Stratospheric Water Vapor Simulations: From Polar Regions to the Tropical Tropopause
Final Report Abstract
Polar stratospheric clouds (PSCs) and cold stratospheric aerosols drive heterogeneous chemistry and play a major role in polar ozone depletion. PSC particles provide sites for heterogeneous reactions that convert stable chlorine reservoir species (mostly HCl and ClONO2 ) to radicals that destroy ozone catalytically. PSCs also prolong ozone depletion by delaying chlorine deactivation through the removal of gas-phase HNO3 and H2 O by sedimentation of large PSC particles. Within the DFG project “Stratospheric Water Vapor Simulations: From Polar Regions to the Tropical Tropopause” , the Chemical Lagrangian Model of the Stratosphere (CLaMS) has been further developed and extended. The advanced microphysical PSC scheme includes now supercooled ternary solution (STS) droplets, nitric acid trihydrate (NAT), and ice PSCs and represents a major improvement of the representation of cloud physics in CLaMS. We studied various Arctic and Antarctic winters on different temporal and spatial scales up to the point of individual cloud details. The overall agreement between CLaMS and PSC measurements from the spaceborne instruments CALIOP and MIPAS is convincing. Also CLaMS simulations of gas-phase HNO3 and H2 O match with trace gas measurements from MLS. The simultaneous observations by CALIOP, MIPAS, and MLS have provided an unprecedented polar vortex-wide climatological view of PSC occurrence and composition in both hemispheres. These data have spurred advances in our understanding of PSC formation and related dynamical processes. The SPARC Polar Stratospheric Cloud initiative became a main part of the DFG project which resulted in the publication of a comprehensive paper in Reviews of Geophysics, an invitation-only reviews journal that provides overviews of recent research in all areas of the Earth and space sciences. For this, we brought together an international team of key scientists representing satellite, ground-based, and airborne measurements, as well as theoreticians and modelers. From synthesizing all information, we now know that STS-NAT mixtures are the predominant composition of PSCs over most of the winter in both hemispheres, and that the uptake of gas-phase nitric acid by the clouds depends strongly on how fast the air is cooling. In rapid cooling situations, uptake is dominated by liquid STS droplets, whereas in cases with slow cooling, thermodynamically favored NAT particles are predominant. We are now confident that NAT particles form at temperatures above the frost point through some selective nucleation mechanism, perhaps involving meteoritic dust. Many models produce reasonable ozone depletion results for very cold Antarctic winters despite having crude representations of PSCs because the chlorine activation process does not depend on the details of chemical reactions on PSCs or denitrification. However, when temperatures are near the PSC formation threshold, an accurate PSC scheme will yield much better model results than simple approximations. Thus, it remains important to better understand and simulate PSC processes in detail, especially when considering PSC effects in the face of a changing climate.
Publications
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A climatology of polar stratospheric cloud composition between 2002 and 2012 based on MIPAS/Envisat observations, Atmos. Chem. Phys., 18, 5089-5113
Spang, R., Hoffmann, L., Müller, R., Grooß, J.-U., Tritscher, I., Höpfner, M., Pitts, M., Orr, A., and Riese, M.
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On the discrepancy of HCl processing in the core of the wintertime polar vortices, Atmos. Chem. Phys., 18, 8647-8666
Grooß, J.-U., Müller, R., Spang, R., Tritscher, I., Wegner, T., Chipperfield, M. P., Feng, W., Kinnison, D. E., and Madronich, S.
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Lagrangian simulation of ice particles and resulting dehydration in the polar winter stratosphere, Atmos. Chem. Phys., 19, 543-563
Tritscher, I., Grooß, J.-U., Spang, R., Pitts, M. C., Poole, L. R., Müller, R., and Riese, M.
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Mountain-wave-induced polar stratospheric clouds and their representation in the global chemistry model ICON-ART, Atmos. Chem. Phys., 21, 9515–9543
Weimer, M., Buchmüller, J., Hoffmann, L., Kirner, O., Luo, B., Ruhnke, R., Steiner, M., Tritscher, I., and Braesicke, P.
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Polar stratospheric clouds: Satellite observations, processes, and role in ozone depletion. Reviews of Geophysics, 59, e2020RG000702
Tritscher, I., Pitts, M. C., Poole, L. R., Alexander, S. P., Cairo, F., Chipperfield, M. P., Grooß, J.-U., Höpfner, M., Lambert, A., Luo, B.P., Molleker, S., Orr, A., Salawitch, R., Snels, M., Spang, R., Woiwode, W. and T. Peter
