Abstract

Polar stratospheric clouds (PSCs) and cold stratospheric aerosols drive heterogeneous chemistry and play a major role in polar ozone depletion. The Chemical Lagrangian Model of the Stratosphere (CLaMS) simulates the nucleation, growth, sedimentation, and evaporation of PSC particles along individual trajectories. Particles consisting of nitric acid trihydrate (NAT), which contain a substantial fraction of the stratospheric nitric acid (HNO3), were the focus of previous modeling work and are known for their potential to denitrify the polar stratosphere. Here, we carried this idea forward and introduced the formation of ice PSCs and related dehydration into the sedimentation module of CLaMS. Both processes change the simulated chemical composition of the lower stratosphere. Due to the Lagrangian transport scheme, NAT and ice particles move freely in three-dimensional space. Heterogeneous NAT and ice nucleation on foreign nuclei as well as homogeneous ice nucleation and NAT nucleation on preexisting ice particles are now implemented into CLaMS and cover major PSC formation pathways.We show results from the Arctic winter 2009/2010 and from the Antarctic winter 2011 to demonstrate the performance of the model over two entire PSC seasons. For both hemispheres, we present CLaMS results in comparison to measurements from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP), the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), and the Microwave Limb Sounder (MLS). Observations and simulations are presented on season-long and vortex-wide scales as well as for single PSC events. The simulations reproduce well both the timing and the extent of PSC occurrence inside the entire vortex. Divided into specific PSC classes, CLaMS results show predominantly good agreement with CALIOP and MIPAS observations, even for specific days and single satellite orbits. CLaMS and CALIOP agree that NAT mixtures are the first type of PSC to be present in both winters. NAT PSC areal coverages over the entire season agree satisfactorily. However, cloud-free areas, next to or surrounded by PSCs in the CALIOP data, are often populated with NAT particles in the CLaMS simulations. Looking at the temporal and vortex-averaged evolution of HNO3, CLaMS shows an uptake of HNO3 from the gas into the particle phase which is too large and happens too early in the simulation of the Arctic winter. In turn, the permanent redistribution of HNO3 is smaller in the simulations than in the observations. The Antarctic model run shows too little denitrification at lower altitudes towards the end of the winter compared to the observations. The occurrence of synoptic-scale ice PSCs agrees satisfactorily between observations and simulations for both hemispheres and the simulated vertical redistribution of water vapor (H2O) is in very good agreement with MLS observations. In summary, a conclusive agreement between CLaMS simulations and a variety of independent measurements is presented.

Highlights

  • The representation of polar stratospheric clouds (PSCs) in global models is often poor despite their importance for ozone chemistry in polar winter and spring

  • nitric acid trihydrate (NAT) and ice parameterizations by Hoyle et al (2013) and Engel et al (2013), respectively, are based on PSC observations during this particular winter. To demonstrate that both parameterizations are working in Chemical Lagrangian Model of the Stratosphere (CLaMS), as well as in the original Lagrangian Zurich Optical and Microphysical box Model (ZOMM), we selected 18 January 2010 for a single cloud comparison because this day was analyzed in detail by Engel et al (2013)

  • We present CLaMS simulations based on a new Lagrangian ice sedimentation scheme focusing on the 2009/2010 Arctic and 2011 Antarctic winters

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Summary

Introduction

The representation of polar stratospheric clouds (PSCs) in global models is often poor despite their importance for ozone chemistry in polar winter and spring. The uptake of nitrogen-containing species in PSCs changes the chemical composition of the lower stratosphere under PSC conditions substantially, with important impact for ozone loss chemistry (Solomon, 1999; Wohltmann et al, 2017; Müller et al, 2018). PSCs are supposed to consist of liquid supercooled ternary solution (STS) droplets, solid nitric acid trihydrate (NAT) particles, and/or solid ice particles (Peter and Grooß, 2012) Their formation mechanisms are still a focus of research, newly motivated by global, high-resolution satellite observations (Spang et al, 2018; Pitts et al, 2018). Global satellite data from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) (Pitts et al, 2018) and the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) (Spang et al, 2018) are available for both winters and allow a comprehensive evaluation of our model results to be conducted

CLaMS model description and setup
Sedimentation module
Parameterization of temperature fluctuations
Comparison to measurements
CALIOP
Results
Conclusions
Full Text
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