The Evolution and Role of Midtropospheric Cyclonic Vortex in the Formation of Super Typhoon Nepartak (2016)

摘要

Based on a successful cloud-resolving simulation with the Weather Research and Forecasting Model, this study examines the evolution and the role of midtropospheric mesoscale cyclonic vortex in the formation of Super Typhoon Nepartak (2016). The midtropospheric vortex is correlated with the convective activity in pre-Nepartak. Once the deep convection outbreaks, the midtropospheric vortex intensifies first via the vertical advection associated with the severe updrafts and then through the midlevel convergence associated with stratiform precipitation. As the stratiform precipitation dissipates, the midlevel vortex weakens slightly in the following shallow convection phase. The above-described processes recur sequentially during the pregenesis of Nepartak, and the midtropospheric vortex demonstrates diurnal variations. Its intensification usually corresponds to the weakening of low-level cyclonic circulation except for the deep convection phase, indicating that the development of midtropospheric vortex can inhibit the development of self-sustained low-level cyclonic circulation. Although the midtropospheric vortex is not always a quasi-balanced perturbation, a cold core can be found in the lower troposphere below it during the most of the pregenesis stage. The appearance of the cold core enhances the low-level temperature gradient around it, which favors convection burst. In addition, the closed cyclonic circulation associated with the midlevel vortex can serve as a pouch protecting the vorticity, moisture, and convection inside from the vertical wind shear and dry air intrusion when the low-level and midlevel vortices are overlapped in the late pregenesis stage, which facilitates the sustained deep convection and the formation of Nepartak.