DOWNSLOPE WINDSTORMS IN THE LEE OF RIDGES ON MARS

The semianalytical theory of downslope windstorms, originally developed to understand large increases in surface winds in the lee of major mountain chains on Earth, such as the famous windstorms in Boulder, Colorado, is applied to martian conditions. The focus of this work is to assess whether this theory can help us understand dark erosional wind streaks downstream of ridges on the martian surface, These streaks provide a well-documented example of dust raising on Mars, However, the formation of the streaks remains a mystery because atmospheric circulation models indicate surface friction velocities far lower than those required for the initiation of eolian activity. Downslope windstorms result from the interaction of topographically launched finite amplitude gravity waves with a level of zero wind, which vertically traps the waves, When this zero-wind level is at the proper height, large increases in downstream surface winds (up to a factor of 4) can be produced, During the martian night, downslope windstorms can occur in the lee of quite small ridges (heights greater than or equal to tens of meters) when a level of zero-wind is present in the lowest few kilometers of the atmosphere, Three independent models of the martian atmospheric circulation indeed show levels of zero wind in the form of wind reversals at the appropriate height and season and at the locations where the ridge-related dark streaks are concentrated. The strong upstream/downstream difference in surface winds produced by the downslope windstorm mechanism when a suitable wind reversal is present, rather than a particularly strong upstream wind, is what controls the occurrence of the ridge-related dark streaks, Downslope windstorms also appear to be potentially useful in understanding the build-up of dust in the atmosphere preceding the martian global dust storms. (C) 1995 Academic Press, Inc.