Incipient Vortex Research Articles

Numerical Study on the Genesis of Incipient Vortex of a Tropical Storm over the South China Sea. Part II Sensitivity of Sea Surface Sensible Heating

The experiments are conducted to determine the impact of the anomalous surface sensible heating (SH) from the South China Sea (SCS) on the incipient vortex of the storm Mindulle (2010) formation by using the Weather Research and Forecasting (WRF) model, including the control-run with and the sensitivity-run without such heating. Significant impact is seen, with the incipient vortex failing to generate without the related SH. Based on the atmospheric thermal adaption theory, the outputs reveal that the genesis of the incipient vortex can be divided into the formation stage and the strengthen stage. At the formation stage, the surface SH favoured the enhancement of the surface convergence. Thus strong westerlies and southerlies along with the topography forcing constrain updrafts over the Luzon Island. The non-uniform condensation latent heating (LH) released by the vertical motion creates the cyclonic vorticity in the lower atmosphere, which was conducive to the incipient vortex formation. At the strengthen stage, the more intense LH due to the enhanced ascending motions mainly contributed to cyclonic vorticity generation. Therefore, the incipient vortex continuously strengthened and attained to the intensity of tropical storm. The simulation results illustrate that the development of the Mindulle (2010) is a “bottom-up” process.

Numerical Study on the Genesis of Incipient Vortex of a Tropical Storm over the South China Sea. Part I Control Simulation

The previous work has found that strong sensible heating (SH) over the South China Sea (SCS) and western Pacific and related condensation latent heating (LH) are of great significance for the genesis and growth of the initial disturbance of Mindulle (2010). The Weather Research and Forecasting (WRF) model is used to simulate the genesis process of the storm Mindulle using grids nested to high resolution without bogusing. The numerical simulations are executed to determine the impact of the anomalous SH from the SCS on the incipient vortex of the storm formation, including the control-run with and the sensitivity-run without such heating. In the control simulation, the genesis process of the incipient vortex of Mindulle was reproduced along with the reasonable simulated SH over the SCS compared with the observations, which shows that the WRF model has an ability to simulate the formation process of tropical cyclones at the pre-WISHE stage without bogusing over the SCS basin.

Three-dimensional off-grid localization of incipient tip vortex cavitation using Bayesian inference

In this paper, we propose a 3-D source localization method to determine the incipient tip vortex cavitation (TVC) positions by constructing a signal model for the broadband off-grid cavitation. Subsequently, the conventional sparse Bayesian learning (SBL), whose performance is diminished owing to an inappropriate signal representation for the off-grid TVC by basis mismatch, was extended to solve the signal model via Taylor series and the block-sparse based broadband off-grid approach (off-grid BSBL). The off-grid BSBL was examined using synthetic and experimental cavitation tunnel data. Furthermore, its localization results were compared with those for conventional and adaptive beamformers and on-grid BSBL, which generally suffer from low resolutions and basis mismatch, respectively. When the off-grid BSBL was applied to chirp- and pop-type signals from incipient TVC, it determined the TVC positions near the top center of the propeller and provided the clearest localization results among the schemes considered.

On the Contributions of Incipient Vortex Circulation and Environmental Moisture to Tropical Cyclone Expansion

AbstractThis study investigates the contributions of incipient vortex circulation and midlevel moisture to tropical cyclone (TC) expansion within an idealized numerical modeling framework. We find that the incipient vortex circulation places the primary constraint on TC expansion. Increasing the midlevel moisture further promotes expansion but mostly expedites the intensification process. The expansion rate for initially large vortices exhibits a stronger response to increasing the midlevel moisture compared to initially small vortices. Previous studies have noted a proclivity for relatively small TCs to stay small and relatively large TCs to stay large; that is, TCs possess a sort of “memory” with respect to their incipient circulation. We reproduce this finding with an independent modeling framework and further demonstrate that an initially large vortex can expand more quickly than its relatively smaller counterpart; therefore, with all other factors contributing to expansion held constant, the contrast in size between the two vortices will increase with time. Varying the incipient vortex circulation is associated with subsequent variations in the amount and scale of outer‐core convection. As the incipient vortex circulation decreases, outer‐core convection is relatively scarce and characterized by small‐scale, isolated convective elements. On the contrary, as the incipient vortex circulation increases, outer‐core convection abounds and is characterized by relatively large rainbands and mesoscale convective systems. A combined increase in the amount and scale of outer‐core convection permits an initially large vortex to converge a substantially greater amount of absolute angular momentum compared to its relatively smaller counterpart, resulting in distinct expansion rates.

Supplemental data for "On the contributions of incipient vortex circulation and environmental moisture to tropical cyclone expansion"

Supplemental data for "On the contributions of incipient vortex circulation and environmental moisture to tropical cyclone expansion"

Block-sparse compressive localization for incipient tip vortex cavitation noise

Marine propeller cavitation is a dominant noise source of ships. Thus, localizing the cavitation noise source is essential for subsequent remedy. Given that incipient tip vortex cavitation (TVC) noise radiates in all directions as a monopole source and a few noise sources exist in the vicinity of the propeller, the localization problem can be considered as a sparse signal reconstruction problem. Compressive sensing (CS) based localization technique utilizes a sparsity promoting constraint and solves the localization problem efficiently with high resolution. Block-sparse CS, based on block-sparsity, is adopted to process multiple frequency components of the sources coherently. Block-sparse CS localization of TVC shows superior performance with high resolution compared to the conventional CS based incoherent multiple frequency processing. To demonstrate the performance of the block-sparse CS localization, both synthetic and real cavitation tunnel experiment data are used.

Spectral Kurtosis를 이용한 프로펠러 날개끝 보텍스 소음의 발생위치추정

One primary concern for marine propeller design is the incipient tip vortex cavitation. The present study elaborates its hyperbolic localization, which is subjected to an estimation of time difference of arrival between measured acoustic signals. The generalized cross-correlation with phase transform, a conventional algorithm for time delay estimation, tends to be susceptible to the noise outside the frequency band of signal. In order to suppress these effects, we propose a weighting function, resembling the rectangular window in frequency domain. By further noting that the signal arisen by the tip vortex cavitation can be fall into the cyclostationary type, the spectral kurtosis is employed to design the optimum band of the frequency weights. Finally, the method is verified by experimental measurements in the water tunnel.

Role of Surface Enthalpy Fluxes in Idealized Simulations of Tropical Depression Spinup

AbstractAn idealized, three-dimensional, cloud-system-resolving model is used to investigate the influence of surface enthalpy flux variations on tropical depression (TD) spinup, an early stage of tropical cyclogenesis in which the role of surface fluxes remains incompletely understood. A range of simulations supports the hypothesis that a negative radial gradient of surface enthalpy flux outside the storm center is necessary for TD spinup but can arise from multiple mechanisms. The negative radial gradient is typically created by the wind speed dependence of surface enthalpy fluxes, consistent with some previous theories for tropical cyclone intensification. However, when surface enthalpy fluxes are prescribed to be independent of wind speed, spinup still occurs, albeit more slowly, with the negative radial gradient of surface enthalpy flux maintained by an enhanced air–sea thermodynamic disequilibrium beneath the cold core of the incipient vortex. Surface enthalpy flux variations seem more important for intensification than initial conditions. For example, a vortex forms and intensifies even from a state of rest when the center of the domain is initialized to be nearly saturated with water vapor, but this intensification is modest in amplitude and transient, lasting less than 12 h, without interactive surface enthalpy flux. Sustained spinup on time scales longer than a day does not occur when surface enthalpy fluxes are horizontally homogeneous or constant, even when fixed at the high value of 200 W m−2. In the ensemble of simulations presented here, the vortex intensification rate scales linearly with the storm-scale surface enthalpy flux anomaly relative to the undisturbed environment.

Hyperbolic localization of incipient tip vortex cavitation in marine propeller using spectral kurtosis

Abstract Incipient tip vortex cavitation is one of the primary concerns for propeller design in commercial- or naval- vessels. This paper attempts its hyperbolic localization, which should be preceded by accurate estimation of time delay between measured acoustic signals. Although the generalized cross-correlation with phase transform can be considered as a conventional algorithm for time delay estimation, it is prone to be vulnerable to the noise dwelling outside the frequency band of signal. In order to nullify these noise effects, a weighting function, resembling the rectangular window in frequency domain, is introduced. By further noting that the signal arisen by the tip vortex cavitation can be categorized into the cyclostationary type, the spectral kurtosis is adhered to design the optimum band of the frequency weights. Finally, the proposed method is shown to be valid by two experimental demonstrations of artificial source and propeller cavitation.

Impact of Madden Julian oscillation (MJO) and other meteorological phenomena on the heavy rainfall event from 19th − 28th December, 2014 over Sri Lanka

This study revealed the influence of Madden Julian oscillation (MJO) and cold surge (CS) on the extreme rainfall events that occurred in Sri Lanka from 19th to 28th December 2014. During this period, the Northeast Monsoon winds over the Bay of Bengal were strengthened by a CS, which intensified convective activity over the Bay of Bengal. Simultaneously, convective phase 3 of the MJO, which was located over the eastern Indian Ocean is associated with a positive rainfall anomaly over Sri Lanka. Westerly wind anomalies associated with MJO phase 3 shifted southward due to the strengthening of northeasterly winds by the CS, forming an incipient vortex to the southeast of Sri Lanka. The low level moisture transport was enhanced by this vortex circulation together with strong cross-equatorial moisture flux transportation. The convectively active phase of MJO provided favourable conditions such as low level convergence, cyclonic relative vorticity, and deep convection for the intensification of vortex into a tropical depression over the southwest Bay of Bengal. Strong upward motion and combination of strong low level convergence with upper-level divergence allowed evacuation of the ascending air mass, aiding the intensification of deep convection. This persistent vertical coupling structure allowed deep convection of moist warm air, which led to heavy rainfall during the period. Combination of a number of factors, such as local terrain effects, CS at low levels, MJO wet phase and a vortex to southeast of Sri Lanka led to an intensification of the tropical depression that caused an extreme rainfall event over the Eastern and Northeastern parts of Sri Lanka.

Vortex reconnection in the late transition in channel flow

Vortex reconnection, as the topological change of vortex lines or surfaces, is a critical process in transitional flows, but is challenging to accurately characterize, particularly in shear flows. We apply the vortex-surface field (VSF), whose isosurface is the vortex surface consisting of vortex lines, to study vortex reconnection in the Klebanoff-type temporal transition in channel flow. The VSF evolution can capture the reconnection of the hairpin-like vortical structures evolving from the initial vortex sheets in opposite halves of the channel. The incipient vortex reconnection is characterized by the vanishing minimum distance between a pair of vortex surfaces and the reduction of vorticity flux through the region enclosed by the wall and the VSF isoline of the channel half-height on the spanwise symmetric plane. We find that the surge of the wall-friction coefficient begins at the identified reconnection time. From the Biot–Savart law, the rapid reconnection of vortex lines can induce a velocity opposed to the mean flow, which partially blocks the flow near the central region and generally accelerates the near-wall fluid motion in the flow with constant mass flux. Therefore, the vortex reconnection appears to play an important role in the sudden increase of wall friction in transitional channel flows.

Impacts of the Diurnal Radiation Cycle on the Formation, Intensity, and Structure of Hurricane Edouard (2014)

Abstract This work examines the impacts of the diurnally varying radiation cycle on the formation, intensity, structure, and track of Hurricane Edouard (2014) at different stages of its life cycle through convection-permitting simulations. During the formation stage, nighttime destabilization through radiative cooling may promote deep moist convection that eventually leads to the genesis of the storm, while a tropical cyclone fails to develop in the absence of the night phase despite a strong incipient vortex under moderately strong vertical wind shear. The nighttime radiative cooling further enhances the primary vortex before the storm undergoes rapid intensification. Thereafter, the nighttime radiative cooling mainly increases convective activities outside of the primary eyewall that lead to stronger/broader rainbands and larger storm size during the mature stage of the hurricane; there is, however, less impact on the hurricane’s peak intensity in terms of maximum 10-m surface wind speed. The control forecast undergoes distinct secondary eyewall formation during the mature stage of Edouard (as observed), while there is no apparent eyewall replacement cycle as simulated in sensitivity experiments without solar insolation and the moat is narrower in those with switch-on solar insolation at night, suggesting the potential role of the diurnally varying radiative impact.

The impact of vertical shear on the sensitivity of tropical cyclogenesis to environmental rotation and thermodynamic state

AbstractThe impact of vertical wind shear on the sensitivity of tropical cyclogenesis to environmental rotation and thermodynamic state is investigated through idealized cloud‐resolving simulations of the intensification of an incipient vortex. With vertical shear, tropical cyclones intensify faster with a higher Coriolis parameter, f, irrespective of the environmental thermodynamic state. The vertical shear develops a vertically tilted vortex, which undergoes a precession process with the midlevel vortices rotating cyclonically around the surface center. With a higher f, the midlevel vortices are able to rotate continuously against the vertical shear, leading to the realignment of the tilted vortex and rapid intensification. With a lower f, the rotation is too slow such that the midlevel vortices are advected away from the surface center and the intensification is suppressed. The parameter, χb, measuring the effect from the low‐entropy downdraft air on the boundary layer entropy, is found to be a good indicator of the environmental thermodynamic favorability for tropical cyclogenesis in vertical shear. Without vertical shear, tropical cyclones are found to intensify faster with a lower f by previous studies. We show this dependency on f is sensitive to the environmental thermodynamic state. The thermodynamical favorability for convection can be measured by χm, which estimates the time it takes for surface fluxes to moisten the midtroposphere. A smaller χm not only leads to a faster intensification due to a shorter period for moist preconditioning of the inner region but also neutralizes the faster intensification with a lower f due to enhanced peripheral convection.

Effects of vertical wind shear on the predictability of tropical cyclones: Practical versus intrinsic limit

AbstractThe effects of environmental shear on the dynamics and predictability of tropical cyclones (TCs) are further explored through a series of cloud‐permitting ensemble sensitivity experiments with small, random initial condition perturbations on the low‐level moisture fields. As an expansion of earlier studies, it is found that larger the shear magnitude, less predictable the TCs, especially the onset time of the rapid intensification (RI), until the shear is too large for the TC formation. Systematic differences amongst the ensemble members begin to arise right after the initial burst of moist convection associated with the incipient vortex. This randomness inherent in moist convection first changes the TC vortex structure subtly, but the location and strength of subsequent moist convection are greatly influential to the precession and alignment of the TC vortex as well as the RI onset time. Additional ensemble sensitivity experiments with different magnitude random perturbations to the mean environmental shear (6 m s−1) show that when the standard deviation of the random shear perturbations among different ensemble members is as small as 0.5 m s−1, the difference in shear magnitude overwhelms the randomness of moist convection in influencing the TC development and rapid intensification (indicative of limited practical predictability). However, for the ensemble with standard deviation of 0.1 m s−1 in random shear perturbations, the uncertainty in TC onset timing is comparable to the ensemble that is perturbed only by small random moisture conditions in the initial moisture field (indicative of the limit in intrinsic predictability).

Numerical analyses of pressure fluctuations induced by interblade vortices in a model Francis turbine

Interblade vortices can greatly influence the stable operations of Francis turbines. As visible interblade vortices are essentially cavitating flows, i.e., the ones to cause interblade vortex cavitations, an unsteady simulation with a method using the RNG k – ɛ turbulence model and the Zwart-Gerber-Belamri (ZGB) cavitation model is carried out to predict the pressure fluctuations induced. Modifications of the turbulence viscosity are made to improve the resolutions. The interblade vortices of two different appearances are observed from the numerical results, namely, the columnar and streamwise vortices, as is consistent with the experimental results. The pressure fluctuations of different frequencies are found to be induced by the interblade vortices on incipient and developed interblade vortex lines, respectively, on the Hill diagram of the model runner's parameters. From the centrifugal Rayleigh instability criterion, it follows that the columnar interblade vortices are stable and the streamwise interblade vortices are unstable in the model Francis turbine.

Projected Twenty-First-Century Changes in the Length of the Tropical Cyclone Season

Abstract This study investigates projected changes in the length of the tropical cyclone season due to greenhouse gas increases. Two sets of simulations are analyzed, both of which capture the relevant features of the observed annual cycle of tropical cyclones in the recent historical record. Both sets use output from the general circulation models (GCMs) of either phase 3 or phase 5 of the CMIP suite (CMIP3 and CMIP5, respectively). In one set, downscaling is performed by randomly seeding incipient vortices into the large-scale atmospheric conditions simulated by each GCM and simulating the vortices’ evolution in an axisymmetric dynamical tropical cyclone model; in the other set, the GCMs’ sea surface temperature (SST) is used as the boundary condition for a high-resolution global atmospheric model (HiRAM). The downscaling model projects a longer season (in the late twenty-first century compared to the twentieth century) in most basins when using CMIP5 data but a slightly shorter season using CMIP3. HiRAM with either CMIP3 or CMIP5 SST anomalies projects a shorter tropical cyclone season in most basins. Season length is measured by the number of consecutive days that the mean cyclone count is greater than a fixed threshold, but other metrics give consistent results. The projected season length changes are also consistent with the large-scale changes, as measured by a genesis index of tropical cyclones. The season length changes are mostly explained by an idealized year-round multiplicative change in tropical cyclone frequency, but additional changes in the transition months also contribute.

Localization of incipient tip vortex cavitation using ray based matched field inversion method

Cavitation of marine propeller is one of the main contributing factors of broadband radiated ship noise. In this research, an algorithm for the source localization of incipient vortex cavitation is suggested. Incipient cavitation is modeled as monopole type source and matched-field inversion method is applied to find the source position by comparing the spatial correlation between measured and replicated pressure fields at the receiver array. The accuracy of source localization is improved by broadband matched-field inversion technique that enhances correlation by incoherently averaging correlations of individual frequencies. Suggested localization algorithm is verified through known virtual source and model test conducted in Samsung ship model basin cavitation tunnel. It is found that suggested localization algorithm enables efficient localization of incipient tip vortex cavitation using a few pressure data measured on the outer hull above the propeller and practically applicable to the typically performed model scale experiment in a cavitation tunnel at the early design stage.

Development of North Atlantic Tropical Disturbances near Upper-Level Potential Vorticity Streamers

Abstract Tropical cyclone (TC) development near upper-level potential vorticity (PV) streamers in the North Atlantic is studied from synoptic climatology, composite, and case study perspectives. Midlatitude anticyclonic wave breaking is instrumental in driving PV streamers into subtropical and tropical latitudes, in particular near the time-mean midocean trough identified previously as the tropical upper-tropospheric trough. Twelve TCs developed within one Rossby radius of PV streamers in the North Atlantic from June through November 2004–08. This study uses composite analysis in the disturbance-relative framework to compare the structural and thermodynamic evolution for developing and nondeveloping cases. The results show that incipient tropical disturbances are embedded in an environment characterized by 850–200-hPa westerly vertical wind shear and mid- and upper-level quasigeostrophic ascent associated with the PV streamer, with minor differences between developing and nondeveloping cases. The key difference in synoptic-scale flow between developing and nondeveloping cases is the strength of the anticyclone north of the incipient tropical disturbance. The developing cases are marked by a stronger near-surface pressure gradient and attendant easterly flow north of the vortex, which drives enhanced surface latent heat fluxes and westward (upshear) water vapor transport. This evolution in water vapor facilitates an upshear propagation of convection, and the diabatically influenced divergent outflow erodes the PV streamer aloft by negative advection of PV by the divergent wind. This result suggests that the PV streamer plays a secondary role in TC development, with the structure and intensity of the synoptic-scale anticyclone north of the incipient vortex playing a primary role.

Stationary cavitation bubbles forming on a delta wing vortex

Vortex cavitation forming in the leading-edge vortices of a delta wing was examined to determine how the individual cavitation bubbles incepted, grew, interacted with the underlying vortical flow and produced acoustic tones. The non-cavitating vortical flow over the delta wing was chosen to be similar to those previously reported in the literature. It was found that vortex breakdown was unaffected by the presence of incipient and developed vortex cavitation bubbles in the vortex core. While some cavitation bubbles incepted, grew, and collapsed relatively quickly, others reached an equilibrium position wherein the bubble tip was stationary in the laboratory frame at a particular location along the vortex axis. For a given attack angle, the equilibrium location moved upstream with a reduction in free stream cavitation number. It is shown that the existence of these stationary vortex bubbles is possible when there is a balance between the axial growth of the bubble along the vortex axis and the opposite motion of the axial jetting flow in the vortex core, and only a single equilibrium position is possible along the axially evolving vortex for a given free stream cavitation number. These transient and stationary vortex bubbles emit significant cavitation noise upon inception, growth, and collapse. The spectral content of the noise produced was expected to be related to the interaction of the bubble with the surrounding vortical flow in a manner similar to that reported in previous studies, where sustained tones were similar to the underlying vortex frequency. However, in the present study, the dominant frequency and higher harmonics of the tones occur at a higher frequency than that of the underlying vortex. Hence, it is likely that the highly elongated stationary bubbles have higher-order volume oscillations compared to the two-dimensional radial mode of the vortex cores of vortex cavitation bubbles with much smaller diameter-to-length ratios.

An examination of two pathways to tropical cyclogenesis occurring in idealized simulations with a cloud-resolving numerical model

Abstract. Simulations are conducted with a cloud-resolving numerical model to examine the transformation of a weak incipient mid-level cyclonic vortex into a tropical cyclone. Results demonstrate that two distinct pathways are possible and that development along a particular pathway is sensitive to model physics and initial conditions. One pathway involves a steady increase of the surface winds to tropical cyclone strength as the radius of maximum winds gradually decreases. A notable feature of this evolution is the creation of small-scale lower tropospheric cyclonic vorticity anomalies by deep convective towers and subsequent merger and convergence by the low-level secondary circulation. The second pathway also begins with a strengthening low-level circulation, but eventually a significantly stronger mid-level circulation develops. Cyclogenesis occurs subsequently when a small-scale surface concentrated vortex forms abruptly near the center of the larger-scale circulation. The small-scale vortex is warm core throughout the troposphere and results in a fall in local surface pressure of a few millibars. It usually develops rapidly, undergoing a modest growth to form a small tropical cyclone. Many of the simulated systems approach or reach tropical cyclone strength prior to development of a prominent mid-level vortex so that the subsequent formation of a strong small-scale surface concentrated vortex in these cases could be considered intensification rather than genesis. Experiments are performed to investigate the dependence on the inclusion of the ice phase, radiation, the size and strength of the incipient mid-level vortex, the amount of moisture present in the initial vortex, and the sea surface temperature. Notably, as the sea surface temperature is raised, the likelihood of development along the second pathway is increased. This appears to be related to an increased production of ice. The sensitivity of the pathway taken to model physics and initial conditions revealed by these experiments raise the possibility that the solution to this initial value problem is near a bifurcation point. Future improvements to model parameterizations and more accurate observations of the transformation of disturbances to tropical cyclones should clarify the conditions that favor a particular pathway when starting from a mid-level vortex.