Aerosol-cloud interaction in the twilight zone

The proposed research project will investigate the “twilight zone” with multiple ground based remote sensors available at CNR-IMAA Atmospheric Observatory (CIAO). Clouds and aerosols play a significant role in the radiative balance of the Earth. One of the main key uncertainties in modeling climate is the aerosol indirect effect (AIE). A relevant part of the study of AIE is related to the effect of the “twilight zone”.

The “twilight zone” is described as a distinct zone (Charlson et al., 2007; Koren et al., 2009; Madonna et al., 2009) characterized by intermediate conditions associated with evaporating cloud fragments and enhanced aerosol. The “twilight zone” has been observed both close to the edge of visible cloud layers or also in apparently clear skies.

The importance of the “twilight zone” arises from the fact that it is estimated that between a significant part of the atmosphere previously considered as cloud-free in satellite observations might actually represented by the “twilight zone” of in-between particles. This fact could have a significant impact on the way the Earth’s radiative budget is estimated. Therefore it is important to study the microphysical processes that occur near clouds, but also in optically “clear” conditions, in order to better understand and characterize this area of the atmosphere.

The approach consists of an analysis of CNR-IMAA CIAO archive data from multiple sensors (MWR, multi-λ LIDAR, ceilometers and sun-photometer) for liquid water signatures in apparently cloud-free datasets (according to a sky camera). Analysis of key variables such as brightness temperatures, liquid water path, relative humidity , water mixing ratio, optical depth, sky radiances will be done in order to address the source (no visible clouds present).

Finally, an estimation of the TOA radiative forcing associated with the “twilight zone” through radiative transfer modeling will be done in cooperation with the University of Reading.