Thermal tides in an assimilation of three years of Thermal Emission Spectromenter data from Mars Global Surveyor

Introduction. Thermal tides are particularly prominent in the Mars atmosphere with the result that temperature and wind fields have a strong dependence on local solar time (LT). Tides include westward propagating migrating (sun-synchronous) waves driven in response to solar heating and additional nonmigrating waves resulting from zonal variations in the thermotidal forcing. Zonal modulation of forcing can arise from longitudinal variations of the boundary (topography and surface thermal inertia) and radiatively active aerosols (dust and water ice clouds). Nonmigrating tides appear as diurnally varying upslope/ downslope circulations within the near-surface boundary layer that, like their migrating counterparts, are also able to propagate vertically to aerobraking altitudes in the lower thermosphere. The Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) has yielded atmospheric temperature profiles with unprecedented latitude and longitude coverage that has provided the basis for characterizing the seasonal evolution of tides and stationary waves [1]. However, the twice-daily observations (2 am and 2 pm LT) are insufficient to unambiguously resolve the sunsynchronous tides. Recently the technique of data assimilation has been sufficiently developed for Mars to yield a dynamically consistent set of thermal and dynamic fields suitable for detailed investigations of various aspects of the martian circulations system [2,3,4,5]. We will refer to this data set an the TES Reanalysis, which represents the current best estimate of the evolving state of the martian atmosphere during the MGS mission. The assimilated thermal and dynamical fields provide a means of assessing circulation variability and transport capability reflecting the variability of the actual Mars atmosphere