Observations of the spectral clear-sky aerosol forcing over the tropical Indian Ocean

• Published in: Journal of Geophysical Research 1999-10, Vol.104 (D20), p.24
• Main Authors: Meywerk, Jens, Ramanathan, V
• Format: Article
• Language: English
• ISSN: 0148-0227

During the first field phase (FFP) of the Indian Ocean Experiment (INDOEX) in February and March, 1998, the spectral global and direct beam irradiance have been measured between 350 and 1050 nm wavelengths using a 512-channel, fixed grating, photodiode array spectroradiometer. The spectral optical depth, the spectral aerosol forcing, and the aerosol forcing for the photosynthetically active radiation have been derived from direct beam measurements and global irradiance measurements. The optical depth at 500 nm wavelength decreases from approximately 0.5 in the northern Arabian Sea to as low as 0.05 south of the Intertropical Convergence Zone (ITCZ) near approximately 15 deg S latitude. The surface aerosol forcing efficiency is defined as the rate of change of net irradiance at the surface due to an increase by 1 in optical depth at 500 nm. The continental aerosol south of the ITCZ shifts the peak in the direct solar radiation from 470 nm (for pristine conditions) to approximately 580 nm for the polluted region. The spectral aerosol forcing efficiency peaks around 460 nm, with -1.2, -0.6, and +0.6 W/sq m/nm for the direct, global, and diffuse irradiance, dropping for the lower and higher wavelengths to about -0.3, -0.25, and 0.05 W/sq m/nm at 350 nm and -0.3, -0.1, and +0.2 at 1050 nm. Integrated over 400-700 nm, the aerosols decrease the noontime solar flux by as much as -38 W/sq m in the Arabian Sea to as little as -2 W/sq m south of the ITCZ. (Author)