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1. Dependence of Tropical Cyclone Weakening Rate in Response to an Imposed Moderate Environmental Vertical Wind Shear on the Warm‐Core Strength and Height of the Initial Vortex

   https://doi.org/10.1029/2023GL107779  

   Gao, Qi; Wang, Yuqing (March 2024, Geophysical Research Letters)

   Abstract This study investigated the dependence of the early tropical cyclone (TC) weakening rate in response to an imposed moderate environmental vertical wind shear (VWS) on the warm‐core strength and height of the TC vortex using idealized numerical simulations. Results show that the weakening of the warm core by upper‐level ventilation is the primary factor leading to the early TC weakening in response to an imposed environmental VWS. The upper‐level ventilation is dominated by eddy radial advection of the warm‐core air. The TC weakening rate is roughly proportional to the warm‐core strength and height of the initial TC vortex. The boundary‐layer ventilation shows no relationship with the early weakening rate of the TC in response to an imposed moderate VWS. The findings suggest that some previous diverse results regarding the TC weakening in environmental VWS could be partly due to the different warm‐core strengths and heights of the initial TC vortex. 

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2. The Effect of Initial Vortex Asymmetric Structure on Tropical Cyclone Intensity Change in Response to an Imposed Environmental Vertical Wind Shear

   https://doi.org/10.1029/2023GL104222  

   Gao, Qi; Wang, Yuqing (August 2023, Geophysical Research Letters)

   Abstract Previous studies have investigated how the environmental vertical wind shear (VWS) may trigger the asymmetric structure in an initially axisymmetric tropical cyclone (TC) vortex and how TC intensity changes in response. In this study, the possible effect of the initial vortex asymmetric structure on the TC intensity change in response to an imposed environmental VWS is investigated based on idealized full‐physics model simulations. Results show that the effect of the asymmetric structure in the initial TC vortex can either enhance or suppress the initial weakening of the TC in response to the imposed environmental VWS. When the initial asymmetric structure is in phase of the VWS‐induced asymmetric structure, the TC weakening will be enhanced and vice versa. Our finding calls for realistic representation of initial TC asymmetric structure in numerical weather prediction models and observations to better resolve the asymmetric structure in TCs. 

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3. Effect of Unidirectional Vertical Wind Shear on Tropical Cyclone Intensity Change—Lower‐Layer Shear Versus Upper‐Layer Shear

   https://doi.org/10.1029/2019JD030586  

   Fu, Hao; Wang, Yuqing; Riemer, Michael; Li, Qingqing (June 2019, Journal of Geophysical Research: Atmospheres)

   Abstract In this study, a quadruply nested, nonhydrostatic tropical cyclone (TC) model is used to investigate how the structure and intensity of a mature TC respond differently to imposed lower‐layer and upper‐layer unidirectional environmental vertical wind shears (VWSs). Results show that TC intensity in both cases decrease shortly after the VWS is imposed but with quite different subsequent evolutions. The TC weakens much more rapidly for a relatively long period in the upper‐layer shear than in the lower‐layer shear, which is found to be related to the stronger storm‐relative asymmetric flow in the middle‐upper troposphere and the larger vertical vortex tilt in the former than in the latter. The stronger storm‐relative flow in the former imposes a greater ventilation of the warm core in the middle‐upper troposphere, leading to a more significant weakening of the storm. The storm in the lower‐layer shear only weakens initially after the VWS is imposed but then experiences a quasi periodic intensity oscillation with a period of about 24 hr. This quasi periodic behavior is found to be closely related to the boundary layer thermodynamic “discharge/recharge” mechanism associated with the activity of shear‐induced outer spiral rainbands. There is no significant intensity oscillation for the storm embedded in the upper‐layer shear, even though outer spiral rainbands develop quasi periodically also. The boundary layer inflow is very weak in that case and the low equivalent potential temperature air induced by downdrafts in outer spiral rainbands therefore cannot penetrate into the inner core but remains in the outer region. 

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4. Factors Affecting the Weakening Rate of Tropical Cyclones over the Western North Pacific

   https://doi.org/10.1175/MWR-D-19-0356.1  

   Fei, Rong; Xu, Jing; Wang, Yuqing; Yang, Chi (September 2020, Monthly Weather Review)

   null (Ed.)

   Abstract In this study, based on the 6-hourly tropical cyclone (TC) best track data and the ERA-Interim reanalysis data, statistical analyses as well as a machine learning approach, XGBoost, are used to identify and quantify factors that affect the overwater weakening rate (WR) of TCs over the western North Pacific (WNP) during 1980–2017. Statistical analyses show that the TC rapid weakening events usually occur when intense TCs cross regions with a sharp decrease in sea surface temperature (DSST) with relatively faster eastward or northward translational speeds, and move into regions with large environmental vertical wind shear (VWS) and dry conditions in the upshear-left quadrant. Results from XGBoost indicate that the relative intensity of TC (TC intensity normalized by its maximum potential intensity), DSST, and VWS are dominant factors determining TC WR, contributing 26.0%, 18.3%, and 14.9% to TC WR, and 9, 5, and 5 m s−1 day−1 to the variability of TC WR, respectively. Relative humidity in the upshear-left quadrant of VWS, zonal translational speed, divergence at 200 hPa, and meridional translational speed contribute 12.1%, 11.8%, 8.8%, and 8.1% to TC WR, respectively, but only contribute 2–3 m s−1 day−1 to the variability of TC WR individually. These findings suggest that the improved accurate analysis and prediction of the dominant factors may lead to substantial improvements in the prediction of TC WR. 

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5. Quantifying the Environmental Effects on Tropical Cyclone Intensity Change Using a Simple Dynamically Based Dynamical System Model

   https://doi.org/10.1175/JAS-D-23-0058.1  

   Xu, Jing; Wang, Yuqing; Yang, Chi (December 2023, Journal of the Atmospheric Sciences)

   Abstract Accurate prediction of tropical cyclone (TC) intensity is quite challenging due to multiple competing processes among the TC internal dynamics and the environment. Most previous studies have evaluated the environmental effects on TC intensity change from both internal dynamics and external influence. This study quantifies the environmental effects on TC intensity change using a simple dynamically based dynamical system (DBDS) model recently developed. In this simple model, the environmental effects are uniquely represented by a ventilation parameterB, which can be expressed as multiplicative of individual ventilation parameters of the corresponding environmental effects. Their individual ventilation parameters imply their relative importance to the bulk environmental ventilation effect and thus to the TC intensity change. Six environmental factors known to affect TC intensity change are evaluated in the DBDS model using machine learning approaches with the best track data for TCs over the North Atlantic, central, eastern, and western North Pacific and the Statistical Hurricane Intensity Prediction Scheme (SHIPS) dataset during 1982–2021. Results show that the deep-layer vertical wind shear (VWS) is the dominant ventilation factor to reduce the intrinsic TC intensification rate or to drive the TC weakening, with its ventilation parameter ranging between 0.5 and 0.8 when environmental VWS between 200 and 850 hPa is larger than 8 m s⁻¹. Other environmental factors are generally secondary, with their respective ventilation parameters over 0.8. An interesting result is the strong dependence of the environmental effects on the stage of TC development. 

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