INDOEX Aerosol Optical Depths and Radiative Forcing Derived from AVHRR

The Indian Ocean Experiment (INDOEX) has as a primary objective determining the radiative forcing due to anthropogenic aerosols over climatologically significant space and time scales: the Indian Ocean during the winter monsoon, January - March. During the winter monsoon, polluted, low-level air from the Asian subcontinent blows over the Arabian Sea and Indian Ocean. As part of INDOEX, aerosol optical depths are derived from Advanced Very High Resolution Radiometer (AVHRR) data for the cloud-free ocean regions. The AVHRR radiances are first calibrated using the interior zone of the Antarctic and Greenland ice sheets, which prove to be radiometrically stable calibration targets. Optical depths are derived by matching the observed radiances to radiances calculated for a wide range of optical depths and viewing geometry. Optical depths derived with the AVHRR are compared with those derived with NASA's Aerosol Robotic Network (AERONET) CIMEL instrument at the Center for Clouds, Chemistry, and Climate's Kaashidhoo Observatory, as well as with other surface and shipboard observations taken in the INDOEX region. The retrieved and surface-based optical depths agree best for an aerosol model in which the AVHRR observations at 0.63 and 0.83 microns are used to determine relative amounts of marine and polluted continental aerosol and then the resulting aerosol mixture is used to derive the optical depths. Broadband radiative transfer calculations for the mixture of marine and polluted continental aerosols are combined with the 0.63 and 0.83-micron AVHRR radiances to determine the radiative forcing due to aerosols in the INDOEX region. Monthly composites of aerosol optical depth and top of the atmosphere, surface, and atmospheric radiative forcing are derived from calibrated AVHRR radiances for January-March 1996-2000. Based on the standard deviation of the daily means, a region with high mean aerosol concentration exhibits inter-annual variability comparable to its daily variability, whereas a region with low mean aerosol concentration exhibits inter-annual variability that is approximately one-third of its daily variability.