Temperature dependence and physical interpretation of the self-continuum absorption of water vapor in the 1.6  μ m window

New measurements of the very weak water vapor self-continuum absorption are reported by cavity ring down spectroscopy (CRDS) in the 1. 6 μm window. For 30 selected spectral points ranging between 5702 and 6855 cm−1, the temperature dependence of the self-continuum cross section is studied by monitoring the absorption during pressure ramps of pure water vapor at 285, 296, 308, 317, and 332 K. For each measurement point, the self-continuum cross section, CS (T), was retrieved from the purely quadratic pressure dependence of the measured absorption coefficient, after subtraction of the monomer lines contribution. The temperature dependence was studied with a highly stable temperature regulated CRDS cell, allowing for a significant gain compared to a first study Mondelain et al. , J. Geophys. Res. Atmos. 119, 5625–5639 (2014). Although some significant deviations are evidenced, an overall good agreement with the MTCKD model of the self-continuum is noted. In the low energy range of the window, the predictions based on the far-wing line function obtained by Ma et al. J. Chem. Phys. 128, 124313 (2008) show an excellent agreement with the measurements. This is not the case in the high-energy range of the window where far-wing predictions underestimate both the cross sections and the temperature dependence. Semi-empirical simulations indicate that the missing absorption in that interval is due to the contribution of the quasi-bound water dimers. The comparison of the simulations with the MTCKD model in the 0–10000 cm−1 range indicates that the hybrid interpretation of the origin of the continuum proposed in the 1. 6 μm window applies to other transparency windows.