Microscale characteristics of homogeneous freezing events in cirrus clouds

Abstract

We investigate homogeneous freezing of aqueous aerosol particles, a fundamental ice formation process in cirrus clouds. We estimate freezing time scales and vertical extensions of freezing layers, demonstrating that such freezing events are highly transient and localized. While time scales decrease with increasing vertical velocity driving ice nucleation, layer depths are weak functions of the vertical velocity. Our results are used to discuss possible effects of turbulent diffusion and entrainment-mixing on homogeneous freezing in cirrus. Large turbulent diffusivity acts to broaden water vapor-depleted freezing layers and facilitate sedimentation of freshly nucleated ice crystals out of them into ice-supersaturated air. Homogeneous freezing events could be affected by microscale turbulence in episodes of intense turbulence dissipation rates, although such episodes are rare. We conjecture that freezing layers are broader in the case of heterogeneous ice nucleation and effects of sedimentation on nucleation increase in importance. Our findings point to the difficulty of inferring nucleated cirrus ice crystal numbers from measurements and place tight constraints on cirrus models with regard to spatial and temporal resolution.