Supersaturation-Dependent Parameterizations of Cirrus-like Particle Growth Rates from Measurements

The early growth of small ice particles in cirrus clouds is poorly understood. Many begin as spherical frozen droplets, but develop into complex shapes, or “habits”. For an ice habit to change, water vapor must deposit more efficiently onto some surfaces than others. This efficiency is represented by a deposition coefficient α. In numerical cloud models, the deposition coefficient is often set to a constant value of unity, meaning that all vapor molecules incorporate into the crystalline lattice, or a constant lower value (0.1 being typical). This would force the ice to remain isometric and compact, which contradicts our recent in-situ observation that showed branched and hollowed particles even at even small sizes (maximum dimension < 100 µm). Such habit complexity is often treated in models with an effective density ρeff, which allows the particle to be treated as a sphere geometrically while increase its growth rate. However, models often use the bulk density of ice for such small particles, which would leave them compact.

Here, we present parameterizations of both the deposition coefficient and effective density, as derived from almost 300 single-particle growth experiments. The particles were typical of cirrus (maximum radius ~ 40 µm) and grew in a thermal-gradient diffusion chamber at cirrus temperatures (-65 to -40 °C) and supersaturations ranging from ice- to liquid-equilibrium. At low supersaturation, the measured growth is kinetically limited compared to a solid sphere with α = 1, but it is enhanced at higher supersaturations. The parameterizations based on these data depend only on the ambient supersaturation and can easily be incorporated into cirrus microphysical models.