Earth Observation Data Group

Abstract:

We use satellite observations of aerosol optical depth tau(a) and cloud effective radius r(e) from the ATSR-2 instrument in 1997 to investigate the Twomey indirect effect ( IE, -partial derivative ln r(e)/partial derivative ln tau(a)) in regions of continental outflow. We generally find a negative correlation between tau(a) and r(e), with the strongest inverse relationships downwind of Africa. North American and eastern Asian continental outflow exhibits a strong seasonal dependence, as expected. Global values for IE range from 0.10 to 0.16, consistent with theoretical predictions. Downwind of Africa, we find that the IE is unphysically high but robust ( r = -0.85) during JJA associated with high aerosol loading, and attribute this tentatively to the Twomey hypothesis accounting only for a limited number of physical properties of aerosols.

Abstract:

An inter-comparison study of the aerosol optical thickness (AOT) at 0.55 mu m retrieved using different satellite instruments and algorithms based on the analysis of backscattered solar light is presented for a single scene over central Europe on October 13th, 2005. For the first time comparisons have been performed for as many as six instruments on multiple satellite platforms. Ten different algorithms are briefly discussed and inter-compared. It was found that on the scale of a single pixel there can be large differences in AOT retrieved over land using different retrieval techniques and instruments. However, these differences are not as pronounced for the average AOT over land. For instance, the average AOT at 0.55 mu m for the area 7-12E, 49-53N was equal to 0.14 for MISR, NASA MODIS and POLDER algorithms. It is smaller by 0.01 for the ESA MERIS aerosol product and larger by 0.04 for the MERIS BAER algorithm. AOT as derived using AATSR gives on average larger values as compared to all other instruments, while SCIAMACHY retrievals underestimate the aerosol loading. These discrepancies are explained by uncertainties in a priori assumptions used in the different algorithms and differences in the sensor characteristics. Validation against AERONET shows that MERIS provides the most accurate AOT retrievals for this scene. (C) 2007 Elsevier B.V. All rights reserved.

Abstract:

The differing path lengths of the forward and nadir views of the Advanced Along-Track Scanning Radiometer (AATSR) are used to separate the contributions from aerosol scattering and surface reflectance in the observed top-of-atmosphere (TOA) radiance. This poster presents an effort to extend the Oxford-RAL retrieval of Aerosols and Clouds (ORAC) scheme used by the GRAPE and GlobAEROSOL projects [1], which currently uses data from the nadir viewing geometry only, to take advantage of the dual-view capabilities AATSR offers. The new algorithm uses optimal estimation to retrieve aerosol optical depth at 550 nm, effective radius and surface albedo at 550 nm for both forward and nadir viewing geometries (with the spectral shape of the surface constrained by a model based on Cox and Munk statistics [2,3] for the sea, and MODIS data [4] for the land).