Modelling of the combined late-winter ice cap edge and slope winds in Mars' Hellas and Argyre regions

Towards the end of southern hemisphere winter (L-s approximate to 180 degrees) the Martian southern polar cap extends equatorward to 40 degrees S and covers at least, the southern slopes of the Hellas and Argyre impact basins. Subsequently, during retreat of the seasonal ice cap, varying configurations of ice coverage on these slopes occur. Since both sloping topography and ice-edge effects can independently drive mesoscale circulations, the superposition of these two processes may then generate interesting wind patterns. A set of numerical experiments has been performed with the University of Helsinki 2-D Mars Mesoscale Circulation Model (MMCM) in order to study the characteristics of circulations driven by these combined forcings. A model-centre latitude of 57 degrees S and a slope angle of 0.6 degrees, both representative of Hellas' southern slope, are used. When compared with the winds arising in the ice-free slope case, ice coverage in the upper extent of the slope results in diminished upslope (daytime) winds, while the down-slope (nighttime) flow is enhanced. Ice coverage in the lower section of the slope in turn causes enhanced upslope (daytime) and attenuated downslope (nocturnal) flows. This arises due to the daytime off-ice near-surface flow induced by the thermal contrast at the ice cap edge. The surface winds are persistently downslope over a fully ice-covered slope. Inclusion of atmospheric dust ( tau = 0.3) reduces the ice-edge forcing. In comparison with the dust-free situation, the resulting circulation is almost unchanged in the case of ice-covered upper part of the slope, in the opposite case the daytime flow is attenuated and the nocturnal downslope flow enhanced. When the entire slope is ice-covered, the flow is amplified due to the increased direct atmospheric heating. Inclusion of a large scale circulation component (7 m/s southerly wind) in conjunction with an ice-covered slope top results in the generation of a downslope windstorm (fohn, or bora-type of event) with near surface winds exceeding 30 m/s. Winds of this magnitude, not realised in any of the other experiments, approach speeds deemed capable of lifting dust from the surface. (C) 1999 Elsevier Science Ltd. All rights reserved.