On the size-dependence in the recent time-dependent theory of tropical cyclone intensification

Abstract

Previous observational studies have shown that the intensification rate (IR) of a tropical cyclone (TC) is often 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 provides a detailed analysis to address how TC real-time size affects its intensification and why no size parameter explicitly appears in the theory based on results from axisymmetric numerical simulations. Results show that the overall correlation between 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-m s−1 wind (R17), is largely related to the correlation between IR and intensity because size and intensity are highly interrelated. As a result, the correlation between TC IR and size for a given intensity is rather weak. Diagnostic analysis shows that TC real-time size (RMW and R17) has two opposing effects on intensification. A larger TC size tends to result in a higher steady-state intensity, but reduce 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 IR on TC size for a given intensity.