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Ann. Geophys., 22, 1885-1902, 2004
www.ann-geophys.net/22/1885/2004/
© European Geosciences Union 2004
Modeling study of mesospheric planetary waves: genesis and characteristics
H. G. Mayr1, J. G. Mengel2, E. R. Talaat3, H. S. Porter4, and K. L. Chan5
1Goddard Space Flight Center, Greenbelt, MD, 20771, USA
2Science Systems & Applications, Inc., Lanham, MD, USA
3Applied Physic Laboratory, Johns Hopkins University, Laurel, MD, USA
4Furman University, Greenville, SC, USA
5Hong Kong University of Science and Technology, Hong Kong, China
Abstract. The Numerical Spectral Model (NSM) extends from the
ground into the thermosphere and incorporates Hines' Doppler Spread
Parameterization for small-scale gravity waves (GWs). In the present version
of the model we account for a tropospheric heat source in the zonal mean (m=0),
which reproduces qualitatively the observed zonal jets near the
tropopause and the accompanying reversal in the latitudinal temperature
variations. In the study presented here, we discuss the planetary waves (PWs) that are solely
generated internally, i.e. without the explicit excitation sources related
to tropospheric convection or topography. Our analysis shows that PWs are
not produced when the zonally averaged heat source into the atmosphere is
artificially suppressed, and that the PWs are generally weaker when the
tropospheric source is not applied. Instabilities associated with the zonal
mean temperature, pressure and wind fields, which still need to be explored,
are exciting PWs that have amplitudes in the mesosphere comparable to those
observed. Three classes of PWs are generated in the NSM. (1) Rossby type
PWs, which slowly propagate westward relative to the mean zonal flow, are
carried by the winds so that they appear (from the ground) to propagate,
respectively, eastward and westward in the winter and summer hemispheres
below 80km. Depending on the zonal wave number and magnitudes of the zonal
winds, and under the influence of the equatorial oscillations, these PWs
typically have periods between 2 and 20 days. Their horizontal wind
amplitudes can exceed 40 m/s in the lower mesosphere. (2) Rossby-gravity
waves, which propagate westward at low latitudes and have periods around 2
days for zonal wave numbers m=2 to 4. (3) Eastward propagating equatorial
Kelvin waves, which are generated in the upper mesosphere with periods
between 1 and 3 days depending on m. A survey of the PWs reveals that the
largest wind amplitudes tend to occur below 80km in the winter hemisphere;
but above that altitude the amplitudes are larger in the summer hemisphere
where the winds can approach 50m/s. This pattern in the seasonal variations
also appears in the baroclinity of the zonal mean (m=0). The nonmigrating
tides in the mesosphere are significantly larger for the model with the
tropospheric heat source, in which PWs are apparently generated by the
instabilities that arise around the tropopause.
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