Shallow cirrus convection – a source for ice supersaturation

Abstract

The origin and persistence of high ice supersaturation is still not well understood. In this study, the impact of local dynamics as source for ice supersaturation inside cirrus clouds is investigated. Nucleation and growth of ice crystals inside potentially unstable layers in the tropopause region might lead to shallow convection inside (layered) cirrus clouds due to latent heat release. The intrinsic updraught inside convective cells constitutes a dominant but transient source for ice supersaturation. A realistic case of shallow cirrus convection is investigated using radiosonde data, meteorological analyses and large-eddy simulations of cirrus clouds. The simulations corroborate the existence of ice supersaturation inside cirrus clouds as a transient phenomenon. Ice supersaturation is frequent, but determined by the life cycle of convective cells in shallow cirrus convection. Cirrus clouds driven by shallow cirrus convection are mostly not in thermodynamic equilibrium; they are usually in a subsaturated or supersaturated state.

• cirrus clouds
• ice supersaturation
• cloud dynamics
• thermodynamics

6. Acknowledgements

I thank Ulrich Achatz, Marcia Baker, Andreas Dörnbrack, Fabian Fusina, Hanna Joos, Ulrich Schumann, Piotr Smolarkiewicz and Heini Wernli for fruitful discussions. I also thank two anonymous reviewers; their comments helped to improve the manuscript significantly. This work was partly supported by the European Commission within the framework of a Marie Curie fellowship (Impact of mesoscale dynamics and aerosols on the life cycle of cirrus clouds). This work contributes to the COST action ‘Water Vapour in the Climate System’ (WaVaCS). The simulations were carried out on the high-performance computing facilities of the European Centre for Medium-Range Forecasts (ECMWF) within the framework of the special project ‘Ice supersaturation and cirrus clouds’ (SPDEISSR).

Notes

¹Note that in most discussions, no definitions on ‘high supersaturation’ and ‘thick cirrus clouds’ are given. There is a kind of common sense on high values (usually S _i >1.3, close to nucleation thresholds), but clear definitions are lacking.