A Look at Amazon Basin Seasonal Dynamics with the Biophysical ...

Atmospheric Attenuation Of Total Solar Flux By Clouds At Six Amazonian Sites: 1999-2001J.S. Schafer 1,2 , B.N. Holben 2 T.F. Eck 2,3 , P. Artaxo 4 , M.A. Yamasoe 5 , A.S. Procopio 41 Science Systems and Applications Inc., Code 923, NASA/Goddard Space Flight Center, Greenbelt, Maryland20771 e-mail: jschafer@aeronet.gsfc.nasa.gov2 Biospheric Sciences Branch, Code 923, NASA/Goddard Space Flight Center, Greenbelt, Maryland.3 Goddard Earth Sciences and Technology Center, University of Maryland – Baltimore County, Baltimore,Maryland.4 Instituto de Física Aplicada, Universidade de São Paulo, São Paulo, Brazil5 Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, São Paulo, BrazilIn Brazil, we now have a data set of pyranometer measurements at several sites distributedacross the Amazon basin, with a record spanning more than 3 years at some locations. Thisnetwork provides an opportunity to characterize the nature of atmospheric effects on surface,broadband irradiance. Sufficient data are now available to assess trends in cloud attenuation on arange of timescales (diurnally, seasonally, and interannually).Cloud-induced fractional and absolute total flux reductions at the surface were evaluated forall years and sites. The fractional reduction, ƒ B was computed as the ratio of received irradianceto the modeled clear-sky irradiance for background (low) aerosol conditions. A distinctdifference was found between cloud attenuation in the wet and dry seasons, particularly in thesouthern Amazon.Histograms of ƒ B for typical wet season months reveal a bi-modal distribution with areduction peak (when the solar beam is obstructed) and an enhancement peak (produced by edgereflections from broken cloud cover). This phenomenon has been noted previously at theAbracos Hill sites during a 2 month study in 1999 (Gu et al., 2001). Our multi-year, multi-sitedata now suggest this is a fairly consistent feature of wet season months in the southern sites inRondônia and northern Mato Grosso, and of most or all months at the equatorial sites (Balbina,Belterra) in Pará and Amazonas. The average reduction by clouds observed at one site was 54%for February and only 15% for June. Such dramatic differences in available surface insolation(due to seasonal cloud variation) need to be quantified for climate modeling applications.