Browsing by Author "Schmunk, Robert Bradley"

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    A comparative study of the early terrestrial atmospheres with interactive cloud formation
    (1990) Schmunk, Robert Bradley; Chamberlain, Joseph W.
    Due to their formation at about the same time in the same region of the early solar nebula, it is reasonable to assume that the primitive atmospheres of Earth, Mars and Venus were similar and that present-day differences have arisen as a result of their differing masses and incident solar fluxes. Using a radiative-convective model, we determine maximum and minimum carbon dioxide levels for the early atmospheres which are consistent with this assumption and with climatic conditions thought to have existed on the three terrestrial planets 4.0 billion years ago. Rather than employ the cloud-free atmosphere approach of earlier studies, we include an interactive water vapor transport and cloud formation scheme in the model. Due to uncertainties about the direction of cloud cover feedback, we treat cloud cover as fixed. For most cases examined, we set the cloud cover at 50%, but the effect of varying cloud cover is also explored.
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    A zonally and annually averaged study of potential early Martian atmospheres
    (1993) Schmunk, Robert Bradley; Few, Arthur A., Jr.
    Observations of the surface of Mars suggest a high probability of surface water activity in that planet's past. Consequently, many studies of Mars' early atmosphere have attempted to estimate the carbon dioxide level by requiring that surface temperatures be high enough to support surface liquid water. In the main, these studies have employed one-dimensional, radiative-convective climate models capable of considering only a single solar zenith angle, typically chosen to represent a global and annual average. Such models are hence not well suited for considering meridional variations in the temperature profile, which are affected by variations in the orbital obliquity and the meridional redistribution of heat by dynamic processes. I describe modifications to a more complex model, the multi-level energy balance model designed at NASA's Goddard Laboratory for Atmospheric Sciences, which make it suitable for study of an atmosphere with varying carbon dioxide levels. Vertically and meridionally defined, the model includes heating and cooling by radiation, mean meridional circulation, large-scale (baroclinic) and small-scale (convective) eddies, and surface turbulent flux. I present annually-averaged results for an examination of potential atmospheres of early Mars, given that its carbon dioxide level may range from 0 to 500 Pa and the orbital obliquity from 0$\sp\circ$ to 50$\sp\circ.$ These results are compared with those obtained from a radiative-convective model.