Dept. of Geosciences Colloquium: The Longwave Cloud Twilight Zone- Clouds' Signature in the So-called Clear Sky
Eshkol Eytan, Weizmann Institute
Clouds are a dominant player in Earth's energy budget as their global coverage is higher than 60% and they strongly modulate the radiation field. The estimation of clouds’ radiative effect demands an accurate quantification of their coverage and optical properties. Both aspects were shown to be ill-defined, as a large portion of the so-called clear sky has a signal that is neither cloudy nor cloud-free. So far, studies of these regions named the "cloud twilight zone" or "Albedo continuum", used only the solar spectrum to identify the source mechanisms of the signal. Here we study the cloud twilight zone in the longwave infrared and give a first estimate of its effect on the energy budget. We analyze the longwave cloud twilight zone in a global dataset of satellite images of shallow cloud fields and estimate a lower bound of its radiative effect to be ~0.75 W/m2. This is equivalent to an addition of ~75 ppm of CO2 to an atmospheric column (60% of the increase since the preindustrial era). The longwave cloud twilight occupies 60% of the “cloud-free” sky in cloud fields and interferes with important remotely sensed variables such as sea surface temperature and water vapor.
Event Organizers: Dr. Roy Barkan and Dr. Asaf Inbal