The response of CAPE and CIN to tropospheric thermal variations

In the absence of moist convection, convective available potential energy (CAPE) and convective inhibition (CIN) both respond to changes in the thermal and humidity profiles of the atmosphere. It is shown that these changes can be understood in terms of a direct effect, involving changes in the profile in the absence of parcel changes, and an indirect effect involving changes in air‐parcel evolution in a developing convective boundary layer. Succinctly, low‐wave‐number (deep) changes in the thermal profile maximize the direct influence on CAPE while higher‐wave‐number (shallower, near‐surface) thermal changes maximize the direct influence on CIN. A simple estimate of the direct influence on CAPE, which is independent of the assumptions relating to choice of parcel ascent, is shown to give accurate results. The indirect influence comes about as a result of changes in the stability of the profile just above the inversion: the stability acts as a control on the entrainment of dry air into the boundary layer under conditions of surface heating, so that a boundary layer growing into a stable lower troposphere is more humid than one growing into a less stable profile, with significantly higher equivalent potential temperature. Thus, a profile of relatively high stability just above the inversion, typically exhibiting high CIN, will also tend to allow CAPE to build up in the boundary layer quite rapidly, through a suppression of the entrainment under conditions of surface heating and boundary‐layer growth. Thus it may be said that high CIN tends to lead to the accumulation of high CAPE even in the absence of convective downdraught feedbacks.