Temperature inversions, thermal tides, and water ice clouds in the Martian tropics

[1] We report new results on the structure and dynamics of the tropical atmosphere of Mars derived from a combination of radio occultation measurements by Mars Global Surveyor and simulations by a Mars general circulation model (MGCM). Radio occultation experiments sounded the equatorial atmosphere at latitudes of 36°N to 30°S during midsummer of the Northern Hemisphere (Ls = 134°–162°), sampling the predawn thermal structure at a local time of ∼0412. Elevated temperature inversions are a conspicuous feature of these observations. They appear at pressures between 30 and 200 Pa, well above the surface, and their magnitude exceeds 6 K in 34% of the temperature profiles in this latitude band. The properties and spatial distribution of these elevated inversions are organized across the tropics on planetary scales. Inversions are strongest and occur most frequently above elevated terrain, achieving a peak magnitude of ∼30 K near Tharsis, and their altitude generally increases toward the south. According to MGCM simulations, which closely resemble the observations, these temperature inversions arise from zonally modulated thermal tides. The best simulation includes an interactive hydrologic cycle, which results in strong coupling between the thermal tides and radiatively active water ice clouds. Prominent clouds form in response to wave-induced adiabatic cooling and evolve in a pattern closely correlated with the thermal structure of the tides. The tides in turn are intensified by radiative forcing from the clouds. This tide-cloud coupling imposes strong diurnal modulation on the properties of clouds in the tropics.