Recorded presentation14.6
A new approach for large-scale tropical convective dynamics based on asymptotic nondivergence
Jun-Ichi Yano, CNRM, Toulouse, France, Toulouse, France; and M. Bonazolla, S. Mulet, K. Delayen, S. Hagas, and C. Zhang
The large-scale tropical atmospheric dynamics are often considered to
be dominated by divergent flows associated with moist convection. No
balance condition exists in the large-scale tropical atmosphere, but
they are reduced to a set of linear equatorial waves in dry limit to
a good approximation. Various large-scale convective coherencies,
among them, the best known example would be the so-called
Madden-Julian oscillation, are considered as under a consequence of
coupling between wave dynamics and deep moist convection.
However, a systematic scale analysis indicates there is an alternative
possibility: a system dictated by a vorticity conservation law with
nondivergence to a leading-order approximation. Such an alternative
regime is identified at a scale smaller than the regime for the
convectively-coupled linear equatorial waves only by factor of three:
the latter is identified at the horizontal scale of 3000km, where as
the former at 1000km. The result reflects a fundamental subtlety in
the tropical scale analysis due to a strong sensitivity of a
nondimensional beta parameter on the horizontal scale.
Probably, the most surprising aspect of this alternative regime is
asymptotic nondivergence: i.e., the tropical large-scale dynamics is
dominated more by the vorticity than the divergence. This tendency is
systematically analyzed by using the TOGA-COARE LSA gridded data set.
It is found that the ratio of the root-mean square divergence for the
transient component to that of the vorticity is the smallest for the
scales of 20-80 days and 1500 km, indicating that the Madden-Julian
oscillation is more dominated by vorticity than divergence. The RMS
ratio goes down close to 0.2 at the Madden-Julian scale. At the
synoptic scale of a day and 1000 km, the RMS ratio is larger with a
value closer to 0.3.
This relatively weak divergence poses, however, an irony when the same
analysis is theoretically repeated both for free and forced linear
equatorial waves with varying wavenumbers and frequencies. Rather
surprisingly, the corresponding RMS ratio between the divergence and
the vorticity is much smaller, and less than 0.1 everywhere, for free
Rossby waves and also for free Kelvin waves in planetary scale limits.
The same conclusion is obtained for the forced waves except for narrow
ranges where a "resonance" of inertial-gravity waves with forcing
occurs. Thus, the observed RMS ratio is not consistent with the linear
wave theories.
The second important aspect of the alternative regime is that the
system can be described by the conservation of the absolute vorticity
in a self-contained manner to the leading order without effects of
divergence. In order to verify this point, a systematic vorticity
budget analysis is performed with use of the global NCEP analysis
data. The analysis overall confirm the scale analysis, but it also
demonstrates a non-negligible contribution of transient eddies in the
budget.
The asymptotic nature of this dynamical regime must be emphasized. It
is argued that the divergencd is negligible to the leading-order, but
it does not at all mean that the divergence is totally negligible. On
the contrary, the catalytic effect of a weak divergence is indeed
taken into account as a higher-order effect. More precisely, by
introducing a two time-scale description under an asymptotic
expansion, a weak convective feedback to the vorticity dynamics is
taken into account as a slow process.
References:
Delayen, K., and J. I. Yano, 2009:
Is Asymptotic Nondivergence of
The Large--Scale Tropical Atmosphere Consistent with
Equatorial Wave Theories?
Submitted to Tellus.
Yano, J. I., and M. Bonazzola, 2009:
Scale analysis for large-scale tropical atmospheric dynamics.
J. Atmos. Sci., 66, 159--172.
Yano, J. I., and S. Mulet, and M. Bonazzola, 2009:
Tropical Large-Scale Circulations: Asymptotically Nondivergent?
accepted to Tellus.
Session 14, Interactions between Tropical Convection and the Large Scale Circulation III
Friday, 12 June 2009, 10:20 AM-12:20 PM, Pinnacle BC
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