Minimum aerosol layer detection sensitivities and their subsequent impacts on aerosol optical thickness retrievals in CALIPSO level 2 data products

Abstract

Abstract. 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 % 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.

How much of the global aerosol optical depth is found in the boundary layer and free troposphere?

“…The European Centre for Medium-range Weather Forecasts Re-Analysis-Interim (ERA-Interim) archive is based on data assimilation of observations, in particular from satellites, and numerical weather prediction modeling Dee et al, 2011). The boundary layer height product is diagnosed using the parcel lifting method (or bulk Richardson method) proposed by Troen and Mahrt (1986). The product has been evaluated and showed good agreement with other methods to determine the BL height (von Engeln and Teixeira, 2013).…”

A Global Analysis of Dust Diurnal Variability Using CATS Observations

“…The spatial variations in DAOD is evaluated against MISR. In light of the larger uncertainty associated with a reported AOD of zero (Toth et al, 2018), any AOD or DAOD that equal to zero is ignored in the current analysis, following Campbell et al (2012). This approach results in ~1000 of DAOD retrievals in any three-hour local time window at each 2˚x2˚ grid cell over the dust source regions, such as North Africa, and fewer than 100 retrievals over remote oceans (Supplemental Figure 1).…”

Answer to Referee 2

“…Fire emissions occur most of the time in the PBL. However, for some large fires, estimated at roughly 15 % of all fires, fire emissions are released in the free troposphere above the PBL (Val Martin et al, 2010;Martin et al, 2012;Sofiev et al, 2012). In some extreme cases, fire emissions can even reach the upper troposphere-lower stratosphere region (Fromm et al, 2006).…”