Why does rapid contraction of the radius of maximum wind precede rapid intensification in tropical cyclones?

2021 ◽  
Author(s):  
Yuanlong Li ◽  
Yuqing Wang ◽  
Yanluan Lin ◽  
Xin Wang
Keyword(s):  
Tropical Cyclones ◽  
Radial Distribution ◽  
Diabatic Heating ◽  
Rate Of Change ◽  
Rapid Intensification ◽  
Absolute Vorticity ◽  
Radial Gradient ◽  
Maximum Wind ◽  
Tangential Wind ◽  
Vorticity Flux

AbstractThe radius of maximum wind (RMW) has been found to contract rapidly well preceding rapid intensification in tropical cyclones (TCs) in recent literature but the understanding of the involved dynamics is incomplete. In this study, this phenomenon is revisited based on ensemble axisymmetric numerical simulations. Consistent with previous studies, because the absolute angular momentum (AAM) is not conserved following the RMW, the phenomenon can not be understood based on the AAM-based dynamics. Both budgets of tangential wind and the rate of change in the RMW are shown to provide dynamical insights into the simulated relationship between the rapid intensification and rapid RMW contraction. During the rapid RMW contraction stage, due to the weak TC intensity and large RMW, the moderate negative radial gradient of radial vorticity flux and small curvature of the radial distribution of tangential wind near the RMW favor rapid RMW contraction but weak diabatic heating far inside the RMW leads to weak low-level inflow and small radial absolute vorticity flux near the RMW and thus a relatively small intensification rate. As RMW contraction continues and TC intensity increases, diabatic heating inside the RMW and radial inflow near the RMW increase, leading to a substantial increase in radial absolute vorticity flux near the RMW and thus the rapid TC intensification. However, the RMW contraction rate decreases rapidly due to the rapid increase in the curvature of the radial distribution of tangential wind near the RMW as the TC intensifies rapidly and RMW decreases.

Revisiting the Relationship between Eyewall Contraction and Intensification

2015 ◽  
Vol 72 (4) ◽  
pp. 1283-1306 ◽  
Author(s):  
Daniel P. Stern ◽  
Jonathan L. Vigh ◽  
David S. Nolan ◽  
Fuqing Zhang
Keyword(s):  
Tropical Cyclones ◽  
Wind Profile ◽  
Diabatic Heating ◽  
Initial Size ◽  
Radial Gradient ◽  
Vortex Model ◽  
Ring Model ◽  
Peak Intensity ◽  
Tangential Wind ◽  
The Relationship

Abstract In the widely accepted convective ring model of tropical cyclone intensification, the intensification of the maximum winds and the contraction of the radius of maximum winds (RMW) occur simultaneously. This study shows that in idealized numerical simulations, contraction and intensification commence at the same time, but that contraction ceases long before peak intensity is achieved. The rate of contraction decreases with increasing initial size, while the rate of intensification does not vary systematically with initial size. Utilizing a diagnostic expression for the rate of contraction, it is shown that contraction is halted in association with a rapid increase in the sharpness of the tangential wind profile near the RMW and is not due to changes in the radial gradient of the tangential wind tendency. It is shown that a number of real storms exhibit a relationship between contraction and intensification that is similar to what is seen in the idealized simulations. In particular, the statistical distribution of intensifying tropical cyclones indicates that, for major hurricanes, most contraction is completed prior to most intensification. By forcing a linearized vortex model with the diabatic heating and frictional tendencies from a simulation, it is possible to qualitatively reproduce the simulated secondary circulation and separately examine the vortex responses to heating and friction. It is shown that heating and friction both contribute substantially to boundary layer inflow. They also both contribute to the contraction of the RMW, as the positive wind tendency from heating-induced inflow is maximized inside of the RMW, while the net negative wind tendency from friction and frictionally induced inflow is maximized outside of the RMW.

scholarly journals Some Implications of Core Regime Wind Structures in Western North Pacific Tropical Cyclones

2011 ◽  
Vol 26 (1) ◽  
pp. 61-75 ◽  
Author(s):  
Delia Yen-Chu Chen ◽  
Kevin K. W. Cheung ◽  
Cheng-Shang Lee
Keyword(s):  
Tropical Cyclone ◽  
Tropical Cyclones ◽  
Quantitative Measure ◽  
Small Region ◽  
Rapid Intensification ◽  
Maximum Wind ◽  
Convective Structures ◽  
Tangential Wind ◽  
Strong Convection ◽  
Infrared Brightness

Abstract In this study, a tropical cyclone (TC) is considered to be compact if 1) the radius of maximum wind or the maximum tangential wind is smaller than what would be expected for an average tropical cyclone of the same intensity or the same radius of maximum wind, and 2) the decrease of tangential wind outside the radius of maximum wind is greater than that of an average TC. A structure parameter S is defined to provide a quantitative measure of the compactness of tropical cyclones. Quick Scatterometer (QuikSCAT) oceanic winds are used to calculate S for 171 tropical cyclones during 2000–07. The S parameters are then used to classify all of the cases as either compact or incompact according to the 33% and 67% percentiles. It is found that the early intensification stage is favorable for the occurrence of compact tropical cyclones, which also have a higher percentage of rapid intensification than incompact cases. Composite infrared brightness temperature shows that compact tropical cyclones have highly axisymmetric convective structures with strong convection concentrated in a small region near the center. Low-level synoptic patterns are important environmental factors that determine the degree of compactness; however, it is believed that compact tropical cyclones maintain their structures mainly through internal dynamics.

The Intensity- and Size-Dependent Response of Tropical Cyclones to Increasing Vertical Wind Shear

2021 ◽  
Author(s):  
Peter M. Finocchio ◽  
Rosimar Rios-Berrios
Keyword(s):  
Wind Field ◽  
Wind Shear ◽  
Diabatic Heating ◽  
Inner Core ◽  
Potential Temperature ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Rapid Intensification ◽  
Vertical Coupling ◽  
Tangential Wind

AbstractThis study describes a set of idealized simulations in which westerly vertical wind shear increases from 3 to 15 m s−1 at different stages in the lifecycle of an intensifying tropical cyclone (TC). The TC response to increasing shear depends on the intensity and size of the TC’s tangential wind field when shear starts to increase. For a weak tropical storm, increasing shear decouples the vortex and prevents intensification. For Category 1 and stronger storms, increasing shear causes a period of weakening during which vortex tilt increases by 10–30 km before the TCs reach a near-steady Category 1–3 intensity at the end of the simulations. TCs exposed to increasing shear during or just after rapid intensification tend to weaken the most. Backward trajectories reveal a lateral ventilation pathway between 8–11 km altitude that is capable of reducing equivalent potential temperature in the inner core of these TCs by nearly 2°C. In addition, these TCs exhibit large reductions in diabatic heating inside the radius of maximum winds (RMW) and lower-entropy air parcels entering downshear updrafts from the boundary layer, which further contributes to their substantial weakening. The TCs exposed to increasing shear after rapid intensification and an expansion of the outer wind field reach the strongest near-steady intensity long after the shear increases because of strong vertical coupling that prevents the development of large vortex tilt, resistance to lateral ventilation through a deep layer of the middle troposphere, and robust diabatic heating within the RMW.

scholarly journals The Experimental HWRF System: A Study on the Influence of Horizontal Resolution on the Structure and Intensity Changes in Tropical Cyclones Using an Idealized Framework

2011 ◽  
Vol 139 (6) ◽  
pp. 1762-1784 ◽  
Author(s):  
Sundararaman G. Gopalakrishnan ◽  
Frank Marks ◽  
Xuejin Zhang ◽  
Jian-Wen Bao ◽  
Kao-San Yeh ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclones ◽  
Horizontal Resolution ◽  
Grid Resolution ◽  
Weather Research And Forecasting ◽  
Maximum Wind ◽  
Tangential Wind ◽  
Intensity Changes ◽  
Horizontal Grid ◽  
Weather Research

Abstract Forecasting intensity changes in tropical cyclones (TCs) is a complex and challenging multiscale problem. While cloud-resolving numerical models using a horizontal grid resolution of 1–3 km are starting to show some skill in predicting the intensity changes in individual cases, it is not clear at this time what may be a reasonable horizontal resolution for forecasting TC intensity changes on a day-to-day-basis. The Experimental Hurricane Weather Research and Forecasting System (HWRFX) was used within an idealized framework to gain a fundamental understanding of the influence of horizontal grid resolution on the dynamics of TC vortex intensification in three dimensions. HWFRX is a version of the National Centers for Environmental Prediction (NCEP) Hurricane Weather Research and Forecasting (HWRF) model specifically adopted and developed jointly at NOAA’s Atlantic Oceanographic and Meteorological Laboratory (AOML) and Earth System Research Laboratory (ESRL) for studying the intensity change problem at a model grid resolution of about 3 km. Based on a series of numerical experiments at the current operating resolution of about 9 km and at a finer resolution of about 3 km, it was found that improved resolution had very little impact on the initial spinup of the vortex. An initial axisymmetric vortex with a maximum wind speed of 20 m s−1 rapidly intensified to 50 m s−1 within about 24 h in either case. During the spinup process, buoyancy appears to have had a pivotal influence on the formation of the warm core and the subsequent rapid intensification of the modeled vortex. The high-resolution simulation at 3 km produced updrafts as large as 48 m s−1. However, these extreme events were rare, and this study indicated that these events may not contribute significantly to rapid deepening. Additionally, although the structure of the buoyant plumes may differ at 9- and 3-km resolution, interestingly, the axisymmetric structure of the simulated TCs exhibited major similarities. Specifically, the similarities included a deep inflow layer extending up to about 2 km in height with a tangentially averaged maximum inflow velocity of about 12–15 m s−1, vertical updrafts with an average velocity of about 2 m s−1, and a very strong outflow produced at both resolutions for a mature storm. It was also found in either case that the spinup of the primary circulation occurred not only due to the weak inflow above the boundary layer but also due to the convergence of vorticity within the boundary layer. Nevertheless, the mature phase of the storm’s evolution exhibited significantly different patterns of behavior at 9 and 3 km. While the minimum pressure at the end of 96 h was 934 hPa for the 9-km simulation, it was about 910 hPa for the 3-km run. The maximum tangential wind at that time showed a difference of about 10 m s−1. Several sensitivity experiments related to the initial vortex intensity, initial radius of the maximum wind, and physics were performed. Based on ensembles of simulations, it appears that radial advection of the tangential wind and, consequently, radial flux of vorticity become important forcing terms in the momentum budget of the mature storm. Stronger convergence in the boundary layer leads to a larger transport of moisture fluxes and, subsequently, a stronger storm at higher resolution.

scholarly journals Reply to “Comments on ‘Revisiting the Relationship between Eyewall Contraction and Intensification’”

2017 ◽  
Vol 74 (12) ◽  
pp. 4275-4286 ◽  
Author(s):  
Daniel P. Stern ◽  
Jonathan L. Vigh ◽  
David S. Nolan ◽  
Fuqing Zhang
Keyword(s):  
Tropical Cyclone ◽  
Wind Profile ◽  
Radial Gradient ◽  
Maximum Wind ◽  
Tangential Wind ◽  
The Relationship

Abstract In their comment, Kieu and Zhang critique the recent study of Stern et al. that examined the contraction of the radius of maximum wind (RMW) and its relationship to tropical cyclone intensification. Stern et al. derived a diagnostic expression for the rate of contraction and used this to show that while RMW contraction begins and accelerates as a result of an increasing negative radial gradient of tangential wind tendency inward of the RMW, contraction slows down and eventually ceases as a result of the increasing sharpness of the wind profile around the RMW during intensification. Kieu and Zhang claim that this kinematic framework does not yield useful understanding, that Stern et al. are mistaken in their favorable comparison of this framework to earlier work by Willoughby et al., and that Stern et al. are mistaken in their conclusion that an equation for the contraction of the RMW derived by Kieu is erroneous. This reply demonstrates that each of these claims by Kieu and Zhang is incorrect.

scholarly journals Observational Study on the Characteristics of the Boundary Layer during Changes in the Intensity of Tropical Cyclones Landing in Guangdong, China

2019 ◽  
Vol 2019 ◽  
pp. 1-14
Author(s):  
Fei Liao ◽  
Ran Su ◽  
Pak-Wai Chan ◽  
Yanbin Qi ◽  
Kai-Kwong Hon
Keyword(s):  
Boundary Layer ◽  
Angular Momentum ◽  
Tropical Cyclone ◽  
Tropical Cyclones ◽  
Wind Velocity ◽  
Guangdong Province ◽  
Height Range ◽  
Maximum Wind ◽  
Tangential Wind ◽  
Radial Outflow

Eleven tropical cyclones that landed in Guangdong Province since 2012 and experienced strengthening or weakening over the offshore area were studied. Since the structure of the tropical cyclone boundary layer significantly influences the variation of the intensity of the cyclone, continuous observations of the wind profile radar at a coastal radar station in Guangdong Province were combined with aircraft observation data of the No. 1604 “Nida” cyclone to analyse the variations in the distributions of the radial wind, tangential wind, and angular momentum in the typhoon boundary layer and the similarities and differences between the boundary layers of the 11 tropical cyclones during the strengthening or weakening of their intensities. The analysis results show that the presence of the supergradient wind and the enhancement effect of the radial inflow play important roles in enhancing the intensity of a tropical cyclone. The observations indicate that when the tangential wind velocity in the maximum wind velocity radius reaches the velocity of the supergradient wind and when the radial inflow either gradually increases towards the centre of the tropical cyclone or gradually covers the entire boundary layer, the angular momentum tends to be shifted towards the centre. At this time, the maximum radial inflow, maximum tangential wind, and maximum angular momentum are in the same height range in the vertical direction. When a strong radial outflow occurs in the boundary layer of a tropical cyclone or the area with maximum wind velocity is located in the air outflow, the angular momentum cannot easily be transported towards the centre of the typhoon. Therefore, the spatial configuration of the three physical quantities will determine future changes in the intensity of tropical cyclones. The scope of the results presented here is limited to the 11 selected cases and suggests extending the analysis to more data.

scholarly journals How Do Outer Spiral Rainbands Affect Tropical Cyclone Structure and Intensity?*

2009 ◽  
Vol 66 (5) ◽  
pp. 1250-1273 ◽  
Author(s):  
Yuqing Wang
Keyword(s):  
Tropical Cyclone ◽  
Tropical Cyclones ◽  
Surface Pressure ◽  
Diabatic Heating ◽  
Inner Core ◽  
Phase Changes ◽  
Wave Radiation ◽  
Atmospheric Column ◽  
Tangential Wind ◽  
Spiral Rainbands

Abstract A long-standing issue on how outer spiral rainbands affect the structure and intensity of tropical cyclones is studied through a series of numerical experiments using the cloud-resolving tropical cyclone model TCM4. Because diabatic heating due to phase changes is the main driving force of outer spiral rainbands, their effect on the tropical cyclone structure and intensity is evaluated by artificially modifying the heating and cooling rate due to cloud microphysical processes in the model. The view proposed here is that the effect of diabatic heating in outer spiral rainbands on the storm structure and intensity results mainly from hydrostatic adjustment; that is, heating (cooling) of an atmospheric column decreases (increases) the surface pressure underneath the column. The change in surface pressure due to heating in the outer spiral rainbands is significant on the inward side of the rainbands where the inertial stability is generally high. Outside the rainbands in the far field, where the inertial stability is low and internal atmospheric heating is mostly lost to gravity wave radiation and little is left to warm the atmospheric column and lower the local surface pressure, the change in surface pressure is relatively small. This strong radially dependent response reduces the horizontal pressure gradient across the radius of maximum wind and thus the storm intensity in terms of the maximum low-level tangential wind while increasing the inner-core size of the storm. The numerical results show that cooling in the outer spiral rainbands maintains both the intensity of a tropical cyclone and the compactness of its inner core, whereas heating in the outer spiral rainbands decreases the intensity but increases the size of a tropical cyclone. Overall, the presence of strong outer spiral rainbands limits the intensity of a tropical cyclone. Because heating or cooling in the outer spiral rainbands depends strongly on the relative humidity in the near-core environment, the results have implications for the formation of the annular hurricane structure, the development of concentric eyewalls, and the size change in tropical cyclones.

scholarly journals Structural Changes Preceding Rapid Intensification in Tropical Cyclones as Shown in a Large Ensemble of Idealized Simulations

2018 ◽  
Vol 75 (2) ◽  
pp. 555-569 ◽  
Author(s):  
Yoshiaki Miyamoto ◽  
David S. Nolan
Keyword(s):  
Tropical Cyclones ◽  
Structural Changes ◽  
Deep Convection ◽  
Vertical Wind Shear ◽  
Rapid Intensification ◽  
Large Ensemble ◽  
Translation Speed ◽  
Budget Analysis ◽  
Tangential Wind ◽  
Vortex Size

Abstract Structural changes that precede rapid intensification (RI) of tropical cyclones (TCs) are examined in a full-physics model by conducting a large ensemble (270) of idealized TC simulations. The processes leading to RI in a representative case with moderate shear are consistent with previous studies for weakly sheared cases. The most distinct changes are that the vortex tilt and the vortex size begin to decrease more rapidly 6 h before the onset of RI. A vorticity budget analysis for the upper layer around the low-level center reveals that the vertical vorticity is increased by vertical advection, stretching, and tilting terms before RI, whereas the horizontal advection is small. Thus, the upright vortex structure is not achieved through a vortex alignment process but rather is built upward by deep convection. The ensemble simulations are generated by changing the intensity and size of the initial vortex, the magnitude of vertical wind shear, and the translation speed. The ensemble members that show RI are consistent with the control case and many previous studies: before the onset of RI, the intensity gradually increases, the radius of maximum tangential wind (RMW) decreases, the flow structure becomes more symmetric, the vortex tilt decreases, and the radius of maximum convergence approaches the radius of maximum winds. A dimensionless parameter representing a tendency for the formation of the vertically upright structure is considered. The product of this parameter and the local Rossby number is significantly larger for TCs that exhibit RI in the next 24 h.

scholarly journals Doppler Radar Analysis of the Rapid Intensification of Typhoon Goni (2015) after Eyewall Replacement

2018 ◽  
Vol 75 (1) ◽  
pp. 143-162 ◽  
Author(s):  
Udai Shimada ◽  
Masahiro Sawada ◽  
Hiroyuki Yamada
Keyword(s):  
Angular Momentum ◽  
Wind Field ◽  
Doppler Radar ◽  
Rapid Intensification ◽  
Wind Fields ◽  
Maximum Wind ◽  
Low Level ◽  
Contraction Speed ◽  
Budget Analysis ◽  
Tangential Wind

A ground-based Doppler radar observed the rapid intensification (RI) of Typhoon Goni (2015) for 24 h immediately after it completed an eyewall replacement cycle. Goni’s RI processes were examined by using radar reflectivity and wind fields retrieved by the ground-based velocity track display (GBVTD) technique. The maximum wind at 2-km altitude increased by 30 m s−1 during the first 6 h of RI, and it further increased by 20 m s−1 during the subsequent 12 h. Around the onset of RI, relatively strong outflow (>2 m s−1) was present both inside and outside the radius of maximum wind (RMW) above the boundary layer (BL), suggesting the existence of supergradient flow in and just above the BL. Despite this outflow, angular momentum increased inside the RMW. The low-level RMW contracted rapidly from 50 to 33 km, causing the RMW to slope greatly outward with height. The radius of maximum reflectivity was a few kilometers inside the RMW. A budget analysis of absolute angular momentum showed that the outflow contributed to the contraction of the tangential wind field. During RI, eyewall convection was enhanced, and a well-defined eye appeared. The low-level outflow changed into inflow immediately outside the RMW. Then the tangential wind field and high inertial stability region expanded radially outward, followed by the formation of an outer reflectivity maximum at twice the RMW. The contraction speed of the low-level RMW slowed down.

Rapid Development of the Tropical Cyclone Warm Core

2009 ◽  
Vol 66 (11) ◽  
pp. 3335-3350 ◽  
Author(s):  
Jonathan L. Vigh ◽  
Wayne H. Schubert
Keyword(s):  
Steady State ◽  
Stability Region ◽  
Diabatic Heating ◽  
Static Stability ◽  
Rapid Intensification ◽  
Theoretical Argument ◽  
Maximum Wind ◽  
Warm Core ◽  
Diabatic Forcing ◽  
Transverse Circulation

Abstract This paper presents a simple theoretical argument to isolate the conditions under which a tropical cyclone can rapidly develop a warm-core thermal structure and subsequently approach a steady state. The theoretical argument is based on the balanced vortex model and, in particular, on the associated transverse circulation equation and the geopotential tendency equation. These second-order partial differential equations contain the diabatic forcing and three spatially varying coefficients: the static stability A, the baroclinity B, and the inertial stability C. Thus, the transverse circulation and the temperature tendency in a tropical vortex depend not only on the diabatic forcing but also on the spatial distributions of A, B, and C. Experience shows that the large radial variations of C are typically the most important effect. Under certain simplifying assumptions as to the vertical structure of the diabatic forcing and the spatial variability of A, B, and C, the transverse circulation equation and the geopotential tendency equation can be solved via separation of variables. The resulting radial structure equations retain the dynamically important radial variation of C and can be solved in terms of Green’s functions. These analytical solutions show that the vortex response to a delta function in the diabatic heating depends critically on whether the heating occurs in the low-inertial-stability region outside the radius of maximum wind or in the high-inertial-stability region inside the radius of maximum wind. This result suggests that rapid intensification is favored for storms that have at least some of the eyewall convection inside the radius of maximum wind. The development of an eye partially removes diabatic heating from the high-inertial-stability region of the storm center; however, rapid intensification may continue if the eyewall heating continues to become more efficient. As the warm core matures and static stability increases over the inner core, conditions there become less favorable for deep upright convection and the storm tends to approach a steady state.

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