Monday, 3 June 2002
Seasonal variation of aerosol direct radiative forcing and optical properties estimated from the ground-based solar radiation measurements
We have analyzed the surface direct radiative forcing and optical properties of aerosols from the ground-based solar radiation measurement, which was operated under clear sky conditions at the Meteorological Research Institute, located at 36.05ĀčN and 140.15ĀčE in Tsukuba, Japan, for two years from April 1997 to March 1999. We have simultaneously measured the global and diffuse irradiances in the visible (VIS) and near-infrared (NIR) regions by using two sets of the total-band (0.305–2.9 mm) and NIR-band (0.715– 2.9mm) pyranometers, respectively. The VIS-band irradiance was estimated by taking differences between the total-band and NIR-band irradiances. We also measured the spectral optical thicknesses of aerosols in the air-column by a sun photometer. From these data, we have estimated the broadband surface radiative forcing, and further retrieved size distributions and the imaginary part of the complex refraction index of the columnar aerosols. We found a close correlation, with similar features of seasonal variations, among these parameters. The columnar aerosols exhibit the minimum surface radiative forcing with the minimum optical thickness, but with the maximum value (0.04) of the imaginary refractive index in winter, and vice versa in summer, when the minimum imaginary index of 0.02 was estimated. The surface radiative forcing in the VIS-band was estimated to be almost four times larger than in the NIR-band. The total-band aerosol forcing efficiency, defined by a change in the surface radiative forcing due to a unit increase of aerosol optical thickness at 500nm, takes a largest magnitude of –160 Wm-2 in winter and a smallest magnitude of about -105 Wm-2 in spring through summer. The results suggest that the correlated seasonal variations between the aerosol radiative forcing and the optical properties are caused by seasonal changes in dominant aerosol components.
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