Simulations of the Extratropical Transition of Tropical Cyclones: Contributions by the Midlatitude Upper-Level Trough to Reintensification

The extratropical cyclone development processes during the reintensification stage of an extratropical transition from a tropical cyclone (TC) are described using numerical simulations. Three control simulations without a tropical cyclone present examine the extratropical cyclogenesis associated with upper-level troughs that are characterized as weak, moderate, and strong. When no tropical cyclone is included in the simulation, the minimum surface pressures attained with the weak, moderate, and strong troughs are 1003, 991, and 977 mb, respectively. In all three cases, the low tilts northwestward with height during intensification, and the rainfall pattern and eventual occlusion are representative of classic extratropical cyclone development. The interactions of a tropical cyclone with each of the three midlatitude circulation patterns are compared with the control simulations to illustrate the contributions to the extratropical transition of the tropical cyclone. In the three trough-with-TC cases, the minimum surface pressures were almost identical (967, 965, and 959 mb). Thus, the final intensity of the extratropical cyclone is not only related to the strength of the upper-level trough but must also be related to the structure of the basic midlatitude environment. The proper phasing of the tropical cyclone with the midlatitude trough results in substantial enhancement of the upper-level divergence. In addition, higher θe values in the lower troposphere associated with the tropical cyclone remnants are absorbed in the developing extratropical cyclone. The lifting of this moist air results in precipitation that is greater in both amount and areal extent, which enhances extratropical development when compared with the control cases. Based on these simulations, an important conclusion is that a weak midlatitude trough interacting with tropical cyclone remnants may have as much potential to intensify, as does a moderate or strong trough, and may have longer periods of rapid intensification. A development potential parameter based on the three main factors in the Petterssen development equation (upper-level divergence, midlevel positive vorticity advection, and low-level temperature advection) is calculated for all simulations. The strength and areal extent of the development parameter has utility in predicting where and whether extratropical cyclogenesis will occur during the reintensification stage of extratropical transition.