Abstract
Abstract. We assess the consistency between instantaneously collocated level-2 aerosol optical depth (AOD) retrievals from MODIS-Aqua (C5) and CALIOP (Version 2 & 3), comparing the standard MODIS AOD (MYD04_L2) data to the AOD calculated from CALIOP aerosol extinction profiles for both the previous release (V2) and the latest release (V3) of CALIOP data. Based on data collected in January 2007, we investigate the most useful criteria for screening the MODIS and CALIOP retrievals to achieve the best agreement between the two data sets. Applying these criteria to eight months of data (Jan, Apr, Jul, Oct 2007 and 2009), we find an order of magnitude increase for the CALIOP V3 data density (by comparison to V2), that is generally accompanied by equal or better agreement with MODIS AOD. Differences in global, monthly mean, over-ocean AOD (532 nm) between CALIOP and MODIS range between 0.03 and 0.04 for CALIOP V3, with CALIOP generally biased low, when all available data from both sensors are considered. Root-mean-squares (RMS) differences in instantaneously collocated AOD retrievals by the two instruments are reduced from values ranging between 0.14 and 0.19 using the unscreened V3 data to values ranging from 0.09 to 0.1 for the screened data. A restriction to scenes with cloud fractions less than 1% (as defined in the MODIS aerosol retrievals) generally results in improved correlation (R2>0.5), except for the month of July when correlations remain relatively lower. Regional assessments show hot spots in disagreement between the two sensors in Asian outflow during April and off the coast of South Africa in July.
Highlights
Aerosols introduce a major uncertainty in predictions of possible future changes to the Earth system in general, and its climate in particular, owing to the incomplete knowledge of aerosol physicochemical properties and their spatial distribution
We conclude that 12 × 12 km2 boxes centered at CALIOP V3 aerosol optical depth (AOD) retrievals provide an optimum sampling of MODIS retrievals for comparison of the two data sets
To illustrate the technical details of our comparison between MODIS and CALIOP AOD, this section discusses the comparison of MODIS and CALIOP AOD at 532 nm for January of 2007
Summary
Aerosols introduce a major uncertainty in predictions of possible future changes to the Earth system in general, and its climate in particular, owing to the incomplete knowledge of aerosol physicochemical properties and their spatial distribution. Estimates of aerosol indirect effects require detailed knowledge of aerosol activation processes in areas difficult to access with satellite sensors, e.g., below opaque clouds It is paramount for the scientific community to develop techniques to combine the A-Train aerosol observations with each other and with auxiliary suborbital observations to improve the estimates of the specific aerosol properties relevant for the various processes governing aerosol-climate interactions. The most comparable observation between the two sensors, that is relevant for aerosol radiative effects, is AOD, which MODIS provides at seven wavelengths over-ocean (one of which is extrapolated) and three wavelengths over-land (of which two hold no independent information), and which can be calculated from CALIOP profiles of aerosol extinction at 532 nm and 1064 nm. Their comparisons were screened very differently from our data set, and the fundamental result of a bias difference of ∼0.064 between MODIS and CALIOP AOD is similar to ours
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