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Abstract. Ever since the observations of Percival Lowell, the annual cycle of Martian water has been a fascinating topic in planetary exploration. Observations by the Viking Orbiter, supplemented by Earth-based microwave and infrared observations, have given us a reasonable picture of this cycle. We are now also able to model the cycle using our Mars Climate Model, a simplified atmospheric general circulation model designed specifically for this purpose. We find that a thin adsorbing layer of the Martian regolith plays a fundamental role in the water cycle, limiting the lower atmospheric relative humidity and preventing the formation of widespread ice dep6sits at low latitudes. We are thus able to estimate a largescale average value of the specific soil surface area of this regolith. Water which evaporates from the permanent north polar ice cap during summer is returned by a process of repeated evaporation and precipitation on the retreating seasonal cap the following spring, so that the global inventory of water outside the polar caps ranges within narrow limits. (There is a small net annual deposition of water ice at the south polar cap which is always at dry ice temperatures.) If ice on the residual south polar cap is exposed during the summer, it rapidly sublimes, generating vapor amounts similar to those observed in northern summer. Recovery to normal dry conditions in the southern atmosphere occurs very rapidly in the next year. Such an event could explain the otherwise anomalous Earth-based pre-Viking observations of a wet southern summer. If southern ice deposits at lower latitudes are exposed, the vapor can be transfered irreversibly through the strong Hadley cell to the north polar cap. We therefore speculate that the asymmetry of Mars' current orbit is responsible for the asymmetry of the present water distribution (with extensive permanent water ice deposits located only in the colder, aphelion summer, northern hemisphere).

We have written a one‐dimensional numerical model of the exchange of H2O between the atmosphere and subsurface of Mars through the planetary boundary layer (PBL). Our goal is to explore the mechanisms of H2O exchange and to elucidate the role played by the regolith in the local H2O budget. The atmospheric model includes effects of Coriolis, pressure gradient, and frictional forces for momentum: radiation, sensible heat flux, and advection for heat. The model differs from Flasar and Goody by use of appropriate Viking‐based physical constants and inclusion of the radiative effects of atmospheric dust. The pressure gradient force is specified or computed from a simple slope model. The subsurface model accounts for conduction of heat and diffusion of H2O through a porous adsorbing medium in response to diurnal forcing. The model is initialized with depth‐independent H2O concentrations (2 kg m−3) in the regolith and a dry atmosphere. The model terminates when the atmospheric H2O column abundance stabilizes to 0.1% per sol. Results suggest that in most cases, the flux through the Martian surface reverses twice in the course of each sol. In the midmorning, the regolith begins to release H2O to the atmosphere and continues to do so until midafternoon, when it once more becomes a sink. It remains an H2O sink throughout the Martian night. In the early morning and late afternoon, while the atmosphere is convective, the atmosphere supplies H2O to the ground at a rapid rate, occasionally resulting in strong pulses of H2O into the ground. The model also predicts that for typical conditions, perhaps 15–20 sols are required for the regolith to supply an initially dry atmosphere with its equilibrium load. The effects of surface albedo, thermal inertia, solar declination, atmospheric optical depth, and regolith pore structure are explored. Increased albedo cools the regolith, so less H2O appears in the atmospheric column above a bright surface. The friction velocity is higher above a dark surface, so there is more diurnal H2O exchange; relative humidities are much higher above a bright surface. Thermal inertia I affects the propagation of energy through a periodically heated homogeneous surface. Our results suggest that higher thermal inertia forces more H2O into the atmosphere because the regolith is warmer at depth. Surface stresses are higher above a low I surface, but there is less diurnal exchange because the atmosphere is dry. The latitude experiment predicts that the total diurnal insolation is more important to the adsorptively controlled H2O column abundance than the peak daytime surface temperature. Fogs and high relative humidity will be far more prevalent in the winter hemisphere. The dust opacity of the atmosphere plays a very significant role; the PBL height, column abundances, relative humidity, and surface stresses all increase very strongly as the optical depth approaches zero. The dust opacity of the atmosphere must be considered in subsequent PBL models.

A combined 3‐dimensional circulation model and aerosol microphysical/transport model is used to simulate the dispersion of the Mt. Pinatubo volcanic cloud in the stratosphere for the first few months following the eruption. Radiative heating of the cloud due to upwelling infrared radiation from the troposphere is shown to be an important factor affecting the transport. Without cloud heating, cloud material stays mostly north of the equator, whereas with cloud heating, the cloud is transported southward across the equator within the first two weeks following the eruption. Generally the simulations agree with TOMS, AVHRR, and SAGE satellite observations showing the latitude distribution of cloud material to be between about 20°S and 30°N within the first few months. Temperature perturbations in the stratosphere induced by the aerosol heating are generally 1–4 K, in the range of those observed.

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