Transient Eddies in the Martian Atmosphere: Representation in Reanalysis and Idealized Modeling

Abstract

We present three projects related to transient eddies in the Martian atmosphere. First we survey eddy activity in the MACDA Mars reanalysis, emphasizing its spatial distribution and propagation. Band-pass-filtered variance and covariance fields indicate a longitude dependence of eddy amplitudes. Northern hemisphere eddy phenomena include a roughly wavenumber 3 pattern of low-level eddy meridional temperature transport evident before and after winter solstice. Southern hemisphere fields tend to have two local maxima, associated with topographic features. We also estimate spatially varying phase velocities of the eddies and find that eddy propagation follows topography, particularly at the surface.

The second project uses an idealized general circulation model (GCM) to investigate the extent to which Martian transient eddy properties are functions of the mean flow. The model can be forced to have mean flows based on MACDA or the GFDL realistic Mars GCM. Analyzing simulations with mean flows based on a segment of MACDA, we find that the

model is moderately successful in reproducing northern hemisphere spatial patterns (but not amplitudes) of four major eddy fields. Examining a smaller set of fields, we show that the model also partially reproduces seasonal variations of horizontal patterns of low-level eddy activity. Finally, using a mean flow based on the GFDL Mars GCM we find that the idealized model reproduces a bias seen in this more realistic GCM's eddy wavenumber distribution, thus indicating a source of the bias.

A very similar idealized GCM is also used in the third project, to study how transient eddies in an atmosphere with a relatively fixed thermal structure are affected by changes to the radiative damping time and planetary rotation rate. This project developed from an unsuccessful attempt to understand why Martian eddies are more regular than their terrestrial counterparts. As the radiative damping time is changed, simulations with the standard Mars rotation rate exhibit a regime transition in eddy properties while simulations with a doubled rotation rate show nonmonotonic eddy amplitude variations. The barotropic jets in the simulations also vary with radiative damping time, which inspired a preliminary investigation of the low-level jets in Mars reanalyses and realistic GCMs.