Atmos. Chem. Phys. Discuss., 12, 24643-24676, 2012
www.atmos-chem-phys-discuss.net/12/24643/2012/
doi:10.5194/acpd-12-24643-2012
© Author(s) 2012. This work is distributed
under the Creative Commons Attribution 3.0 License.
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
An assessment of CALIOP polar stratospheric cloud composition classification
M. C. Pitts1, L. R. Poole2, A. Lambert3, and L. W. Thomason1
1NASA Langley Research Center, Hampton, Virginia 23681, USA
2Science Systems and Applications, Incorporated, Hampton, Virginia 23666, USA
3Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA

Abstract. This study assesses the robustness of the CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) polar stratospheric cloud (PSC) composition classification algorithm – which is based solely on the spaceborne lidar data – through the use of nearly coincident gas-phase HNO3 data from the Microwave Limb Sounder (MLS) on Aura and Goddard Earth Observing System Model, Version 5 (GEOS-5) temperature analyses. Following the approach of Lambert et al. (2012), we compared the observed temperature-dependent HNO3 uptake by CALIOP PSCs with modeled uptake for equilibrium STS (supercooled ternary solution) and NAT (nitric acid trihydrate), which indicates how well PSCs in the various composition classes conform to expected temperature existence regimes and also offers some insight into PSC growth kinetics. We examined the CALIOP PSC data record from both polar regions over the period from 2006 through 2011 and over a range of potential temperature levels spanning the 15–30 km altitude range. We found that most PSCs identified as STS exhibit gas phase uptake of HNO3 consistent with theory, but with a small temperature bias, similar to Lambert et al. (2012). Ice PSC classification is also robust in the CALIOP optical data, with the mode in the ice observations occurring about 0.5 K below the frost point. We found that CALIOP PSCs identified as liquid/NAT mixtures exhibit two distinct preferred modes. One mode is significantly out of thermodynamic equilibrium with respect to NAT (4–5 K below the equilibrium NAT existence temperature), with HNO3 uptake dominated by the more numerous liquid droplets. The other liquid/NAT mixture mode is much closer to NAT thermodynamic equilibrium, indicating that the particles have been exposed to temperatures below the NAT existence temperature for extended periods of time. The CALIOP PSC composition classification scheme was found to be excellent in an overall sense, and we have a good understanding of the cause of the minor misclassifications that do occur. We will investigate means to correct these deficiencies in our next generation algorithm.

Citation: Pitts, M. C., Poole, L. R., Lambert, A., and Thomason, L. W.: An assessment of CALIOP polar stratospheric cloud composition classification, Atmos. Chem. Phys. Discuss., 12, 24643-24676, doi:10.5194/acpd-12-24643-2012, 2012.
 
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