| Abstract: | Previous calculations of the surface wind stress required to raise dust on Mars are reconsidered and the threshold friction velocity is found to be about 2.0 m sec?1 with particles of 200–300 μm being the most easily lifted. With this friction velocity, the planetary resistance law yields a corresponding wind at the top of the Ekman layer of 60 m sec?1, and the logarithmic wind law yields a corresponding wind at the top of the Prandtl layer of 38 m sec?1. These speeds are somewhat lower than those used by previous investigators.Various mechanisms for producing such strong winds are examined and it is concluded that the general circulation, thermal effects of topography, mechanical effects of topography and dust devils are all capable of doing so.Dust storms associated with small-scale disturbances are found to be incapable of growth. A scaling analysis of the equations of horizontal motion and of hydrostatic balance shows that a dust cloud at least 10 km thick and several tens of km in radius can, by absorption of sunlight, generate temperature gradients that, in turn, produce winds capable of raising more dust. Thus, a feedback mechanism is suggested in which an initial dust cloud exceeding certain critical dimensions can grown to planetary size. The preference of large dust storms to occur at southern hemisphere summer solstice is attributed to the maximum of insolation at that time. It is suggested that the frequent origin in the Noachis-Hellas region may be due to orographic features of the right scale and to low height in that area. |
