TRANSPORT TIMESCALES IN THE MARTIAN ATMOSPHERE - GENERAL-CIRCULATION MODEL SIMULATIONS

Simulations with a Mars general circulation model (GCM) are used to pe rform idealized tracer transport experiments, which are analyzed to yi eld estimates of eddy mixing coefficients as well as ''stratospheric'' ventilation timescales for the zonal-mean circulation. The results in dicate that relatively moderate values of the vertical eddy mixing coe fficient, K-z similar to N 20-100 m(2)/s, may be most appropriate for the 10 to 45-km altitude region of the Martian atmosphere. Under dusty solstice conditions, somewhat stronger eddy mixing is present, but th e transport is dominated by advection by the mean meridional circulati on which acts to ventilate the atmosphere above 1 mbar (similar to 20 km) in only about 7 days. Such a mean circulation has transport effect s which are roughly comparable to those produced by eddy mixing with a much larger K-z value, similar to 1500 m(2)/s. In contrast, the compu ted mean ventilation timescale for a nondusty equinox circulation is a pproximately 180 days. In this case, vertical eddy mixing is the domin ant transport process. In an intermediate nondusty solstice case, the effects of mean advection and eddy mixing are of comparable importance . An estimate of a mean ventilation timescale from the GCM for a late northern winter seasonal date (similar to 45 days) is in very good agr eement with the value of similar to 38 days recently inferred from Mar iner 9 infrared imaging spectrometer (IRIS) data [Santee and Crisp, 19 95].