DIAGNOSTIC CALCULATIONS OF THE CIRCULATION IN THE MARTIAN ATMOSPHERE

The circulation of the Martian atmosphere during late southern summer is derived from atmospheric temperature and dust distributions retriev ed from a subset of the Mariner 9 infrared interferometer spectrometer (IRIS) thermal emission spectra (L(S) = 343 degrees-348 degrees) (San tee and Crisp, 1993). Zonal-mean zonal winds are calculated by assumin g gradient wind balance and zero surface zonal wind. Both hemispheres have intense midlatitude westerly jets with velocities of 80-90 m/s ne ar 50 km; in the southern tropics the winds are easterly with velociti es of 40 m/s near 50 km. The net effect of the zonal-mean meridional c irculation and large-scale waves can be approximated by the diabatic c irculation, which is defined from the atmospheric thermal structure an d net radiative heating rates. The radiative transfer model described by Crisp (1990) and Santee (1993) is used to compute solar heating and thermal cooling rates from diurnal averages of the retrieved IRIS tem perature and dust distributions. At pressures below 4 mbar, there are large net radiative heating rates (up to 5 K/d) in the equatorial regi on and large net radiative cooling rates (up to 12 K/d) in the polar r egions. These net radiative heating rates are used in a diagnostic str eam function model which solves for the meridional and vertical compon ents of the diabatic circulation simultaneously. We find a two-cell ci rculation, with rising motion over the equator, poleward flow in both hemispheres, sinking motion over both polar regions, and return flow i n the lowest atmospheric levels. The maximum poleward velocity is 3 m/ s in the tropics at similar to 55 km altitude, and the maximum vertica l velocity is 2.5 cm/s downward over the north pole at similar to 60 k m altitude. If these large transport rates are sustained for an entire season, the Martian atmosphere above the l-mbar level is overturned i n about 38 days. This diabatic circulation is qualitatively similar to the terrestrial diabatic circulation at the comparable season, but is more vigorous.