Abstract
Recent laboratory measurements show that absorption by the water vapour continuum in near‐infrared windows may be about an order of magnitude higher than assumed in many radiation codes. The radiative impact of the continuum at visible and near‐infrared wavelengths is examined for the present day and for a possible future warmer climate (with a global‐mean total column water increase of 33%). The calculations use a continuum model frequently used in climate models (‘CKD’) and a continuum model where absorption is enhanced at wavelengths greater than 1 µm based on recent measurements (‘CAVIAR’). The continuum predominantly changes the partitioning between solar radiation absorbed by the surface and the atmosphere; changes in top‐of‐atmosphere net irradiances are smaller. The global‐mean clear‐sky atmospheric absorption is enhanced by 1.5 W m−2 (about 2%) and 2.8 W m−2 (about 3.5%) for CKD and CAVIAR respectively, relative to a hypothetical no‐continuum case, with all‐sky enhancements about 80% of these values. The continuum is, in relative terms, more important for radiation budget changes between the present day and a possible future climate. Relative to the no‐continuum case, the increase in global‐mean clear‐sky absorption is 8% higher using CKD and almost 20% higher using CAVIAR; all‐sky enhancements are about half these values. The effect of the continuum is estimated for the solar component of the water vapour feedback, the reduction in downward surface irradiance and precipitation change in a warmer world. For CKD and CAVIAR respectively, and relative to the no‐continuum case, the solar component of the water vapour feedback is enhanced by about 4 and 9%, the change in clear‐sky downward surface irradiance is 7 and 18% more negative, and the global‐mean precipitation response decreases by 1 and 4%. There is a continued need for improved continuum measurements, especially at atmospheric temperatures and at wavelengths below 2 µm.
Highlights
Water vapour is the most important gaseous absorber in the Earth’s atmosphere (e.g. Kiehl and Trenberth, 1997)
Journal of the Royal Meteorological Society published by John Wiley & Sons Ltd on behalf of the Royal Meteorological Society
This study has examined the role of the water vapour continuum at solar wavelengths for both the present-day radiation budget, and in a possible future warmer and moister climate
Summary
Water vapour is the most important gaseous absorber in the Earth’s atmosphere (e.g. Kiehl and Trenberth, 1997). An extensive series of laboratory measurements of the near-infrared continuum has been reported, as part of the UK-based CAVIAR (Continuum Absorption at Visible and Infrared wavelengths and its Atmospheric Relevance) consortium, for the self-continuum (i.e. due to interactions between water vapour molecules: Ptashnik et al, 2011a) and the foreign-continuum (i.e. due to the interaction of water vapour with other molecules: Ptashnik et al, 2012). These measurements indicate that the continuum in the near-infrared windows around 1.25, 1.6, 2.1 and 3.8 μm is significantly c 2014 The Authors.
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