Improving the CALIOP aerosol optical depth using combined MODIS-CALIOP observations and CALIOP integrated attenuated total color ratio

This paper aims to evaluate CALIOP aerosol optical depth (AOD) retrieval using MODIS AOD with the goal of improving the CALIOP selection of the lidar ratio leveraging the vertical resolved CALIOP and multispectral MODIS observations. Comparing the MODIS fine mode ratio to CALIOP, we find that the CALIOP integrated attenuated total color ratio provides sensitivity to the aerosol size and type. This finding can be used to better constrain the lidar ratio and improve the CALIOP AOD independent from MODIS. To retrieve the aerosol optical depth from CALIOP requires knowledge of the aerosol lidar ratio that varies significantly as a function of aerosol type. For most CALIOP retrievals the lidar ratio is estimated by correlating CALIOP observables (depolarization and backscatter) with precomputed lidar ratio climatologies. Applying a lidar ratio not representative of the observed aerosols can result in significant AOD biases and is one of the primary sources of uncertainty in the current CALIOP AOD. We demonstrate that over ocean the MODIS sensitivity to the fine-and coarse-mode aerosol mixing ratios provides additional constraints to the aerosol lidar ratio. When MODIS fine-mode retrievals are collocated with CALIOP, the improved lidar ratio significantly reduces the CALIOP AOD mean biases from vertical bar 0.064 vertical bar to vertical bar 0.020 vertical bar when compared to MODIS. In addition, we demonstrate that the CALIOP integrated attenuated total color ratio is correlated with the MODIS fine and coarse mixing ratios in marine environments. This finding suggests that for a CALIOP-only AOD retrieval the integrated attenuated total color ratio can be used to better constrain the lidar ratio. Using the CALIOP integrated attenuated total color ratio, the CALIOP-only AOD retrieval improves the AOD mean biases (vertical bar 0.064 vertical bar to vertical bar 0.007 vertical bar) when compared to the MODIS AOD.