12C.8 Evaluation of a Time-Dependent Model for the Intensification of Tropical Cyclones

Thursday, 19 April 2018: 9:45 AM
Champions ABC (Sawgrass Marriott)
Ke Peng, Nanjing Univ., Nanjing, China; and R. Rotunno and G. H. Bryan
Manuscript (631.6 kB)

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 (Ck) matches the theoretical result, IR~ Ck, but the relationship between IR and the surface-exchange coefficient for momentum (Cd) 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 Cd between E-IR and the numerical model. Other aspects of E-IR in the context of recent research on TC intensification are discussed.
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