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

2018 ◽  
Vol 75 (10) ◽  
pp. 3541-3563 ◽  
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.

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

2016 ◽  
Vol 73 (10) ◽  
pp. 3911-3930 ◽  
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.

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

2016 ◽  
Vol 73 (3) ◽  
pp. 1315-1333 ◽  
Author(s):  
Junyao Heng ◽  
Yuqing Wang
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Layer Structure ◽  
Critical Role ◽  
Surface Friction ◽  
Frictional Loss ◽  
Major Mechanism ◽  
Tangential Wind ◽  
Negative Effect ◽  
Recent Debate

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.

Departures from Axisymmetric Balance Dynamics during Secondary Eyewall Formation

2014 ◽  
Vol 71 (10) ◽  
pp. 3723-3738 ◽  
Author(s):  
Sergio F. Abarca ◽  
Michael T. Montgomery
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Three Dimensional ◽  
Balance Model ◽  
Wind Maximum ◽  
Secondary Eyewall Formation ◽  
Tangential Wind ◽  
Eyewall Replacement Cycle ◽  
Tangential Momentum ◽  
Balance Dynamics

Abstract Departures from axisymmetric balance dynamics are quantified during a case of secondary eyewall formation. The case occurred in a three-dimensional mesoscale convection-permitting numerical simulation of a tropical cyclone, integrated from an initial weak mesoscale vortex in an idealized quiescent environment. The simulation exhibits a canonical eyewall replacement cycle. Departures from balance dynamics are quantified by comparing the azimuthally averaged secondary circulation and corresponding tangential wind tendencies of the mesoscale integration with those diagnosed as the axisymmetric balanced response of a vortex subject to diabatic and tangential momentum forcing. Balance dynamics is defined here, following the tropical cyclone literature, as those processes that maintain a vortex in axisymmetric thermal wind balance. The dynamical and thermodynamical fields needed to characterize the background vortex for the Sawyer–Eliassen inversion are obtained by azimuthally averaging the relevant quantities in the mesoscale integration and by computing their corresponding balanced fields. Substantial differences between azimuthal averages and their homologous balance-derived fields are found in the boundary layer. These differences illustrate the inappropriateness of the balance assumption in this region of the vortex (where the secondary eyewall tangential wind maximum emerges). Although the balance model does broadly capture the sense of the forced transverse (overturning) circulation, the balance model is shown to significantly underestimate the inflow in the boundary layer. This difference translates to unexpected qualitative differences in the tangential wind tendency. The main finding is that balance dynamics does not capture the tangential wind spinup during the simulated secondary eyewall formation event.

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.

Essential Dynamics of Secondary Eyewall Formation

2013 ◽  
Vol 70 (10) ◽  
pp. 3216-3230 ◽  
Author(s):  
Sergio F. Abarca ◽  
Michael T. Montgomery
Keyword(s):  
Boundary Layer ◽  
Time Dependent ◽  
Layer Model ◽  
Mesoscale Simulation ◽  
Wind Maximum ◽  
Boundary Layer Model ◽  
Secondary Eyewall Formation ◽  
Tangential Wind ◽  
Eyewall Replacement Cycle ◽  
Replacement Cycle

Abstract The authors conduct an analysis of the dynamics of secondary eyewall formation in two modeling frameworks to obtain a more complete understanding of the phenomenon. The first is a full-physics, three-dimensional mesoscale model in which the authors examine an idealized hurricane simulation that undergoes a canonical eyewall replacement cycle. Analysis of the mesoscale simulation shows that secondary eyewall formation occurs in a conditionally unstable environment, questioning the applicability of moist-neutral viewpoints and related mathematical formulations thereto for studying this process of tropical cyclone intensity change. The analysis offers also new evidence in support of a recent hypothesis that secondary eyewalls form via a progressive boundary layer control of the vortex dynamics in response to a radial broadening of the tangential wind field. The second analysis framework is an axisymmetric, nonlinear, time-dependent, slab boundary layer model with radial diffusion. When this boundary layer model is forced with the aforementioned mesoscale model's radial profile of pressure at the top of the boundary layer, it generates a secondary tangential wind maximum consistent with that from the full-physics, mesoscale simulation. These findings demonstrate that the boundary layer dynamics alone are capable of developing secondary wind maxima without prescribed secondary heat sources and/or invocation of special inertial stability properties of the swirling flow either within or above the boundary layer. Finally, the time-dependent slab model reveals that the simulated secondary wind maximum contracts inward, as secondary eyewalls do in mesoscale models and in nature, pointing to a hitherto unrecognized role of unbalanced dynamics in the eyewall replacement cycle.

scholarly journals How Much Does the Upward Advection of the Supergradient Component of Boundary Layer Wind Contribute to Tropical Cyclone Intensification and Maximum Intensity?

2020 ◽  
Vol 77 (8) ◽  
pp. 2649-2664 ◽  
Author(s):  
Yuanlong Li ◽  
Yuqing Wang ◽  
Yanluan Lin
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Numerical Experiments ◽  
Inner Core ◽  
Maximum Intensity ◽  
The Other ◽  
Wind Component ◽  
Tangential Wind ◽  
Tangential Momentum ◽  
Tc Model

Abstract Although the development of supergradient winds is well understood, the importance of supergradient winds in tropical cyclone (TC) intensification is still under debate. One view is that the spinup of the eyewall occurs by the upward advection of high tangential momentum associated with supergradient winds from the boundary layer. The other view argues that the upward advection of supergradient winds by eyewall updrafts results in an outward agradient force, leading to the formation of a shallow outflow layer immediately above the inflow boundary layer. As a result, the spinup of tangential wind in the eyewall by the upward advection of supergradient wind from the boundary layer is largely offset by the spindown of tangential wind due to the outflow resulting from the agradient force. In this study, the net contribution by the upward advection of the supergradient wind component from the boundary layer to the intensification rate and final intensity of a TC are quantified through ensemble sensitivity numerical experiments using an axisymmetric TC model. Results show that consistent with the second view above, the positive upward advection of the supergradient wind component from the boundary layer by eyewall updrafts is largely offset by the negative radial advection due to the outflow resulting from the outward agradient force. As a result, the upward advection of the supergradient wind component contributes little (often less than 4%) to the intensification rate and but it contributes about 10%–15% to the final intensity of the simulated TC due to the enhanced inner-core air–sea thermodynamic disequilibrium.

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

2018 ◽  
Vol 75 (7) ◽  
pp. 2497-2505 ◽  
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.

A theoretical decomposition of mean skin friction generation into physical phenomena across the boundary layer

2016 ◽  
Vol 790 ◽  
pp. 339-367 ◽  
Author(s):  
Nicolas Renard ◽  
Sébastien Deck
Keyword(s):  
Boundary Layer ◽  
Skin Friction ◽  
Reynolds Shear Stress ◽  
Reynolds Numbers ◽  
Skin Friction Coefficient ◽  
Momentum Budget ◽  
The Mean ◽  
Logarithmic Layer ◽  
High Reynolds ◽  
Physical Phenomena

A theoretical decomposition of mean skin friction generation into physical phenomena across the whole profile of the incompressible zero-pressure-gradient smooth-flat-plate boundary layer is derived from a mean streamwise kinetic-energy budget in an absolute reference frame (in which the undisturbed fluid is not moving). The Reynolds-number dependences in the laminar and turbulent cases are investigated from direct numerical simulation datasets and Reynolds-averaged Navier–Stokes simulations, and the asymptotic trends are consistently predicted by theory. The generation of the difference between the mean friction in the turbulent and laminar cases is identified with the total production of turbulent kinetic energy (TKE) in the boundary layer, represented by the second term of the proposed decomposition of the mean skin friction coefficient. In contrast, the analysis introduced by Fukagataet al.(Phys. Fluids, vol. 14 (11), 2002, pp. 73–76), based on a streamwise momentum budget in the wall reference frame, relates the turbulence-induced excess friction to the Reynolds shear stress weighted by a linear function of the wall distance. The wall-normal distribution of the linearly-weighted Reynolds shear stress differs from the distribution of TKE production involved in the present discussion, which consequently draws different conclusions on the contribution of each layer to the mean skin friction coefficient. At low Reynolds numbers, the importance of the buffer-layer dynamics is confirmed. At high Reynolds numbers, the present decomposition quantitatively shows for the first time that the generation of the turbulence-induced excess friction is dominated by the logarithmic layer. This is caused by the well-known decay of the relative contributions of the buffer layer and wake region to TKE production with increasing Reynolds numbers. This result on mean skin friction, with a physical interpretation relying on an energy budget, is consistent with the well-established general importance of the logarithmic layer at high Reynolds numbers, contrary to the friction breakdown obtained from the approach of Fukagataet al.(Phys. Fluids, vol. 14 (11), 2002, pp. 73–76), essentially based on a momentum budget. The new decomposition suggests that it may be worth investigating new drag reduction strategies focusing on TKE production and on the nature of the logarithmic layer dynamics. The decomposition is finally extended to the pressure-gradient case and to channel and pipe flows.

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.

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