CALIPSO level 3 stratospheric aerosol profile product: version 1.00 algorithm description and initial assessment

Atmos. Meas. Tech., 12, 6173–6191, 2019 https://doi.org/10.5194/amt-12-6173-2019 © Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License. CALIPSO level 3 stratospheric aerosol profile product: version 1.00 algorithm description and initial assessment Jayanta Kar1,2 , Kam-Pui Lee1,2 , Mark A. Vaughan2 , Jason L. Tackett2 , Charles R. Trepte2 , David M. Winker2 , Patricia L. Lucker1,2 , and Brian J. Getzewich1,2 1 Science Systems and Applications Inc., Hampton, VA, USA 2 Science Directorate, NASA Langley Research Center, Hampton, VA, USA Correspondence: Jayanta Kar () Received: 13 June 2019 – Discussion started: 25 June 2019 Revised: 26 September 2019 – Accepted: 7 October 2019 – Published: 25 November 2019 Abstract. In August 2018, the Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) project released a new level 3 stratospheric aerosol profile data product derived from nearly 12 years of measurements acquired by the spaceborne Cloud–Aerosol Lidar with Orthogonal Polarization (CALIOP). This monthly averaged, gridded level 3 product is based on version 4 of the CALIOP level 1B and level 2 data products, which feature significantly improved calibration that now makes it possible to reliably retrieve profiles of stratospheric aerosol extinction and backscatter coefficients at 532 nm. This paper describes the science algorithm and data handling techniques that were developed to generate the CALIPSO version 1.00 level 3 stratospheric aerosol profile product. Further, we show that the extinction profiles (retrieved using a constant lidar ratio of 50 sr) capture the major stratospheric perturbations in both hemispheres over the last decade resulting from volcanic eruptions, extreme smoke events, and signatures of stratospheric dynamics. Initial assessment of the product by intercomparison with the stratospheric aerosol retrievals from the Stratospheric Aerosol and Gas Experiment III (SAGE III) on the International Space Station (ISS) indicates good agreement in the tropical stratospheric aerosol layer (30◦ N–30◦ S), where the average difference between zonal mean extinction profiles is typically less than 25 % between 20 and 30 km (CALIPSO biased high). However, differences can exceed 100 % in the very low aerosol loading regimes found above 25 km at higher latitudes. Similarly, there are large differences (≥ 100 %) within 2 to 3 km above the tropopause that might be due to cloud contamination issues. 1 Introduction While the bulk of the global distribution of atmospheric aerosols is concentrated within the planetary boundary layer and free troposphere, the persistent aerosol burden in the stratosphere has long been known to have important implications for Earth’s climate (Turco et al., 1980). Techniques for the reliable detection of a background aerosol layer in the stratosphere date back to the early 1960s (Junge and Manson, 1961). These aerosols are mostly liquid sulfate particles that are derived from precursor gases like SO2 and carbonyl sulfide (OCS) transported from the troposphere (Thomason and Peter, 2006; Kremser et al., 2016; Thomason et al., 2018). In addition, intermittent volcanic eruptions and strong biomass burning events can inject sulfates, ash, and smoke into the stratosphere, which can last for long periods of time and exert significant climatic influences. For example, stratospheric perturbations from the Pinatubo volcano in 1991 lasted for several years (Chazette et al., 1995; Robock, 2000; Deshler, 2008). While eruptions of the same scale as Pinatubo have not taken place in the last 25 years or so, there is evidence that a large number of smaller eruptions has been significantly affecting the stratosphere with implications for the climate system (Vernier et al., 2011a; Solomon et al., 2011). Thus it is very important to monitor stratospheric aerosol loading over the long term. In pursuit of this goal, stratospheric aerosol measurements have been made using numerous techniques, including ground-based lidars, balloon-borne in situ samplers, and multi-sensor aircraft measurements, since the mid-twentieth century (Junge and Manson, 1961; Northam et al., 1974; Hoffman et al., 1975; McCormick et al., 1984; Published by Copernicus Publications on behalf of the European Geosciences Union. 6174 Grams and Fiocco, 1986; Brock et al., 1993; Beyerle et al., 1994; Jaeger and Deshler, 2002). Most of our current knowledge of the global distribution of stratospheric aerosols comes from satellite measurements. The earliest of such measurements were carried out by the Stratospheric Aerosol Measurement II (SAM II) onboard the Nimbus 7 spacecraft, which provided vertical profiles of aerosol extinction at 1 µm, and were followed by the Stratospheric Aerosol and Gas Experiment (SAGE) series of instruments (Chu and McCormick, 1979; Kent and McCormick, 1984; Mauldin III et al., 1985; Chu et al., 1989; Damadeo et al., 2013). The basic principle employed in these instruments is solar occultation, whereby the vertical profile of stratospheric aerosols is retrieved from the measurement of sunlight as the rays pass through the atmosphere during sunrise and sunset events as observed from the orbiting spacecraft. Stratospheric aerosols have been characterized using this technique from SAGE instruments on the Earth Radiation Budget Satellite (ERBS) and Meteor-3M as well as from the International Space Station (ISS). Among other spaceborne instruments that have used this technique are the Polar Ozone and Aerosol Measurement (POAM II, POAM III; Glaccum et al., 1996; Lucke et al., 1999) and Measurement of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation (MAESTRO; McElroy et al., 2007). In addition, the Optical Spectrograph and InfraRed Imager System (OSIRIS) and the Ozone Mapping and Profiler Suite (OMPS) have used a limb scatter technique to obtain aerosol extinction profiles (Bourassa et al., 2012; Chen et al., 2018). A novel and pioneering technique to retrieve aerosol profiles from space came about with the launch of the Cloud– Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) mission in April 2006, with a two-wavelength, polarization-sensitive elastic backscatter lidar as the primary payload (Winker et al., 2010). For over 12 years the Cloud– Aerosol Lidar with Orthogonal Polarization (CALIOP) has been providing vertically resolved profiles of aerosol and cloud extinction globally. The primary measurement from a spaceborne elastic backscatter lidar consists of the attenuated backscatter coefficients of the aerosols and clouds in the atmosphere. The strong backscatter from the tropospheric aerosols, combined with CALIOP’s relatively strong signalto-noise ratio (SNR), has been exploited to provide accurate extinction profiles in the troposphere (Young and Vaughan, 2009; Winker et al., 2013; Young et al., 2013, 2016, 2018). In comparison, the aerosol loading in the (...truncated)