Monday 11 Jun 2012 Investigating a simple model of nonlinear baroclinic equilibration (Taylor and Francis sponsored)

Dr Gavin Esler - UCL

Harrison 209 15:00-16:00

Unstable fluid flows at high Reynolds number often equilibrate
by stirring fluid parcels across a region encompassing one throughout
which a local instability criterion is satisfied. Analytical solutions describing such
behaviour are rare. Warn and Gauthier, however, have given just such a
solution for the equilibration of baroclinic instability in a two-layer
quasi-geostrophic model. In Warn and Gauthier's solution the lower layer
develops as a nonlinear Rossby wave critical layer and at long times the
layer becomes completely stirred. The case where Warn and Gauthier's
solution is perturbed by weak Ekman friction is considered here. The new
situation is surprisingly complex, with the introduction of an entirely
new instability mechanism. It is shown that the idea, popular in dynamical
meteorology, of `baroclinic adjustment' in which the forced-dissipative
system is expected to adjust to a state neutral to baroclinic instability,
does not work even in this relatively simple model.

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