Abstract

The structure and circulations of the cirrus uncinus generating head were determined from aircraft measurements of the temperatures, horizontal wind velocities and particle spectra at different altitudes. Stable layers were found to exist directly above and below the head. The head was found to exist in a region with a dry adiabatic lapse rate. Waves were observed in the stable layer below the head. The head was found to he divided into two regions in active cirrus uncinus. The upshear part of the head is the updraft region, and the downshear part the downdraft region. A region containing almost no crystals was found to separate the up- and downdraft regions. This “hole” was typically 150 m across.

The vertical velocities in cirrus uncinus were determined from aircraft and Doppler radar measurements. Typical vertical velocities were estimated to range from 100–200 cm s−1 from aircraft particle measurements, and determined from Doppler radar measurements to range from 120–180 cm s s−1 Typical downdraft velocities of 50 cm s s−1 were determined from the aircraft measurements and from the Doppler measurements to be a maximum of 80 cm s−1, with 20–40 cm s−1 typical velocity.

Two mechanisms are suggested for the formation of cirrus uncinus clouds. For cirrus uncinus oriented in lines almost perpendicular to the wind direction, it is suggested that there is layer lifting and that convective cells develop along the lifting line. In the case of isolated cirrus uncinus, it is suggested that a wave in the stable layer below the head region causes a perturbation in the head region which results in convection in the layer. Two mechanisms are suggested for the formation of new generating cells upwind or downwind of the original cell, which significantly increases the lifetime of the cloud. Evaporative cooling in the trail region may induce the formation of new turrets above the trail of an original cell. A second possible mechanism is the formation of a convergent and divergent region at the stable layer below the head region induced by the downdraft in the trail region of the head.

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