Lagrangian simulation of ice particles and resulting dehydration in the polar winter stratosphere
Authors: Tritscher Ines , Grooß Jens-Uwe , Spang Reinhold , Pitts Michael C. , Poole Lamont R. , Müller Rolf , Riese Martin 
Published: January 2019

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.