Associates

Charles McLandress

University of Toronto


Main questions

Main questions:

  • Can variations in gravity wave drag, modulated by mean-flow filtering, account for observed variations in tidal structure?

  • How much does a model of the gravity wave/tide interaction depend on the GW parameterization?

  • Description of the dynamics of the gravity wave/tide interaction

  • Tidal amplitude modification by GWs?

  • Seasonal variation of tidal amplitude.

Outline:


Gravity wave parameterizations

Gravity wave parameterizations

  • Source spectrum: pseudo-momentum flux density at source level: F(c,z=0)

  • Saturation criterion: each wave in the spectrum propagates conservatively until saturated.

  • Spectrum modified at each level: either saturated waves are obliterated (F(c,z)=0) or propagate at the saturated bound (F(c,z)=Fsat(c,z)) (e.g. Lindzen parameterization)


Saturation criteria

Saturation criteria

Mean flow forcing

  • For all parameterizations where saturated waves are assumed to be obliterated, the forcing may be expressed as:

This shows how forcing strength is related to density, slope of the cutoff curve and the shape of the source spectrum.


Mean flow forcing

Mean flow forcing

  • For all parameterizations where saturated waves are assumed to be obliterated, the forcing may be expressed as:

This shows how forcing strength is related to density, slope of the cutoff curve and the shape of the source spectrum.


David a ortland northwest research associates charles mclandress university of toronto

Cutoff phase speed profile (red). Two curves for westward and eastward propagating waves.

Source spectrum

Saturation spectra for different altitudes. The cutoff phase speed at each altitude is given by the intersection of this curve with the source spectrum.


David a ortland northwest research associates charles mclandress university of toronto

Saturation for different spectraThe shape of the forcing profile will depend on the altitude dependence of the cutoff phase speed, which, in turn, depends on the shape of the source spectrum. Three examples are shown here and the next figure.

Saturation spectra for different altitudes (blue)


Source spectrum determines shape of forcing profile

Source spectrum determines shape of forcing profile

Algebraic source profile (green), as used in the Hines param, produces a force profile that rapidly increases with altitude. Is this realistic?


David a ortland northwest research associates charles mclandress university of toronto

Shape of source spectrum important for determining altitude where significant tidal interaction occurs

Sample tide wind profile (green)

Notice that power law (red)

causes forcing to occur

more in phase with the tide.

This explains why the Hines parameterization amplifies tidal amplitude.


Comparison of hines and ad parameterization using the same source spectrum

Comparison of Hines and AD parameterization using the same source spectrum

Doppler spreading causes waves to saturate sooner than they

would individually. At each level, saturated part of the spectrum

has smaller flux than for AD (AD forcing shown with intermittency=.5)


David a ortland northwest research associates charles mclandress university of toronto

Equivalent Rayleigh Friction

For gravity wave drag, a has real and imaginary part with Im(a)<0. A complex a implies that the relative phase difference between the GW force and the tide is not 180°


David a ortland northwest research associates charles mclandress university of toronto

Effect of real part of damping coefficient on tide structure

  • Only factor that has a strong influence on tidal amplitude

  • Small effect on horizontal amplitude structure

  • Introduces latitudinal phase variation


David a ortland northwest research associates charles mclandress university of toronto

Efect of imaginary part of damping coefficient(black=classical mode structure, red=damped structure)

  • Im(a)>0 (Diffusion):

    Longer wavelength

  • Im(a)<0 (GW drag)

    Shorter wavelength

    In this example:

    Im(a) = -1

    Vertical wavelength= 20km


David a ortland northwest research associates charles mclandress university of toronto

Phase of forcing relative to the tide depends on source spectrum. The phase shift controls relative magnitude of real and imaginary part of the equivalent Rayleigh friction coefficient


Phase lag depends on saturation criterion

Phase lag depends on saturation criterion


David a ortland northwest research associates charles mclandress university of toronto

Experiments with a time-dependent linearized primitive equation model

Model ingredients:

  • Background winds taken from UARS Reference Atmosphere Project (URAP) or Canadian Middle Atmosphere Model (CMAM);

  • (1,1) Hough mode forcing in troposphere derived from CMAM annual mean;

  • Eddy and molecular diffusion in MLT;

  • Alexander-Dunkerton or Hines gravity wave parameterization;

  • Only forcing from momentum deposition due to GW breaking (not parameterized eddy diffusion)


David a ortland northwest research associates charles mclandress university of toronto

GW forcing of the mean flow winds: URAP (UARS reference atmosphere) for Januaryradiative equilibrium temperatures from MIDRAD

GW force required

to maintain

climatology

GW force computed

from AD parameterization


David a ortland northwest research associates charles mclandress university of toronto

Background zonal mean zonal winds used in the tidal model


David a ortland northwest research associates charles mclandress university of toronto

Meridional wind amplitudeURAP background, Alexander-Dunkerton GW parameterization


David a ortland northwest research associates charles mclandress university of toronto

Gravity wave effects on tidal structure:narrower horizontal structureshorter vertical wavelength


Comparison of direct ep flux divergence and diffusive gravity wave forcing

Comparison of direct (EP flux divergence) and diffusive gravity wave forcing

Relatively weak below 90 km and therefore not likely to have much effect on tidal amplitude

Note similar lat-alt structure of diffusive and direct forcing

Diurnal component of GW force

Time-mean component of GW force


David a ortland northwest research associates charles mclandress university of toronto

Adding GW parameterization enhances seasonal variability with URAP winds


David a ortland northwest research associates charles mclandress university of toronto

Adding GW parameterization enhances seasonal variability with URAP windsNote: Alexander-Dunkerton GW parameterization reduces amplitude


David a ortland northwest research associates charles mclandress university of toronto

Including GW forcing does not enhance seasonal variability with CMAM winds


David a ortland northwest research associates charles mclandress university of toronto

Including GW forcing does not enhance seasonal variability with CMAM winds


Seasonal variation of gw forcing

Seasonal variation of GW forcing

Solstice winds cause a relatively larger in-phase component of forcing, leading to enhanc0ed damping of the tide amplitude. The force is also confined to the winter hemisphere.


David a ortland northwest research associates charles mclandress university of toronto

Seasonal variation of GW forcingSeasonal variability depends more on the background winds used and not the GQ parameterization


David a ortland northwest research associates charles mclandress university of toronto

Mean wind modulation of GW forcing(Alexander-Dunkerton parameterization)

This GW force enhancement, responsible for the seasonal amplitude variability using URAP background winds is absent using CMAM winds

Latitude

Latitude


David a ortland northwest research associates charles mclandress university of toronto

Mean flow modulation of GW breakingWhy do URAP and CMAM winds cause different behavior in GW forcing?Answer appears to be that CMAM does not produce an equatorward tilt of the winter jet.

Jet is weak at mid-latitudes

Low phase speed waves near peak of spectrum break higher, causing larger acceleration,

in a stronger winter jet


David a ortland northwest research associates charles mclandress university of toronto

Mean flow modulation of GW breaking

Peak of spectrum shielded by westerly tropospheric jet


David a ortland northwest research associates charles mclandress university of toronto

Conclusions

  • Seasonal wind variations modulate the gravity wave forcing and eddy diffusion

    • Winter hemisphere jet causes waves near the peak of the GW spectrum to break in the mesosphere. Very strong GW drag and eddy diffusion can occur in mid-latitudes at the top of a strong jet.

    • GW drag has stronger effect on the tide than eddy diffusion

    • Effect of gravity wave drag likely depends somewhat on GW parameterization, but mostly on the shape of the source spectrum

    • When the GW effective friction has a large negative imaginary component, GW interaction will reduce the vertical wavelength of the tide and thereby enhance the effects of any ambient diffusion

    • Seasonal variation of the GW/tide interaction appears to be very sensitive to the structure of the mean flow, and may require a westerly jet tilted equatorward for this to be an effective mechanism of seasonal variability.


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