Does big CAPE equate to big tornado potential in supercell thunderstorms?

Abstract

<p>Convective available potential energy (CAPE), when considered by itself, is not a skillful discriminator of tornadic supercells from their nontornadic counterparts.  However, there is a longstanding notion that large CAPE may allow strong-to-violent tornadoes to occur in environments with otherwise weak vertical wind shear.  There is also anecdotal evidence that the largest tornadoes often occur on days with moderate-to-extreme CAPE.  These notions and anecdotes prompt us to ask whether large CAPE is <em>conditionally</em> supportive for significant tornado formation?</p> <p>We use a matrix of large-eddy simulations of supercells to address this question, wherein the vertical wind shear and the magnitude and vertical distribution of CAPE are independently varied.  Our analysis of these simulations identifies two influences of CAPE on storm evolution that potentially facilitate both tornadogenesis, and the formation of large and intense tornadoes.  Large CAPE generally fostered more negatively buoyant and expansive cold pools.  In environments with strong shear and low-level storm-relative flow, a more negatively buoyant rear-flank downdraft amid large CAPE fortified the rear-flank convergence zone beneath the updraft, leading to a more efficient projection of initially horizontal streamwise vorticity into the vertical.  This led to more intense mesocyclones and tornadoes, when compared to simulations with the same wind profile but smaller CAPE.  In environments with weak vertical wind shear and large CAPE, storms were more resistant to the negative effects of entrainment and thereby more prone to sustaining mesocyclones and producing tornadoes, when compared to environments with both weak shear and CAPE.  We argue that these processes explain past anecdotes about the role of CAPE in tornadogenesis.</p>