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Tang Hawii 0085O 10974.Pdf A DIAGNOSTIC STUDY OF RAINFALL EVOLUTION IN A WEAK LANDFALLING TROPICAL CYCLONE OVER EAST CHINA A THESIS SUBMITTED TO THE GRADUATE DIVISION OF THE UNIVERSITY OF HAWAI‘I IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN ATMOSPHERIC SCIENCES May 2021 By Lichun Tang Thesis Committee: Yuqing Wang, Chairperson Pao-Shin Chu Torri Giuseppe All rights reserved ACKNOWLEDGEMENTS It is my honor to have this opportunity to conduct this research work as a master’s student of the University of Hawai‘i at Manoa. Here I would like to thank my advisor Prof. Yuqing Wang and Zifeng Yu, for their great patience and extraordinary support in instructing me on how to describe phenomena and diagnose and explain the mechanisms. This thesis would have been impossible without their advice and support. Additionally, I want to express my thanks to the committee members, Prof. Pao-Shin Chu and Prof. Torri Giuseppe, for their kind participation and valuable discussions and suggestions. Lastly, I would also like to express my appreciation to my parents and friends, Yingxin Zhang and Rong Fei, for their passion and patience when I was having trouble preparing this thesis. They supported me and were always willing to help me during my study. 1 ABSTRACT Tropical storm Rumbia (2018) made landfall over East China on 16 August 2018 with a moderate intensity but led to long-lasting and heavy landfall, causing causality and tremendous economic loss to East China. In this study, the fifth generation European Centre for Medium- Range Weather Forecasting (ECMWF) reanalysis (ERA-5) data, the best-track tropical cyclone data, and rainfall observations from China Meteorological Administration (CMA) were used to diagnose the rainfall evolution of Rumbia after its landfall. Results showed that when it approached and made landfall over East China, Rumbia was embedded in an environment with a deep-layer (300–850hPa) southwesterly vertical wind shear (VWS), heavy rainfall mostly occurring downshear-left in its inner-core region and downshear-right in the outer-core region. The translation of Rumbia also contributed to the rainfall distribution to some extent. The strong southwesterly-southeasterly summer monsoon flow transported water vapor from the tropical ocean and the East China Sea to the storm’s core region, providing moisture and convective instability conditions in the mid-lower troposphere for the sustained rainfall even after Rumbia moved well inland. The low-level convective instability and the deep-layer environmental VWS played an important role in deepening the inflow boundary layer and the development of the secondary circulation and the heavy rainfall in the northeast quadrant of Rumbia. It is concluded that the environmental VWS and the storm translation are key to the asymmetric rainfall distribution, and the southwesterly-southeasterly summer monsoon flow transported warm and moist air from the tropical oceans and provided the moisture and convective instability conditions to the sustained rainfall of Rumbia after its landfall. 2 Key words: Landfalling tropical cyclone, heavy rainfall, asymmetric structure, vertical wind shear 3 Contents ACKNOWLEDGEMENTS .......................................................................................................................... 1 ABSTRACT .................................................................................................................................................. 2 LIST OF FIGURES ...................................................................................................................................... 5 1. Introduction ............................................................................................................................................... 8 2. Data ......................................................................................................................................................... 13 3. Results ..................................................................................................................................................... 15 3.1 An overview of Rumbia (2018) ........................................................................................................ 15 a. Rainfall evolution ................................................................................................................................ 15 b. Synoptic evolution ............................................................................................................................... 17 3.2 Effects of vertical wind shear and storm motion. ............................................................................. 22 3.3 Water vapor supply ........................................................................................................................... 28 3.4 Convective instability ....................................................................................................................... 30 3.5 The secondary circulation ................................................................................................................. 34 5. Summary and discussion ......................................................................................................................... 39 References ................................................................................................................................................... 41 4 LIST OF FIGURES Figure 1. (a) Observed storm track (solid line) and accumulated total rainfall (shading, mm). (b) Observed evolution of the central pressure (blue polyline) and maximum 10-m height wind (red polyline) from 0600 UTC 14 August to 1800 UTC 20 August. .................... 16 Figure 2. Evolution of the observed 6-hourly rainfall distribution (mm) from 1400 UTC 16 August 16 to 1400 18 August 2018. .............................................................................. 18 Figure 3. Geopotential height (shading, geopotential meters) and wind vectors (m·s-1) at 200 hPa: (a) 2000 UTC 16 August, (b) 0200, (c) 0800, (d) 1400, (e) 2000 UTC 17 August, (f) 0200 UTC 18 August 2018. ........................................................................................... 19 Figure 4. Same as Fig. 3, but at 500 hPa. .............................................................................. 20 Figure 5. South-north vertical cross sections of potential vorticity (shading, PVU) and potential temperature (contours, K) across the center of Rumbia: (a) 2000 UTC 16 August, (b) 0200, (c) 0800, (d) 1400, (e) 2000 UTC 17 August, (f) 0200 UTC 18 August 2018. .............................................................................................................................. 21 Figure 6. Time evolution of VWS magnitude (a) and direction (b) in different vertical layers over Rumbia. Here the VWS is defined as the horizontal mean wind vector averaged within a radius of 800 km from the storm center at the pressure levels. ....................... 22 Figure 7. Hourly rainfall evolution (shading, mm) from 0200–1300 UTC August 17. Grey dashed concentric circles are 150, 300, and 500 km from the typhoon center. Black arrows denote the storm motion vector, blue arrows refer to the deep layer VWS between 300–850 hPa. The cyclone symbol indicates the storm center at the given time. ......... 23 Figure 8. Same as Fig.7, but from 1400 UTC August 17 to 0100 UTC August 18............... 24 5 Figure 9. The evolution of the lower tropospheric relative humidity (shading, %) and wind vectors (m·s-1), both averaged between 700 and 1000 hPa: (a) 2000 UTC 16 August, (b) 0200, (c) 0800, (d) 1400, (e) 2000 UTC 17 August, (f) 0200 UTC 18 August 2018. The cyclone symbol indicates the storm center at the given time. ....................................... 27 Figure 10. The lower tropospheric water vapor flux (arrows, m-1·Pa-1·s-1) and vapor flux divergence (shading, m -2·Pa-1), both averaged between 700 and 1000 hPa: (a) 2000 UTC 16 August, (b) 0200, (c) 0800, (d) 1400, (e) 2000 UTC 17 August, (f) 0200 UTC 18 August 2018. The cyclone symbol indicates the storm center at the given time. .......... 29 Figure 11. Convective Available Potential Energy (CAPE, shading, J·kg-1) with the deep layer VWS vectors given at the storm center (red arrows): (a) 2000 UTC 16 August, (b) 0200, (c) 0800, (d) 1400, (e) 2000 UTC 17 August, (f) 0200 UTC 18 August 2018. The cyclone symbol indicates the storm center at the given time. ..................................................... 31 Figure 12. The vertical cross-section of equivalent potential temperature (shading, K) averaged in an annulus between the radii of 150 and 300 km from the storm center: (a) 2000 UTC 16 August, (b) 0200, (c) 0800, (d) 1400, (e) 2000 UTC 17 August, (f) 0200 UTC 18 August 2018. .................................................................................................... 32 Figure 13. Vertical-radius cross section of equivalent potential temperature (contours, K) and its vertical gradient (∂ϑe/∂z) (shading, K/m). Both averaged in the northeast quadrant of the storm: (a) 2000 UTC 16 August, (b) 0200, (c) 0800, (d) 1400, (e) 2000 UTC 17 August, (f) 0200 UTC 18 August 2018. ........................................................................ 33 Figure 14. Northeast quadrant mean radial wind (shading, m·s-1) and vertical motion (contours, m·s-1): (a) 2000 UTC 16 August, (b) 0200, (c) 0800, (d) 1400, (e) 2000 UTC 17 August, (f) 0200 UTC 18 August 2018. .....................................................................................
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