Minimum Aerosol Layer Detection Sensitivities and their Subsequent Impacts on Aerosol Optical Thickness Retrievals in Calipso Level 2 Data Products
Journal Article - Open Access
University of North Dakota Grand Forks United States
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Due to instrument sensitivities and algorithm detection limits, level 2 L2 Cloud-Aerosol Lidar with Orthogonal Polarization CALIOP 532 nm aerosol extinction profile retrievals are often populated with retrieval fill values RFVs, which indicate the absence of detectable levels of aerosol within the profile. In this study, using 4 years 2007-2008 and 2010-2011 of CALIOP version 3 L2 aerosol data, the occurrence frequency of daytime CALIOP profiles containing all RFVs all-RFV profiles is studied. In the CALIOP data products, the aerosol optical thickness AOT of any all-RFV profile is reported as being zero, which may introduce a bias in CALIOP-based AOT climatologies. For this study, we derive revised estimates of AOT for all-RFV profiles using collocated Moderate Resolution Imaging Spectroradiometer MODIS Dark Target DT and, where available, AErosol RObotic NEtwork AERONET data. Globally, all-RFV profiles comprise roughly 71 percent of all daytime CALIOP L2 aerosol profiles i.e., including completely attenuated profiles, accounting for nearly half 45 percent of all daytime cloud-free L2 aerosol profiles. The mean collocated MODIS DT AERONET 550 nm AOT is found to be near 0.06 0.08 for CALIOP all-RFV profiles. We further estimate a global mean aerosol extinction profile, a so-called noise floor, for CALIOP all-RFV profiles. The global mean CALIOP AOT is then recomputed by replacing RFV values with the derived noise-floor values for both all-RFV and non-all-RFV profiles. This process yields an improvement in the agreement of CALIOP and MODIS over-ocean AOT.
- Atmospheric Physics