Many research studies using satellite-derived aerosol and shortwave flux information have shown that aerosols scatter shortwave radiation back into space, cooling the atmosphere under clear sky conditions. However, significant questions remain as to the magnitude of this effect, especially over land due to much greater uncertainties in the land aerosol optical thickness (AOT) retrieval algorithms. Recent improvements in these algorithms present in the Collection 5 MODIS data product allow for an improved assessment of overland aerosol radiative effects. This research uses 6 months of Collection 5 MODIS AOT data between June and November 2006 combined with shortwave flux measurements from the CERES instrument aboard the Terra and Aqua satellites. As of May 2006, MODIS Collection 5 and CERES flux data are available in the CERES-SSF product, which uses a point spread weighting function to apply the level 2 AOT data onto each CERES-level pixel. Direct radiative effect is defined as the difference between cloud and aerosol free shortwave flux and observed shortwave flux. Using this process, we produce the first estimates of radaitive effects over land from a primarily observational perspective using the latest available AOT and shortwave flux products. For this study period, we observed DRE values up to –10.0 Wm-2, which depends on total aerosol concentration, aerosol type, and assumed background conditions. Similar results were observed from both Terra and Aqua data. Radiative efficiency varies significantly from region to region, and additional research is underway to determine the precise cause.

Uncertainty remains high, especially with the clear sky background which must be estimated for conditions where AOT = 0. Resulting radiative effect values can vary in excess of ±50% depending on the assumptions used to derive this background. As more improved CERES-SSF data becomes available, this research will be expanded to encompass multiple years of data, while also incorporating different sensors to improve the ability to distinguish between various aerosol types over land and their relative effects to the climate.