scholarly journals Why Do Model Tropical Cyclones Grow Progressively in Size and Decay in Intensity after Reaching Maturity?

2016 ◽  
Vol 73 (2) ◽  
pp. 487-503 ◽  
Author(s):  
Gerard Kilroy ◽  
Roger K. Smith ◽  
Michael T. Montgomery
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclones ◽  
Diabatic Heating ◽  
Inner Core ◽  
Three Dimensional ◽  
Lower Troposphere ◽  
Core Convection ◽  
Tangential Wind ◽  
Hurricane Isabel ◽  
Boundary Layer Dynamics

Abstract The long-term behavior of tropical cyclones in the prototype problem for cyclone intensification on an f plane is examined using a nonhydrostatic, three-dimensional numerical model. After reaching a mature intensity, the model storms progressively decay while both the inner-core size, characterized by the radius of the eyewall, and the size of the outer circulation—measured, for example, by the radius of the gale-force winds—progressively increase. This behavior is explained in terms of a boundary layer control mechanism in which the expansion of the swirling wind in the lower troposphere leads through boundary layer dynamics to an increase in the radii of forced eyewall ascent as well as to a reduction in the maximum tangential wind speed in the layer. These changes are accompanied by changes in the radial and vertical distribution of diabatic heating. As long as the aggregate effects of inner-core convection, characterized by the distribution of diabatic heating, are able to draw absolute angular momentum surfaces inward, the outer circulation will continue to expand. The quantitative effects of latitude on the foregoing processes are investigated also. The study provides new insight into the factors controlling the evolution of the size and intensity of a tropical cyclone. It provides also a plausible, and arguably simpler, explanation for the expansion of the inner core of Hurricane Isabel (2003) and Typhoon Megi (2010) than that given previously.

Revisiting the Dynamics of Eyewall Contraction of Tropical Cyclones

2019 ◽  
Vol 76 (10) ◽  
pp. 3229-3245 ◽  
Author(s):  
Yuanlong Li ◽  
Yuqing Wang ◽  
Yanluan Lin
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclones ◽  
Directional Derivative ◽  
Three Dimensional ◽  
Lower Boundary ◽  
Lower Troposphere ◽  
Surface Friction ◽  
Three Dimensions ◽  
Tangential Wind ◽  
Lower Boundary Layer

Abstract The dynamics of eyewall contraction of tropical cyclones (TCs) has been revisited in this study based on both three-dimensional and axisymmetric simulations and dynamical diagnostics. Because eyewall contraction is closely related to the contraction of the radius of maximum wind (RMW), its dynamics is thus often studied by examining the RMW tendency in previous studies. Recently, Kieu and Stern et al. proposed two different frameworks to diagnose the RMW tendency but had different conclusions. In this study, the two frameworks are evaluated first based on theoretical analysis and idealized numerical simulations. It is shown that the framework of Kieu is a special case of the earlier framework of Willoughby et al. if the directional derivative is applied. An extension of Stern et al.’s approach not only can reproduce but also can predict the RMW tendency. A budget of the azimuthal-mean tangential wind tendency indicates that the contributions by radial and vertical advections to the RMW tendency vary with height. Namely, radial advection dominates the RMW contraction in the lower boundary layer, and vertical advection favors the RMW contraction in the upper boundary layer and lower troposphere. In addition to the curvature, the increase of the radial gradient of horizontal mixing (including the resolved eddy mixing in three dimensions) near the eyewall prohibits eyewall contraction in the lower boundary layer. Besides, the vertical mixing including surface friction also plays an important role in the cessation of eyewall contraction in the lower boundary layer.

scholarly journals Why Do Model Tropical Cyclones Intensify More Rapidly at Low Latitudes?

2015 ◽  
Vol 72 (5) ◽  
pp. 1783-1804 ◽  
Author(s):  
Roger K. Smith ◽  
Gerard Kilroy ◽  
Michael T. Montgomery
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclones ◽  
Three Dimensional ◽  
Feedback Mechanism ◽  
Meridional Overturning Circulation ◽  
Time Dependent ◽  
Lower Latitude ◽  
Tangential Wind ◽  
Low Latitudes ◽  
Diabatic Forcing

Abstract The authors examine the problem of why model tropical cyclones intensify more rapidly at low latitudes. The answer to this question touches on practically all facets of the dynamics and thermodynamics of tropical cyclones. The answer invokes the conventional spin-up mechanism, as articulated in classical and recent work, together with a boundary layer feedback mechanism linking the strength of the boundary layer inflow to that of the diabatic forcing of the meridional overturning circulation. The specific role of the frictional boundary layer in regulating the dependence of the intensification rate on latitude is discussed. It is shown that, even if the tangential wind profile at the top of the boundary layer is held fixed, a simple, steady boundary layer model produces stronger low-level inflow and stronger, more confined ascent out of the boundary layer as the latitude is decreased, similar to the behavior found in a time-dependent, three-dimensional numerical model. In an azimuthally averaged view of the problem, the most prominent quantitative differences between the time-dependent simulations at 10° and 30°N are the stronger boundary layer inflow and the stronger ascent of air exiting the boundary layer, together with the much larger diabatic heating rate and its radial gradient above the boundary layer at the lower latitude. These differences, in conjunction with the convectively induced convergence of absolute angular momentum, greatly surpass the effects of rotational stiffness (inertial stability) and evaporative-wind feedback that have been proposed in some prior explanations.

Does the Rotating Convection Paradigm Describe Secondary Eyewall Formation in Idealized Three-dimensional Simulations?

2021 ◽  
Keyword(s):  
Boundary Layer ◽  
Wind Field ◽  
Three Dimensional ◽  
Lower Troposphere ◽  
Rotating Convection ◽  
Secondary Eyewall Formation ◽  
A Value ◽  
Tangential Wind ◽  
Rain Fall ◽  
Three Dimensional Simulations

Abstract The formation of a plausible secondary eyewall is examined with two principal simulation experiments that differ only in the fixed value of rain fall speed, one with a value of 70 m s−1 (approaching the pseudo-adiabatic limit) that simulates a secondary eyewall, and one with a value of 7 m s−1 that does not simulate a secondary eyewall. Key differences are sought between these idealized three-dimensional simulations. A notable expansion of the lower-tropospheric tangential wind field to approximately 400 km radius is found associated with the precursor period of the secondary eyewall. The wind field expansion is traced to an enhanced vertical mass flux across the 5.25-km height level, which leads, in turn, to enhanced radial inflow in the lower troposphere and above the boundary layer. The inflow spins up the tangential wind outside the primary eyewall via the conventional spin-up mechanism. This amplified tangential wind field is linked to a broad region of outwardly-directed agradient force in the upper boundary layer. Whereas scattered convection is found outside the primary eyewall in both simulations, the agradient force is shown to promote a ring-like organization of this convection when boundary layer convergence occurs in a persistent, localized region of super-gradient winds. The results support prior work highlighting a new model of secondary eyewall formation emphasizing a boundary layer control pathway for initiating the outer eyewall as part of the rotating convection paradigm of tropical cyclone evolution.

scholarly journals Balanced Contribution to the Intensification of a Tropical Cyclone Simulated in TCM4: Outer-Core Spinup Process*

2011 ◽  
Vol 68 (3) ◽  
pp. 430-449 ◽  
Author(s):  
Hironori Fudeyasu ◽  
Yuqing Wang
Keyword(s):  
Angular Momentum ◽  
Tropical Cyclone ◽  
Diabatic Heating ◽  
Inner Core ◽  
Three Dimensional ◽  
Outer Core ◽  
Middle Troposphere ◽  
Tangential Wind ◽  
Spiral Rainbands ◽  
Balance Dynamics

Abstract The balanced contribution to the intensification of a tropical cyclone simulated in the three-dimensional, nonhydrostatic, full-physics tropical cyclone model version 4 (TCM4), in particular the spinup of the outer-core circulation, is investigated by solving the Sawyer–Eliassen equation and by computing terms in the azimuthal-mean tangential wind tendency equation. Results demonstrate that the azimuthal-mean secondary circulation (radial and vertical circulation) and the spinup of the midtropospheric outer-core circulation in the simulated tropical cyclone are well captured by balance dynamics. The midtropospheric inflow develops in response to diabatic heating in mid–upper-tropospheric stratiform (anvil) clouds outside the eyewall in active spiral rainbands and transports absolute angular momentum inward to spin up the outer-core circulation. Although the azimuthal-mean diabatic heating rate in the eyewall is the largest, its contribution to radial winds and thus the spinup of outer-core circulation in the middle troposphere is rather weak. This is because the high inertial stability in the inner-core region resists the radial inflow in the middle troposphere, limiting the inward transport of absolute angular momentum. The result thus suggests that diabatic heating in spiral rainbands is the key to the continued growth of the storm-scale circulation.

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.

Potential Vorticity Diagnosis of a Simulated Hurricane. Part I: Formulation and Quasi-Balanced Flow

2003 ◽  
Vol 60 (13) ◽  
pp. 1593-1607 ◽  
Author(s):  
Xingbao Wang ◽  
Da-Lin Zhang
Keyword(s):  
Boundary Layer ◽  
Potential Vorticity ◽  
Diabatic Heating ◽  
Inner Core ◽  
Three Dimensional ◽  
Hurricane Andrew ◽  
High Resolution Data ◽  
Balanced Flow ◽  
Upper Level ◽  
Balanced Dynamics

Abstract Because of the lack of three-dimensional (3D) high-resolution data and the existence of highly nonelliptic flows, few studies have been conducted to investigate the inner-core quasi-balanced characteristics of hurricanes. In this study, a potential vorticity (PV) inversion system is developed, which includes the nonconservative processes of friction, diabatic heating, and water loading. It requires hurricane flows to be statically and inertially stable but allows for the presence of small negative PV. To facilitate the PV inversion with the nonlinear balance (NLB) equation, hurricane flows are decomposed into an axisymmetric, gradient-balanced reference state and asymmetric perturbations. Meanwhile, the nonellipticity of the NLB equation is circumvented by multiplying a small parameter ɛ and combining it with the PV equation, which effectively reduces the influence of anticyclonic vorticity. A quasi-balanced ω equation in pseudoheight coordinates is derived, which includes the effects of friction and diabatic heating as well as differential vorticity advection and the Laplacians of thermal advection by both nondivergent and divergent winds. This quasi-balanced PV–ω inversion system is tested with an explicit simulation of Hurricane Andrew (1992) with the finest grid size of 6 km. It is shown that (a) the PV–ω inversion system could recover almost all typical features in a hurricane, and (b) a sizeable portion of the 3D hurricane flows are quasi-balanced, such as the intense rotational winds, organized eyewall updrafts and subsidence in the eye, cyclonic inflow in the boundary layer, and upper-level anticyclonic outflow. It is found, however, that the boundary layer cyclonic inflow and upper-level anticyclonic outflow also contain significant unbalanced components. In particular, a low-level outflow jet near the top of the boundary layer is found to be highly unbalanced (and supergradient). These findings are supported by both locally calculated momentum budgets and globally inverted winds. The results indicate that this PV inversion system could be utilized as a tool to separate the unbalanced from quasi-balanced flows for studies of balanced dynamics and propagating inertial gravity waves in hurricane vortices.

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 The Boundary Layer Dynamics of Tropical Cyclone Rainbands

2018 ◽  
Vol 75 (11) ◽  
pp. 3777-3795 ◽  
Author(s):  
Jeffrey D. Kepert
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Tropical Cyclone ◽  
Tropical Cyclones ◽  
Surface Wind ◽  
Three Dimensional ◽  
Surface Wind Speed ◽  
Wind Maximum ◽  
Boundary Layer Dynamics ◽  
Spiral Bands

Abstract Spiral bands are ubiquitous features in tropical cyclones and significantly affect boundary layer thermodynamics, yet knowledge of their boundary layer dynamics is lacking. Prompted by recent work that has shown that relatively weak axisymmetric vorticity perturbations outside of the radius of maximum winds in tropical cyclones can produce remarkably strong frictional convergence, and by the observation that most secondary eyewalls appear to form by the “wrapping up” of a spiral rainband, the effect of asymmetric vorticity features that mimic spiral bands is studied. The mass field corresponding to an axisymmetric vortex with added spiral vorticity band is constructed using the nonlinear balance equation, and supplied to a three-dimensional boundary layer model. The resulting flow has strong low-level convergence and a marked updraft extending along the vorticity band and some distance downwind. There is a marked along-band wind maximum in the upper boundary layer, similar to observations, which is up to about 20% stronger than the balanced flow. A marked gradient in the inflow-layer depth exists across the band and there is an increase in the surface wind factor (the ratio of surface wind speed to nonlinear-balanced wind speed) near the band. The boundary layer dynamics near a rainband therefore form a continuum with the flow near a secondary eyewall. None of these features are due to convective momentum transports, which are absent from the model. The sensitivities of the flow to band length, width, location, crossing angle, and amplitude are examined, and the possible contribution of boundary layer dynamics to the formation of the tropical cyclone rainbands discussed.

Fluctuations in inner-core structure during the rapid intensification of Super Typhoon Nepartak (2016)

2020 ◽  
Author(s):  
Sam Hardy ◽  
Juliane Schwendike ◽  
Roger K. Smith ◽  
Chris J. Short ◽  
Michael J. Reeder ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Structural Changes ◽  
Inner Core ◽  
Single Case ◽  
Three Dimensional ◽  
Core Structure ◽  
Rapid Intensification ◽  
Budget Analysis ◽  
Tangential Wind ◽  
The Mean

AbstractThe key physical processes responsible for inner-core structural changes and associated fluctuations in the intensification rate for a recent, high-impact western North Pacific tropical cyclone that underwent rapid intensification (Nepartak, 2016) are investigated using a set of convection-permitting ensemble simulations. Fluctuations in the inner-core structure between ring-like and monopole states develop in 60% of simulations. A tangential momentum budget analysis of a single fluctuation reveals that during the ring-like phase, the tangential wind generally intensifies, whereas during the monopole phase, the tangential wind remains mostly constant. In both phases, the mean advection terms spin up the tangential wind in the boundary layer, whereas the eddy advection terms deepen the storm’s cyclonic circulation by spinning up the tangential wind between 1.5 and 4 km. Further calculations of the azimuthally-averaged, radially-integrated vertical mass flux suggest that periods of near-constant tangential wind tendency are accompanied by a weaker eyewall updraft, which is unable to evacuate all the mass converging in the boundary layer. Composite analyses calculated from 18 simulations produce qualitatively similar results to those from the single case, a finding that is also in agreement with some previous observational and modelling studies. Above the boundary layer, the integrated contribution of the eddy term to the tangential wind tendency is over 80% of the contribution from the mean term, irrespective of inner-core structure. Our results strongly indicate that to fully understand the storm’s three-dimensional evolution, the contribution of the eddies must be quantified.

Sign in / Sign up

Export Citation Format

Share Document