Other
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p51e..01h&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P51E-01
Other
[0321] Atmospheric Composition And Structure / Cloud/Radiation Interaction, [3346] Atmospheric Processes / Planetary Meteorology, [5405] Planetary Sciences: Solid Surface Planets / Atmospheres, [5445] Planetary Sciences: Solid Surface Planets / Meteorology
Scientific paper
Water vapor in the Martian atmosphere varies in space and time. These variations are the result of the complex interplay of atmospheric transport, cloud microphysical processes, and exchange with surface and subsurface reservoirs. General circulation models (GCMs) can help interpret the relative contributions of these processes if they successfully simulate the observed behavior. This talk focuses on the efforts of the Ames Mars GCM group at simulating the present seasonal water cycle. The ultimate goal of these efforts is to identify the nature and distribution of surface sources and sinks. An obvious source of atmospheric water is the north polar residual cap (NPRC), which is a vast deposit of water ice that is exposed during summer. The question we seek to answer with the model is: Are other sources of water needed to explain the observations? Previous modeling studies have shown that most of the main features of the observed water cycle can be explained without the need for additional sources and indeed, we find similar results. However, there are at least two elements of these modeling studies that render this conclusion tentative. The first is the representation of the NPRC itself. The models generally assume that the NPRC is a continuous ice sheet that exists everywhere poleward of ~ 80N (depending on resolution). In reality, the NPRC is not continuous as there are outliers, longitudinal variations, and sizeable dark lanes interspersed throughout. Thus, the models do not accurately represent the area of exposed ice. The second concern with the models is how they treat clouds. Clouds, though a minor reservoir for water, can have a significant effect on the meridional transport and overall abundance of water in the atmosphere. Yet, as for Earth, they are very difficult to model for two reasons: cloud microphysics is complicated, and clouds are radiatively active. The Ames GCM now includes a non-uniform representation of the NPRC, a fairly sophisticated cloud microphysics package, and the option to allow clouds to interact with solar and infrared radiation. Not surprisingly the non-uniform representation of the NPRC tends to dry out the water cycle, though we need to carefully compare our predicted ice temperatures with observations to have confidence in this result. Our cloud microphysics scheme predicts particle sizes somewhat larger than previous models, particularly for low level polar clouds, and this too tends to dry out the water cycle though only by modest amounts. However, the biggest impact on the simulated water cycle occurs when the clouds are radiatively active. In these simulations the water cycle dries out by more than a factor of two. Thus, all three of these model improvements lead to a drier water cycle. By far the most important of these is the radiative effects of clouds. There are several reasons radiatively active clouds dry out the water cycle, but the main one is the formation of optically thick low-level clouds over the NPRC that increase reflected sunlight, cool the surface, and reduce sublimation. Other GCMs get similar results so this effect is not model dependent. Since observations do not show optically thick clouds over the NPRC during summer, it is clear that the models are missing some process. Until this issue is resolved, we cannot yet answer the question posed above.
Haberle Robert M.
NASA/Ames Mars Circulation Modeling Group General
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