Abstract for EGS conference 1998
MECHANISMS FOR MID-LATITUDE CYCLONE DEVELOPMENT
Jake Badger and Brian Hoskins
The initial development of localized cyclonic perturbations is
investigated in a hierarchy of models. The work presented explores
the dependence of the linear evolution on the perturbation scale,
structure and location. The mechanisms involved in the rapid kinetic
energy growth and formation of deep systems are explained in terms of
potential vorticity, the effects of changing perturbation phase tilt
and Rossby wave propagation. Perturbations that are confined both
vertically and horizontally in streamfunction and located away from
boundaries yield the largest transient kinetic energy
growth. Transient growth rates in excess of the fastest growing normal
mode can be achieved for perturbations having no phase tilt with
height. Basic states with positive meridional gradient of potential
vorticity give rise to perturbation growth accompanied by upward
propagation. Perturbations located at low levels benefit from this
propagation, exhibiting earlier deep system formation downstream and
sustained growth, whereas perturbations located at upper levels take
much longer to engage the surface boundary. This work offers a
fundamental physical basis for understanding optimal perturbations;
many aspects of the structure and evolution of singular vectors
calculated by the ECMWF can be understood in the context of
this idealized work. The role of diabatic processes as well as the
relevance of these mechanisms in storm track organization using more
realistic models of the atmosphere will be presented.