Abstract
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 and H2O 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 PSCs in the various CALIOP composition classes with modeled uptake for supercooled ternary solutions (STS) and equilibrium nitric acid trihydrate (NAT). 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 NAT mixtures exhibit two distinct preferred modes which reflect the fact that the growth of NAT particles is kinetically limited. One mode is significantly out of thermodynamic equilibrium with respect to NAT due to short exposure times to temperatures below the NAT existence temperature, TNAT, with HNO3 uptake dominated by the more numerous liquid droplets. The other NAT mixture mode is much closer to NAT thermodynamic equilibrium, indicating that the particles have been exposed to temperatures below TNAT for extended periods of time. With a few notable exceptions, PSCs in the various composition classes conform well to their expected temperature existence regimes. We have a good understanding of the cause of the minor misclassifications that do occur and will investigate means to correct these deficiencies in our next generation algorithm.
Highlights
We have restricted our analyses to scenes in which the Microwave Limb Sounder (MLS) field of view (FOV) is at least 75 % filled with PSC observations
One is the variability in PSC composition over the MLS FOV, which we addressed by analyzing the subset of scenes in which the MLS FOV was > 75 % filled with PSC observations and at least 2/3 of those observations were of a single PSC composition
Our approach was to examine the uptake of HNO3 as a function of T − Tice to determine if the assigned PSC composition is consistent with the thermodynamic existence regimes for STS, nitric acid trihydrate (NAT), and ice
Summary
1 Introduction supercooled ternary solutions (STS) and equilibrium nitric acid trihydrate (NAT). A recent study of the early 2008 Antarctic PSC season by Lambert et al (2012) demonstrated that one can gain valuable insight into PSC processes by analyzing the CALIOP data in combination with nearly coincident gas phase HNO3 and H2O measurements from the Microwave Limb Sounder (MLS) on the Aura satellite. Since HNO3 and/or H2O are the major constituents of all PSC particles (STS, NAT, and H2O ice), tracking their uptake by PSCs as a function of temperature using MLS data provides constraints on particle composition and volume density. Comparison of the observed uptake of HNO3 by CALIOP PSCs with modeled uptake for equilibrium STS and NAT indicates how well PSCs in the various composition classes conform to expected temperature existence regimes and offers some insight into the kinetics of PSC growth. The overarching goals of this paper are to assess the robustness of the P09 composition classification algorithm and identify potential improvements that can be implemented in the generation algorithm
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