Nonlinear Response of a Tropical Cyclone Vortex to Prescribed Eyewall Heating with and without Surface Friction in TCM4: Implications for Tropical Cyclone Intensification

Abstract The recent debate on whether surface friction contributes positively or negatively to tropical cyclone (TC) intensification has been clarified based on two idealized numerical experiments, one without and the other with surface friction, using the fully compressible, nonhydrostatic TC model, version 4 (TCM4), with prescribed eyewall heating. The results show that with surface friction included, the intensification rate of the TC vortex is largely reduced, indicating that surface friction contributes negatively to TC intensification. Results from tangential wind budgets demonstrate that although surface friction largely enhances the boundary layer inflow and the contraction of the radius of maximum wind (RMW), the positive tangential wind tendency resulting from the frictionally induced inward absolute angular momentum (AAM) transport in the boundary layer is not large enough to offset the negative tendency due to the direct frictional loss of AAM to the surface. Results from the Sawyer–Eliassen equation suggest that the balanced response to eyewall heating is the major mechanism for TC intensification and the unbalanced dynamics due to the presence of surface friction seem to spin up tangential wind in the surface layer near the RMW where the flow is strongly subgradient and spin down tangential wind immediately above where the flow is strongly supergradient. Although surface friction shows an overall net negative effect on TC intensification, it plays a critical role in producing the realistic boundary layer structure with enhanced inflow, a low-level jet in tangential wind with supergradient nature, and a shallow outflow layer at the top of the inflow boundary layer.

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

Atmosphere ◽

10.3390/atmos11020158 ◽

2020 ◽

Vol 11 (2) ◽

pp. 158 ◽

Cited By ~ 1

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.

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Impact of Parameterized Boundary Layer Structure on Tropical Cyclone Rapid Intensification Forecasts in HWRF

Monthly Weather Review ◽

10.1175/mwr-d-16-0129.1 ◽

2017 ◽

Vol 145 (4) ◽

pp. 1413-1426 ◽

Cited By ~ 24

Author(s):

Jun A. Zhang ◽

Robert F. Rogers ◽

Vijay Tallapragada

Keyword(s):

Boundary Layer ◽

Tropical Cyclone ◽

Inner Core ◽

Layer Structure ◽

Intensity Change ◽

Rapid Intensification ◽

Near Surface ◽

Boundary Layer Structure ◽

Tangential Wind ◽

The Impact

Abstract This study evaluates the impact of the modification of the vertical eddy diffusivity (Km) in the boundary layer parameterization of the Hurricane Weather Research and Forecasting (HWRF) Model on forecasts of tropical cyclone (TC) rapid intensification (RI). Composites of HWRF forecasts of Hurricanes Earl (2010) and Karl (2010) were compared for two versions of the planetary boundary layer (PBL) scheme in HWRF. The results show that using a smaller value of Km, in better agreement with observations, improves RI forecasts. The composite-mean, inner-core structures for the two sets of runs at the time of RI onset are compared with observational, theoretical, and modeling studies of RI to determine why the runs with reduced Km are more likely to undergo RI. It is found that the forecasts with reduced Km at the RI onset have a shallower boundary layer with stronger inflow, more unstable near-surface air outside the eyewall, stronger and deeper updrafts in regions farther inward from the radius of maximum wind (RMW), and stronger boundary layer convergence closer to the storm center, although the mean storm intensity (as measured by the 10-m winds) is similar for the two groups. Finally, it is found that the departure of the maximum tangential wind from the gradient wind at the eyewall, and the inward advection of angular momentum outside the eyewall, is much larger in the forecasts with reduced Km. This study emphasizes the important role of the boundary layer structure and dynamics in TC intensity change, supporting recent studies emphasizing boundary layer spinup mechanism, and recommends further improvement to the HWRF PBL physics.

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Reply to “Comments on ‘Revisiting the Balanced and Unbalanced Aspects of Tropical Cyclone Intensification’”

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-18-0020.1 ◽

2018 ◽

Vol 75 (7) ◽

pp. 2497-2505 ◽

Cited By ~ 1

Author(s):

Junyao Heng ◽

Yuqing Wang ◽

Weican Zhou

Keyword(s):

Boundary Layer ◽

Tropical Cyclone ◽

Inner Core ◽

Model Simulation ◽

Surface Friction ◽

Fast Response ◽

Tangential Wind ◽

Physics Model ◽

Balanced Dynamics ◽

Balanced Solution

Abstract In their comment, Montgomery and Smith critique the recent study of Heng et al. that revisited the balanced and unbalanced aspects of tropical cyclone (TC) intensification based on diagnostics of a full-physics model simulation using the Sawyer–Eliassen equation. Heng et al. showed that the balanced dynamics reproduced to a large extent the secondary circulation in the full-physics model simulation and concluded that balanced dynamics can well explain TC intensification in their full-physics model simulation. Montgomery and Smith suspect the balanced solution in Heng et al. because the basic-state vortex is not exactly in thermal wind balance in the boundary layer and possibly a too-large diffusivity in the numerical model was used. In this reply, we first indicate that the boundary layer spinup mechanism proposed by Smith et al. is a fast response of the TC boundary layer to surface friction and should not be a major mechanism of TC intensification. We then evaluate the possible effect of imbalance in the basic state in the boundary layer on the balanced solution. The results show that although the removal of the imbalance in the boundary layer leads to about a one-third reduction in the maximum inflow near the surface in the inner-core region, the overall effect on the tangential wind budget is marginal because of other compensations. We also show that both the horizontal and vertical diffusivities in the model used in Heng et al. are reasonable based on previous observational studies. Therefore, we conclude that all results in Heng et al. are valid. Some related issues are also discussed.

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Concentric Eyewall Formation in Typhoon Sinlaku (2008). Part III: Horizontal Momentum Budget Analyses

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-18-0037.1 ◽

2018 ◽

Vol 75 (10) ◽

pp. 3541-3563 ◽

Cited By ~ 5

Author(s):

Yi-Hsuan Huang ◽

Chun-Chieh Wu ◽

Michael T. Montgomery

Keyword(s):

Boundary Layer ◽

Surface Friction ◽

Absolute Vorticity ◽

Frictional Loss ◽

Secondary Eyewall Formation ◽

Positive Tendency ◽

Momentum Budget ◽

Tangential Wind ◽

Tangential Momentum ◽

The Mean

This is a follow-up work to two prior studies examining secondary eyewall formation (SEF) in Typhoon Sinlaku (2008). This study shows that, in the SEF region, the majority of the elevated winds are supergradient. About two-thirds of the rapid increase in tangential wind tendencies immediately prior to SEF are attributed to agradient wind tendencies. This suggests the importance of nonlinear, unbalanced dynamical processes in SEF in addition to the classical axisymmetric balanced response to forcings of heating and momentum. In the SEF region, analyses show two distinct responsible processes for the increasing azimuthal tangential wind in two vertical intervals. Within the boundary inflow layer, the competing effect between the mean radial influx of absolute vorticity and deceleration caused by surface friction and subgrid diffusion yields a secondary maximum of positive tendency. Analyses further demonstrate the major impact of the mean radial influx of absolute vorticity on SEF. Above the boundary inflow layer, the vertical advection acts to vertically extend the tangential wind jet via the lofting of the enhanced tangential momentum farther upward. The roles of the nonlinear unbalanced dynamics in these two processes are discussed in this paper. From a Lagrangian perspective, the persistently increasing agradient force outweighs the frictional loss, effectively decelerating boundary layer inflowing air across the SEF region. This explains the sharpening of the radial gradient of boundary layer inflow, which is shown to be responsible for the buildup of a zone with concentrated boundary layer convergence. The previously proposed unbalanced dynamical pathway to SEF is elaborated upon and supported by the current results and discussion.

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Secondary Eyewall Formation in an Idealized Tropical Cyclone Simulation: Balanced and Unbalanced Dynamics

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-15-0146.1 ◽

2016 ◽

Vol 73 (10) ◽

pp. 3911-3930 ◽

Cited By ~ 19

Author(s):

Hui Wang ◽

Chun-Chieh Wu ◽

Yuqing Wang

Keyword(s):

Boundary Layer ◽

Tropical Cyclone ◽

Model Simulation ◽

Surface Friction ◽

Secondary Eyewall Formation ◽

Budget Analysis ◽

Tangential Wind ◽

Spiral Rainbands ◽

Physics Model ◽

Balanced Dynamics

Abstract The secondary eyewall formation (SEF) in an idealized simulation of a tropical cyclone (TC) is examined from the perspective of both the balanced and unbalanced dynamics and through the tangential wind (Vt) budget analysis. It is found that the expansion of the azimuthal-mean Vt above the boundary layer occurs prior to the development of radial moisture convergence in the boundary layer. The Vt expansion results primarily from the inward angular momentum transport by the mid- to lower-tropospheric inflow induced by both convective and stratiform heating in the spiral rainbands. In response to the Vt broadening is the development of radial inflow convergence and the supergradient flow near the top of the inflow boundary layer. Results from the Vt budget analysis show that the combined effect of the mean advection and the surface friction is to spin down Vt in the boundary layer, while the eddy processes (eddy radial and vertical advection) contribute positively to the spinup of Vt in the SEF region in the boundary layer. Therefore, eddies play an important role in the spinup of Vt in the boundary layer during SEF. The balanced Sawyer–Eliassen solution can well capture the secondary circulation in the full-physics model simulation. The radial inflow diagnosed from the Sawyer–Eliassen equation is shown to spin up Vt and maintain the vortex above the boundary layer. However, the axisymmetric balanced dynamics cannot explain the spinup of Vt in the boundary layer, which results mainly from the eddy processes.

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Revisiting the Balanced and Unbalanced Aspects of Tropical Cyclone Intensification

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-17-0046.1 ◽

2017 ◽

Vol 74 (8) ◽

pp. 2575-2591 ◽

Cited By ~ 10

Author(s):

Junyao Heng ◽

Yuqing Wang ◽

Weican Zhou

Keyword(s):

Boundary Layer ◽

Tropical Cyclone ◽

Inner Core ◽

Model Simulation ◽

Surface Friction ◽

Core Region ◽

Physics Model ◽

The One ◽

Balanced Dynamics ◽

Balanced Solution

Abstract The balanced and unbalanced aspects of tropical cyclone (TC) intensification are revisited with the balanced contribution diagnosed with the outputs from a full-physics model simulation of a TC using the Sawyer–Eliassen (SE) equation. The results show that the balanced dynamics can well capture the secondary circulation in the full-physics model simulation even in the inner-core region in the boundary layer. The balanced dynamics can largely explain the intensification of the simulated TC. The unbalanced dynamics mainly acts to prevent the boundary layer agradient flow in the inner-core region from further intensification. Although surface friction can enhance the boundary layer inflow and make the inflow penetrate more inward into the eye region, contributing to the eyewall contraction, the net dynamical effect of surface friction on TC intensification is negative. The sensitivity of the balanced solution to the procedure used to ensure the ellipticity condition for the SE equation is also examined. The results show that the boundary layer inflow in the balanced response is very sensitive to the adjustment to inertial stability in the upper troposphere and the calculation of radial wind at the surface with relatively coarse vertical resolution in the balanced solution. Both the use of the so-called global regularization and the one-sided finite-differencing scheme used to calculate the surface radial wind in the balanced solution as utilized in some previous studies can significantly underestimate the boundary layer inflow. This explains why the boundary layer inflow in the balanced response is too weak in some previous studies.

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A New Time-dependent Theory of Tropical Cyclone Intensification

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-21-0169.1 ◽

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.

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

Advances in Meteorology ◽

10.1155/2019/8072914 ◽

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.

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An Observational Study of Tropical Cyclone Spinup in Supertyphoon Jangmi (2008) from 24 to 27 September

Monthly Weather Review ◽

10.1175/mwr-d-12-00306.1 ◽

2014 ◽

Vol 142 (1) ◽

pp. 3-28 ◽

Cited By ~ 36

Author(s):

Neil T. Sanger ◽

Michael T. Montgomery ◽

Roger K. Smith ◽

Michael M. Bell

Keyword(s):

Boundary Layer ◽

Tropical Cyclone ◽

Observational Study ◽

Satellite Imagery ◽

Doppler Radar ◽

Relative Vorticity ◽

Tropical Storm ◽

Level Data ◽

Tangential Wind ◽

Research Aircraft

Abstract An observational study of tropical cyclone intensification is performed using dropsondes, in situ flight-level data, satellite imagery, and Electra Doppler Radar (ELDORA) during the spinup of Tropical Storm Jangmi (2008) in the western North Pacific. This event was observed with research aircraft during the Tropical Cyclone Structure 2008 (TCS08) field experiment over the course of 3 days as Jangmi intensified rapidly from a tropical storm to a supertyphoon. The dropsonde analysis indicates that the peak azimuthally averaged storm-relative tangential wind speed occurs persistently within the boundary layer throughout the spinup period and suggests that significant supergradient winds are present near and just within the radius of maximum tangential winds. An examination of the ELDORA data in Tropical Storm Jangmi reveals multiple rotating updrafts near the developing eye beneath cold cloud top temperatures ≤−65°C. In particular, there is a 12-km-wide, upright updraft with a peak velocity of 9 m s−1 with collocated strong low-level (z < 2 km) convergence of 2 × 10−3 s−1 and intense relative vorticity of 4 × 10−3 s−1. The analysis of the corresponding infrared satellite imagery suggests that vortical updrafts are common before and during rapid intensification. The findings of this study support a recent paradigm of tropical cyclone intensification in which rotating convective clouds are important elements in the spinup process. In a system-scale view of this process, the maximum tangential wind is found within the boundary layer, where the tangential wind becomes supergradient before the air ascends into the eyewall updraft.

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