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Keywords:

  • Climate sensitivity;
  • Radiative transfer;
  • Two-stream methods

Abstract

A comprehensive spectrum-resolving radiative transfer model (SRM) was used to simulate the average, present-day, solar radiation field on Mars. A CO2-only 6 hPa Martian atmosphere absorbs about 1% of zenith solar radiation, producing a modest heating rate of 4–5 K day−1 in the lowest 10 km. The trace gases have an insignificant effect but airborne dust reduces the downwelling solar flux effectively, and the reflected flux somewhat less. This produces an anti-greenhouse trend (cooling at the surface, warming within the atmosphere, reflection at the top), which increases strongly with the dust load. For instance, with dust visible optical depth of unity and sun in zenith, the surface solar irradiation is attenuated by 26% and the solar heating rate increases to about 70 K day−1 in the lowest 25 km. The numbers are however strongly dependent on the optical properties of the dust, which are not known very well.

Several fast two-stream methods for dust were compared with the SRM results. Their common systematic errors were reduced by a simple, physically-based correction. The global albedo of Mars was then studied as a function of dust load, dust optics and surface albedo. The crossover from added airborne dust tending to make the whole planet look whiter or darker occurred at surface albedo of about 35%, nearly independently of the dust load. We demonstrate, however, that this value is sensitive to the optical properties of the assumed dust. Copyright © 2005 Royal Meteorological Society