Conditional symmetric instability ‐ a possible explanation for frontal rainbands

Abstract In order to study the possible importance of symmetric baroclinic instability in the formation of frontal rainbands, the existing theory is reviewed and the inclusion of the effects of latent heat release attempted. When an atmosphere is rendered symmetrically unstable by latent heat release, it may be said to be conditionally symmetrically unstable. Simple numerical experiments support the extended theory and describe the structure of the finite amplitude cells. These exhibit conditional gravitational instability in preferred linear regions. It is shown that such a gravitational destabilization is possible only when a wet bulb potential vorticity is initially negative. This latter is a necessary and possibly sufficient condition for ‘conditional symmetric instability’ (CSI). Limited comparison with observed frontal rainbands lends some support to the hypothesis that CSI can be a dominant formative mechanism, though more sophisticated numerical modelling and observational studies are required.