Atmospheric mesoscale modeling of water and clouds during northern summer on Mars

Daniel Tyler, Jeffrey R. Barnes
2014 Icarus (New York, N.Y. 1962)  
47 48 The North Polar Layered Deposits and the outlier ices are believed to be the 49 primary sources of atmospheric water in the present-day water cycle on Mars. 50 These permanent (residual) ices play a key role in the annual water cycle; hereafter 51 they will be collectively referred to as the North Polar Residual Cap (NPRC). In the 52 absence of seasonal CO2 frost deposits, the NPRC region is extremely complex, with 53 dramatic and fine-scale transitions between bright and dark surfaces
more » ... nd dark surfaces (ices and/or 54 regolith/fines). The topography of the region itself is highly complex. Moreover, 55 since fine-scale changes in surface type lead to very sharp gradients in ground 56 temperature, strong smaller-scale circulations must have an important role in the 57 regional circulation. For any season, mesoscale models are powerful tools for 58 studying the various circulations in the NPRC region. When a mesoscale model is 59 carefully configured for the season of interest, high-resolution simulations will 60 provide a realistic depiction of circulations over a very wide range of scales. In an 61 analysis of the results from such simulations, subtle aspects of the water cycle can 62 be understood more completely, a primary and long-term goal of this work. 63 Global Climate Models (GCMs), which simulate the interactions over the 64 entire planet between atmospheric dynamics and microphysical water processes, 65 are themselves extremely important tools. GCMs have greatly benefited our 66 understanding of atmospheric circulations and the water cycle on Mars [e.g., Houben 67 et al., 1997; Richardson and Wilson, 2002; Montmessin et al., 2004; Bottger et al., 68 2005]. Regarding the present-day water cycle, GCM studies often consider two 69 unresolved issues: 1) whether the NPRC is gaining or losing ice mass, and 2) 70 whether the high latitude regolith is an important source/sink term in the annual 71 water cycle. In general, GCMs are capable of producing good agreement with the 72 observed water cycle. However, due to the fine-scale complexity of the polar region, 73 a typically configured GCM (with at best a spatial resolution of a few degrees, that is 74 subject to the "pole problem" -a meridionally stretched grid in polar latitudes that 75 requires non-physical filtering to maintain computational stability) simply cannot 76 resolve the smaller-scale circulations. These smaller-scale circulations may be very 77 important. When GCM results are compared with observations of summertime 78 clouds, discrepancies are typically seen. GCMs tend to simulate water ice clouds 79 that are too thick [Haberle et al., 2011; Madeleine et al., 2011 , Madeleine et al., 2012 . 80 It is possible that a large part of the reason for this is that GCMs cannot realistically 81 simulate the net effect of all the important subgrid circulations in the northern polar 82 summertime atmosphere. To study this specific issue, mesoscale modeling can be 83 used to refine our understanding, which will eventually lead to a more quantitative 84 study of the subtle scientific questions regarding the water cycle. 85 When mesoscale models are configured with multiple levels of nesting over a 86 location of interest, the circulation is simulated over a very large part of the planet, 87 and also for some specific region at very high resolution (often to ~5 km or better). 88 In this study, the specific season/region of interest is northern polar summertime 89 and the exposed NPRC ices. When configured on a polar stereographic projection, 90 model dynamics are not subject to the "pole problem", and non-physical filtering is 91 not required. Mesoscale models cannot be run like GCMs (where a number of years 92
doi:10.1016/j.icarus.2014.04.020 fatcat:sl47mmzvmnapfjvulr4m7mhu2a