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

Abstract

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.

Sensitivity of near‐infrared transmittance calculations for remote sensing applications to recent changes in spectroscopic information

“…In order to validate the water vapour continuum absorption in the near‐IR spectral region, it is essential to carry out a radiative closure experiment, such as those performed by, for example, Mlawer et al . () and Liuzzi et al . () at far‐infrared wavelengths.…”

“…There is therefore the need to further evaluate the MT_CKD 2.5 water vapour continuum model that is usually adopted for transmittance calculations in high-precision remote sensing applications in the near-IR. In order to validate the water vapour continuum absorption in the near-IR spectral region, it is essential to carry out a radiative closure experiment, such as those performed by, for example, Mlawer et al (2019) and Liuzzi et al (2014) at far-infrared wavelengths.…”

FORUM: Unique Far-Infrared Satellite Observations to Better Understand How Earth Radiates Energy to Space

“…Any estimate of its radiative impact is, of course, dependent on the quality of the underlying spectroscopy and knowledge of its vertical distribution. A number of measurement campaigns have substantially improved our knowledge of water vapor spectroscopy within the FIR (Mlawer et al 2019;Fox 2015;Green et al 2012;Turner et al 2012;Tobin et al 1999).…”

“…FIRST participated in the RHUBC-II campaign, whose data were used to recommend changes to the water vapor continuum absorption in the FIR (Mast et al 2017) which have since been adopted (Mlawer et al 2019). FIRST underwent a substantial recalibration after the RHUBC-II campaign (Latvakoski et al 2014(Latvakoski et al , 2013 and conducted a ground-based measurement campaign at Absolute Radiance and Refractivity Observatory (CLARREO) mission (Wielicki et al 2013), and will be addressed by the FORUM mission.…”

“…), providing continuous, unattended operations. These ground-based campaigns have shown the capability to measure the atmospheric downwelling radiation in a very wide spectral band (seeFigure 3), contributing in such a way to update the water vapor spectroscopy in the FIR region(Liuzzi et al 2014;Mlawer et al 2019), which was used for studies performed to increase thescience readiness level of FORUM. Another FORUM precursor is the Far-Infrared Spectroscopy of the Troposphere (FIRST) instrument that was developed through NASA's Instrument Incubator Program beginning in 2001 to demonstrate technologies needed to measure the FIR with a space-based instrument (Wellard et al 12 Accepted for publication in Bulletin of the American Meteorological ociety.…”