ALBERT

Description: We present simulations of the interannual variability of martian global dust storms (GDSs) with a simplified low-order model (LOM) of the general circulation. The simplified model allows one to conduct computationally fast long-term simulations of the martian climate system. The LOM is constructed by Galerkin projection of a 2D (zonally averaged) general circulation model (GCM) onto a truncated set of basis functions. The resulting LOM consists of 12 coupled nonlinear ordinary differential equations describing atmospheric dynamics and dust transport within the Hadley cell. The forcing of the model is described by simplified physics based on Newtonian cooling and Rayleigh friction. The atmosphere and surface are coupled: atmospheric heating depends on the dustiness of the atmosphere, and the surface dust source depends on the strength of the atmospheric winds. Parameters of the model are tuned to fit the output of the NASA AMES GCM and the fit is generally very good. Interannual variability of GDSs is possible in the IBM, but only when stochastic forcing is added to the model. The stochastic forcing could be provided by transient weather systems or some surface process such as redistribution of the sand particles in storm generating zones on the surface. The results are sensitive to the value of the saltation threshold, which hints at a possible feedback between saltation threshold and dust storm activity. According to this hypothesis, erodable material builds up its a result of a local process, whose effect is to lower the saltation threshold until a GDS occurs. The saltation threshold adjusts its value so that dust storms are barely able to occur.

Description: The collision of Comet Shoemaker-Levy 9 (SL 9) with Jupiter created crescent-shaped ejecta patterns around impact sites. Although the observed impact plumes rose through a similar height of approx.3000 km, the radii of the created ejects patterns differ from impact to impact and generally are larger for larger impacts. The azimuthal angle of the symmetry axis of the ejects pattern is larger than that predicted by the models of oblique impacts, due to the action of the Coriolis force that rotates ejecta patterns counterclockwise from the south. We study the formation of ejects patterns using a simple model of ballistic plume above a rotating plane. The ejected particles follow ballistic trajectories and slide horizontally for about an hour after reentry into the jovian atmosphere. The lateral expansion of the plume is stopped by the friction force, which is assumed to be proportional to the square of the horizontal velocity. Two different mass-velocity distributions used in the simulations produce qualitatively similar results. The simulated ejecta patterns fit very well the "crescents" observed at the impact sites. The sizes and azimuthal angles of symmetry axis of ejects patterns depend on a parameter L, which has dimension of length and is related to the mass of the fragment. Thus more massive impacts produce larger ejects patterns that are rotated through a wider angle.