OBSERVATIONS OF THE MESOSCALE SUBSTRUCTURE IN THE COLD-AIR OF A DEVELOPING FRONTAL CYCLONE

Observations from 58 dropwindsondes released in a mesoscale array duri ng the FRONTS 92 experiment are interpreted in the context of satellit e imagery to derive the mesoscale structure and evolution of parts of a frontal cyclone developing over the eastern North Atlantic. A concep tual model involving the intertwining of 'dry intrusion' and 'cloud he ad' flows is corroborated and is used to provide the framework for int erpreting the detailed mesoscale behaviour. In the cold air, two disti nct dry-intrusions were responsible for two cold fronts, trailing sout h-westwards from the tip of the cloud head. Both were surface features at the beginning of our study but the leading one evolved into an upp er front with mid-level convection as the dry intrusion responsible fo r it overran the warm sector. Areas of both high and low potential vor ticity were indicated within the dry intrusions. Upon encountering a c ritical level of zero system-relative velocity at the top of the moist boundary layer, the dry-intrusions' arrival was associated with the d evelopment of multiple dry and moist laminae near the top of the bound ary layer. The vertical wavelength of the laminae was about 1 km and t hey extended over 200 km in the front-normal direction, with a slope o f typically 1 in 60. Although most parts of the laminae were subsatura ted, their circulations combined with the double structure of the dry intrusions to produce multiple shallow cloud-lines within the boundary layer. These formed as an extension of the south-western tip of the c loud head associated with the developing cyclone. Possible mechanisms for generating the observed structures are discussed.