Geoscience Reference
In-Depth Information
9
Numerical Simulation (DNS, LES) of Geophysical Laboratory
Experiments: Quasi-Biennial Oscillation (QBO) Analogue and
Simulations Toward Madden-Julian Oscillation (MJO) Analogue
Nils P. Wedi
9.1. INTRODUCTION
the finding in
Zagar et al.
[2005] that convectively coupled
equatorial waves
1
match a dominant portion of the hori-
zontal structure of statistical deviations from a linear wave
model that is employed for optimizing the multivariate use
of observations in the ECMWF global data assimilation
for numerical weather prediction (NWP). However, low-
frequency planetary-scale waves in the tropics, with rela-
tively small meridional-to-zonal wavelength aspect ratio,
can be described by the same barotropic vorticity equation
as obtained by
Charney
[1963] for synoptic-scale motions
[
Maicun
, 1987].
The prevalent mix of Rossby, Kelvin, and gravity
waves in the equatorial atmosphere propagate horizon-
tally as well as vertically and substantially influence strato-
spheric motions and subsequently the global atmospheric
circulation. Multiscale convective heating, extratropical
wavedriving, and orographic forcings provide important
sources of small-scale (yet influential) vertically propagat-
ing gravity waves [
Fritts and Alexander
, 2003;
Alexander
,
2010]. A recent review of the current theoretical under-
standing of stratospheric equatorial dynamics including
the QBO can be found in the work of
Gray
[2010].
In this chapter the direct numerical simulation (DNS)
of the laboratory analogue of the QBO and the large-
eddy simulation (LES) of a proposed laboratory analogue
of MJO-like tropical dynamics serve as examples of a
deeper underlying concept. By using the same numerical
apparatus, it is possible to show the formation of coher-
ent spatiotemporal structures similar to the ones observed
in the tropical atmosphere: Either by vertical oscillations
It is human nature to seek and be fascinated by
the self-organization of coherent spatiotemporal struc-
tures, common in many disciplines such as meteorology,
astrophysics, biology, human sciences, and economy
[
Kondepudi and Prigogine
, 1998]. Among many other
interesting phenomena, the equatorial atmosphere offers
two distinct intraseasonal (ir)regularities, the quasi-
biennial oscillation (QBO) and the Madden-Julian oscil-
lation (MJO). Both phenomena provide intriguing clues
into some of the fundamental mechanisms underlying
Earth's climate system and thus provide excellent topics
for laboratory-scale experimentation as well as associated
numerical simulations.
Previous chapters considered specific aspects of baro-
clinic and barotropic flows that can be reproduced in
laboratory studies and that are ubiquitous in global
atmospheric flows. In this chapter, we consider two con-
spicuous phenomena that are specific to the equatorial
zone of rotating planets, where the quasi-horizontal “bal-
ance” model obtained from geostrophic theory does not
generally apply [
Saujani and Shepherd
, 2006]. Instead,
planetary scales
L
3000 km in the tropics are com-
monly viewed as dominated by linear waves, described by
the linear shallow-water theory with nonlinear advective
terms neglected, with characteristic time scale signifi-
cantly shorter than that of synoptic-scale systems [
Mat-
suno
, 1966;
Gill
, 1980;
Wheeler and Kiladis
, 1999;
Yano
and Bonazzola
, 2009]. Consistently, a substantial part of
the observed large-scale tropical variability is explained
by linear equatorial wave motions. This is illustrated by
1
After
Wheeler and Kiladis
[1999], equatorial wave patterns in the
spectral analysis of satellite-observed data records have been termed
“convectively coupled equatorial waves.”
Numerical Aspects Section, European Centre for Medium-
Range Weather Forecasts, Reading, United Kingdom.
