Journal of Conference Abstracts

Perspectives on Baroclinic Instability and its Relevance to the Atmosphere

Brian Hoskins (B.J.Hoskins@rdg.ac.uk)¹, Eyal Heifitz (eyalh@cyclone.tau.ac.il)² & John Methven¹

¹Dept of Meteorology, University of Reading, Reading, RG6 6BB, U.K.

²Dept of Geophysics and Planetary Sciences, Tel Aviv University, Israel

The idea that the classical normal mode theory of baroclinic instability is relevant to the Earth's atmosphere has been challenged in recent years. Indeed it has been questioned whether the atmosphere is actually unstable or whether a better model is that of a damped system subjected to white noise forcing. Much emphasis has also been placed on structures that would grow by the largest amount in a specified finite time. These "singular vector" structures have little similarity with the baroclininc normal modes. The European Centre for Medium Range Weather Forecasts use singular vectors in defining the range of initial conditions for their ensemble of 10-day forecasts.

In this talk an attempt will be made to reconcile these views with the normal mode theory in describing the storm-tracks found in the N Atlantic and Pacific in winter. The growth of singular vectors will be discussed in terms of potential vorticity inversion and Rossby wave propagation ideas.

It will also be shown how initial value problems describable in terms of a growing and decaying normal mode pair can be viewed in terms of the propagation and interaction of two Rossby waves. Each acts on the other to change its amplitude and phase. This interaction produces the phase-locking and mutual amplification of the unstable normal mode.