Abstract
We assess the CALIPSO Version 4.2 (V4) aerosol typing and assigned lidar ratios over ocean using aerosol optical depth (AOD) retrievals from the Synergized Optical Depth of Aerosols (SODA) algorithm and retrieved columnar lidar ratio estimated by combining SODA AOD and CALIPSO attenuated backscatter (CALIPSO-SODA). Six aerosol types – clean marine, dusty marine, dust, polluted continental/smoke, polluted dust, and elevated smoke – are characterized using CALIPSO-SODA over ocean and the results are compared against the prescribed V4 lidar ratios, when only one aerosol type is present in the atmospheric column. For samples detected at 5-km or 20-km spatial resolutions and having AOD > 0.05, the CALIPSO-SODA lidar ratios are significantly different between different aerosol types, and are consistent with the type-specific values assigned in V4 to within 10 sr (except for polluted continental/smoke). This implies that the CALIPSO classification scheme generally categorizes aerosols correctly. We find remarkable daytime/nighttime regional agreement for clean marine aerosol over the open ocean (CALIPSO-SODA = 20–25 sr, V4 = 23 sr), elevated smoke over the southeast Atlantic (CALIPSO-SODA = 65–75 sr, V4 = 70 sr), and dust over the subtropical Atlantic adjacent to the African continent (CALIPSO-SODA = 40–50 sr, V4 = 44 sr). In contrast, daytime polluted continental/smoke lidar ratio is more than 20 sr smaller than the constant V4 vaue for that type, attributed in part to the challenge of classifying tenuous aerosol with low signal-to-noise ratio. Dust over most of the Atlantic Ocean features CALIPSO-SODA lidar ratios less than 40 sr, possibly suggesting the presence of dust mixed with marine aerosols or lidar ratio values that depend on source and evolution of the aerosol plume. The new dusty marine type introduced in V4 features similar magnitudes and spatial distribution as its clean marine counterpart with lidar ratio differences of less than 3 sr, and nearly identical values over the open ocean, implying that some modification of the classification scheme for the marine subtypes is warranted.
Highlights
For more than 15 years, the Cloud Aerosol Lidar with Orthogonal Polarization (CALIOP), onboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) platform, has revolutionized our understanding of the role of aerosols in the climate system, revealing little known aspects of long-range aerosol transport, as well as the aerosol structure in the boundary layer and the free troposphere (e.g., Adams et al, 2012; Winker et al, 2013; Yu et al, 2015; Kacenelenbogen et al, 2019; Jumelet et al, 2020)
Following Kim et al, (2016), we look at the 150 iteration height in Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO)-Synergized Optical Depth of Aerosols (SODA) by comparing our VFMMAX+2 km assumption against retrievals that make use of SODA aerosol optical depth (AOD) but estimated using as the initial height: a) the tropopause height according to GEOS-5, and b) 36km height, right below the 36-39 km calibration layer used in Version 4.2 (V4)
CALIPSO-SODA lidar ratio is representative of the atmospheric column, we assess CALIPSO V4 for retrievals in which the profiles are characterized by only one aerosol type
Summary
For more than 15 years, the Cloud Aerosol Lidar with Orthogonal Polarization (CALIOP), onboard the Cloud-. The primary challenge of deriving aerosol extinction coefficient and aerosol optical depth using CALIOP (or any other elastic-backscatter lidar) is to separate the particulate backscatter and extinction coefficients with only one direct measurement: the attenuated backscatter coefficients. To resolve this ambiguity, CALIPSO adopts the standard procedure of reducing the inversion problem to one unknown by relating extinction and backscatter coefficients in the lidar equation via an assumed extinction-to-backscatter ratio (or lidar ratio, Young and Vaughan, 2009). Boundary layer and lower tropospheric aerosols over the ocean are classified into nondepolarizing clean marine, weakly scattering and mildly depolarizing polluted continental/smoke, and dusty marine (a moderately depolarizing mixture). Our goal is to determine how well the prescribed CALIPSO V4 lidar ratios compare to the retrieved CALIPSO-SODA lidar ratios for each CALIPSO aerosol type over the ocean
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