Observational constraints on non-Lorentzian continuum effects in the near-infrared solar spectrum using ARM ARESE data

A. M. Vogelmann, V. Ramanathan, W. C. Conant, W. E. Hunter

Research output: Contribution to journalArticle

23 Scopus citations

Abstract

Uncertainties exist in the magnitude of the water vapor continuum at solar wavelengths and many models do not include a solar continuum. We assess whether the neglect of the continuum in some models could explain a significant amount of the excess solar absorption found by several recent studies, in which the observed atmospheric solar absorption is significantly greater than that modeled. Towards this goal, we constrain the magnitude of the near-infrared water vapor continuum absorption using observations from the Atmospheric Radiation Measurement (ARM) Enhanced Shortwave Experiment (ARESE). Narrowband irradiances measured by two independent Multifilter Rotating Shadowband Radiometers (MFRSRs) are used to infer the clear-sky transmission by water vapor in the 0.94 μm band. Over 16000 such observations are compared to non-continuum (i.e., a pure Lorentzian model) and continuum calculations using a correlated-k distribution model, which shows excellent agreement with a line-by-line model and uses coincident measurements of the pressure, temperature and water vapor profiles. Continuum calculations use the CKD super-Lorentzian formulation. The data suggest the need for a far wing continuum in the 0.94 μm band with an absorption that falls between that computed for pure Lorentzian lines and the CKD continuum. A sensitivity analysis presents the effects of uncertainties in parameters affecting the calculations, including those in the line parameters, continuum magnitude and atmospheric state. Upper estimates for the absorption of broadband solar radiation by the continuum, beyond that computed for pure Lorentzian far wings, are only 1 to 2 W m-2 for a diurnal (24 h) average, and 4 to 6 W m-2 for local noon. Thus, uncertainties in or the neglect of the water vapor continuum at solar wavelengths are not likely explanations for excess absorption of the order of 15 to 30 W m-2 (diurnal average).

Original languageEnglish (US)
Pages (from-to)231-246
Number of pages16
JournalJournal of Quantitative Spectroscopy and Radiative Transfer
Volume60
Issue number2
DOIs
StatePublished - Aug 1998

ASJC Scopus subject areas

  • Radiation
  • Atomic and Molecular Physics, and Optics
  • Spectroscopy

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