EMPIRICAL INVESTIGATION OF CLOUD STREETS OVER NORTHERN GERMANY BY USE OF ROUTINE AEROLOGICAL DATA

Abstract

Cloud streets are a quite frequently observed phenomenon in the atmosphere as has been revealed by satellite pictures during the last two decades. Cumulus clouds are organized in longitudinal bands, which may extend up to 500 km with a spacing of approximately 2-8 km. Generally, street spacing is between 2 and 4 times the height of the convection layer; observations over easlern Asia during cold air outbreaks gave an aspect ratio up to 18. Cloud streets are orientated roughly along the mean wind of the convection layer, and the phase speed in the transverse direction is usually small to the direction of the lower pressure. Under convective conditions and moderate wind speeds, cloud streets can be found over land as well as over the oceans. The organization of clouds into streets is explained by the existence of horizontal roll vortices in the atmospheric boundary layer. Near the surface, the vortices collect convective elements into their upward vertical velocity regions. In this paper, are presented some results of an investigation of cloud streets over northern Germany with the help of routine aerological radiosonde data. Because this area is rather flat, cloud streets are not very much effected by orographic structures. The results suggest that the inflection point insability may serve as a trigger for vortex roll development in the planetary boundary layer.  The roll development is enhanced and modified through the action of buoyancy for unstable stratification.