Atmospheric waves are another major contributor to Mars' atmospheric circulation. Stationary waves, also called Rossby waves, remain fixed in the rotating reference frame of the planet and are associated with strong eastward jets (the jet stream is a familiar example). They propagate vertically in response to topographic variations or from heating over thermal continents. They tend to occur at high latitudes because of the strength of the Coriolis force and, while strongest in the winter hemisphere, occur at all seasons. On Mars, they influence the stability of the atmosphere and are a major contributor to the distribution of heat from the equatorial to the polar regions. MGS's TES and Radio Occultation investigations were the first to unambiguously detect stationary waves in the martian atmosphere (Banfield et al., 2000, 2003; Hinson et al., 2001, 2003; Fukuhara and Imamura, 2005). Radio occultation results suggest that stationary waves dominate at altitudes below ~75 km (Cahoy et al., 2006).
Traveling planetary waves are produced by temperature and pressure (baroclinic) variations and are commonly associated with weather fronts. Clouds associated with traveling waves were detected in Mariner 9 and Viking data (Conrath, 1981; Murphy et al., 1990), but the most detailed view of traveling waves has been obtained from MGS analysis (Hinson and Wilson, 2002; Wilson et al., 2002). Banfield et al. (2004) analyzed two years of MGS TES data to determine that traveling waves are strongest in late northern fall and early northern winter, with much weaker waves detected in the southern fall and winter seasons. They also found strong annual repeatability in traveling waves.
The atmospheric circulation is modeled using Mars Global Circulation Models (MGCMs), which grew out of terrestrial GCMs beginning in 1969. MGCMs primarily model the circulation occurring within the lower atmosphere, although some have been extended to the middle and upper atmosphere. MGCMs parameterize the physical processes affecting atmospheric circulation and include contributions from Hadley circulation, Coriolis effect, atmospheric dust, radiative heating and cooling, clouds, convection, turbulence, waves, drag forces from interaction with surface roughness (i.e., the planetary boundary layer), and the seasonal condensation/ sublimation flow (Read and Lewis, 2004). The four major MGCMs have been developed at NASA Ames Research Center (Pollack et al., 1990, 1993; Barnes et al., 1993, 1996; Haberle et al., 1993; Murphy et al., 1995; Joshi et al., 1997), the French Laboratoire de Météorologie Dynamique (Hourdin et al., 1995; Forget et al., 1999), Oxford University (Joshi etal., 1995;Collins etal., 1996; Lewis etal., 1997; Newman et al., 2002a, b), and Princeton University (Wilson, 1997; Richardson and Wilson, 2002; Richardson et al., 2002). These MGCMs have become increasingly robust and successful in modeling the observed martian atmospheric circulation.
GCMs provide insights into the global circulation patterns but computational limitations prevent them from providing much detail. In recent years mesoscale models have been developed which provide detailed studies of small-scale processes on a regional scale (Rafkin et al., 2001; Toigo and Richardson, 2002). Mesoscale models are particularly useful when investigating processes such as dust lifting (Toigo et al., 2003) and conditions at landing sites (Tyler et al, 2002; Kass et al., 2003; Toigo and Richardson, 2003).
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