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A.V. Rodin (Space Research Institute, Moscow), R.J. Wilson (GFDL/NOAA)
The current Mars climate is characterized by two distinct seasons associated with the strong orbital modulation of insolation; a relatively warm, dusty, perihelion season characterized by episodic dust storms, and a stable, cold, dust-free but cloudy aphelion season. During the aphelion season, the relatively large global water column and cool tropical temperatures imply a particularly low (\lesssim 10 km) level of saturation. The ascending branch of the global Hadley cell in the northern tropics (0-30\circ NH) forms a prominent cloud belt that persists throughout the aphelion season from Ls\approx 60\circ and quickly decays at Ls\approx 145\circ. This season has been simulated by the GFDL Mars GCM which incorporates water ice microphysics, the radiative effects of dust and water ice clouds, and surface and subsurface sources and sinks of water. GCM simulations result in the formation of a tropical cloud system very similar to that observed by Mars Global Surveyor. The simulations suggest that dust may be confined to low altitudes in the NH due to water ice condensation on dust nuclei and the asymmetry of the Hadley cell. This, in turn, may lead to a stronger seasonal modulation of global temperature. We propose that the decrease in the water vapor supply to the tropics at Ls\approx 145\circ and the gradually increasing insolation leads to the disintegration of the tropical cloud system which may permit enhanced dust loading followed by a transition to the perihelion season climate. The transition between the two seasons may be accompanied by a short-term warming and slow (\approx 15-30 days) oscillations associated with the thermal feedback between the clouds and the Hadley circulation. The GCM simulations also suggest that the cloud system may contribute to the interhemispheric transport of water in the aphelion season despite the relatively low mass of the cloud system. This work is supported by grants from NASA's Planetary Atmospheres and JURRISS programs.
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