Assessing the Vertical Distribution of Convective Available Potential Energy

Comparisons of convective available potential energy (CAPE) with standard instability indices for evaluating the convective potential of the atmosphere such as the lifted index (LI) reveal only moderate correlations. This is because the LI is a measure of single-level buoyancy while CAPE is a measure of both integration depth and the buoyancy. Normalizing the CAPE values by the depth over which the integration takes place provides an index (NCAPE) that is independent of the depth and is a convenient measure of the mean parcel buoyancy. This normalization effectively distinguishes between environments with similar CAPE but exhibiting different buoyancy and integration depth. Also, because the vertical distribution of CAPE can have an important effect on convective updraft strength, it is advantageous to vertically partition CAPE and NCAPE into multiple layers. NCAPE may provide a more useful indicator of buoyancy in environments in which the depth of free convection is shallow and total CAPE is small. It is suggested that NCAPE computations be used in combination with CAPE for evaluation of convective potential.