Far-IR Water Vapor Spectroscopy from Analysis of Field Campaign Measurements

Abstract:

Accurate water vapor spectroscopy is critical to utilization of far-infrared radiances from upcoming missions such as FORUM and PREFIRE. This presentation will detail the derivation of improved far-IR water vapor spectroscopic parameters based on observations from a 2009 campaign conducted at an extremely dry high-altitude location in Chile. The low column water vapor amounts during this campaign, the second Radiative Heating in Underexplored Bands Campaign (RHUBC-II), sponsored by the U.S. Department of Energy Atmospheric Radiation Measurement (ARM), resulted in low opacities in the far-IR, allowing the evaluation of the spectroscopy in this region. Comparisons between far-IR radiances measured by the REFIR-PAD interferometer and calculations by the LBLRTM radiative transfer model led to the derivation of improved water vapor foreign continuum absorption coefficients and air-broadened widths of numerous water vapor lines. Due to the challenging conditions of the campaign, the water vapor fields used as input to LBLRTM had significant uncertainties, which led to the derived continuum coefficients also having uncertainties as high as 20-30%.

We will present an overview of the RHUBC-II campaign and the radiative closure analysis that resulted in the derivation of the improved spectroscopic parameters. We will also show the impact of these modifications, once implemented in LBLRTM and the MT_CKD water vapor continuum model, on calculated fluxes and heating rates. The derived continuum values will be presented in the context of values derived from analyses of previous field deployments. Also, we will show that results from recent laboratory experiments provide validation for the continuum values derived in the RHUBC-II analysis and, therefore, the use of this improved spectroscopy will advance the goals of the FORUM and PREFIRE missions.