| Two aspects of the dust cycle on Mars are examined: the seasonal variation of dust aerosols in the atmosphere as observed by spacecraft and dust lifting by high wind stress at the south pole during late spring employing a specially developed mesoscale atmospheric model. Reanalysis of Viking mission optical depth measurements shows that the visible to infrared ratio of total extinction opacity varies with season, and is due to seasonally varying water ice haze. The Martian atmosphere is clearer of dust, especially during northern spring and summer, than previously thought. Water ice hazes can provide roughly 50% of the total visible opacity in these seasons, and that they represent only 1--5% of the total water column. Next, the conversion for use on Mars of a terrestrial mesoscale atmospheric model (the Mars MM5) is presented and described. Validation of the Mars MM5 is conducted by comparison with a general circulation model on scales of a few hundred kilometers and with Martian surface landers (Viking Lander 1, Viking Lander 2, and Mars Pathfinder) on scales of a few kilometers, and in both cases there is good agreement in the meteorological variables of temperature, pressure, and wind. Tides are found to be at least as important as slopes in generating the diurnal cycle of winds at the lander sites, in contrast to previous one-dimensional studies. Finally, assuming that dust injection is related to the movement of sand-sized grains or aggregates, the Mars MM5 predicts wind stresses of sufficient strength to initiate movement of sand-sized particles, and hence dust lifting, during late southern spring in the south polar region. It is found that the direct cap edge thermal contrast provides the primary drive for high surface wind stresses at the cap edge at this season while sublimation flow is not found to be particularly important. Comparison between simulations, in which dust is injected when wind stresses are high and those with inactive dust injection, show no signs of consistent feedback due to dust clouds on the surface wind stress fields during the late spring season examined here. |
