MMM SEMINAR NCAR
Laboratory for
internal gravity-wave dynamics: The numerical equivalent
to the
quasi-biennial oscillation (QBO) analogue
Nils P.Wedi
European Centre for Medium Range Weather Forecasts (ECMWF)
Abstract
The
quasi-biennial oscillation (QBO) represents the dominant variability in the
equatorial stratosphere [1]. It
exhibits a fundamental dynamical mechanism with challenging detail. Holton and
Lindzen [3,2] were among the first to present a conceptual model of the QBO
describing the oscillation as an interaction between the mean flow and
propagating waves. The principal mechanism of the QBO was demonstrated in the
laboratory experiment of Plumb and McEwan [4].
The laboratory analogue of the stratospheric equatorial oscillation
consists of a cylindrical annulus filled with density-stratified salty water,
forced at the lower (upper) boundary by an oscillating membrane. At
sufficiently large forcing amplitude the wave motion generates a longer period
zonal mean-flow oscillation. The laboratory experiment is often employed to
explain the basic mechanism of the atmospheric QBO.
We
have applied our numerical modeling framework developed in [5], that allows the
simulation of stratified flows with intricate geometric, time-dependent
boundary forcings, either at the top or at the bottom of the domain, in the
direct numerical simulation of this quasi-biennial oscillation (QBO) analogue.
A series of 2D and 3D simulations
demonstrate the ability to reproduce the laboratory results. We are able
to address a number of details, difficult to deduce from experimental evidence
alone. The numerical experiments identify the developing periodically reversing
mean flow pattern as an entirely wave-interaction driven phenomena, in contrast
to the original interpretation of a weakly-nonlinear damped wave problem.
Furthermore, we find an oscillation mechanism different to the one originally
described. The results not only enhance the confidence in the numerical
approach but further elevate the importance of
the laboratory setup in its fundamental similarity to the atmosphere,
while allowing the study of the principal atmospheric mechanisms and their
numerical realizability in a confined 'laboratory' environment.
[1]
Baldwin MP, Gray LJ, Dunkerton TJ, Hamilton K, Haynes PH, Randel WJ,
Holton
JR, Alexander MJ, Hirota I, Horinouchi T, Jones DBA, Kinnersley JS, Marquardt
C, Sato K, Takahashi M. The quasi-biennial oscillation. Reviews Geophys.
2001; 39(2):179-229.
[2]
Holton JR, Lindzen RS.An updated theory for the quasi-biennial cycle of the
tropical stratosphere. J. Atmos. Sc 1972; 29:1076-1079.
[3]
Lindzen RS, Holton JR.A theory of the quasi-biennial oscillation. J. Atmos.
Sci. 1968; 25:1095-1107.
[4]
Plumb RA, McEwan D. The instability of a forced standing wave in a viscous
stratified fluid: A laboratory analogue of the quasi-biennial oscillation.
J. Atmos. Sci 1978; 35:1827-1839.
[5]
Wedi NP, Smolarkiewicz PK. Extending Gal-Chen \& Somerville
terrain-following coordinate transformation on time-dependent curvilinear
boundaries. J. Comput. Phys. 2004; 193:1-20.
Thursday, July 8, 2004
Foothills Laboratory,
Building 2, Room 1022
3:30-4:30
Refreshments at 3:15