Cirrus Uncinus Generating Cells and the Evolution of Cirriform Clouds. Part I: Aircraft Observations of the Growth of the Ice Phase

Abstract

The growth of the ice phase in cirrus uncinus and cirrostratus clouds was studied through aircraft measurement of cloud particle spectra at different altitudes. Five different cirrus uncinus clouds were studied; one of the cirrus uncinus evolved into cirrostratus. The temperature range of sampling was ?19 to ?58°C. In cirrus uncinus heads, crystals were determined to be nucleated and grown in the upshear region, before being carried into the trail region of the head downshear as a result of wind shear. The updraft region is upshear, and the downdraft region downshear. A ?hole? was found to separate the up-and downshear regions of the head, with a horizontal extent of about 150 m. The concentrations of crystals in the head region were on the order of 0.5 cm ?3, with 0.025-0.05 cm ?3 longer than 100 µm. Accumulation of particles in the updraft region was noted. The mean length of crystals longer than 100 µm (precipitation size particles) ranged between 0.5 and 1.0 mm, and crystals as long as 2 mm were found at temperatures as low as ?56°C. The average ice water content was found to be 0.15?0.3 gm ?3 in the head. The cirrostratus clouds sampled had their nucleation regions near the top of the clouds; crystals sedimented and grew from this source region near the top to near the base, and then evaporated to the base. The crystal concentrations were about 0.2 cm?3, WITH 0.01 ?3 longer than 100 µm. The mean length of crystals larger than 100 µm ranged between 0.2?0.5mm. The ice water content ranged between 0.01?0.16 g m?3.