NAT-rock formation by mother clouds: a microphysical model study S. Fueglistaler1, B.P. Luo1, C. Voigt1, K.S. Carslaw2, and Th. Peter1 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.
Citation: 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.