The environments and associated physical mechanisms that cause size and structure changes in a tropical cyclone

Tropical cyclones (TCs) can make significant size changes during their lifetime and the amount of size change depends on the environmental conditions surrounding the TC and internal TC dynamics. Focusing on the former, this study explores the impacts that environmental temperature and air-sea temperature difference has on TC size and structure changes. The study is conducted in two parts: the first part uses the WRF-ARW model to test the sensitivity of TC size changes to simple changes in the environment; the second part validates the results from the first part by characterizing the environments associated with real cases of TC size change in the North Atlantic basin.;It is found that when the simulated atmosphere is cooled, the initial specific humidity and convective available potential energy (CAPE) decrease but the surface energy fluxes from the ocean increase. The higher surface fluxes support a larger inner- and outer-core precipitation field, which leads to a larger wind field through the diabatic production of potential vorticity. In contrast, a warmer atmosphere with reduced surface energy fluxes inhibits the growth of the TC wind field. The higher initial CAPE and moisture content, however, allow the TC to spin up more rapidly with a compact core of intense precipitation. Thus, it is not the temperature of the atmosphere that is causing the size changes, but instead it is the higher surface energy fluxes that arise from the increased air-sea temperature difference.;In the second part of the study, the environments associated with real cases of TC size change in the North Atlantic Basin were characterized. Environmental composites, created using the ERA-Interim reanalysis dataset, show that TCs that made size changes in the deep tropics were typically associated with more environmental, mid-level humidity and higher air-sea temperature difference. The TCs that made large size changes in the extratropics were associated with highly-baroclinic environments and high mid-level moisture south of the TC-circulation center. The results are generally consistent with the size change environments simulated in the first part of this study and with others that have modeled the impact of environmental moisture on TC size changes.