Climate-Relevant Gas Absorption Properties from AWARE and Other ARM Spectral Measurements

Principal Investigator(s):
Eli Mlawer, Atmospheric and Environmental Research

Co-Investigator(s):
Vivienne Payne, Jet Propulsion Lab of the California Institute of Technology

Collaborator(s):
David D. Turner, NOAA Earth System Research Lab
Daniel Feldman, Lawrence Berkeley Lab

Detailed knowledge of important atmospheric radiative processes is critical to our ability to predict future climate. In particular, water vapor continuum absorption in the infrared ‘atmospheric window’ (8.3-12.5 μm) plays a key role with respect to the change in the planetary energy balance due to ‘water vapor feedback’, i.e. the additional radiative forcing caused by the expected increase in water vapor abundances in response to the higher temperatures caused by increases in greenhouse gases. In addition, accurate knowledge of the absorption properties of water vapor and methane is needed in order to exploit information about these gases encoded in satellite observations.

The overarching objective of this research is to use measurements from the Atmospheric Emitted Radiance Interferometer (AERI) deployed at the ARM West Antarctic Radiation Experiment (AWARE) and other ARM sites to improve our knowledge of uncertain infrared spectroscopic parameters of importance to climate, remote sensing, and data assimilation. A primary focus will be the water vapor continuum in the infrared atmospheric window. Radiation codes used to predict climate and weather base their representation of the window water vapor continuum on the MT_CKD model, but in this spectral region the MT_CKD water vapor continuum absorption coefficients were derived more than a decade ago by an analysis of AERI measurements for conditions with a limited range of precipitable water vapor (PWV) and temperature values, leading to increased uncertainty in the derived coefficients.

We will perform a new, comprehensive analysis of all aspects of the water vapor continuum in the atmospheric window -- the self continuum, the foreign continuum, and the self continuum temperature dependence, all resolved spectrally. Such an analysis is only possible if it is based on spectrally resolved radiometric measurements that span the entire range of relevant PWV and temperature values. The low PWV amounts and cold temperatures of AWARE, coupled with observations from more than a decade of AERI measurements at TWP/Darwin and at SGP, including many at high PWV and temperatures, will provide an appropriate foundation for such a comprehensive analysis.

We also propose to exploit the low PWV values associated with the AWARE AERI measurements to analyze and improve spectroscopic parameters in the key infrared water vapor absorption band used for remote sensing and data assimilation of water vapor. In addition, we will also use AWARE AERI measurements to improve infrared spectroscopic parameters in the main infrared methane absorption band used for the remote sensing of methane and to determine the radiative forcing of this gas. Previous analyses have demonstrated issues with our knowledge of the properties of these absorption bands that we aim to resolve with our study.

All improved absorption parameters will be implemented in the MT_CKD continuum model, the highly used Line-By-Line Radiative Transfer Model (LBLRTM), and the fast radiation codes RRTMG and RRTMGP. In addition, the accuracy of a number of standard ARM products will be enhanced by the improvements in spectroscopy resulting from our study, including the MWRRET PWV/LWP retrieval and AERIoe retrievals. RRTMG is used in WRF, WRF-Chem, CESM, ACME, as well as other dynamical models, and its next-generation successor code, RRTMGP, will be implemented in ACME. This proposed work will thereby lead to important improvements in the treatment of radiation in DOE-supported regional and global climate models.