The Martian Hadley circulation: Comparison of “viscous” model predictions to observations

The predictions of an analytical steady thermally forced viscous model for the zonally averaged global circulation of the Martian atmosphere are compared to observations of atmospheric temperatures and eolian feature directions. The temperature of the winter polar atmosphere is a sensitive indicator of the value of the mean eddy diffusivity v of the atmosphere. For global dust storm season, the observed large warming of the atmosphere over the winter (north) pole during the 1977b storm as well as the distribution and azimuths of Type I(b) dust streaks can be well reproduced if v = 107−108cm2sec−1. Observed northeasterly surface winds at high northern latitudes could be produced by plausible fluxes of CO2 from the northern polar cap. The thermal structure at southern latitudes during this storm is satisfactorily produced, but a major discrepancy exists between observations and predictions of the temperature structure at middle northern latitudes. By comparing these results with those from previous inviscid models of the zonal mean circulation on Mars, we suggest v > 107cm2sec−1 at heights above 30 km and v ⪡ 107cm2sec−1 at lower altitudes may yield a meridional circulation which produces the dynamical warming necessary to account for the observed thermal structure at 0.6 mbar at all latitudes. For late southern summer when the dust content of the atmosphere has decreased to levels characteristic of much of the Martian year, the lack of dynamical warming of the winter pole implies v ≤ 106cm2sec−1. In addition, the predicted surface wind azimuths at this season agree well with the observed azimuths of Type I(d) dust streaks, which are observed to form at this season. The observed latitudinal concentration of the streaks is not explained by the model and may require additional processes which have not been included.