3-DIMENSIONAL NUMERICAL-SIMULATION OF MARTIAN GLOBAL DUST STORMS

We present results from the first numerical simulations of simultaneou sly evolving three-dimensional thermal, dynamical, and radiatively act ive suspended dust fields in the Martian atmosphere. Simulations of so uthern summer dust storms (arising from a prescribed southern subtropi cal surface dust source) conducted with a Mars general circulation mod el (GCM) illustrate the important role of dust transport by atmospheri c eddies. Both traveling and stationary eddies contribute to dust tran sport to high latitudes in both hemispheres. These hemispheric differe nces arise from seasonal and topographic effects. Transport into the s outh polar regions is accomplished primarily by thermally and topograp hically forced standing eddies. Both traveling and stationary eddies t ransport dust to middle and high northern (winter) latitudes. Atmosphe ric wave motions are affected by the developing storms. Thermal tidal amplitudes increase at storm onset, with the calculated pressure respo nse at a model grid point corresponding to the location of the Viking Lander 1 site in good agreement with observations. In qualitative agre ement with observations, winter hemisphere baroclinic waves weaken dur ing the early stages of the storm, but as the storm wanes, amplitudes of these waves increase. A slowly westward propagating (9 degrees of l ongitude per sol) zonal wavenumber one feature in the temperature and geopotential fields at middle northern latitudes amplifies rapidly dur ing the initial sols (Martian solar days) of the simulated storms. Thi s feature is suggestive of the observed north polar warming which occu rred during the 1977B global dust storm, but the simulations produce a much weaker polar warming (similar to 10 K at 0.5 mbar) than was obse rved (40-50 K). The globally integrated CO2 condensation rate decrease s by 15-20% during the simulated dust storm onset and would likely be decreased more if a stronger polar warming were produced. During the i nitial stages of the simulated storms, surface stress values in the so uthern subtropics intensify due primarily to the intensification of th e Hadley circulation and thermally driven tides. This supports the hyp othesis that these components of the general circulation contribute st rong positive feedbacks to the developing storms.