Observational evidence on the effects of clouds and aerosols on net ecosystem exchange and evapotranspiration

Abstract

Clouds and aerosols control variations in the ratio of diffuse to total incident solar radiation (Rd/Rs) and so determine to a large extent variations in both Light-Use Efficiency (LUE) and in the Evaporative Fraction (EF). Five years (2002-2007) of continuous ground measurements collected at twenty-three sites in the U.S. were analyzed to quantify this effect. Our results show that LUE is 19.4% and 203% larger for patchy clouds, and thick clouds than those for clear skies while LUE is about -6% for aerosols or thin clouds than those for clear skies. EF is 15.4%, 17.9% and 23.2% larger for aerosols or thin clouds, patchy clouds, and thick clouds than those for the clear sky when Eddy Covariance (ECOR) data are used, respectively. The values are 9.0%, 11.3% and 23.3% when Energy Balance Bowen Ratio (EBBR) data are used. This is the first systematic observational study on effects of Rd/Rs on EF and further study is needed. Copyright 2008 by the American Geophysical Union.