Cloud-aerosol transition zone radiative effects from modeling and observational perspectives

Aerosols and clouds, as two particular cases of a single phenomenon (i.e., a suspension of particles in the air), are important components in the climate system. They play a crucial role in determination of Earth’s energy budget, as they strongly affect the balance between the incoming shortwave solar radiation absorbed by Earth’s atmosphere and surface, and the thermal longwave radiation emitted from the Earth. Although aerosols and clouds interact and affect each other's properties, their radiative properties and effects are usually treated separately in climate, meteorological, and weather forecasting studies and models. Thus, a discrimination between the cloudy and noncloudy skies is often required in such contexts. Traditionally, the algorithms used for performing this discrimination assume that the state of the sky is either cloudy or noncloudy (but containing a certain aerosol load), leaving no space for an intermediate phase. However, the change in the state of sky from cloudy to cloudless (or vice versa) occurs gradually, and it comprises an additional phase called “transition zone” (or “twilight zone”), which may represent a variety of atmospheric processes: hydration/dehydration of aerosols, cloud fragments shearing off from the adjacent clouds, decaying and incipient clouds, etc. As a result of this simplified assumption about the state of sky, the area corresponding to the transition zone is often labeled as an area containing optically thin layers of cloud or aerosol. However, the microphysical and radiative characteristics of the transition zone are expected to lay on the border between those corresponding to a cloud and those corresponding to an atmospheric aerosol. In other words, radiative and optical properties corresponding to clear (noncloudy) or cloudy skies are misleadingly used to characterize such transition zone conditions. In the present thesis we contribute to the knowledge available about the transition zone from an energy balance perspective ​
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