Evaluation of recent updates to the spectroscopy of CO2 and CH4 in the thermal infrared using observations from IASI

Author: Mark W. Shephard, Matthew J. Alvarado, Eli J. Mlawer, Gennady Uymin, Karen Cady-Pereira and Jean-Luc Moncet
December 6, 2012
AGU Fall Meeting 2012

Mark W. Shephard; Matthew J. Alvarado; Vivienne Payne; Eli J. Mlawer; Gennady Uymin; Karen E. Cady-Pereira; Jennifer S. Delamere; Jean-Luc Moncet (2012) Evaluation of recent updates to the spectroscopy of CO2 and CH4 in the thermal infrared using observations from IASI. AGU Fall Meeting 2012, San Francisco, CA.

Here we describe recent updates to the spectroscopic parameters for the greenhouse gases CO2 and CH4 in AER’s line-by-line radiative transfer model LBLRTM. AER’s line-by-line models are widely regarded as a reference standard within the atmospheric community, with users across a range of disciplines in government agencies, industry, and academia. LBLRTM has been used as the basis of the forward models for the Infrared Atmospheric Sounding Interferometer (IASI) and the NASA Tropospheric Emission Spectrometer (TES).

The accuracy with which it is possible to model gaseous absorption with line-by-line radiative transfer models is currently limited mainly by uncertainties in the knowledge of spectroscopic parameters, lineshape and continua. Therefore, continual improvements to spectroscopic parameters and continua are crucial to ensure future scientific progress. Spectral line parameters used in LBLRTM are now based on the HITRAN 2008 compilation, with selected notable exceptions, made only after extensive validation. Exceptions to HITRAN in the infrared include updated CO2 line positions and intensities, updated line mixing coefficients for CO2 and CH4 and improvements to the H2O line positions and intensities. Recent updates to the MT_CKD continuum used by LBLRTM include updates to the CO2 and self-broadened H2O continua in the 2400 cm^(-1) region.

We will show the results of a rigorous validation of these updates to LBLRTM against a global dataset of 128 clear-sky, nighttime, ocean, near-nadir IASI measurements during April 2008. We compare the results of the latest version of LBLRTM (v12.1) to a previous version (v9.4+) to determine the impact of these spectroscopic updates on the spectral residuals as well as the retrieved profiles of temperature and H2O. We find that the spectroscopy in the CO2 ν2 and ν3 bands is significantly improved in LBLRTM v12.1 relative to v9.4+, and that the spectroscopy of these two CO2 bands is remarkably consistent in LBLRTM v12.1. The updated H2O spectroscopy in LBLRTM v12.1 substantially improved the residuals in the P-branch of the H2O ν2 band, while the improvements in the R-branch are more modest. The retrieved H2O mixing ratios are on average 15% lower between 100 and 200 mbar and 10 to 30% higher between 400 and 800 mbar with the new spectroscopy, but the variability among the cases is very large in the lower troposphere (over a factor of 2). Significant systematic residuals remain in the ν4 band of CH4, but the magnitude of the positive bias in the retrieved mixing ratios is reduced in LBLRTM v12.1, suggesting that the updated spectroscopy could improve retrievals of CH4 from satellite observations.

The AER radiative transfer models and the associated databases (e.g., line parameters, continua, and molecular cross-sections) are publicly available from AER (http://www.rtweb.aer.com).