Abstract:
A variety of general circulation model experiments are performed to
investigate the influence of seasonality and topography on the strength of baroclinic
eddies in the Martian atmosphere. Three different models are used: a full physics
model, a simplified physics model, and a zonally symmetric simplified physics
model. All three models are sigma coordinate, finite difference global atmospheric
circulation models that have been adapted to the Martian regime. The full physics
model has previously been tested extensively by researchers at the NASA Ames
Research Center. The simplified physics model replaces many of the atmospheric
physics routines with simple parameterizations; most importantly, the radiation
code is replaced by Newtonian cooling. A Newtonian cooling code with a radiative
time constant that varies in height and latitude produces superior results to one with
a radiative time constant that is the same everywhere throughout the atmosphere.
It is found that baroclinic eddy activity is extremely sensitive to the mean
meridional temperature gradient in the simplified model. A power law fit gives an
exponent of approximately six. The baroclinic eddy activity is also sensitive to the
maximum growth rate in the Eady model of baroclinic activity. This is due to the
close connection between the meridional temperature gradient and the maximum
growth rate. Baroclinic adjustment theory, which predicts how baroclinic eddies
will react to changes in the mean circulation, does not appear to be valid in the
Martian regime, according to the simplified model. This finding may be related to
the differences in the relative strengths of the baroclinic eddies and the mean
circulation on Earth and Mars.
The simplified model indicates that seasonality is more important than
topography in creating stronger eddies in the northern hemisphere winter than in
the southern hemisphere winter. However, the effects of topography in the
simplified model may not be adequately matching the effects of topography in the
full physics model, particularly in the southern hemisphere.
