24 Oct 2022
24 Oct 2022
Status: this preprint is currently under review for the journal AMT.

The CALIPSO version 4.5 stratospheric aerosol subtyping algorithm

Jason L. Tackett1, Jayanta Kar2, Mark A. Vaughan1, Brian J. Getzewich1, Man-Hae Kim3, Jean-Paul Vernier4, Ali H. Omar1, Brian E. Magill2, Michael C. Pitts1, and David M. Winker1 Jason L. Tackett et al.
  • 1NASA Langley Research Center, Hampton, VA, USA
  • 2Science Systems and Applications, Inc., Hampton, VA, USA
  • 3Seoul National University, Seoul, South Korea
  • 4National Institute of Aerospace Associates, Hampton, VA, USA

Abstract. The accurate classification of aerosol types injected into the stratosphere is important to properly characterize their chemical and radiative impacts within the Earth climate system. The updated stratospheric aerosol subtyping algorithm used in the Version 4.5 (V4.5) release of the Cloud Aerosol Lidar with Orthogonal Polarization (CALIOP) level 2 data products now delivers more comprehensive and accurate classifications than its predecessor. The original algorithm identified four aerosol subtypes for layers detected above the tropopause: volcanic ash, smoke, sulfate/other, and polar stratospheric aerosol (PSA). In the revised algorithm, sulfates are separately identified as a distinct, homogeneous subtype and the diffuse, weakly scattering layers previously assigned to the sulfate/other class are recategorized as a fifth “unclassified” subtype. By making two structural changes to the algorithm and revising two thresholds, the V4.5 algorithm improves the ability to discriminate between volcanic ash and smoke from pyrocumulonimbus injections, improves the fidelity of the sulfate subtype, and more accurately reflects the uncertainties inherent in the classification process. The 532 nm lidar ratio for volcanic ash was also revised to a value more consistent with the current state of knowledge. This paper briefly reviews the previous version of the algorithm (V4.1/V4.2), then fully details the rationale and impact of the V4.5 changes on subtype classification frequency for specific events where the dominant aerosol type is known based on literature. Classification accuracy is best for volcanic ash due to its characteristically high depolarization ratio. Smoke layers in the stratosphere are also classified with reasonable accuracy, though during the daytime a substantial fraction are misclassified as ash. It is also possible for mixtures of ash and sulfate to be misclassified as smoke. The V4.5 sulfate subtype accuracy is less than that for ash or smoke, with sulfates being misclassified as smoke about one-third of the time. However, because exceptionally tenuous layers are now assigned to the unclassified subtype and the revised algorithm levies more stringent criteria for identifying an aerosol as sulfate, it is more likely that layers labeled as this subtype are in fact sulfate.

Jason L. Tackett et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on amt-2022-289', Anonymous Referee #1, 23 Nov 2022
  • RC2: 'Comment on amt-2022-289', Anonymous Referee #2, 23 Nov 2022
  • RC3: 'Comment on amt-2022-289', Anonymous Referee #3, 25 Nov 2022

Jason L. Tackett et al.

Jason L. Tackett et al.


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Short summary
The accurate identification of aerosol types in the stratosphere is important to characterize their impacts on the Earth climate system. The space-borne lidar on board CALIPSO is well-posed to identify aerosol in the stratosphere from volcanic eruptions and major wildfire events. This manuscript describes improvements implemented in the version 4.5 CALIPSO data release to more accurately discriminate between volcanic ash, sulfate, and smoke within the stratosphere.