Monday, 3 June 2002
Aerosol radiative forcing estimated from ground-based radiation measurements at GAME/AAN sites
Focusing on radiative forcing of aerosol, we first investigated optical characteristics of aerosol using radiation measurements at 8 stations over East Asia [Sri-Samrong (17.17N, 99.87E) in Thailand, Mandalgovi (45.59N, 106.19E), Dunhuang (40.16N, 94.80E), Hefei (31.90N, 117.16E), Shouxian (32.55N, 116.78E), Yinchuan (38.48N, 106.22E) in China, Anmyon-Do (36.52N, 126.32E) in Korea and Amami-Oshima (28.44N, 129.70E) in Japan], as a part of the GAME/AAN scientific experiment. An algorithm of Nakajima et al. (1996) is used for retrieving aerosol parameters such as optical thickness, Ångström exponent, single scattering albedo, and size distribution from sky-radiation measurements, which then can be used for examining spatial and temporal variations of aerosol. Obtaining aerosol radiative forcing at TOA and surface, a radiative transfer model is used with inputs of obtained aerosol parameters and GMS-5 satellite-based cloud optical properties. Results show that there is a good agreement of simulated downwelling radiative flux at the surface with observation within 10 W m-2 rms error under the clear sky condition. However, a relatively large difference up to 40 W m-2 rms error is found under the cloudy sky condition.
The computed aerosol radiative forcing at the surface shows downward flux changes ranging from -30 to -110 W m-2 per unit aerosol optical thickness at 0.5 mm. The magnitudes are dependent upon geographical locations of observation sites where aerosol characteristics are much different from each other. The surface and TOA flux changes at Sri-Samrong in Thailand, which is located at tropical zone, are much smaller than those from others, indicating that the absorption by aerosol is larger than scattering effect. On the other hand, the scattering is more important than absorption at Anmyon and Amami-Oshima.