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Closes: 23 Oct 2018

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21 May 2019

Distinguished Lecture

Distinguished Lecture - AS

Zhe-Min TAN
Nanjing University

"Dynamics of Tropical Cyclone Intensity Change: External and Internal Influences"

Abstract
Tropical cyclones (TCs) are among the deadliest and costliest of disasters, causing destructions due to the strong winds, the flooding and mudslides associated with storm surges and heavy rainfall. Despite the continuing improved forecast of TC’s track, the prediction of TC intensity and structure still faces the great challenges in atmospheric sciences today, and progresses marginally due to the large uncertainties of our understanding on the dynamics and physics that govern the changes of TC’s structure and intensity, both internally and externally.

Large-scale environmental vertical wind shear is one of the most important external factors that regulate TC intensity and structure. Our recent study suggests a new thermodynamic pathway that explains the detrimental role of deep-layer shear on TC intensity. The persistent patterns of shear-organized convection outside the eyewall transport the moist entropy upward effectively. This pathway works collectively with the classic mid-level and low-level ventilation to reduce the radial gradient of moist entropy across the eyewall. As a result, the heat engine efficiency is reduced so as to weaken TC intensity. In addition to the axisymmetric point of view, the weakening of TC intensity begins with a quadrant-dependent evolution in terms of low-level tangential wind through the multi-scale processes (vortex scale and convective scale). Given that tropical cyclones could still intensify under strong vertical wind shear, it is not only the deep-layer shear, but also the vertical profile of environmental flows, need to be considered to explain the large variability of TC intensity change. In the presence of complicated environmental flows, it is the relative configuration of low-level vortex tilt and the overall vortex tilt that determines the regions where upward motions dominant with respect to the deep-layer shear. This configuration, together with TC movement, primarily controls the convection organization in the azimuthal direction. This further determines whether a positive or a negative feedback would be established through the coupling between vortex tilt and convection to affect the precession of overall vortex tilt that is important for TC’s intensification variability.

The concentric eyewall structures are common in the intense TCs, which are related to the pronounced changes of TC intensity and structure. Thus, the second eyewall formation (SEF) and subsequent eyewall replacement cycle (ERC) are the one of important internal process influenced TC intensity and structure change. Although substantial advances have been made in understanding SEF and ERC, some key disagreements remain, and there no consensus on any single theory of SEF. Our recent study suggests that in the intense TCs, the SEF generally occurs in association with the asymmetric outer rainbands. The enhancement of convection in the SEF region follows the formation and inward contraction of an outer rainband. The descending radial inflow in the middle and downwind portions of the outer rainband initiates/maintains a strong inflow in the boundary layer. The latter is able to penetrate into the inner-core region, sharpens the gradient of radial velocity, and reinforces convergence. Consequently, warm and moist air is continuously lifted up at the leading edge of the strong inflow to support deep convection. Moreover, the inflow from the outer rainband creates strong supergradient winds that are ejected outward downwind, thereby enhancing convergence and convection on the other side of the storm. The interaction between outer rainband forcing and boundary layer dynamics is the key factor for the SEF. These also imply that the favorable environment for outer rainband formation is necessary condition for the SEF. A relatively large size for TC wind structure is conducive to the SEF.

In this talk I will discuss the dynamical process related to both internal and external influence on the tropical cyclone intensity and structure change and their implications for the improvement of tropical cyclone intensity prediction.

Biography
Dr. Zhe-Min Tan is currently the Cheung-Kong Chair Professor of Meteorology in the School of Atmospheric Sciences at Nanjing University. He has been a full professor of atmospheric science since 1999 and the Vice-President of Nanjing University since 2009. He was the Head of Department of Meteorology at Nanjing University from 1998-2006, and the Director of the Key Laboratory of Mesoscale Severe Weather, Ministry of Education of China and Nanjing University from 2000-2010. He was the Associate Vice-President of Nanjing University from 2006 to 2009.

He graduated in Meteorology at Nanjing University in 1982, and obtained a Ph.D. in Atmospheric Science at Nanjing University in 2000. He has dedicated himself to atmospheric research for an extended period of time. His research interests cover a range of problems in mesoscale atmospheric dynamics and predictability, tropical cyclone physics and dynamics, and atmospheric boundary layer dynamics. He has published more than 100 peer-reviewed articles published in high-quality journals and book chapters, and has served on many national and international advisory committees. He has received a number of awards. As an outstanding atmospheric scientist, he also dedicates himself in education, and was awarded the Special Prize for National Higher Education and Teaching Achievement Award by the State Council of China in 2014.