Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 2 1 2002 10.5194/acpd-2-29-2002 http://www.atmos-chem-phys-discuss.net/2/29/2002/ http://www.atmos-chem-phys-discuss.net/2/29/2002/acpd-2-29-2002.html http://www.atmos-chem-phys-discuss.net/2/29/2002/acpd-2-29-2002.pdf 29 42 2002-01-14 NAT-rock formation by mother clouds: a microphysical model study S. Fueglistaler B. P. Luo C. Voigt K. S. Carslaw Th. Peter Atmospheric and Climate Science, ETH Zürich, Switzerland School of the Environment, University of Leeds, Leeds, UK Polar stratospheric clouds (PSCs) of type 1a or 1a-enh containing high number densities of nitric acid trihydrate (NAT) particles, can act as mother clouds for extremely large NAT particles, termed NAT-rocks, provided the air below the clouds is supersaturated with respect to NAT. Individual NAT particles at the cloud base fall into undepleted gas phase and rapidly accelerate due to a positive feedback between their growth and sedimentation. The resulting reduction in number density is further enhanced by the strong HNO₃ depletion within a thin layer below the mother cloud, which delays subsequent particles. This paper introduces the basic microphysical principles behind this mother cloud/NAT-rock mechanism, which produces 10^-4 cm^-3 NAT-rocks with radii around 10<font face="Symbol">m</font>m some kilometers below the mother cloud. The mechanism requires neither selective nucleation nor additional atmospheric dilution and works even for a monodisperse particle size distribution in the mother cloud.