Abstract
Abstract. This work provides a comparison of satellite retrievals of Saharan desert dust aerosol optical depth (AOD) during a strong dust event through March 2006. In this event, a large dust plume was transported over desert, vegetated, and ocean surfaces. The aim is to identify the differences between current datasets. The satellite instruments considered are AATSR, AIRS, MERIS, MISR, MODIS, OMI, POLDER, and SEVIRI. An interesting aspect is that the different algorithms make use of different instrument characteristics to obtain retrievals over bright surfaces. These include multi-angle approaches (MISR, AATSR), polarisation measurements (POLDER), single-view approaches using solar wavelengths (OMI, MODIS), and the thermal infrared spectral region (SEVIRI, AIRS). Differences between instruments, together with the comparison of different retrieval algorithms applied to measurements from the same instrument, provide a unique insight into the performance and characteristics of the various techniques employed. As well as the intercomparison between different satellite products, the AODs have also been compared to co-located AERONET data. Despite the fact that the agreement between satellite and AERONET AODs is reasonably good for all of the datasets, there are significant differences between them when compared to each other, especially over land. These differences are partially due to differences in the algorithms, such as assumptions about aerosol model and surface properties. However, in this comparison of spatially and temporally averaged data, it is important to note that differences in sampling, related to the actual footprint of each instrument on the heterogeneous aerosol field, cloud identification and the quality control flags of each dataset can be an important issue.
Highlights
Desert dust is one of the most abundant and important aerosols in the atmosphere
We report results from the Desert dust Retrieval Intercomparison (DRI) project, which performed a comparison of retrievals for a Saharan desert dust episode in March, 2006 using data from a wide range of state-ofthe-art schemes
If dust is modelled assuming non-spherical particles, additional decisions need to be made as to the specific distribution of particle shape(s) to use. These differences, which can be significant, will affect the calculated phase function and so the retrieved aerosol optical depth (AOD) (e.g. Kalashnikova and Sokolik, 2002; Kalashnikova et al, 2005). These effects are not analysed in the present paper, but for future research we suggest that an analysis of AOD differences between datasets as a function of scattering angle could help isolate phase function effects
Summary
Desert dust is one of the most abundant and important aerosols in the atmosphere. Dust grain size and composition make it radiatively active over a wide spectral range (from the ultraviolet to the thermal infrared), and so airborne dust has a significant direct radiative forcing on climate (IPCC, 2007 – ar4 2.4.1). Aerosol retrievals for satellite radiometers have typically made use of visible and near-infrared measurements, the interpretation of which becomes difficult over bright surfaces such as deserts (Kaufman et al, 1997) To overcome these difficulties, more recent algorithms make use of additional information available from certain instruments, for example multi-angle observations, shorter (ultraviolet) wavelengths, thermal infrared wavelengths, and polarisation. We report results from the Desert dust Retrieval Intercomparison (DRI) project, which performed a comparison of retrievals for a Saharan desert dust episode in March, 2006 using data from a wide range of state-ofthe-art schemes This comparison reveals differences related to a range of factors including details of retrieval schemes, sensitivity of different instruments, accuracy of the aerosol model assumed, and the importance of good quality control. The project compiled a database of retrieval results which can be used in future work to test algorithm improvements
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