Determination of Near-IR Water Vapor Self Continuum from Observations

There is still significant uncertainty in the strength of water vapor continuum absorption in the windows between near-infrared water vapor bands. Many radiation codes obtain their water vapor continuum absorption coefficients from the MT CKD continuum model, which derives its near-infrared values from a water vapor line-shape function that has been constrained by continuum measurements in other spectral regions. A number of recent laboratory studies have shown that the strength of the near-IR self continuum in the MT CKD model is too low, but these laboratory studies disagree with each other by more than an order of magnitude. At the high end of the range of measured strengths, the self continuum absorbs a significant amount of solar radiation, so it is crucial that other observational studies are performed to establish the actual strength of this absorption source. We describe here an analysis of measurements from a solar FTS in Lamont, OK, part of the Total Carbon Column Observing Network (TCCON). Thirteen periods on individual days in 2012 were identified as being reasonably stable with respect to aerosol optical depth and precipitable water vapor. For each period, the FTS measurements, coincident measurements from a Normal Incidence Multi-Filter Radiometer (NIMFR) at the DOE ARM site, and calculations from the Line-By-Line Radiative Transfer Model (LBLRTM) were used to obtain aerosol optical depths in window regions from 8000-20000 cm⁻¹, which were then extended to lower wavenumbers through a generalized Angstrom relationship. In windows between 4000-7000 cm⁻¹, these aerosol optical depths, and the LBLRTM optical depths without any self continuum, were subtracted from FTS-derived total optical depths. The self continuum absorption coefficients that were derived from this procedure were at the low end of the recent laboratory studies, but higher than the current version of MT CKD. The implications of the results of this study with respect to the absorption of solar irradiance will be discussed.