DYNAMIC STRUCTURE AND EVOLUTION OF THE PREFRONTAL BOUNDARY-LAYER - 3 CASE-STUDIES OF FRONTS 87

Data provided by soundings during the FRONTS 87 experiment are used to study the structure and evolution of the prefrontal planetary boundar y layer for three case studies. Hodographs of the observed winds are c ompared to theoretical ones. The presence of a low-level jet in the pr efrontal atmosphere strongly enhances low-level convergence within the boundary layer. The maximum of frontward wind in the cross-front dire ction is observed just below the maximum of the low-level jet. The mag nitude of convergence and of the associated upward motions strongly de pends on the magnitude of the low-level jet. Observations seem to conf irm that the more active convective motions correspond to the case stu dies characterised by the strongest low-level jets.