A high-spectral resolution study of the near-infrared solar flux disposition in clear and overcast atmospheres

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Abstract

The sensitivity of the near-infrared spectral atmospheric and surface fluxes to the vertical location of clouds is investigated, including a study of factors (drop-size distribution, drop optical depth, solar zenith angle, cloud geometrical thickness, atmospheric profiles) which govern this dependence. Because of the effects of the abovecloud, in-cloud and below-cloud water vapor the atmospheric absorbed flux in each spectral band depends critically on the cloud location, with a high cloud resulting in lesser absorption and greater reflection than a low one having the same drop optical depth. The difference between a high and a low cloud forcing of atmospheric absorption increases with drop optical depth. For any optical depth, clouds with larger drops cause a greater forcing of the spectral atmospheric absorption than those with smaller ones, so high clouds can even cause an increase rather than a decrease of the atmospheric absorption relative to clear skies. In contrast, the spectral and total surface fluxes are relatively insensitive to cloud vertical location. Instead, they are determined by the drop characteristics, notably drop optical depth. This near-invariance characteristic is attributable to the fact that most of the insolation reaching the surface is in the weak water vapor spectral absorption regions; here drops dominate the radiative interactions and thus there is little dependence on cloud height. In addition, the overlap of the drop spectral features with the moderate-to-strong vapor absorption bands ensures that insolation in these regimes fails to reach the surface no matter where the cloud is located; instead, these bands contribute the most to atmospheric absorption. The near-invariant behavior of the spectrål and total surface flux holds separately for a wide variety of conditions studied. As a consequence, the difference in reflection, between two columns containing clouds with the same optical depth but located at different altitudes, is approximately balanced in magnitude by the difference in the atmospheric absorption; this holds for every spectral interval whether it be a weak, moderate, or strong vapor/drop absorption band. It also follows that the net fluxes at the top and surface of overcast atmospheres do not have a general, unambiguous relationship; this is in sharp contrast to a linear relation between them in clear skies. However, under certain overcast conditions (e.g., specific vertical location of clouds and solar zenith angle), a simple linear relationship is plausible.

Original languageEnglish (US)
Article number98JD02379
Pages (from-to)23255-23273
Number of pages19
JournalJournal of Geophysical Research Atmospheres
Volume103
Issue numberD18
DOIs
StatePublished - Jan 1 1998

All Science Journal Classification (ASJC) codes

  • Geophysics
  • Forestry
  • Oceanography
  • Aquatic Science
  • Ecology
  • Water Science and Technology
  • Soil Science
  • Geochemistry and Petrology
  • Earth-Surface Processes
  • Atmospheric Science
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science
  • Palaeontology

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