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[1] The Fifth-Generation Penn State/NCAR Mesoscale Model (MM5) has been adapted to study circulations in the Martian atmosphere. The NASA Ames Mars general circulation model (GCM) provides initial and boundary conditions. The meteorology of this Mars MM5 (the OSU MMM5) is compared with Pathfinder and Viking Lander 1 (VL1) data for late northern summer. The MMM5 uses an equator-crossing semiglobal polar stereographic mother domain, significantly reducing the boundary reflections inherent in Martian mesoscale simulations. Using two-way nests, simultaneous simulations of two regions are performed: (1) Chryse Planitia (Mars Pathfinder (MPF)/VL1) and (2) the central chasmas of Valles Marineris. Simulations are hydrostatic and dry. The MMM5 uses the same atmospheric radiation package as the GCM but a much enhanced nearsurface vertical resolution and a different Planetary Boundary Layer (PBL) scheme. Model topography, thermal inertia, and surface albedo maps have all been developed using the most recent data from the Mars Global Surveyor (MGS) Mars Orbiter Laser Altimeter (MOLA) and Thermal Emission Spectrometer (TES) experiments. The diurnal cycles of surface air temperature, surface pressure, and surface wind all show improvement in comparison with the GCM. For certain regions the local surface pressure tidal amplitudes are strongly dependent on the resolution of the model and/or the topography; VL1 and MPF are in such a region. The diurnal cycles of wind are complex, and for many locations the near-surface winds are dominated by slope flows of multiple scales. Comparison with data indicates that resolving these slope flows is very important for simulating the diurnal wind cycle. At specific locations these slope flows dramatically influence the diurnal surface pressure cycle.

[1] The Oregon State University Mars MM5 was used in a comprehensive highresolution study of northern polar summertime circulations. Three simulations (L s = 120, L s = 135, and L s = 150) characterize the changing circulation. The atmosphere is dry, and model dynamics are hydrostatic. A modified TES thermal inertia map provides a realistic simulation of the polar thermal environment. The highest-resolution nest (18 km) resolves complex flows near the cap; zonal-mean easterlies ($10 m/s) and zonal-mean katabatic winds ($5 m/s) near the surface are relatively steady during this study. Katabatic flows are shallow ($300 m); the easterlies are deeper ($1.5 km). Transient eddies are very important within the first scale height; they are excited by mechanisms, and at locations, that change dramatically during this short study period. At L s = 120 they form along the residual cap edge with a zonal wave number one structure, producing strong excursion winds (10-15 m/s) that blow consistently across the cap. By L s = 135, strong eddies are seen to form on the northern slopes of Alba Patera and Tharsis. These eddies are quite suggestive of the large annular cloud structures seen in Hubble Space Telescope and Mars Orbital Camera imagery at this location and season and can traverse the high latitudes to reach the residual cap before dissipating. Eddies in the earlier two simulations appear to be primarily excited by energetic flows near the surface. By L s = 150 an early fall polar jet causes strong winter-like baroclinic eddies to develop. The transient eddies found in this study are probably important in the water cycle of the northern residual cap.Citation: Tyler, D., Jr., and J. R. Barnes (2005), A mesoscale model study of summertime atmospheric circulations in the north polar region of Mars,

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[1] In late May of 2008, the NASA/JPL Phoenix spacecraft will touch down near its targeted landing site on Mars (68.2°N, 126.6°W). Entry, descent, and landing (EDL) occurs in the late afternoon ($1630 hours local solar time (LST)) during late northern spring (L s $ 78°). Using a mesoscale and a large-eddy simulation (LES) model, we have investigated the range of conditions that might be encountered in the lower atmosphere during EDL. High-resolution ($18 km) results from the Oregon State University Mars MM5 (OSU MMM5) are used to understand the hazards from the transient circulations prominent during this season. Poleward of $80°N these storms produce strong winds ($35 m s À1 ) near the ground; however, owing to the synoptic structure of these storms, and the deep convective mixed layer equatorward of the seasonal cap boundary during EDL, our modeling suggests the spacecraft would not be in winds stronger than $20 m s À1 at parachute separation. The storm-driven variability is much weaker at Phoenix latitudes than it is poleward of the seasonal cap edge (result from an extensive sensitivity study). The OSU MLES model is used to explicitly simulate the hazards of convection and atmospheric turbulence at very high resolution (100 m). This modeling suggests that an upper bound for the maximum expected horizontal-mean atmospheric turbulent kinetic energy (TKE) is $12 m 2 s À2 , seen $3 km above the ground at $1430 hours LST. TKE amplitudes are greatest when the horizontal mean wind is large (shear production) and/or the surface albedo is low (a lower albedo enhances buoyancy production, mimicking decreased atmospheric stability after a storm advects colder air into the region). LES simulations predict deep mixed layers ($6-7 km), $1.5 km deeper than the mesoscale model ($5 km). Mesoscale modeling suggests that the actual landing site differs meteorologically from other longitudes (larger-amplitude diurnal wind cycle), a consequence of the strong thermal circulations that are excited by the very large regional topography. The OSU MLES model was modified for this work to utilize time-and height-dependent geostrophic wind forcing (constructed from OSU MMM5 results). With this forcing, the OSU MLES provides a site-specific simulation, where the time/height variability of the horizontal mean LES wind field is in good agreement with the OSU MMM5. On the basis of some statistical analysis, we have good confidence that the ''fullspectrum'' wind field is within engineering guidelines for Phoenix EDL.