Atmospheric mesoscale modeling of water and clouds during northern summer on Mars
Daniel Tyler, Jeffrey R. Barnes
Icarus (New York, N.Y. 1962)
For a key season in the annual water cycle (L s $ 120°) a mesoscale model is used to study atmospheric water vapor and water ice clouds in the northern polar region of Mars. Model results at high-resolution (15 km) allow the examination of various mesoscale aspects of the circulation in this complex (topography, albedo and thermal inertia) region. A simple cloud scheme is used, where only the mean cloud particle size is carried, and nucleation is not explicitly treated. For this study, new
... resolution maps of albedo and thermal inertia were developed (poleward of 60°N), and model ground temperatures are in good agreement with observations at high resolution, typically within $5 K of TES (for ice and non-ice locations at AM and PM times of day). Diurnal mean sublimation rates are greatest along the edges of the polar dome and the largest outliers ($25-50 lm/sol). This is a consequence of widespread stability (atmospheric inversion) over the cold interiors of the largest ice surfaces, as well as strong ventilating winds that are modeled around the polar dome with sufficient spatial resolution. The structure of high latitude atmospheric water vapor is complex, especially so near Phoenix. Dynamically, two factors are responsible: (1) the transient circulations that form in the baroclinic zone around the polar dome and (2) a "storm zone" that forms on the poleward slopes of Alba Patera where there is additional transient activity that has a sizeable effect on the Phoenix region. This "storm zone" forms because of a rapidly evolving aspect of the regional circulation, and it plays a key role in the seasonally recurring annular cloud (that is simulated in this study). Also simulated are observations made during the Phoenix mission that seem to be dynamically related to the appearance of the annular cloud. Together this may signify a seasonal transition in the region. To simulate realistic clouds over the polar region (compared with opacity observations and imagery), a sufficiently realistic circulation appears to be important, and relatively high spatial resolution is needed for this. If a low-resolution run (135 km, no nests) is compared to a highresolution run (two levels of nesting to 15 km in the polar region), we find that the high-resolution case produces ten times less cloud ice over the most polar latitudes. The activation of the first nest (45 km) produces a sufficiently realistic circulation, such that excess vapor and cloud ice are readily ventilated equatorward from polar latitudes. A more sophisticated cloud scheme might serve to reduce the sensitivity seen in this study. However, sufficient spatial resolution is what causes the circulation to become realistic, and in this regard microphysics is not involved.