THE ROLE OF THE ENVIRONMENTAL FLOW IN THE DEVELOPMENT OF SECONDARY FRONTAL CYCLONES

The impact of the environmental flow on the development of secondary f rontal cyclones is investigated. Several case-studies are examined as examples of secondary frontal-cyclone events observed in the North Atl antic-western Europe sector. A simple measure of growth is defined to chart their development. The vorticity attribution technique of Bishop is utilized to calculate the action of the large-scale (environmental ) flow on the fronts. In particular the environmental along-front stre tching-shown to be important in theoretical models of frontal instabil ities-is calculated. The role of the environmental deformation appears to be crucial: as part of a baroclinic life cycle, stretching deforma tion acts to build up a front but suppress along-front waves; if the s tretching rate diminishes, barotropic instabilities may then break out . Diagnostics are examined to try to ascertain the growth mechanisms a t work in each frontal-cyclone case. A range of values for the commonl y prescribed deformation-frontogenesis and shearing-frontogenesis para meters are calculated.