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Matthew H. Hitchman
,
Conway B. Leovy
,
John C. Gille
, and
Paul L. Bailey

Abstract

Data from the Limb Infrared Monitor of the Stratosphere (LIMS) are used to identify a new type of planetary scale disturbance in the equatorial lower mesosphere during northern winter 1978/79. The disturbances consist of two or three vertically stacked temperature extrema of alternating sign. They persist for as long as two weeks and do not propagate. Their occurrence is confined to regions of very weak or negative inertial stability, and their meridional to vertical aspect ratio, meridional structure and zonal spectrum are consistent with disturbances predicted by inertial instability theory. However, they are found only when there is strong forcing of the subtropical mesosphere by zonal wavenumber one and two Rossby waves. This fact, together with the absence of zonal propagation, suggests that stationary Rossby waves determine their occurrence and longitudinal structure. These structures can significantly modify the zonal mean flow and should be taken into account in dynamical models of the equatorial mesosphere.

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Varavut Limpasuvan
,
Conway B. Leovy
,
Yvan J. Orsolini
, and
Byron A. Boville

Abstract

The middle atmosphere version of the National Center for Atmospheric Research Community Climate Model 2 (MACCM2) mechanistic model is used to simulate several aspects of the observed two-day wave near the stratopause and its possible connection with inertial instability. Model experiments show that, for a prescribed initial wind condition with strong horizontal wind variation in the Tropics, inertial instability can trigger the two-day wave. An increase in horizontal wind curvature fostered by inertial instability circulation barotropically destabilizes the easterly jet in low summer latitudes and allows first the growth of the wavenumber-4 and then the wavenumber-3 component of the two-day wave. Near the stratopause, the two-day wave Eliassen–Palm flux is directed equatorward away from the wave’s critical line source and westerly momentum is transported into the easterly jet core by the wave. While much of the wavenumber-4 energy is confined near the stratopause, the wavenumber-3 energy can propagate upward well into the mesosphere where strong Rayleigh damping is imposed. The model results suggest that the observed disparity in the wavenumber-3 amplitude between the austral and boreal summers is a consequence of the difference in summer easterly jet strength.

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Ruth S. Lieberman
,
Conway B. Leovy
,
Byron A. Boville
, and
Bruce P. Briegleb

Abstract

In this paper, the authors assess the suitability of the heating fields in the latest version of the NCAR Community Climate Model (CCM2) for modeling the thermal forcing of atmospheric tides. Accordingly, diurnal variations of the surface pressure, outgoing longwave radiation, cloudiness, and precipitation are examined in the CCM2. The fields of radiative, sensible, and latent beating are similarly analyzed. These results are subjectively compared with available data.

Equatorial diurnal surface pressure tides are fairly well simulated by CCM2. The model successfully reproduces the semidiurnal surface pressure tides; however, this may result in part from reflection of wave energy at the upper boundary. The CCM2 large-scale diurnal OLR is generally consistent with observations. The moist-convective scheme in the model is able to reproduce the diurnally varying cloudiness and precipitation patterns associated with land-sea contrasts; however, the amplitudes of CCM2 diurnal continental convective cloudiness are weaker than observations. The CCM2 boundary-layer sensible heating is consistent with a very limited set of observations, and with estimates obtained from simple models of diffusive heating. Although the CCM2 tropospheric solar radiative heating is similar in magnitude to previous estimates, there are substantial differences in the vertical structures. A definitive assessment of the validity of the CCM2 diurnal cycle is precluded by the lack of detailed observations and the limitations of our CCM2 sample. Nevertheless, the authors conclude that the global-scale components of the CCM2 diurnal heating are useful proxies for the true diurnal forcing of the tides.

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