Exploring the Upper Bound of Tropical Cyclone Intensification

We introduce a new mode of thought on tropical cyclone (TC) intensification, namely, how can one ascertain the theoretical and practical upper bounds of TC intensification, given an initial vortex structure and expected environmental conditions. The expectation of an upper bound of intensification is partially motivated by observations showing that several tropical cyclones have intensified on the order of 100-105 kt over a 24-h period, but that this magnitude of intensification has never been observed to occur over time periods much shorter than a day. Dynamically, one can expect that the upper limit on intensification is controlled by the dynamical efficiency at which the vortex converts diabatic heating to kinetic energy. Thus the radial structure of the wind is key, as is the potential intensity. Finally, this upper bound may be strongly constrained by scaling considerations relating the timescales of inflowing parcel trajectories and the evolving secondary circulation. Numerically, most state-of-the-art full physics 3D numerical models struggle to predict extreme rates of intensification, yet one real-time forecast aid, the CHP6 member of the Coupled Hurricane Intensity Prediction Scheme (CHIPS), has sometimes captured the correct magnitude and timing of extreme rapid intensification events. Examples of recent successful predictions of very rapid intensification (defined as >= 50 kt intensity increase in 24 hours) or extreme rapid intensification (ERI, defined as a >= 85 kt increase in intensity in 24-h or a >= 50-kt increase in 12 hours) include:

• PATRICIA (EP20): 00 UTC 22 Oct 2015: 60 kt -> 150 kt in 24 hours (CHP6 predicted ~150 kt)
• MERANTI (WP16): 06 UTC 10 Sep 2016: 35 kt -> 155 kt in 60 hours (CHP6 predicted ~160 kt)
• MARIA (AL15): 12 UTC 17 Sep 2017: 60 kt -> 140 kt in 36 hours (CHP6 predicted ~135 kt)

While CHP6 is not a forecast model per se, since it is meant to show the sensitivity of the TC to very favorable environmental conditions relative to the other members of the CHIPS ensemble, the fact that it sometimes captures ERI events with 36-72 hour lead time when no other full physics model does, suggests that the dynamics of ERI are primarily axisymmetric and do not require a 3D full-physics framework. These examples of correct predictions of extreme RI events suggest that the general pathway of extreme rapid intensification can be captured by an axisymmetric numerical model, and that the MPIR may be achieved when the storm structure and latent heating distribution are axisymmetric in a favorable environment. When the MPIR limit is high for a given storm, extreme rapid intensification becomes possible.

The goals of this work are twofold: 1) to create and evaluate an initial version of forecast guidance for the upper bound rate at which TCs may intensify and examine the verification statistics for such a model, and 2) to examine high-resolution flight-level aircraft observations for several well-observed ERI cases to learn how the TC’s physical scale (radius of maximum winds), inertial stability, dynamic efficiency, and column-integrated diabatic heating evolve during ERI. Development of effective MPIR-based guidance could be extremely useful to operational forecasters who need to assess what the upper-bound intensification risk is for marginal tropical storms that are a few days away from landfall.