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Mamoudou B. Ba, Robert Frouin, Sharon E. Nicholson, and Gérard Dedieu

Abstract

Downward surface solar irradiance and albedo of the African continent are estimated from Meteosat B2 data at 30-km spatial resolution. The algorithm, based on Dedieu et al.’s approach, is verified against other satellite estimates and ground-based measurements. In the computations, the International Satellite Cloud Climatology Project’s (ISCCP) radiometric calibration is adjusted using the Libyan desert as a reference target of constant reflectance properties. Surface albedo is corrected for sun zenith angle effects, allowing for better detection of seasonal changes due to the vegetation cycle. The estimates obtained with Meteosat B2 data agree generally well with other satellite estimates, although biases of 20 W m−2 (downward surface solar irradiance) and 0.15 (surface albedo) are obtained in some cases. There is evidence, from comparisons with surface measurements, that the clear-sky downward surface solar irradiance is overestimated over semiarid regions of Africa because of uncertainties in aerosol characteristics. In the Sahel region, where spatial albedo gradients are high, it is advantageous to use 30-km Meteosat B2 products instead of the current, coarser 280-km-resolution ISCCP products.

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Beth Chertock, Robert Frouin, and Richard C. J. Somerville

Abstract

A new method has been used to generate the first satellite-based long-term climatology of surface solar irradiance over the world oceans. These monthly mean data cover the period November 1978 through October 1985 on a global, 9° latitude-longitude spatial grid. The large-scale variability of surface solar irradiance is assessed over the world oceans for the entire (84-month) record. The results demonstrate the ability of the method to reveal large-scale seasonal and interannual phenomena. The reduction in surface solar irradiance due to clouds is evaluated globally both on monthly and long-term climatological scales. Monthly cloud forcing anomalies are found to display eastward propagation over the course of the 1982–1983 El Niño event. The mean January climatology is found to he consistent with the climatology obtained from a general circulation model run in perpetual January mode. This study marks the first large-scale observation-based examination of cloud solar forcing at the ocean surface. In addition, empirical orthogonal function (EOF) analysis is employed to investigate modes of seasonal and nonseasonal variability. Nonseasonal EOF modes of surface solar irradiance are related to nonseasonal EOF modes of outgoing longwave radiation (OLR). The dominant modes during the 1982–1983 El Niño are associated with eastward propagation in both the shortwave and longwave fields. These dominant nonseasonal EOF modes of surface solar irradiance are found to display features and amplitude variations that are nearly identical to those of the corresponding nonseasonal EOF modes of OLR. The association of these modes with El Niño is quantified using the correlation of the mode amplitudes with the Southern Oscillation index (SOI). In each case modes 1 and 2 are positively correlated with the SOI, and mode I has a strong correlation of 0.75 for the shortwave and 0.76 for the longwave field. Finally, a study of the regionally averaged behavior of surface solar irradiance and sea surface temperature (SST) in a section of the tropical Pacific (9°N–9°5, 117°– 144°W) during this same period indicates that fluctuations of surface solar irradiance in the tropical Pacific are sometimes a regional response to underlying changes in SST (and associated changes in cloudiness), rather than a driving mechanism responsible for variations in SST.

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Akiko Higurashi, Teruyuki Nakajima, Brent N. Holben, Alexander Smirnov, Robert Frouin, and Bernadette Chatenet

Abstract

Global distributions of the aerosol optical thickness and Ångström exponent are estimated from National Oceanic and Atmospheric Administration Advanced Very High Resolution Radiometer channel-1 and -2 radiances for four months in 1990. Global distributions of those Ångström parameters are consistent with present knowledge on the distributions of desert-derived, biomass-burning, and anthropogenic pollutant aerosols obtained by ground-based and aircraft measurements. Especially, it is found that thin anthropogenic aerosols can be identified with large Ångström exponent values around the east coast of North America, Europe, and eastern Asia. Satellite-retrieved values of Ångström parameters are further compared with measured spectral optical thickness obtained by the National Aeronautics and Space Administration Aerosol Robotic Network sky radiometer network.

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Arthur J. Miller, Michael A. Alexander, George J. Boer, Fei Chai, Ken Denman, David J. Erickson III, Robert Frouin, Albert J. Gabric, Edward A. Laws, Marlon R. Lewis, Zhengyu Liu, Ragu Murtugudde, Shoichiro Nakamoto, Douglas J. Neilson, Joel R. Norris, J. Carter Ohlmann, R. Ian Perry, Niklas Schneider, Karen M. Shell, and Axel Timmermann
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