Deepening Process of an Incipient Vortex of Tropical Cyclone Simulated in a Global Nonhydrostatic Model

The tropical cyclogenesis (TCG) in the western north Pacific (WNP) occurs under a wide range of spatio-temporal variabilities, such as the El Nino Southern Oscillation, East Asian monsoon, intraseasonal variabilities, and synoptic scale wave disturbances. In view of mesoscale processes, moist physics play a critical role as well as vorticity dynamics. A global nonhydrostatic model is a useful tool to investigate such multiscale convective processes of the tropical cyclogenesis in a seamless framework. The objective of this study to specify typical scenarios of TCG in the WNP by numerical simulations of TCG using a global nonhydrostatic model with a cloud permitting resolution. Here we focus on deepening process of an incipient vortex in a case study of TY0806 (Fengshen), which developed in middle June over the Philippine Sea. The environmental condition was generally favorable just after the onset of WNP summer monsoon coinciding with the active phase of intraseasonal variability. The incipient vortex of TY0806 was traceable as far as central Pacific around two weeks before the TCG in the analysis data. The vortex was loosely coupled with convection as it traveled westward along 5N, interacted with middle tropospheric vorticity anomaly in the WNP, and rapidly deepened after 19 June 2008 (approximately 30 hPa in a day). The numerical simulation was initialized on 15 June 2008. The early developing stage of the vorticity in association with the deep convective events was successfully simulated in comparison with in situ observations. In the model, the rapid deepening started after successive formation of deep convection within a several tens of kilometers of the vorticity center for more than a day, and a ring of intense convection was formed around the center. A detailed analysis of the latent heating and adiabatic effects associated with the deep convection exhibited a marked difference between the 10-20 km from the center and the outside region including the eyewall. The simulation results suggested that the compensating subsidence associated with the successive formation of deep convection contributed to the formation of a warm core and deepening. The subsidence and warming started from the lower troposphere and got stronger in the upper levels. Ensemble simulations with modification in physical processes indicated that the deepening is more rapid and greater in magnitude when the convective growth and compensating subsidence are more concentrated near the center for a day or more.