A New Look to the Generalized CAPE

Abstract

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 pseudoadiabatic processes, in contrast, increase the availability of convective potential energy considerably. It is further increased significantly when an irreversible redistribution of water in the atmosphere due to precipitation and its corresponding evaporation is considered. Finally, the contributions of the ice phase in clouds and solid precipitation evaporation to GCAPE are 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, comparisons of numerical values of CAPE and GCAPE for an idealized and a real atmospheric profile are carried out. Although the mechanism and fraction of realization of convective available potential energy may be very different in individual weather systems, the pseudoadiabatic 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-yr period, GCAPE was positive, while a considerable fraction of these days showed zero CAPE.