Prof. Peter Read

Abstract:

We investigate the production of equatorial jets which demostrate strong local superrotation in an atmospheric general circulation model of Mars. These westerly jets are driven by diurnal thermal tides, and their strength is shown to be closely related to the amount of dust in the atmosphere. The superrotating jets are strongest near to equinox and under conditions of high atmospheric dust loading. If there is sufficient dust, in amounts corresponding to dust storm conditions, the westerly equatorial jets can occur at any time of year and reach speeds of over 40 m/s, peaking between 10 and 20 km altitude. For more moderate dust amounts, typical of background levels on Mars, the jets are still strong when the subsolar point is close to the equator and latitudinally symmetric tidal modes are forced. Strong easterly retrograde winds are also found high above the equator, and it is shown that the thermal tides play a major role in their formation. This process is especially relevant close to equinox when the cross-equatorial meridional circulation is weak.

Abstract:

A dust transport scheme has been developed for a general circulation model of the Martian atmosphere. This enables radiatively active dust transport, with the atmospheric state responding to changes in the dust distribution via atmospheric heating, as well as dust transport being determined by atmospheric conditions. The scheme includes dust lifting, advection by model winds, atmospheric mixing, and gravitational sedimentation. Parameterizations of lifting initiated by (1) near-surface wind stress and (2) convective vortices known as dust devils are considered. Two parameterizations are defined for each mechanism and are first investigated offline using data previously output from the non-dust- transporting model. The threshold-insensitive parameterizations predict some lifting over most regions, varying smoothly in space and time. The threshold-sensitive parameterizations predict lifting only during extreme atmospheric conditions (such as exceptionally strong winds), so lifting is rarer and more confined to specific regions and times. Wind stress lifting is predicted to peak during southern summer, largely between latitudes 15° and 35°S, with maxima also in regions of strong slope winds or thermal contrast flows. These areas are consistent with observed storm onset regions and dark streak surface features. Dust devil lifting is also predicted to peak during southern summer, with a moderate peak during northern summer. The greatest dust devil lifting occurs in early afternoon, particularly in the Noachis, Arcadia/Amazonis, Sirenum, and Thaumasia regions. Radiatively active dust transport experiments reveal strong positive feedbacks on lifting by near-surface wind stress and negative feedbacks on lifting by dust devils.