Analysis of Water Vapor Absorption in the Far-Infrared and Submillimeter Regions Using Surface Radiometric Measurements From Extremely Dry Locations

The second Radiative Heating in Underexplored Bands Campaign (RHUBC-II) was conducted in 2009 by the U.S. Department of Energy Atmospheric Radiation Measurement program to improve water vapor spectroscopy in the far-infrared spectral region. RHUBC-II was located in an extremely dry region of Chile to ensure very low opacities in this spectral region. Spectrally resolved measurements by a far-infrared spectrometer and a submillimeter interferometer from RHUBC-II are compared with line-by-line radiative transfer model calculations. Water vapor amounts and temperatures used in the calculations come from collocated radiosondes, with extensive adjustments to correct for issues due to the campaign's dry conditions and mountainous terrain. A reanalysis is also performed of far-infrared measurements taken at the Atmospheric Radiation Measurement North Slope of Alaska site before and during the first RHUBC campaign. These analyses determine that differences between the measurements and model calculations using existing spectroscopic parameters are significant in the far-infrared and submillimeter regions, leading to the derivation of improved water vapor continuum absorption coefficients and air-broadened widths of 74 water vapor lines. The foreign continuum is increased by more than 50% in part of the far-infrared and the widths of more than 20 lines are changed by more than 10%. The uncertainty in the foreign continuum coefficients is estimated as greater than 20% in some spectral regions, primarily a consequence of the uncertainty in the specification of water vapor. The improved far-infrared spectroscopic parameters have a notable impact on calculated spectral radiances and a modest impact on broadband radiative fluxes and heating rates.