Featured: A new look to the generalized CAPE

Journal of Atmospheric Sciences | Steinacker [2016]


A determination of the generalized CAPE (GCAPE) is suggested, where, instead of an adiabatic reversible vertical redistribution of air in a conditionally unstable atmosphere, irreversible processes with regard to the water cycle are taken into account. Irreversible processes like entrainment or atmospheric dissipation due to precipitation generally reduce the generation of kinetic energy, irreversible pseudo-adiabatic processes, in contrast, increase the availability of convective potential energy considerably. It is further increased significantly, when we consider an irreversible redistribution of water in the atmosphere due to precipitation and its corresponding evaporation. Finally, the contribution of the ice phase in clouds and solid precipitation evaporation to GCAPE is investigated. The second aim of this paper is to compare CAPE and GCAPE in an unstable atmosphere. As an important result it is shown, that CAPE and GCAPE are inequivalent metrics of stability. It is even possible that an atmospheric profile with a considerable GCAPE shows a zero CAPE value. With the aid of different models with increasing complexity of thermodynamic processes a comparison of numerical values of CAPE and GCAPE for an idealized as well as a real atmospheric profile is carried out. Although the mechanism and fraction of realization of convective potential available energy may be very different in individual weather systems, the pseudo-adiabatic GCAPE arguably seems to be the better quantity than CAPE to compare or calibrate energy conversions in the atmosphere during organized deep convection with precipitation, because convection requires GCAPE, but not necessarily CAPE. Evaluations have shown, that on all days with thunderstorms in Vienna during a 9 year period GCAPE was positive, while a considerable fraction of these days showed a zero CAPE.

Full text can be found here.


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