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.
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