Abstract
Due to instrument sensitivities and algorithm detection limits, level 2 (L2) Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) 532nm 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% of all daytime CALIOP L2 aerosol profiles (i.e., including completely attenuated profiles), accounting for nearly half (45 %) 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.
Highlights
The first approach introduces an artificial underestimation of mean aerosol optical thickness (AOT) by including profiles in which AOT was not retrieved. The latter, presumably leads to an overestimation, since it is likely that all-retrieval fill values (RFVs) profiles reflect relatively low AOT cases in which Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) layer detection exhibits a lack of sensitivity to diffuse aerosol presence that caused nothing to be reported within the column
Both plots were obtained from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) Lidar Browse Images website, and the data were collected from CALIOP during the daytime on 2 July 2010 over the Arctic
The vertical feature mask (VFM) shows that the range bins within the white box are classified as either surface or clear air features, and the corresponding level 2 (L2) aerosol extinction coefficient profiles are all-RFVs
Summary
Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) measurements provide critical information on aerosol vertical distribution for studies involving aerosol modeling (e.g., Campbell et al, 2010; Sekiyama et al, 2010; Yu et al, 2010; Zhang et al, 2011, 2014), air quality (e.g., Martin, 2008; Prados et al, 2010; Toth et al, 2014), aerosol climatic effects (e.g., Huang et al, 2007; Chand et al, 2009; Tesche et al, 2014; Thorsen and Fu, 2015; Alfaro-Contreras et al, 2016), and aerosol climatologies (Pappalardo et al, 2010; Wandinger et al, 2011; Amiridis et al, 2015; Toth et al, 2016). Toth et al.: Minimum CALIPSO aerosol layer detection sensitivities domains (e.g., Kacenelenbogen et al, 2011; Kittaka et al, 2011; Redemann et al, 2012; Kim et al, 2013; Ma et al, 2013) These studies tend to attribute the AOT differences to either uncertainties/cloud contamination in the MODIS retrieval or incorrect selection of the lidar ratio (extinctionto-backscatter ratio; Campbell et al, 2013) when deriving CALIOP aerosol extinction and subsequent AOT. The first approach introduces an artificial underestimation of mean AOT by including profiles in which AOT was not retrieved The latter, presumably leads to an overestimation, since it is likely that all-RFV profiles reflect relatively low AOT cases (i.e., lower than any apparent mean sample value) in which CALIOP layer detection exhibits a lack of sensitivity to diffuse aerosol presence that caused nothing to be reported within the column. 2 years of CALIOP aerosol data are analyzed prior to, and after, the switch to investigate any discernible difference in RFV statistics between the two lidar profiles
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