Wednesday, 6 November 2002: 4:30 PM

Explicit modeling of gravity-wave breaking above deep convection

Todd P. Lane, NCAR, Boulder, CO; and R. D. Sharman and T. L. Clark

Deep convective clouds generate gravity waves throughout their development,
maturity, and dissipation. During their vertical propagation, these waves may
be dissipated in some way, and subsequently exert a drag on the mean flow via a
divergence of the vertical flux of horizontal momentum. This drag is thought to
shape the mean flow of the middle atmosphere, and in the tropics may be
important in driving the Quasi-Biennial Oscillation. Arguably the most
important mechanism contributing to this drag is the breakdown of the
vertically-propagating gravity waves. However, the spatial and temporal scales
of such gravity waves and their subsequent breakdown are typically smaller than
the scales that can be resolved by current general circulation models;
therefore their effects must be parameterized. The development of an accurate
and physically-realistic convectively-generated gravity-wave drag
parameterization requires an understanding of the way convective clouds
generate gravity waves, and the way these waves are dissipated. As an
incremental step towards this goal, this study examines the breaking of high
frequency gravity waves above deep convection.

Results from a very high resolution cloud resolving model will be
presented. The numerical model explicitly resolves the deep convective cloud,
the gravity waves that are generated, and the subsequent breakdown of those
waves. In a systematic series of model integrations, the effect of background
wind shear on the wave breaking is investigated. Gravity-wave breaking is
attributed to the interaction between the propagating waves and a critical
level. The height of this critical level is reduced by the nonlinear
cloud-induced circulations. These results also show that the background shear
has a profound effect on the wave spectrum through critical level dissipation
and wave trapping. Subsequently, in the lower stratosphere the highest
frequency, shortest wavelength waves are removed from the spectrum. Thus, it
will be shown that in cases of moderate wind shear, the net effect of such
waves is to exert a drag in the lowest part of the stratosphere. Also, other
issues involving resolution dependence will be briefly discussed.

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