Summary of Convectively Coupled Kelvin Waves in an Idealized Moist General Circulation Model.
by Frierson, which appears
in Journal of the Atmospheric Sciences.
Primary arguments:
- The speed of convectively coupled Kelvin waves can be tuned to
essentially any value within this idealized moist general circulation
model, by changing convection scheme parameters.
-
The speed is set by the gross moist stability, which is determined
internally by convection scheme parameters. When the gross moist
stability becomes negative, the Kelvin wave variability disappears,
and is replaced with variance in the tropical storm band.
Discussion:
In the Frierson 2007a
paper, we studied the effect of different convective scheme parameters on
the tropical general circulation within the idealized GCM from my
thesis.
In this paper, we study the structure of the tropical variability
within the model, and its sensitivity to the same convection scheme
parameter variations. The equatorial waves that we study in this paper
are particularly interesting because they are generally not well-simulated
by state-of-the-art GCMs, which often contain waves that are too fast and
too weak. Understanding the behavior of the equatorial waves within this
relatively simple framework, with a simplified physical package but full
vertical resolution, may help us interpret why GCMs simulate these waves
poorly.
In Frierson 2007a,
we show that changes in the zonally averaged circulation that occur with
convection scheme can be related to changes in the gross moist stability
of the atmosphere. In simple theories, the gross moist stability of the
atmosphere controls the phase speed of moist equatorial waves as well, as
this sets the effective static stability felt by the wave. Does it affect
wave speeds in this model? If so, does a zonally averaged gross moist
stability have any bearing on the wave speeds, or does the wave create its
own stability?
In this paper, we demonstrate that decreases in the zonal mean gross moist
stability are accompanied by decreases in the wavespeed and increases in
Kelvin wave variance. There are some caveats however, as other factors
that influence the phasing of the different components of the wave can
affect the phase speeds as well, and the gross moist stability created by
the wave is different than the zonal mean value as well. However, the zonal
mean GMS gives a good approximation to the wave speed.
As seen in the figure above (an unfiltered space-time diagram of precipitation
with time on the equator), the eastward-propagating Kelvin waves clearly
dominate the variability of precipitation within the tropics. However, when
parameter changes are made to eliminate the Kelvin waves from the model,
the zonal mean circulation stays remarkably similar. This indicates that the
Kelvin waves do not influence the zonally averaged circulation.
We've identified a similar sensitivity of convectively coupled waves to the
convective trigger has been identified in a full general circulation model
with realistic geography in the study of
Lin et al 2007.
In that model (the Seoul National University general circulation model),
the variance of all convectively coupled equatorial waves can be increased
and their speeds can be decreased by increasing the convective trigger.
As in the idealized GCM, this is associated with an increase in the
fraction of large-scale versus convective precipitation, and a decrease
in the gross moist stability of the atmosphere.
Full citation:
Frierson, D. M. W.
Convectively Coupled Kelvin Waves in an Idealized Moist General Circulation Model.
Journal of the Atmospheric Sciences, 64, 2076-2090,
2007.
The official journal link can be found
here.
A PDF download of the full paper can be found here.
This download is courtesy of the American Meteorological Society, who owns sole
rights to it.
The download is subject to copyright laws and statutes. For more
information, please visit the Allen Press/AMS Journals Online website.
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