The Modulation of TC Size Configuration on Rapid Intensification

The wind structure is considered to be one of the critical internal dynamical factors that affect the intensification rate (IR) of a TC. Both the inner- and outer-core sizes are found to have certain linkages to TC rapid intensification (RI) on their own. However, in almost all theoretical paradigms of TC intensification, the connection between TC size configuration, which refers to the feature in the confirmation of TC inner and outer core sizes, and intensification has rarely been explored.

In this study, a new concept of TC fullness is raised to quantitatively measure the feature in TC size configuration, which is defined as the ratio of the extent of the outer-core wind skirt to the outer-core size of the TC. The higher TC fullness often promotes a larger storm intensity regardless of inner- and outer-core size. Through TC fullness, the coherent connections among TC size confirmation, intensity, and intensification are discussed. A simplified theoretical analysis is performed based on a two-dimensional vortex, which reveals an intrinsic relationship between storm intensity and size configuration. Based on this relationship, the TC fullness can be expressed as the product of two parts: the critical fullness, TCF₀, and the ratio of fullness, Rf (Figure 1). The TCF₀ represents the fundamental constraint on size configuration shared by TCs at a given intensity and is the lower limit of fullness, whereas Rf reveals a TC's unique wind structural feature.

There is a clear connection between the occurrence of RI and TC fullness. It is found that the upper limit of TC IR peaks when TCF₀ is around 0.55 (consistent with the intermediate intensity of 35–40 m s⁻¹). Meanwhile, moderate Rf (∼1.4) is also important for the occurrence of RI. As a result, under the combined effect of TCF₀ and Rf, RI TCs (i.e., the RI and ERI events) are more likely to be associated with relatively high fullness (TCF ∼ 0.8). Obtaining a stable and moderate Rf at low TCF₀ favors the subsequent intensification of a TC. This implies that a proper size configuration at the weak stage may raise the potential of achieving RI for TCs. The TCF-TCF₀ diagram is proposed to illustrate the co-evolution of TC size configuration and intensity and to reveal how they modulate intensification. It can be used in the forecast of RI events. This study could help us better understand the impact of TC wind structure on RI. Meanwhile, the intrinsic connections between TC size configuration and intensity have now been validated in both observations and simulations.