Effects of the Mean Flow on the Properties of Martian Transient Eddies

A key question in atmospheric dynamics is the extent to which the climatology of transient eddy activity can be understood primarily as a function of the time mean flow, with minimal reference to the details of the diabatic processes that are the ultimate cause of the atmospheric circulation. We address this problem in the context of Mars by constructing an idealized general circulation model with physical parameterizations consisting of Newtonian thermal forcing and a Rayleigh drag surface boundary layer. The radiative equilibrium temperature field is constructed using an iterative technique (adopted from previous terrestrial studies) so that the idealized model’s three-dimensional time mean temperatures are similar to climatologies computed from segments of a Mars reanalysis (the Mars Analysis Correction Data Assimilation, MACDA) or the Geophysical Fluid Dynamics Laboratory (GFDL) Mars GCM.

Focusing on a northern hemisphere (NH) seasonal interval with strong eddy activity prior to winter solstice, we find that the idealized model can reproduce at least some aspects of the spatial structures of four MACDA eddy variance-covariance fields. The idealized model is also partially successful in simulating seasonal changes in the horizontal patterns of NH low-level eddy temperature and meridional wind variances. However, neither the absolute values nor the seasonal variations of eddy amplitudes are very realistic.

When the idealized model is run with a time mean temperature field like that of the GFDL Mars GCM, it reproduces an eddy zonal wavenumber bias found in the latter model. This finding indicates the significance of the mean flow as a control on the wavenumber distribution of transient eddies and implies that the GFDL Mars GCM’s simulation of eddies would be improved if its mean flow became more realistic.