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The Effects of Changes in HITRAN and Uncertainties in the Spectroscopy on Infrared Irradiance Calculations

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  • 1 Department of Meteorology, University of Reading, Reading, United Kingdom
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Abstract

Atmospheric radiative transfer calculations are susceptible to a number of uncertainties, such as in the assumed atmospheric composition, cloudiness, and spectroscopy. Using both line-by-line and narrowband schemes the authors look at the uncertainties in calculations of infrared irradiances, heating rates, and radiative forcing (due to changes in greenhouse gas concentration), which are due solely to uncertainties in the spectroscopic parameters of H2O, CO2, O3, N2O, and CH4 in the HITRAN database. The uncertainties in line widths are found to be more important than the uncertainties in line strengths. Spectrally integrated irradiances are estimated to be accurate to within about 0.5%, while radiative forcing values, such as those due to a doubling of CO2, are estimated to be accurate to within 5%. Uncertainties in heating rates are generally small, except in the upper stratosphere where they can reach 5%–10%. The authors also investigate the effect of changes in HITRAN between 1986 and 1996, in particular to assess the importance of updating transmittance calculations in radiative transfer schemes as spectroscopic databases are updated. Although the uncertainties in spectrally integrated quantities are small, in narrow spectral intervals the uncertainties are larger; this work does not obviate the need for further work to reduce the uncertainty in the spectroscopic parameters, nor for observational verification of radiative transfer codes.

Corresponding author address: Dr. Keith Shine, Dept. of Meteorology, University of Reading, Reading RG6 6BB, United Kingdom.

Email: K.P.Shine@reading.ac.uk

Abstract

Atmospheric radiative transfer calculations are susceptible to a number of uncertainties, such as in the assumed atmospheric composition, cloudiness, and spectroscopy. Using both line-by-line and narrowband schemes the authors look at the uncertainties in calculations of infrared irradiances, heating rates, and radiative forcing (due to changes in greenhouse gas concentration), which are due solely to uncertainties in the spectroscopic parameters of H2O, CO2, O3, N2O, and CH4 in the HITRAN database. The uncertainties in line widths are found to be more important than the uncertainties in line strengths. Spectrally integrated irradiances are estimated to be accurate to within about 0.5%, while radiative forcing values, such as those due to a doubling of CO2, are estimated to be accurate to within 5%. Uncertainties in heating rates are generally small, except in the upper stratosphere where they can reach 5%–10%. The authors also investigate the effect of changes in HITRAN between 1986 and 1996, in particular to assess the importance of updating transmittance calculations in radiative transfer schemes as spectroscopic databases are updated. Although the uncertainties in spectrally integrated quantities are small, in narrow spectral intervals the uncertainties are larger; this work does not obviate the need for further work to reduce the uncertainty in the spectroscopic parameters, nor for observational verification of radiative transfer codes.

Corresponding author address: Dr. Keith Shine, Dept. of Meteorology, University of Reading, Reading RG6 6BB, United Kingdom.

Email: K.P.Shine@reading.ac.uk

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