Physics
Scientific paper
Feb 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995jatp...57..105e&link_type=abstract
Journal of Atmospheric and Terrestrial Physics (ISSN 0021-9169), vol. 57, no. 2, p. 105-134
Physics
37
Annual Variations, Atmospheric Circulation, Equatorial Atmosphere, Gravity Waves, Morphology, Stratosphere, Temperature Distribution, Velocity Distribution, Waves, Atmospheric Attenuation, Atmospheric Stratification, Brunt-Vaisala Frequency, Normal Density Functions, Wave Attenuation, Wind Velocity
Scientific paper
A first systematic attempt is made to explain the variance of horizontal velocities u(sup '2) + v(sup '2) and relative temperature fluctuations T(sup '2) produced by grvaity waves and equatorial waves in the 20-60 km height range of the atmosphere. Single-wave and spectral theories of the wave field are applied to derive simplified quantities which parameterize both dissipative and nondissipative effects controlling wave variances. The major simplifications is the omission of variations in source strengths and background winds, which can modify wave variances. The success or otherwise of the resulting simulations gives some measure of the relative importance of the retained terms compared to the neglected terms. The simplifications produce terms independent of individual wave parameters, making them valid over the entire wave field, and the neglect of background winds enables their computation from background temperatures alone. This approach accurately models both the phase and depth of the observed annual variation of u(sup '2) + v(sup '2) and T(sup '2) at high latitudes, and its attenuation on moving equatorward. The simulated annual cycle arises principally from seasonal variations in the density stratification of the atmosphere below 60 km, although dependence on background Brunt-Vaisala frequency N accounts for the deeper annual T(sup '2) variation. The observed decrease in T(sup '2) above 40 km is also consistent with the decrease in N at upper heights. An observed Gaussian distribution about the equator of variance at large vertical wavelengths agrees with equatorial-wave theory. Several prominent features are not simulated, and so are probably due to processes omitted during initial simplification of the theory. The semiannual and quasi-biennial variation of equatorial-wave variances are consistent with their perceived role in driving similar oscillations of the background atmosphere in these regions. The nonsimulated peak in upper-stratospheric variances during July-September at sites in the United States coincides with weakening westward flow. It is agrued that this will reduce vertical wavenumbers, and so increase variances in order to conserve the vertical flux of wave action. The hypothesis can explain the confinement of the peak to July-September, and its attenuation on moving to higher latitude. However, it cannot explain a similar feature observed over Wales (52 deg N), nor the absence of a peak in data from Japanese rocket station. Appreciable differences in annual-mean variances among sites in and around North America indicate geographical differences in source intensities. The accurate modeling of seasonal variations at these sites suggests a dominant source that is geogrpahically variable but temporally constant, consistent with topographic forcing and supporting previous lower-atmosphere studies over the United States.
Eckermann Stephen D.
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