Tropical Cyclone Vortex Research Articles

Rapid intensification of tropical cyclones in the context of the solar wind-magnetosphere-ionosphere-atmosphere coupling

Rapid intensification of tropical storms is examined in the context of solar wind coupling to the magnetosphere-ionosphere-atmosphere system. Tropical cyclone “best tracks” in the southern and northern hemispheres are used in the superposed epoch analysis of time series of solar wind parameters. The results indicate that rapid intensification of tropical storms tends to follow arrivals of high-speed streams from coronal holes or interplanetary coronal mass ejections, which can trigger geomagnetic storms. The ensuing auroral and polar cap activity including ionospheric currents and ionospheric convection generates atmospheric gravity waves that propagate from the high-latitude lower thermosphere both upward and downward. If ducted in the lower atmosphere, they can reach tropical troposphere. Despite significantly reduced wave amplitudes, but subject to amplification upon reflection in the upper troposphere, these gravity waves can trigger/release moist instabilities to initiate convective bursts, with the latent heat release leading to intensification of storms. Convective bursts have been linked to rapid intensification of tropical cyclones. Cases of tropical cyclone intensification closely correlated with the solar wind structure are found to be preceded by atmospheric gravity waves generated by the solar wind magnetosphere-ionosphere-atmosphere coupling process. The gravity waves are observed in the ionosphere as traveling ionospheric disturbances. Their propagation in the lower atmosphere is examined by ray tracing in a model atmosphere to show that they can reach tropical cyclones. It is suggested that the interaction of aurorally-generated gravity waves with the tropical cyclone vortex and the inner primary eyewall could play a role in the intensification process. Assuming that quasi-periodic convective bursts lead to vortex waves, a two-dimensional barotropic approximation is used to obtain asymptotic solutions representing propagation of vortex waves and their absorption by the mean flow in the critical layer.

Assimilation of All-Sky Infrared Radiances from Himawari-8 and Impacts of Moisture and Hydrometer Initialization on Convection-Permitting Tropical Cyclone Prediction

Abstract This study explores the impacts of assimilating all-sky infrared satellite radiances from Himawari-8, a new-generation geostationary satellite that shares similar remote sensing technology with the U.S. geostationary satellite GOES-16, for convection-permitting initialization and prediction of tropical cyclones with an ensemble Kalman filter (EnKF). This case studies the rapid intensification stages of Supertyphoon Soudelor (2015), one of the most intense tropical cyclones ever observed by Himawari-8. It is found that hourly cycling assimilation of the infrared radiance improves not only the estimate of the initial intensity, but also the spatial distribution of essential convective activity associated with the incipient tropical cyclone vortex. Deterministic convection-permitting forecasts initialized from the EnKF analyses are capable of simulating the early development of Soudelor, which demonstrates encouraging prospects for future improvement in tropical cyclone prediction through assimilating all-sky radiances from geostationary satellites such as Himawari-8 and GOES-16. A series of forecast sensitivity experiments are designed to systematically explore the impacts of moisture updates in the data assimilation cycles on the development and prediction of Soudelor. It is found that the assimilation of the brightness temperatures contributes not only to better constraining moist convection within the inner-core region, but also to developing a more resilient initial vortex, both of which are necessary to properly capture the rapid intensification process of tropical cyclones.

Statistical Tropical Cyclone Wind Radii Prediction Using Climatology and Persistence: Updates for the Western North Pacific

Abstract This note describes an updated tropical cyclone vortex climatology for the western North Pacific version of the operational wind radii climatology and persistence (i.e., CLIPER) model. The update addresses known shortcomings of the existing formulation, namely, that the wind radii used to develop the original model were too small and symmetric. The underlying formulation of the CLIPER model has not changed, but the larger and more realistic vortex climatology produces improved forecast biases. Other applications that make use of the vortex climatology and CLIPER model forecasts should also benefit from the bias improvements.

Dynamics and Predictability of the Rapid Intensification of Super Typhoon Usagi (2013)

AbstractThis study explores the dynamics and predictability of the rapid intensification (RI) of Super Typhoon Usagi (2013) through a 60‐member convection‐permitting ensemble using the Weather Research and Forecasting (WRF) model and an ensemble Kalman filter (EnKF) data assimilation method. The surface maximum wind speed of Usagi, which was an intense category 4 western North Pacific tropical cyclone (TC), increased by 33 m s−1 over a 24‐hr period. The RI process was captured by the WRF simulation initialized with the global analysis but with a unique forecast challenge of early prediction. We improved the intensity forecasts by assimilating satellite‐derived atmospheric motion vectors into the WRF‐EnKF, which primarily reduced the strength of both the primary and secondary circulations in the TC vortex. Nevertheless, our ensemble forecasts initialized with the EnKF analysis ensemble predicted a significant spread in the intensity with considerable differences in the RI onset timing among individual members. Our analyses show that variation in the RI timing is most sensitive to differences in the initial TC vortex intensity and inner‐core moisture. Ensemble members with similar initial intensities but greater tropospheric moisture content exhibited earlier vortex axisymmetrization and consequently earlier RI. Further sensitivity experiments showed that variations in the inner‐core moisture content have an immediate impact on the structure and strength of inner‐core convection. These variations in inner‐core convection gradually caused differences in intensity between the TC vortices. In this study, we highlight the importance of accurate estimates of the inner‐core moisture content in the modeling and forecasting of TC intensity.

A Dynamical Initialization Scheme for Tropical Cyclones under the Influence of Terrain

Abstract This study extends an earlier dynamical initialization (DI) scheme for tropical cyclones (TCs) to situations under the influence of terrain. When any terrain lower than 1 km exists between 150 and 450 km from the TC center, topographic variables are defined and a filtering algorithm is used to remove noise due to the presence of terrain before the vortex separation is conducted. When any terrain higher than 1 km exists between 150 and 300 km from the TC center, or the TC center is within 150 km of land, a semi-idealized integration without the terrain is conducted to spin up an axisymmetric TC vortex before the inclusion of the terrain and the merging of the TC vortex with the large-scale analysis field. In addition, a procedure for the vortex size/intensity adjustment is introduced to reduce the initial errors before the forecast run. Two sets of hindcasts, one without (CTRL run) and one with the new DI scheme (DI run), are conducted for nine TCs affected by terrain over the western North Pacific in 2015. Results show that the new DI scheme largely reduces the initial position and intensity errors. The 72-h position errors and the intensity errors up to the 36-h forecasts are smaller in DI runs than in CTRL runs and smaller than those from the HWRF forecasts for the same TCs as well. The new DI scheme is also shown to produce the TC inner-core structure and rainbands more consistent with satellite and radar observations.

Evaluation of Tropical Cyclone Structure Forecasts in a High-Resolution Version of the Multiscale GFDL fvGFS Model

Abstract A nested version of the cubed-sphere finite-volume dynamical core (FV3) with GFS physics (fvGFS) is capable of tropical cyclone (TC) prediction across multiple space and time scales, from subseasonal prediction to high-resolution structure and intensity forecasting. Here, a version of fvGFS with 2-km resolution covering most of the North Atlantic is evaluated for its ability to simulate TC track, intensity, and finescale structure. TC structure is evaluated through a comparison of forecasts with three-dimensional Doppler radar from P-3 flights by NOAA’s Hurricane Research Division (HRD), and the structural metrics evaluated include the 2-km radius of maximum wind (RMW), slope of the RMW, depth of the TC vortex, and horizontal vortex decay rate. Seven TCs from the 2010–16 seasons are evaluated, including 10 separate model runs and 38 individual flights. The model had some success in producing rapid intensification (RI) forecasts for Earl, Edouard, and Matthew. The fvGFS model successfully predicts RMWs in the 25–50-km range but tends to have a small bias at very large radii and a large bias at very small radii. The wind peak also tends to be somewhat too sharp, and the vortex depth occasionally has a high bias, especially for storms that are observed to be shallow. Composite radial wind shows that the boundary layer tends to be too deep, although the outflow structure aloft is relatively consistent with observations. These results highlight the utility of the structural evaluation of TC forecasts and also show the promise of fvGFS for forecasting TCs.

A Numerical Study on Rapid Intensification of Typhoon Vicente (2012) in the South China Sea. Part II: Roles of Inner-Core Processes

In Part I of this study, the role of environmental monsoon flow in the onset of rapid intensification (RI) of Typhoon Vicente (2012) was discussed. In this Part II, key inner-core processes that effectively resist environmental vertical wind shear during RI onset are investigated. The convective precipitation shield (CPS) embedded in the downshear convergence zone plays a vital role in preconditioning the tropical cyclone (TC) vortex before RI. The CPS induces a mesoscale positive vorticity band (PVB) characterized by vortical hot tower structures upstream and shallower structures (~4 km) downstream. Multiple mesovortices form successively along the PVB and are detached from the PVB at its downstream end, rotating cyclonically around the TC center. The sufficient amount of vorticity anomalies in the PVB facilitates the upscale growth of a mesovortex into a reformed inner vortex, which eventually replaces the parent TC vortex (i.e., downshear reformation), leading to RI onset. The timing of downshear reformation is closely related to the gradually enhancing convective activity in the CPS, which is likely triggered/enhanced by increased surface heat fluxes in the downshear-left quadrant. Results from vorticity budget analyses suggest that convection in the CPS contributes to the vertical development of the tilted reformed inner vortex largely through tilting horizontal vorticity and advecting vorticity upward. The enhanced midlevel inner vortex precesses more quickly into the upshear flank and is concurrently advected toward the low-level inner vortex, resulting in vertical alignment of the reformed inner vortex and parent TC vortex at the end of downshear reformation.

Effect of Boundary Layer Roll Vortices on the Development of an Axisymmetric Tropical Cyclone

AbstractIn this study, the authors numerically investigate the response of an axisymmetric tropical cyclone (TC) vortex to the vertical fluxes of momentum, heat, and moisture induced by roll vortices (rolls) in the boundary layer. To represent the vertical fluxes induced by rolls, a two-dimensional high-resolution Single-Grid Roll-Resolving Model (SRM) is embedded at multiple horizontal grid points in the mesoscale COAMPS for Tropical Cyclones (COAMPS-TC) model domain. Idealized experiments are conducted with the SRM embedded within 3 times the radius of maximum wind of an axisymmetric TC. The results indicate that the rolls induce changes in the boundary layer wind distribution and cause a moderate (approximately 15%) increase in the TC intensification rate by increasing the boundary layer convergence in the eyewall region and induce more active eyewall convection. The numerical experiments also suggest that the roll-induced tangential momentum flux is most important in contributing to the TC intensification process, and the rolls generated at different radii (within the range considered in this study) all have positive contributions. The results are not qualitatively impacted by the initial TC vortex or the setup of the vertical diffusivity in COAMPS-TC.

An analytical model of maximum potential intensity for tropical cyclones incorporating the effect of ocean mixing

AbstractAn analytical model of maximum potential intensity (PI) for tropical cyclones (TCs) incorporating wind‐induced ocean cooling is developed on the basis of Emanuel's PI theory. The model consists of a one‐dimensional ocean and an axisymmetric TC vortex that is translating at a constant speed. The model advances upon previous approaches by accounting for TC size and shape. The PI is determined at the radius of maximum winds by radially integrating the effect of ocean mixing using a specified wind profile. The resulting analytic ocean cooling agrees well with numerical solutions, and the new PI model better captures maximum intensities of observed TCs compared with Emanuel's original PI. It is also shown that the degree of cooling (i.e., ocean's feedback to atmosphere) can be quantified by a dimensionless parameter, representing the ratio of atmospheric forcing to ocean stability.

Looping tracks associated with tropical cyclones approaching an isolated mountain. Part I: Essential parameters

Essential parameters for making a looping track when a westward-moving tropical cyclone (TC) approaches a mesoscale mountain are investigated by examining several key nondimensional control parameters with a series of systematic, idealized numerical experiments, such as U/Nh, V max/Nh, U/fL x , V max/fR, h/L x , and R/L y . Here U is the uniform zonal wind velocity, N the Brunt–Vaisala frequency, h the mountain height, f the Coriolis parameter, V max the maximum tangential velocity at a radius of R from the cyclone center and L x is the halfwidth of the mountain in the east–west direction. It is found that looping tracks (a) tend to occur under small U/Nh and U/fL x , moderate h/L x , and large V max/Nh, which correspond to slow movement (leading to subgeostrophic flow associated with strong orographic blocking), moderate steepness, and strong tangential wind associated with TC vortex; (b) are often accompanied by an area of perturbation high pressure to the northeast of the mountain, which lasts for only a short period; and (c) do not require the existence of a northerly jet. The nondimensional control parameters are consolidated into a TC looping index (LI), $$ \frac{{U^{2} R^{2} }}{{V_{\hbox{max} }^{2} hL_{y} }} $$ , which is tested by several historical looping and non-looping typhoons approaching Taiwan’s Central Mountain Range (CMR) from east or southeast. It is found that LI < 0.0125 may serve as a criterion for looping track to occur.

Impact of different climatic flows on typhoon tracks

A tropical cyclone (TC) vortex is considered to be embedded in and steered by a large-scale environmental flow. The environmental flow can be decomposed into two parts: temporal climatic flow and anomaly. The former is defined according to the calendar climatology with a diurnal cycle and a seasonal cycle. Thus, the temporal climatic flow of the atmosphere, which can be estimated using reanalysis data, varies with regions, altitudes, and hours. The impact of different climatic flows on TC tracks in the Northwest Pacific is examined using a simple generalized beta-advection model. Results show that the predicted tracks of two TC cases have large deviations from their best tracks in the following 1–2 days if the temporal climatic wind is replaced by other hourly climatic winds on the same calendar day or by a several-day-mean climatic wind. The track deviation is more significant when the climatic wind difference is larger than 2 m s−1. This experiment reconfirms that a TC track is influenced by temporal climatic flow and interaction with other disturbances in the vicinity.

Assessing sensitivities in algorithmic detection of tropical cyclones in climate data

AbstractThis study applies a sensitivity analysis (SA) technique (the Morris method, MM) to an automated Lagrangian tropical cyclone (TC) tracking algorithm used on gridded climate data. MM demonstrates the ability to screen for input parameters defining TCs (such as minimum intensity and lifetime) that contribute significantly to sensitivity in output metrics (such as storm count). The SA is performed by tracking TCs in four different reanalyses. Tracked TC trajectories are compared to a pointwise observational record. Results show that using thermally integrated metrics for isolating TC warm cores is superior to single‐temperature levels. Input thresholds defining TC vortex strength during tracking contribute the most variance in all output metrics. Integrated output metrics (such as accumulated cyclone energy) are less variable than “counting” metrics such as TC frequency. MM greatly reduces the computational requirements for tracker optimization, with tracked TCs demonstrating better hit and false alarm rates than previous studies.

Recent Developments in the Fluid Dynamics of Tropical Cyclones

This article reviews progress in understanding the fluid dynamics and moist thermodynamics of tropical cyclone vortices. The focus is on the dynamics and moist thermodynamics of vortex intensification and structure. We discuss previous ideas on many facets of the subject and articulate also some open questions. The advances reviewed herein provide new insight and tools for interpreting complex vortex-convective phenomenology in simulated and observed tropical cyclones.

Reply to “Comments on ‘Nonlinear Response of a Tropical Cyclone Vortex to Prescribed Eyewall Heating with and without Surface Friction in TCM4: Implications for Tropical Cyclone Intensification’”

Reply to “Comments on ‘Nonlinear Response of a Tropical Cyclone Vortex to Prescribed Eyewall Heating with and without Surface Friction in TCM4: Implications for Tropical Cyclone Intensification’”

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

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

A diagnostic study on heavy rainfall induced by landfalling Typhoon Utor (2013) in South China: 2. Postlandfall rainfall

AbstractIn this part, mechanisms responsible for the maintenance of Typhoon Utor (2013) after landfall and its associated heavy rainfall in South China were investigated with methods including piecewise potential vorticity (PV) inversion. The focus was on the monsoonal influence and the interaction between Utor and the mesoscale convective systems (MCSs) embedded in the southwesterly monsoon flow. The results show that after landfall Utor underwent coalescence with the cyclonic monsoon gyres, related to the quasi‐biweekly oscillation (QBWO) and the Madden‐Julian oscillation (MJO). The QBWO cyclonic gyre, which better developed and coalesced with the tropical cyclone (TC) vortex, enhanced the large‐scale southwesterly monsoon flow southeast of the TC, providing favorable condition for initiation of convection and organization of MCSs in the eastern outer rainband of the TC. Latent heat release in the MCSs provided a positive feedback to enhance the TC circulation and the southwesterly monsoon flow, slowing down the decay of Utor and sustaining heavy rainfall. Piecewise PV inversion confirmed that the nonlinear balanced flow inverted from the PV anomaly associated with latent heating in MCSs led to an increase of the low‐level southwesterly flow south of the TC by over 4 m s−1, which contributed notably to the formation of the low‐level jet and moisture convergence in the eastern outer rainband. It is suggested that the positive feedback between the outer rainband MCSs and the southwesterly monsoon flow is a major mechanism responsible for the maintenance of Typhoon Utor after its landfall and the associated postlandfall heavy rainfall over South China.

Kinetic Energy Budget during the Genesis Period of Tropical Cyclone Durian (2001) in the South China Sea

Abstract A set of kinetic energy (KE) budget equations associated with four horizontal flow components was derived to study the KE characteristics during the genesis of Tropical Cyclone (TC) Durian (2001) in the South China Sea using numerical simulation data. The genesis process was divided into three stages: the monsoon trough stage (stage 1), the midlevel mesoscale convective vortex (MCV) stage (stage 2), and the establishment stage of the TC vortex (stage 3). Analysis showed that the KE of the symmetric rotational flow (SRF) was the largest and kept increasing, especially in stages 2 and 3, representing the symmetrization process during TC genesis. The KE of the SRF was mainly converted from the KE of the symmetric divergent flow (SDF), largely transformed from the available potential energy (APE). It was found that vortical hot towers (VHTs) emerged abundantly, aggregated, and merged within the MCV region in stages 1 and 2. From the energy budget perspective, massive moist-convection-produced latent heat was concentrated and accumulated within the MCV region, especially in stage 2, and further warmed the atmosphere, benefiting the accumulation of APE and the transformation from APE to KE. As a result, the midlevel circulation (or MCV) grew strong rapidly. In stage 3, the intensity and number of VHTs both decreased. However, affected by increasing lower-level inward radial wind, latent heat released by the organized convection, instead of disorganized VHTs in the first two stages, continuously contributed to the strengthening of the surface TC circulation as well as the warm core.

Meso‐β‐scale environment for the stationary band complex of vertically sheared tropical cyclones

The stationary band complex (SBC) is a distinct, quasi‐stationary convective asymmetry outside the eyewall of a tropical cyclone (TC) and plays an important role for TC intensity. Seminal work by Willoughby et al. related the SBC to the kinematic boundary between the TC and its environment and proposed a formation mechanism for the SBC. These results have been derived from observational data that are limited in radial and azimuthal coverage. The current study revisits these results using data from an idealized numerical experiment, which are not subject to such limitations. Consistent with Willoughby et al., the SBC in the idealized experiment is located near the boundary between the TC's ‘moist envelope’ and the lower θe environment. In contrast, however, the SBC is not associated with the kinematic boundary between the TC and the environmental flow. Furthermore, the proposed formation mechanism does not operate in the idealized experiment.I propose an alternative formation mechanism that links the SBC to two fundamental consequences of environmental vertical wind shear: (i) the distortion of the ‘moist envelope’ and (ii) the tilt of the TC vortex. In the idealized experiment, the respective regions of high‐θe air and a positive vorticity anomaly overlap at low levels in the semicircle to the right of the shear vector well outside the eyewall. A simple slab boundary‐layer model indicates that frictional convergence associated with the vorticity asymmetry is a viable, dynamical forcing mechanism for deep convection. While ascent out of the boundary layer occurs over a radially broad region, the large radial shear of the TC's swirling winds promotes the organization of the ensuing convection into a band‐like pattern. I emphasize that this formation mechanism considers only the meso‐β‐scale envelope of the SBC. As shown previously by others, convective‐scale processes contribute further to the organization of the SBC within this envelope.

Idealized Tropical Cyclone Responses to the Height and Depth of Environmental Vertical Wind Shear

Abstract Three sets of idealized, cloud-resolving simulations are performed to investigate the sensitivity of tropical cyclone (TC) structure and intensity to the height and depth of environmental vertical wind shear. In the first two sets of simulations, shear height and depth are varied independently; in the third set, orthogonal polynomial expansions are used to facilitate a joint sensitivity analysis. Despite all simulations having the same westerly deep-layer (200–850 hPa) shear of 10 m s−1, different intensity and structural evolutions are observed, suggesting the deep-layer shear alone may not be sufficient for understanding or predicting the impact of vertical wind shear on TCs. In general, vertical wind shear that is shallower and lower in the troposphere is more destructive to model TCs because it tilts the TC vortex farther into the downshear-left quadrant. The vortices that tilt the most are unable to precess upshear and realign, resulting in their failure to intensify. Shear height appears to modulate this tilt response by modifying the thermodynamic environment above the developing vortex early in the simulations, while shear depth modulates the tilt response by controlling the vertical extent of the convective vortex. It is also found that TC intensity predictability is reduced in a narrow range of shear heights and depths. This result underscores the importance of accurately observing the large-scale environmental flow for improving TC intensity forecasts, and for anticipating when such forecasts are likely to have large errors.

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.