A study of the thermal and dynamical structure of the martian lower atmosphere

Abstract

The scaling of the equations of motion in the lowest one or two scale heights of the martian atmosphere is discussed; it is shown that zonal motions should be of the order of 40 m/sec, diurnal motions of the order of 2 m/sec, and that non-linear interactions for both zonal and diurnal fields of motion are governed by a Rossby number calculated from the largest velocity, and equal to 0.17. The first consequence of this result is that seasonal and diurnal temperature changes can probably be computed with satisfactory precision without taking account of the advection and convection of heat by planetary-scale winds. Small-scale thermal convection is, however, important because temperature gradients computed for radiative transfer alone can be superadiabatic. Priestley's theory for the free atmosphere and Kraichnan's theory for the boundary layer are adopted with due regard to their possible errors. The upper boundary condition on the convective layer is shown to be approximate continuity of temperature. A complete calculation of the temperature profile in the lowest 30 km is made, including condensation in a simple cloud model, for three latitudes. It is concluded that the tropopause will be about 15 km in the tropics; that there is a radiative-convective boundary layer in the lowest kilometer above a thin convective boundary layer; that the diurnal changes are important to the mean profile as well as the time-dependent profile; that turbulent diffusivities may be 10 8 cm 2/sec or higher and therefore important to the planetary dynamics; that very large dust particles could be held in suspension during the daytime; that carbon dioxide condensation can occur as frost but not as cloud except in the polar night; and that visible clouds could be H 2O ice. These calculations are preliminary to a computation of planetary-scale motions in the lowest one or two scale heights. A different dynamics, more analogous to tidal theory, may be required at higher levels.