NAT-rock formation by mother clouds: a microphysical model study
1Atmospheric and Climate Science, ETH Zürich, Switzerland
2School of the Environment, University of Leeds, Leeds, UK
Abstract. 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 mm some kilometers below the mother cloud. The mechanism does not require selective nucleation and works even for a monodisperse particle size distribution in the mother cloud.
Fueglistaler, S., Luo, B. P., Voigt, C., Carslaw, K. S., and Peter, Th.: NAT-rock formation by mother clouds: a microphysical model study, Atmos. Chem. Phys., 2, 93-98, doi:10.5194/acp-2-93-2002, 2002.