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R.J. Wilson (GFDL/OAR/NOAA), M.D. Smith (NASA GSFC), B.J. Conrath ( Cornell U.)
The recent observations by Mars Global Surveyor of a planet-encircling dust storm that developed in July of this year provide an unprecedented opportunity to investigate how small dust storms may evolve into major dust storms. It is thought that the strong coupling between the radiative heating due to the absorption of solar radiation by dust and the resulting atmospheric circulation can provide an explosive mechanism for storm growth. A critical aspect of investigating such mechanisms is the assessment of the evolving thermal forcing due the aerosol. Observations of the two 1977 dust storms by the Viking orbiters indicated large increases in the diurnal range of midlevel (~25 km) temperatures associated with dust storm events. Thermal tides represent the global-scale thermal and dynamical response of the atmosphere to the diurnal cycle of solar heating. Consideration of tides provides a means of relating thermal forcing to observed temperatures. MGS TES temperatures are restricted to morning (0200) and afternoon (1400) local solar time, thereby compromising estimates of the true amplitude of the diurnal tide. By contrast, temperatures derived from the 15 micron channel (T15) of the Viking IRTM have relatively broad local time coverage. In addition, the Viking landers have provided a record of the semidiurnal surface pressure tide, which is closely related to globally integrated dust heating. We use the GFDL Mars general circulation model (MGCM) to aid in the estimation of tides using TES morning and afternoon temperatures and compare these with the Viking dust storm tides. The model employs a dust transport scheme that enables the calculation of solar and IR heating rates based on the evolving aerosol distribution. The comparison of observed and simulated thermal tides provides the basis for the assessment of both the formulation of thermotidal forcing in the MGCM simulation and the resulting dynamical response. We can then examine the influence of the circulation on the evolution of the simulated aerosol fields and investigate the role of radiative feedback for the growth of dust storms.
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