On the behaviour of baroclinic waves undergoing horizontal deformation. I: The ‘ RT ’ phase diagram

Non‐modal baroclinic wave growth in an idealizsed flow comprising baroclinic shear and horizontal confluence is considered. It is shown how a general semi‐analytic solution may be derived for the evolution of non‐modal waves with uniform potential vorticity in this ‘mixed’ basic state. The solution is new. It represents a synthesis of baroclinic waves with pure deformation solutions. For a given initial wavelength the solution is capable of describing an infinite number of distinct non‐modal developments. To overview this large variety, phase diagrams, dubbed ‘ RT ’ diagrams, have been developed. The diagrams depict the evolution of vertical structure for a wide range of initial conditions. Some specific solutions with relevance to type B cyclogenesis and the potentially destablizing effect of deformation are discussed.