Thesis Abstract

The development of secondary frontal cyclones is investigated, with attention focussed upon the role of the environmental flow.

Theory indicates that the environmental flow can play a crucial role in the evolution of a front; however quantifying the environmental effects is difficult as the background flow and frontal flow overlap and mask one another.

A way of partitioning the wind field (via a partitioning of the vorticity and divergence fields) is described. This extends the domain independent attribution technique developed by Bishop (1995I&II). It allows an identification between the frontal and environmental winds, and the perturbation and basic state winds in theoretical models.

A simple barotropic model is used to examine finite amplitude perturbations to a strip of uniform vorticity. An instability develops on the strip, although this can be suppressed by a sufficiently large external deformation flow. When a vorticity forcing of the strip is added there is an increase in maximum growth rate for all deformation rates. These results are combined with published studies to define a critical threshold zone for along-front stretching (0.6-0.8x10-5s-1 for a typical low-level front); above which barotropic frontal instabilities are suppressed.

The attribution technique is tested on a new idealised analytic frontal model which incorporates both frontogenetical deformation and horizontal shearing. To calculate the horizontal shearing a 2 level extension of the attribution method is introduced.

A broad range of observed frontal cyclones are investigated and the results compared to recent theoretical models. 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 instabilities may then break out. Diagnostics are examined to determine probable growth mechanisms. Values are found for the deformation rate, and horizontal shearing frontogenesis parameter commonly prescribed in frontogenesis theory.