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 degre es) 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 configurati ons of ice coverage on these slopes occur. Since both sloping topography an d ice-edge effects can independently drive mesoscale circulations, the supe rposition of these two processes may then generate interesting wind pattern s. 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 rep resentative of Hellas' southern slope, are used. When compared with the win ds 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-slo pe (nighttime) flow is enhanced. Ice coverage in the lower section of the s lope in turn causes enhanced upslope (daytime) and attenuated downslope (no cturnal) flows. This arises due to the daytime off-ice near-surface flow in duced by the thermal contrast at the ice cap edge. The surface winds are pe rsistently downslope over a fully ice-covered slope. Inclusion of atmospher ic dust ( tau = 0.3) reduces the ice-edge forcing. In comparison with the d ust-free situation, the resulting circulation is almost unchanged in the ca se 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 ent ire 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.