Post-processing to remove residual clouds from aerosol optical depth retrieved using the Advanced Along Track Scanning Radiometer
Atmospheric Measurement Techniques Discussions
Clouds reflect solar light much stronger than aerosol particles. Therefore, the retrieval of aerosol optical depth (AOD) is usually only attempted over cloud-free areas. To achieve this, a strict cloud detection scheme needs to be applied consisting of several tests. However, often not all clouds are detected which results in cloud-contaminated retrieval areas with reflectance higher than that for aerosols. The AOD retrieved from reflectances measured with the (Advanced) Along Track Scanning
... g Track Scanning Radiometers (ATSR-2 and AATSR), using the ATSR dual view algorithm (ADV) over land or the ATSR single view algorithm (ASV) over ocean, shows such areas with locally enhanced AOD values. To remove such cloud-contaminated areas, a cloud post-processing (CPP) scheme has been developed at the Finnish Meteorological Institute (FMI) as described in Kolmonen et al. (2015). The application of this scheme indeed results in the removal of cloud-contaminated areas over most of the globe, providing spatially smoother AOD maps and favorable comparison with AOD obtained from independent ground-based measurements from the AERONET sun photometer network. However, closer inspection shows that the CPP also removes areas with elevated AOD not due to cloud contamination, as shown in this manuscript. An improved CPP scheme is presented which better discriminates between valid and cloud-contaminated areas. Also the CPP thresholds have been further evaluated and adjusted according to the findings. Retaining elevated AOD while effectively removing cloud contaminated pixels affects the resulting global and regional mean AOD values as well as coverage. Effects of the CPP scheme on both spatial and temporal variation for the period 2002&ndash;2012 are discussed. With the modified CPP, the AOD coverage increases by 10&ndash;15&thinsp;% with respect to the old scheme. The validation versus AERONET shows an improvement of the correlation coefficient from 0.84 to 0.86 for the global data set for the period 2002&ndash;2012. The global aggregated AOD over land for the period 2003&ndash;2011 is 0.163 with the improved CPP as compared to 0.144 with the old scheme. The globally aggregated AOD over ocean and over the whole globe (land and ocean together) is 0.164 with the improved CPP scheme (as compared to 0.152 and 0.150 with the old scheme, for ocean and whole globe respectively). Effects of the improved CPP scheme on the 10-year time series are illustrated and seasonal and temporal changes are discussed.