83rd Annual

Thursday, 13 February 2003
Short wave aerosol direct forcing over cloud free oceans from Terra
Jianglong Zhang, University of Alabama, Huntsville, AL; and S. A. Christopher
Poster PDF (67.3 kB)
To date, aerosols remain one of the largest sources of uncertainty in climate change studies. The recent IPCC 2001 has categorized the understanding levels of all aerosol types as either low or very low. Previous studies have addressed the issues of aerosol climate forcing using either radiative transfer models and chemical transport model or General Circulation Models (GCM). However, the accurate prediction of the aerosol climate effect using current models is a difficult task. This is because of the wide spatial distribution of aerosols and their large variations in chemical composition and microphysical properties. In this paper, satellite measurements from Terra are used to examine short wave (SW) aerosol climate forcing (SWARF) over cloud free oceans from an observational perspective. Broadband measurements from the Clouds and Earth's Radiant Energy System (CERES) instruments are used to study the short wave aerosol climate effect (SWARF) with and without the present of aerosols. Narrow band measurements from the Moderate-Resolution Imaging Spectroradiometer (MODIS) are used to discriminate aerosol scenes from totally clear and cloudy scenes of CERES pixels. This study shows that for September 2000, the averaged aerosol optical thickness and SWARF are 0.07 and -6Wm-^2 respectively for CERES cloud free regions over global ocean. Dust aerosols from the West Coast of Africa are also found to have higher SWARF values compared with aerosols from biomass burning regions.

Preliminary results in cloudy conditions show that for observed cloud fraction values less than 20% within a CERES footprint, aerosol direct and indirect effects are dependent upon aerosol optical thickness (AOT) ranges. While aerosol indirect effect dominates at the low aerosol loading regions, the aerosol direct effect dominates at the high aerosol loading scenes. Major sources of uncertainties in this study, such as cloud contamination and Angular Distribution Models (ADMs), must be carefully examined in future studies.

Supplementary URL: http://vortex.nsstc.uah.edu/~sundar/papers/grl_2002_revise.pdf