Comparison and analysis of two aerosol retrievals over the ocean in the Terra/Clouds and the Earth's Radiant Energy System–Moderate Resolution Imaging Spectroradiometer single scanner footprint data: 2. Regional evaluation

Abstract

The advanced multichannel Moderate Resolution Imaging Spectroradiometer (MODIS) and simple independent two‐channel Advanced Very High Resolution Radiometer (AVHRR) aerosol retrieval algorithms were compared regionally using the Terra/CERES‐MODIS Single Scanner Footprint (SSF) data. On average, it was found that the two methods tend to overestimate 0.66‐μm aerosol optical thickness (AOT) compared to AERONET surface observations in the original SSF data. If the most cloud‐free data are used, the mean satellite retrievals agree to within ±10% of the AERONET data. The MODIS near‐infrared (1.60‐μm) AOTs are in better agreement with the surface data than the AVHRR‐type retrievals. The satellite‐derived aerosol size parameters are 20–30% smaller than the surface‐based values with the MODIS values closer to the AERONET values than that of AVHRR‐type. The effects of aerosol model assumptions, cloud contamination, and surface roughness on the two aerosol retrievals were analyzed in detail with the careful classification of clear‐sky and surface roughness conditions. For most of the regions examined, the annual mean AOTs from the MODIS retrieval are 0.03 and 0.02 less than their AVHRR‐type counterparts at 0.66 and 1.60 μm, respectively. However, the MODIS values may exceed the AVHRR‐type values in regions where the prevailing aerosol type varies with season or is under an apparent influence of cloud or surface disturbance. Examination of the surface treatments used by the two retrieval methods indicates the need for improvement over very rough ocean surfaces, especially for the AVHRR method. The results indicate that aerosol model assumptions become important for regional retrievals and the dynamic aerosol models used in the MODIS retrieval are better suited for simultaneously measuring the regional variations in aerosol optical properties and their global mean values.