Fundamental Measurements of Ice Particle Growth Rates at Cirrus-like Temperatures and Improved Microphysics Parameterizations*

Cirrus clouds cover about 30% of Earth's surface, and their impact on Earth's radiative balance is one of the largest sources of uncertainty in climate prediction. Much of this uncertainty originates from a lack of information regarding fundamental cirrus microphysics. The vapor growth rates of the ice particles contained within cirrus clouds control the particle shapes, their fall speeds, and the rate of vapor depletion. However, there are few measurements of ice particle growth rates at temperatures relevant to cirrus clouds (< -40 C), and almost none for the tropical tropopause (down to -90 C). Presented here are mass and dimensional growth rate time series of ice crystals grown within thermal-gradient diffusion chambers at temperatures from -67 to -40 C. From these data, we derive parameterizations to describe the growth of ice more realistically than the capacitance theory that is currently used in cloud models. The capacitance theory treats small cirrus particles as solid spheres that grow from vapor with perfect efficiency, but these data show that the particles develop complex, faceted shapes. At high supersaturation, hollowed columnar polycrystals are common, whose growth cannot be described by capacitance theory, but can be modeled with faceted growth theory. To match the data, processes on the crystal surface must be accounted for, otherwise the remaining microphysics will be misrepresented in models.