First principles tell us that deep convection will not develop unless Convective Available Potential Energy (CAPE) exists. Yet, in most environments with CAPE, there is a layer of negative buoyancy, or convective inhibition (CIN), that parcels must overcome before they can respond to CAPE. Over certain regions of the world, such as the Great Plains of the U. S., both CIN and CAPE can become quite large. Under these circumstances, forecasters commonly note that severe convection may develop if parcels can "break the cap" and become positively buoyant.

Predicting whether the cap will be broken can be quite challenging. Part of this challenge arises from the fact that several physical processes have the potential to erode this stable layer. For example, horizontal temperature advection, adiabatic temperature changes associated with vertical motion, and evaporative cooling associated with precipitation falling into a dryer layer of air from above can all act to change cap strength. Yet, it is usually very difficult to quantify the effects of these processes, even in retrospect.

In this study, we examine a case in which the erosion of the cap may be studied quantitatively. A strong cap was revealed in a Norman, OK sounding valid at 1200 UTC on 22 April 2001. During the ensuing 6 hours, widely scattered light rain showers fell through an elevated dry layer and a short wave trough impinged on the area from the southwest. An 1800 UTC sounding revealed a dramatic disappearance of the cap. A 6 h forecast sounding from the experimental version of the Eta model, initialized at 1200 UTC, showed remarkable (and unusual) agreement with the observed 1800 UTC sounding. This agreement, and the fact that the model also generated light precipitation prior to 1800 UTC, suggests that the model may provide significant insight into the complex physical processes leading to the cap erosion.

The cap erosion process is examined by extracting temperature tendencies associated with all physical processes from the 6 h model integration. The magnitudes of individual terms are compared and the mesoscale circulations associated with, and responsible for, the temperature changes are examined. Results are discussed in the context of improving the understanding and operational prediction of the cap erosion process.