Cirrus Uncinus Generating Cells and the Evolution of Cirriform Clouds. Part III: Numerical Computations of the Growth of the Ice Phase

Abstract

Equations were developed to calculate the growth of the ice phase in cirrus clouds. Calculations indicated that nucleation of ice crystals in cirrus uncinus heads forming at temperatures lower than ?35°C generally should occur near the upwind base of the head, and in cirrostratus clouds at the top of the cloud. The growth of ice crystals and the resulting shape of cirrus uncinus clouds with an updraft velocity of 100 cm s?1 were calculated. With an initial crystal concentration of 0.025 cm ?2 and a nucleation temperature of ?40°C, crystals of 0.45 mm length, and a maximum ice water content of 0.3 g m ?3 were predicted. Latent beat release due to the ice crystal growth increased the initial updraft velocity only slightly. A downdraft velocity comparable in magnitude to the original updraft velocity was calculated to occur in the downshear part of the cirrus uncinus head.