Joint Centre for Mesoscale Meteorology (JCMM)
The role of the environmental flow in the development of secondary frontal cyclones
by Ian A Renfrew, Alan J Thorpe and Craig H Bishop
The impact of the environmental flow on the development of secondary frontal cyclones is investigated. Several case studies are examined as examples of secondary frontal cyclone events observed in the North Atlantic - Western Europe sector. A simple measure of growth is defined to chart their development. The vorticity attribution technique of Bishop (1996 I&II) is utilised to calculate the action of the large scale (environmental) flow on the fronts. In particular, the environmental along-front stretching - shown to be important in theoretical models of frontal instabilities - is calculated. The role of the environmental deformation appears to be crucial: as part of a baroclinic lifecycle, stretching deformation acts to build up a front but suppress along-front waves; if the stretching drops off, barotropic instabilities may then break out. Diagnostics are examined to try to ascertain the growth mechanisms at work in each frontal cyclone case. A range of values for the commonly prescribed deformation frontogenesis and shearing frontogenesis parameters are calculated.