Temperature fluctuations force low ice crystal concentration and sustained high supersaturation in tropopause cirrus

Low ice crystal concentration and sustained in-cloud supersaturation, commonly found in observations of cirrus near the tropopause are inconsistent with the conventional picture of cirrus formation from homogeneous freezing of supercooled droplets. Heterogeneous freezing from effloresced ammonium sulfate and glassy aerosol are currently proposed to cause these cirrus features. Such hypotheses however rely on the assumption that only heterogeneous freezing triggered by weak updrafts takes place at the tropopause level. This is however at odds with the gravity wave spectrum in the upper troposphere and reported measurements of the freezing threshold relative humidity in cirrus clouds.

We show that if the effect of background temperature fluctuations and local competition between ice nucleation and water vapor deposition is accounted for, a "dynamical equilibrium” establishes between ice production and sedimentation loss. These newly-discovered states are favored at low temperatures, explain the low temperature cirrus properties, and do not require heterogeneous nuclei to occur. It is shown that the type of dynamical forcing will set cirrus clouds in one of two "preferred" microphysical regimes with very different susceptibility to aerosol emissions.