Global component aerosol direct radiative effect at the top of atmosphere

Abstract

The two-step approach of combining Clouds and the Earth's Radiant Energy System (CERES)/Moderate Resolution Imaging Spectroradiometer (MODIS) shortwave (SW) flux and aerosol optical thickness (AOT) at 0.55 μm with the component AOT fractions from the Goddard Space Flight Centre (GSFC)/Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model to derive top of atmosphere (TOA) component aerosol direct radiative effect (ADRE) over the global cloud-free oceans proposed by the first author in a previous publication has been extended to cloud-free land areas for nearly global coverage. Validation has also been performed by comparing the ADRE computation with calculations from the Fu–Liou radiative transfer model at globally distributed AErosol RObotic NETwork (AERONET) sites by using the aerosol optical properties observed from AERONET and surface reflectance obtained from MODIS observations as the model inputs. The promising validation results provide support for extending the two-step approach from global clear-sky oceans to global clear-sky land areas. The global annual mean values of ADRE for clear-sky condition are +0.3 ± 0.2 W m⁻² for black carbon, −1.0 ± 0.6 W m⁻² for organic carbon; −2.3 ± 0.7 W m⁻² for sulphate; −1.6 ± 0.5 W m⁻² for dust; −2.2 ± 0.6 W m⁻² for sea salt; −2.4 ± 0.8 W m⁻² for anthropogenic aerosol; −4.5 ± 1.2 W m⁻² for natural aerosol; and −6.8 ± 1.7 W m⁻² for total aerosols. For global average cloudy skies, the all-sky values of component ADRE are about 42% of their clear-sky counterparts. The major sources of uncertainty in the estimates are also discussed.

Acknowledgements

We would like to acknowledge the NASA CERES project for providing the CERES/MODIS data and Dr Mian Chin at the NASA GSFC for providing GOCART model data. Proofreading of the manuscript by Drs Ken Knapp and Huai-min Zhang at the NCDC is greatly appreciated. Dr Zhao is supported by the CDR program at the NCDC and the NASA Radiation Program managed by Dr Hal Maring through grant RSP-0022-0005. Dr Laszlo also acknowledges the funding support by the NASA Radiation Program. Constructive comments and encouragement from two anonymous reviewers are gratefully acknowledged. We also appreciate the effort of Editor Costas Varotsos for organizing the special issue of International Journal of Remote Sensing on the role of remote sensing in monitoring the climate change.