New Observations of the Water Vapour Continuum in Near-Infrared Windows

The water vapour continuum is the smoothly-varying wavelength-dependent absorption not attributable to local spectral lines of the water vapour molecule. This absorption is weak in comparison to line absorption; however in the near-infrared windows at 2.5 and 1.6 μm it is the largest gaseous contributor to absorption in the atmosphere. While it is currently parameterised in models (usually using the MT_CKD continuum), its strength and temperature dependence are not well constrained. Laboratory observations differ by a factor of 500 in the 1.6 μm window and a factor of 50 in the 2.5 μm window, with no agreement within the respective measurement uncertainty. These laboratory measurements are taken at or above room temperature; given the difficulty in extrapolating these measurements to lower temperature, the true continuum strength in the atmosphere is even more difficult to ascertain. Because of this disagreement, there is a corresponding uncertainty in how much influence the continuum has for energy budget and climate studies, as well as in remote sensing using these windows (for example in carbon dioxide monitoring). This work presents an analysis which aims to extract the continuum strength from atmospheric measurements by the National Physical Laboratory of the continuum, using an absolutely calibrated sun-pointing Fourier Transform spectrometer. This is a challenging task, due to the need to correct for aerosol scattering, time-varying water vapour amounts and measurement uncertainties. Despite these uncertainties, we find continuum strength in these windows with central values several times larger than that predicted by MT_CKD, and present an estimate of the upper limit on the absorption in a standard mid-latitude summer atmosphere, based on our analysis. Advances that will be needed to better constrain the uncertainty in the near-infrared continuum in future measurements will also be discussed.