The Longwave Radiative Effect of Warm Clouds’ Twilight Zone

Abstract:

It was shown that the binary classification of the sky to cloudy and clear is ill-defined due to the presence of the twilight zone. The radiative properties of this, so called, “cloud-free” area within cloud fields are affected by several features and processes. The shortwave radiation can interact with clouds with weak optical signature (forming and dissipating clouds), clouds that are much smaller than the measured pixel and with humidified aerosol. The shortwave signal can be affected also by lateral illumination by nearby clouds (3D effect). In this work, we explore the twilight interaction with longwave radiation (considering undetectable clouds and high humidity pockets). To do so we analyze global satellite observations of maritime shallow cloud fields and estimate a lower bound of the twilight zone’s radiative effect in the longwave. The average effect is found to be ~0.75 W/m², which is equivalent to the addition of ~75 ppm of CO₂ to an atmospheric column.

Our findings show that 60% of the so called “clear-sky”, within warm cloud fields have a significant twilight effect in the longwave. Moreover, deeper or higher cloud fields (with colder tops) are likely to have a much stronger twilight effect in the longwave, implying that this effect should be considered when estimating the global radiative budget.