Evaluation of a Time-Dependent Model for the Intensification of Tropical Cyclones

An axisymmetric numerical cloud model is used to evaluate the theory of tropical cyclone (TC) intensification proposed by Emanuel (2012, herein E-IR) which is based on gradient-wind balance and moist neutral ascent along angular-momentum (M) surfaces. According to the numerical-model results, the intensification of the TC can be divided into two periods, Phase I and Phase II. During Phase I, the TC intensifies while the M and S* (saturation-entropy) surfaces evolve from nearly orthogonal to almost congruent. During Phase II, the M and S^* surfaces are congruent as the TC intensifies, which is consistent with E-IR. Therefore, the condition of moist slantwise neutrality of E-IR is sufficiently satisfied throughout the intensification in Phase II. It is also found that the sensitivity of the intensification rates (IR) to the surface-exchange coefficient for entropy (C_k) matches the theoretical result, IR~ C_k, but the relationship between IR and the surface-exchange coefficient for momentum (C_d) is essentially opposite to the theoretical result. Furthermore, the tendency diagnosed by E-IR is qualitatively similar to the numerical-model result during Phase II, but not quantitatively similar. The present analysis finds the inclusion of non-gradient-wind effects in the theoretical framework of E-IR produces an IR that is quantitatively similar to the numerical-model results. The neglect of non-gradient-wind effects in E-IR may be the reason for the inconsistency of the sensitivity of IR to C_d between E-IR and the numerical model. Other aspects of E-IR in the context of recent research on TC intensification are discussed.