On the role of convective available potential energy (CAPE) in tropical cyclone intensification

Abstract

This study addresses the role of convective available potential energy (CAPE) in the intensification of simulated tropical cyclones. Additionally, it also examines the 'wind-induced surface heat exchange' (WISHE) theory in which CAPE is non-existent during intensification. We use a hierarchy of models with different complexity. A low-order tropical cyclone model forms the simplest model. It is found that the damping of CAPE by fast convective exchange as assumed in the WISHE theory inhibits substantial intensification in the model. This result can be explained by the dominance of the secondary circulation over surface heat transfer in the growth stage. It leads to entrainment of low entropy air into the eyewall resulting in the weakening of the cyclone. Other simulations reveal that the intensification rate increases with increasing initial CAPE and that the inner core CAPE is smaller than that of the ambient region. Investigations with the more complex Ooyama model yield qualitatively similar results. In this model, two types of convection are considered. The first one is based on frictional convergence in the boundary layer and the second one describes a convective adjustment including a precipitation efficiency. Only frictionally induced convection supports tropical cyclone intensification while the second one strongly acts to dampen the cyclone. Finally, the complex nonhydrostatic cloud model CM1 is used where the initial CAPE is varied. This model also exposes the existence of radially increasing CAPE during intensification. The experiments of this study indicate a positive relationship between the radial CAPE gradient and the intensification rate which disagrees with the basic assumption of WISHE models. The results emphasise the role of the secondary circulation for transporting high entropy air into the tropical cyclone inner core, and therefore should be considered in a proper intensification theory as has been done in the rotating convection paradigm by Montgomery and Smith.

Advances in the Subseasonal Prediction of Extreme Events: Relevant Case Studies across the Globe

“…The medicane is further evaluated using Convective Available Potential Energy (CAPE), an indicator of atmospheric instability, which is a necessary condition for the development of severe weather events. CAPE has been found to be a prominent indicator and potential predictor for tropical cyclones (Huang and Liang 2010;Lee and Frisius 2018;Mylonas et al 2018) but has not been prominently used for medicanes.…”

Tropical cyclogenesis associated with premonsoon climatological dryline over the Bay of Bengal

“…In this research work, the primary objective was to understand the MCSs associated with pre-TCs. Thus, the distribution of the pre-cyclone environmental CAPE was veri ed as it is directly related to the initiation of deep convection (e.g., Williams and Renno 1993;Bhat et al 1996;May and Rajopadhyaya 1999;Rapp et al 2011), including that in the TC environment (Molinari and Vollaro 2009;Molinari et al 2012;Lee and Frisius 2018). The simulated CAPE in all cases was found to be almost similar in distribution to that in the reanalysis data; the larger values of CAPE in the north and northwestern areas of the BoB are especially well projected.…”

Assessing Atmospheric Instability over the Bay of Bengal during October and November Months between 2007 – 2018

“…There are implications of an increasing atmospheric instability to the intensification of tropical cyclones (Lee and Frisius, 2018;Seeley and Romps, 2015). The analyses and data in this research can delineate a pattern of increasing atmospheric instability and rising numbers of TCs.…”