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 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 Potential Uncertainties in the Analysis of Low-Wavenumber Asymmetries Caused by Aliasing Center in Tropical Cyclones

Atmosphere ◽  
2019 ◽  
Vol 10 (6) ◽  
pp. 300 ◽  
Author(s):  
Chengwu Zhao ◽  
Junqiang Song ◽  
Hongze Leng ◽  
Juan Zhao
Keyword(s):  
Angular Momentum ◽  
Tropical Cyclone ◽  
Tropical Cyclones ◽  
Small Displacement ◽  
Sensitivity Tests ◽  
Tangential Wind ◽  
Radial Structure ◽  
Direction Sensitivity ◽  
Asymmetric Wave ◽  
Cyclone Prediction

Variations in both symmetric wind components and asymmetric wave amplitudes of a tropical cyclone depend on the location of its center. Because the radial structure of asymmetries is critical to the wave–mean interaction, this study, under idealized conditions, examines the influences of a center location on the radial structure of the diagnosed asymmetries. It has been found that the amplitudes of aliasing asymmetries are mainly affected by the initial symmetric fields. Meanwhile, the radial structure of asymmetry is controlled by the aliasing direction. Sensitivity tests on the location of the center were employed to emphasize the importance of the aliasing direction using angular momentum equations. With a small displacement, the tendencies of azimuthal tangential wind are found to reverse completely when the center shifts to a different direction. This work concludes that the diagnostic results related to asymmetric decomposition should be treated rigorously, as they are prone to inaccuracies, which in turn affect cyclone prediction.

scholarly journals A Diagnostic Study of the Intensity of Three Tropical Cyclones in the Australian Region. Part II: An Analytic Method for Determining the Time Variation of the Intensity of a Tropical Cyclone*

2010 ◽  
Vol 138 (1) ◽  
pp. 22-41 ◽  
Author(s):  
France Lajoie ◽  
Kevin Walsh
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Tropical Cyclones ◽  
Surface Pressure ◽  
Time Variation ◽  
Surface Wind ◽  
Ground Truth Data ◽  
Maximum Wind ◽  
Australian Region ◽  
Central Surface

Abstract The observed features discussed in Part I of this paper, regarding the intensification and dissipation of Tropical Cyclone Kathy, have been integrated in a simple mathematical model that can produce a reliable 15–30-h forecast of (i) the central surface pressure of a tropical cyclone, (ii) the sustained maximum surface wind and gust around the cyclone, (iii) the radial distribution of the sustained mean surface wind along different directions, and (iv) the time variation of the three intensity parameters previously mentioned. For three tropical cyclones in the Australian region that have some reliable ground truth data, the computed central surface pressure, the predicted maximum mean surface wind, and maximum gust were, respectively, within ±3 hPa and ±2 m s−1 of the observations. Since the model is only based on the circulation in the boundary layer and on the variation of the cloud structure in and around the cyclone, its accuracy strongly suggests that (i) the maximum wind is partly dependent on the large-scale environmental circulation within the boundary layer and partly on the size of the radius of maximum wind and (ii) that all factors that contribute one way or another to the intensity of a tropical cyclone act together to control the size of the eye radius and the central surface pressure.

scholarly journals The Axisymmetric and Asymmetric Aspects of the Secondary Eyewall Formation in a Numerically Simulated Tropical Cyclone under Idealized Conditions on an f Plane

2019 ◽  
Vol 76 (1) ◽  
pp. 357-378 ◽  
Author(s):  
Hui Wang ◽  
Yuqing Wang ◽  
Jing Xu ◽  
Yihong Duan
Keyword(s):  
Boundary Layer ◽  
Angular Momentum ◽  
Tropical Cyclone ◽  
Diabatic Heating ◽  
Lower Troposphere ◽  
Outer Edge ◽  
Top Down ◽  
Secondary Eyewall Formation ◽  
Tangential Wind ◽  
Surface Enthalpy

Abstract The axisymmetric and asymmetric aspects of the secondary eyewall formation (SEF) in a numerically simulated tropical cyclone (TC) under idealized conditions were analyzed. Consistent with previous findings, prior to the SEF, the tangential wind of the TC experienced an outward expansion both above and within the boundary layer near and outside the region of the SEF later. This outward expansion was found to be closely related to the top-down development and inward propagation of a strong outer rainband, which was characterized by deeper and more intense convection upwind and shallower and weaker convection downwind. In response to diabatic heating in the outer rainband was inflow in the mid- to lower troposphere, which brought the absolute angular momentum inward and spun up tangential wind in the inflow region and also in the convective region because of vertical advection. As a result, as the outer rainband intensified and spiraled cyclonically inward, perturbation tangential and radial winds also spiraled cyclonically inward and downward along the rainband. As it approached the outer edge of the rapid filamentation zone outside the primary eyewall, the downwind sector of the rainband in the boundary layer was rapidly axisymmetrized. Continuous inward propagation and axisymmetrization and secondarily the merging with inner rainbands led to the spinup of tangential wind in the boundary layer, enhancing surface enthalpy flux and convection and eventually leading to the simulated SEF. Our results demonstrate that the simulated SEF was a top-down process and was mainly triggered by asymmetric dynamics.

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.

scholarly journals Evaluation of the Assumptions in the Steady-State Tropical Cyclone Self-Stratified Outflow Using Three-Dimensional Convection-Allowing Simulations

2019 ◽  
Vol 76 (9) ◽  
pp. 2995-3009 ◽  
Author(s):  
Dandan Tao ◽  
Kerry Emanuel ◽  
Fuqing Zhang ◽  
Richard Rotunno ◽  
Michael M. Bell ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Angular Momentum ◽  
Steady State ◽  
Tropical Cyclone ◽  
Richardson Number ◽  
Three Dimensional ◽  
Small Scale ◽  
Maximum Wind ◽  
Gradient Wind ◽  
Outflow Region

Abstract The criteria and assumptions that were used to derive the steady-state tropical cyclone intensity and structure theory of Emanuel and Rotunno are assessed using three-dimensional convection-allowing simulations using the Weather Research and Forecasting Model. One real-data case of Hurricane Patricia (2015) and two idealized simulations with and without vertical wind shear are examined. In all three simulations, the gradient wind balance is valid in the inner-core region above the boundary layer. The angular momentum M and saturation entropy surfaces s* near the top of the boundary layer, in the outflow region and along the angular momentum surface that passes the low-level radius of maximum wind MRMW are nearly congruent, satisfying the criterion of slantwise moist neutrality in the vicinity of MRMW. The theoretically derived maximum wind magnitude above the boundary layer compares well with the simulated maximum tangential wind and gradient wind using the azimuthally averaged pressure field during the intensification and quasi-steady state of the simulated storms. The Richardson number analysis of the simulated storms shows that small Richardson number (0 < Ri ≤1) exists in the outflow region, related to both large local shear and small static stability. This criticality of the Richardson number indicates the existence of small-scale turbulence in the outflow region. We also show that the stratification of temperature along the M surfaces at the outflow region for steady-state hurricanes is approximately applicable in these three-dimensional simulations, while the radial distribution of gradient wind is qualitatively comparable to the theoretical radial profiles. Some caveats regarding the theory are also discussed.

A New Time-dependent Theory of Tropical Cyclone Intensification

2021 ◽  
Author(s):  
Yuqing Wang ◽  
Yuanlong Li ◽  
Jing Xu
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Numerical Simulations ◽  
Strong Dependence ◽  
Surface Wind ◽  
Time Dependent ◽  
Quasi Equilibrium ◽  
Free Atmosphere ◽  
Near Surface ◽  
Tangential Wind

AbstractIn this study, the boundary-layer tangential wind budget equation following the radius of maximum wind, together with an assumed thermodynamical quasi-equilibrium boundary layer is used to derive a new equation for tropical cyclone (TC) intensification rate (IR). A TC is assumed to be axisymmetric in thermal wind balance with eyewall convection becoming in moist slantwise neutrality in the free atmosphere above the boundary layer as the storm intensifies as found recently based on idealized numerical simulations. An ad-hoc parameter is introduced to measure the degree of congruence of the absolute angular momentum and the entropy surfaces. The new IR equation is evaluated using results from idealized ensemble full-physics axisymmetric numerical simulations. Results show that the new IR equation can reproduce the time evolution of the simulated TC intensity. The new IR equation indicates a strong dependence of IR on both TC intensity and the corresponding maximum potential intensity (MPI). A new finding is the dependence of TC IR on the square of the MPI in terms of the near-surface wind speed for any given relative intensity. Results from some numerical integrations of the new IR equation also suggest the finite-amplitude nature of TC genesis. In addition, the new IR theory is also supported by some preliminary results based on best-track TC data over the North Atlantic and eastern and western North Pacific. Compared with the available time-dependent theories of TC intensification, the new IR equation can provide a realistic intensity-dependent IR during weak intensity stage as in observations.

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.

scholarly journals An Observational Study of the Symmetric Boundary Layer Structure and Tropical Cyclone Intensity

Atmosphere ◽  
2020 ◽  
Vol 11 (2) ◽  
pp. 158 ◽  
Author(s):  
Yifang Ren ◽  
Jun A. Zhang ◽  
Jonathan L. Vigh ◽  
Ping Zhu ◽  
Hailong Liu ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Tropical Cyclone ◽  
Layer Structure ◽  
Mixed Layer Depth ◽  
Layer Depth ◽  
Storm Intensity ◽  
Boundary Layer Structure ◽  
Warm Core ◽  
Tangential Wind

This study analyses Global Positioning System dropsondes to document the axisymmetric tropical cyclone (TC) boundary-layer structure, based on storm intensity. A total of 2608 dropsondes from 42 named TCs in the Atlantic basin from 1998 to 2017 are used in the composite analyses. The results show that the axisymmetric inflow layer depth, the height of maximum tangential wind speed, and the thermodynamic mixed layer depth are all shallower in more intense TCs. The results also show that more intense TCs tend to have a deep layer of the near-saturated air inside the radius of maximum wind speed (RMW). The magnitude of the radial gradient of equivalent potential temperature (θe) near the RMW correlates positively with storm intensity. Above the inflow layer, composite structures of TCs with different intensities all possess a ring of anomalously cool temperatures surrounding the warm-core, with the magnitude of the warm-core anomaly proportional to TC intensity. The boundary layer composites presented here provide a climatology of how axisymmetric TC boundary layer structure changes with intensity.

scholarly journals Comments on “Revisiting the Relationship between Eyewall Contraction and Intensification”

2017 ◽  
Vol 74 (12) ◽  
pp. 4265-4274 ◽  
Author(s):  
Chanh Q. Kieu ◽  
Da-Lin Zhang
Keyword(s):  
Tropical Cyclone ◽  
Tropical Cyclones ◽  
Governing Equation ◽  
Directional Derivative ◽  
Maximum Wind ◽  
The Relationship ◽  
Derivative Concept

Abstract This comment presents some concerns with the study of Stern et al. and their misinterpretation of the contraction of the radius of the maximum wind (RMW) in tropical cyclones. It is shown that their geometrical RMW contraction model provides little dynamical understanding of the RMW contraction during tropical cyclone intensification, and it differs fundamentally from the RMW contraction model of Willoughby et al. that was derived from the directional derivative concept. Moreover, it is demonstrated that Stern et al. were mistaken in commenting on the derivation of the governing equation for the RMW contraction in Kieu.

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