Observations of the mesoscale sub‐structure in the cold air of a developing frontal cyclone

Abstract

AbstractObservations from 58 dropwindsondes released in a mesoscale array during the FRONTS 92 experiment are interpreted in the context of satellite imagery to derive the mesoscale structure and evolution of parts of a frontal cyclone developing over the eastern North Atlantic. A conceptual model involving the intertwining of ‘dry intrusion’ and ‘cloud head’ flows is corroborated and is used to provide the framework for interpreting the detailed mesoscale behaviour. In the cold air, two distinct dry‐intrusions were responsible for two cold fronts, trailing south‐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 upper front with mid‐level convection as the dry intrusion responsible for it overran the warm sector. Areas of both high and low potential vorticity were indicated within the dry intrusions. Upon encountering a critical level of zero system‐relative velocity at the top of the moist boundary layer, the dry‐intrusions' arrival was associated with the development of multiple dry and moist laminae near the top of the boundary layer. the vertical wavelength of the laminae was about 1 km and they extended over 200 km in the front‐normal direction, with a slope of typically 1 in 60. Although most parts of the laminae were subsaturated, 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 cloud head associated with the developing cyclone. Possible mechanisms for generating the observed structures are discussed.