On the Size Dependence in the Recent Time-Dependent Theory of Tropical Cyclone Intensification

Previous observational studies have shown that the intensification rate (IR) of a tropical cyclone (TC) is en correlated with its real-time size. However, no any size parameter explicitly appears in the recent time-dependent theory of TC intensification, while the theory can still well capture the intensity evolution of simulated TCs. This study pro- vides a detailed analysis to address how TC real-time size affects its intensification and why no size parameter explicitly ap pears in the theory based on the results from axisymmetric numerical simulations. The results show that the overall coffrelation between the TC IR and real-time size as reported in previous observational studies, in terms of both the radius of maximum wind (RMW) and the radius of 17 ms(-1) wind (R17), is largely related to the correlation between the IR and intensity because the size and intensity are highly interrelated. As a result, the correlation between the TC IR and size for a given intensity is rather weak. Diagnostic analysis shows that the TC real-time size (RMW and #17) has two opposing ef- feds on intensification. A larger TC size tends to result in a higher steady state intensity but reluce the conversion efficiency of thermodynamic energy to inner-core kinetic energy or the degree of moist neutrality of the eyewall ascent for a given intensity. The former is favorable, while the latter is unfavorable for intensification. The two effects are implicitly included in the theory and largely offset, resulting in the weak dependence of the IR on TC size for given intensity.