Modeling and Observing Near-Infrared Methane Surface Radiative Forcing
Poster PDF
Authors
Daniel Feldman — Lawrence Berkeley National Laboratory
William D. Collins — Lawrence Berkeley National Laboratory
Chaincy Kuo — Lawrence Berkeley National Laboratory
Connor J. Flynn — University of Oklahoma School of Meteorology
Margaret S. Torn — Lawrence Berkeley National Laboratory
Description
Approximately 30% of the radiative forcing from methane arises from near-infrared absorption features, which results in decreased solar insolation throughout the troposphere. These features arise due to multiple vibrational rotational modes that result in groups of absorption lines known as polyads, and they have been characterized under Earth-like conditions that span most of the near-infrared wavelengths from 1.1 to 4.6 μm.
There are several reasons to suspect that the enumeration and description of these polyads is incomplete, and therefore that the surface forcing by methane is underestimated both in line-by-line radiative transfer and in current climate models. First, additional uncharacterized polyads exist at shorter visible wavelengths, where the sun’s spectral insolation is maximized; second, spectral measurements of the methane-rich planetary atmospheres reveal CH4 absorption up through blue and near-UV; and third, the number of CH4 lines in spectroscopic data bases has been increasing exponentially in time with each successive version of the HITRAN database.
The magnitude, implications, and prospects for resolution of the uncertain CH4 absorption and surface forcing are discussed with analyses supported by massively parallel line-by-line (LBL) radiative transfer models that include realistic atmospheric and surface boundary condition inputs. We also present techniques and preliminary results for using observations from the SAS-HE instrument at the ARM SGP site for resolving uncertainty in near-infrared methane radiative forcing.
