6TROPICAL Tropical Cyclone Intensification and Fullness: The Role of Storm Size Configuration

The wind structure is considered to be one of the important internal dynamical factors that affect the intensification rate (IR) of a TC. Being one aspect of TC wind structure, the intensity plays a critical role in modulating TC IR. However, in almost all the theoretical paradigms of TC intensification, the impact of TC size configuration on intensification has not yet been discussed. The relationship between TC intensification and size configuration is explored based on the TC fullness in this study. 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 is the intensity-related part of the fullness, and has a one-to-one relationship with storm intensity. Theoretically, TCF₀ reveals the fundamental constraint between the intensity and size configuration of a TC, which could approximately be regarded as the lower limit of fullness at a given intensity. From the other aspect, the TCF₀ also describes the maximum intensity that a TC could achieve given the TCF. The Rf is an integrated parameter related to the relative vorticity and size configuration. Unlike TCF₀, Rf describes the individualized structural characteristics of TCs, which could vary significantly among TCs with similar intensity (or TCF₀).

There is a clear relationship between IR 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 large IR. As a result, under the combined effect of TCF0 and Rf, fast intensifying TCs (i.e., the RI and ERI events) are more likely to be associated with relatively high fullness (TCF ∼ 0.8). In addition, the T-T diagram is proposed to depict the co-evolution of TCF₀ and TCF during TC development, which is a useful tool to quantitativly measure the feature of TC wind structure. It is found that the evolution of TCF, TCF₀, and Rf may affect the IR of TCs during the intensification stage. Meanwhile, obtaining a stable and moderate Rf at an early stage when TCF₀ is low appears to favor the subsequent intensification, implying that a proper size configuration may establish at the weak stage for fast-intensifying TCs. The size configuration of TCs is expected to play an important role in modulating storm intensification. Moreover, it is suggested that, when analyzing TC fullness, it would be helpful to take both TCF₀ and Rf into consideration to obtain a complete map of TC wind structure. Furthermore, TCF₀ could be adopted as a simple but practical indicator in evaluating the wind-field parameters in TC datasets. And TC events with fullness less than its TCF₀ should be used with extra caution since they might imply a wind structure that is physically impossible to maintain. This study could help us better understand the impact of TC wind structure on intensification.