This is a past event.

PhD Candidate Gwenore Pokrifka of the Department of Meteorology and Atmospheric Science at Pennsylvania State University will give a talk on Monday, Sept 25 at 4:00 pm In M&M U113.  

Title: Measurements of small ice particle growth rates to be used in cirrus cloud models.

Authors: Gwenore F. Pokrifka
With: Al Moyle, Nathan Magee, Jerry Y. Harrington

Cirrus clouds are often composed of small ice crystals and their growth is poorly understood. Many begin as spherical frozen droplets, but develop into complex shapes, or “habits”. For an ice habit to form, water vapor must deposit more efficiently onto some surfaces than others. This growth efficiency is called a deposition coefficient, α. In numerical cloud models, the deposition coefficient is often assumed to be unity, meaning that all vapor molecules incorporate into the crystalline lattice. This would force the small crystals to remain spherical, which contradicts our recent in-cloud observations that showed branched and hollowed particles even at small sizes (maximum dimension < 100 µm). Here, we present data-driven approximations for the deposition coefficient and the processes that produce hollowing and branching. These approximations are derived from almost 300 single-particle growth experiments of small, cirrus-like, ice crystals. The particles 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 well represented by a supersaturation-dependent deposition coefficient function. At high supersaturation the influence of hollows and branches is characterized by a simple power-law. The approximations based on these data can easily be incorporated into cloud and weather forecasting microphysical models.