NAT-rock formation by mother clouds: a microphysical model study

S. Fueglistaler (1) , B. P. Luo (1) , C. Voigt (1) , K. S. Carslaw (2) , Th. Peter (1)

1 IAC - Institute for Atmospheric and Climate Science [Zürich]
2 SEE - School of Earth and Environment [Leeds]

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 10mm 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.

Domaines: Océan, Atmosphère

S. Fueglistaler, B. P. Luo, C. Voigt, K. S. Carslaw, Th. Peter. NAT-rock formation by mother clouds: a microphysical model study. Atmospheric Chemistry and Physics Discussions, 2002, 2 (1), pp.29-42. ⟨hal-00300819⟩