Longwave radiative effect of the cloud twilight zone

Clouds play a key role in Earth's radiation budget, covering more than 50% of the planet. However, the binary delineation of cloudy and clear sky is not clearly defined due to the presence of a transitionary zone, known as the cloud twilight zone, consisting of liquid droplets and humidified to dry aerosols. The twilight zone is an inherent component of cloud fields, yet its influence on longwave-infrared radiation remains unknown. Here we analyse spectral data from global satellite observations of shallow cloud fields over the ocean to estimate a lower bound on the twilight zone's effect on longwave radiation. We find that the average longwave radiative effect of the twilight zone is similar to 0.75 W m(-2), which is equivalent to the radiative forcing from increasing atmospheric CO2 by 75 ppm. We also find that the twilight zone in the longwave occupies over 60% of the apparent clear sky within the analysed low-level cloud fields. As low-level clouds are relatively warm, the overall longwave radiative contribution from the twilight zone is likely to be higher. We suggest that the twilight zone needs to be accounted for to accurately quantify cloud radiative effects and close the global energy budget.