Origin of Precipitation and Dynamic Organization in Wavelike Precipitation Bands

Abstract

Wavelike parallel precipitation bands, embedded in a layer of cirrus clouds, were studied with a vertically pointing millimetric Doppler radar. Their mean dimensions were 17 km between bands, a 50 km band length and a 6 km band width. Their translation velocity of 20 ms?1 corresponded to the wind velocity at the altitude of their tops. The bands were located 200 km ahead of a surface cold front and were approximately perpendicular to it. The formation of precipitation in the bands took place through a seeder-feeder process. The seeder-zone at the top of the cloud was 1.1 km thick; it was constituted by generating cells (with maximum updrafts of 2 ms?1) associated with turbulent motions whose turbulent kinetic energy dissipation rate ? reached 3 10?4 m2 s?3. An oscillation of the vertical air velocity, corresponding to 15 km horizontal distance between absolute maxima, was observed. This oscillation appeared in the whole cirrus layer but only showed a mean amplitude of 0.7 ms?1, at the most, in a thermally stable layer located at the base of the cloud. Two other characteristic lengths of 1.3 and 0.7 km were observed. They corresponded, respectively, to the horizontal distance between the generating cells and to a dynamic substructure present only in the zone of precipitation generation. In the Appendix, the method developed to compute the vertical air velocity from the velocity data of the vertically pointing Doppler radar is described.