Aerosol optical depths over peninsular India and adjoining oceans during the INDOEX campaigns: Spatial, temporal, and spectral characteristics

The spatial, temporal, and spectral characteristics of aerosol optical dept hs tau (p lambda) for the Indian Ocean Experiment (INDOEX) study period (Ja nuary to April) are examined using data collected through a ground-based ne twork of multiwavelength solar radiometers (MWR) over coastal regions of pe ninsular India; two island locations, one in the Arabian Sea and another in the southern Indian Ocean at 20 degreesS; in conjunction with estimates ma de over various locations over the Arabian Sea and Indian Ocean during the INDOEX cruises of 1996, 1998, and 1999. Spatial variations show extremely l ow values of tau (p) at the shorter (visible) wavelengths (lambda < 750 nm) to the south of the Intertropical Convergence Zone (ITCZ), but increases s ubstantially at locations due north of the ITCZ due to increased source imp act and advection. by favorable winds. An enhancement in tau (p) is seen in the central Arabian Sea, which is attributed to air trajectory effects. An gstrom parameters, deduced from optical depth spectra, reveal a high value of alpha (similar to0.9) for north of the ITCZ, while for the south alpha i s negative, indicating a change in the aerosol size distribution. Accumulat ion aerosols dominate in the north, while concentration of coarse aerosols remain nearly about the same, except very close to the coast. A north-south gradient in aerosol optical depth, with scaling distance of similar to 100 0 to 2000 kin at shorter wavelengths and much higher at longer wavelengths, is observed. The gradient becomes shallower at high wind speeds. The large -scale dynamics associated with the movement of the ITCZ and its interannua l variation appears to significantly influence the aerosol characteristics. As the southwest monsoon sets in over India, considerable wet removal and change in air mass characteristics cause a significant depletion in optical depths, which then became comparable to those prevailing in the southern h emisphere.