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GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L08805, doi:10.1029/2007GL029253, 2007

On the twilight zone between clouds and aerosols

Ilan Koren

Department of Environmental Sciences Weizmann Institute, Rehovot, Israel


Lorraine A. Remer

NASA Goddard Space Flight Center, Greenbelt, Maryland, USA


Yoram J. Kaufman

NASA Goddard Space Flight Center, Greenbelt, Maryland, USA


Yinon Rudich

Department of Environmental Sciences Weizmann Institute, Rehovot, Israel


J. Vanderlei Martins

NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
Department of Physics and Joint Center for Earth Systems Technology, University of Maryland Baltimore County, Baltimore, Maryland, USA


Abstract

Cloud and aerosols interact and form a complex system leading to high uncertainty in understanding climate change. To simplify this non-linear system it is customary to distinguish between “cloudy” and “cloud-free” areas and measure them separately. However, we find that clouds are surrounded by a “twilight zone” – a belt of forming and evaporating cloud fragments and hydrated aerosols extending tens of kilometers from the clouds into the so-called cloud-free zone. The gradual transition from cloudy to dry atmosphere is proportional to the aerosol loading, suggesting an additional aerosol effect on the composition and radiation fluxes of the atmosphere. Using AERONET data, we find that the measured aerosol optical depth is higher by 13% ± 2% in the visible and 22% ± 2% in the NIR in measurements taken near clouds relative to its value in the measurements taken before or after, and that 30%−60% of the free atmosphere is affected by this phenomenon.

Received 6 January 2007; accepted 20 March 2007; published 18 April 2007.

Keywords: clouds; aerosols; twilight.

Index Terms: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801, 4906); 0319 Atmospheric Composition and Structure: Cloud optics; 0320 Atmospheric Composition and Structure: Cloud physics and chemistry; 0321 Atmospheric Composition and Structure: Cloud/radiation interaction; 0345 Atmospheric Composition and Structure: Pollution: urban and regional (0305, 0478, 4251).

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Citation: Koren, I., L. A. Remer, Y. J. Kaufman, Y. Rudich, and J. V. Martins (2007), On the twilight zone between clouds and aerosols, Geophys. Res. Lett., 34, L08805, doi:10.1029/2007GL029253.