A More Transparent Infrared Window

Absorption by the water vapor continuum in the infrared (IR) window is a key element in the Earth’s radiative balance. The strength of this absorption has implications for our climate, impacting the magnitude of water vapor feedback from the temperature change due to increased greenhouse gases as well as the potential for a runaway greenhouse. The dominant source of absorption in the IR window is the water vapor self continuum. Self continuum values in the MT_CKD water vapor continuum model had been fit to coefficients derived in a 2004 analysis of field measurements and, subsequently, the uncertainty of these values was thought to be low. This perspective was reinforced by a 2008 laboratory study of the window self continuum, which showed close agreement with MT_CKD. For these reasons, the continuum absorption coefficients in MT_CKD in the IR window has not changed in two decades.

We present a recent analysis of water vapor continuum absorption in the infrared window that utilizes measurements by the Atmospheric Emitted Radiance Interferometer (AERI) at Atmospheric Radiation Measurement (ARM) program sites (Southern Great Plain (SGP), 2016-2018; GoAmazon (MAO), 2014-2015). The extensive data set ensures a wide range of precipitable water vapor (PWV) values, which is essential for deriving both self and foreign continuum values. Spectral continuum absorption coefficient values were retrieved using the SGP observations, which were then validated using the high-PWV MAO observations, providing a stringent test of the derived self continuum coefficients.

The results of this study show that the IR window self continuum currently in MT_CKD is significantly overestimated and the foreign continuum is significantly underestimated. The net effect is that the infrared window is substantially more transparent than is currently thought and modeled in atmospheric prediction codes. This presentation will detail the methodology of this study, its results, and the implications for radiative balance and climate. Preliminary results will be shown of the impact of these water vapor continuum changes on the climate of the ECMWF weather forecast model.