Intensification of tropical-cyclone-like vortices in uniform zonal background-flows

Dengler, Klaus; Keyser, Daniel

doi:10.1256/smsqj.56307

Cited by 4 publications

(4 citation statements)

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“…Previous studies noted uniform environmental flows around TCs and their influences on structure and intensity changes. The interactions between the TC and its uniform environmental flow reduced the vortex intensity by introducing vortex asymmetry resulting from surface friction, asymmetric moisture convergence and surface flux, and vertical motion (Dengler and Keyser 2000;Peng et al 1999;Kepert 2001;Kwok and Chan 2005). However, Cyclone Sidr moved through a heterogeneous environment; air to the northwest of the TC was cool and dry, whereas that to the southeast was warm and moist.…”

Section: Differences From Other Tcsmentioning

confidence: 99%

Numerical Simulation of Cyclone Sidr Using a Cloud-Resolving Model: Characteristics and Formation Process of an Outer Rainband

Akter

Tsuboki

2012

Monthly Weather Review

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Cyclone Sidr, one of the most devastating tropical cyclones that resulted in several thousand deaths and substantial damages, developed in the north Indian Ocean and made landfall over the Bangladesh coast on 15 November 2007. Observation and simulation results show that Sidr was embedded in a nonuniform environment and contained an intense outer rainband to the east of its center and a significant frontal band to the northwest. A detailed study of the outer rainband is performed by numerical simulation.The eastern band was a long, quasi-straight shape in the meridional direction that remained stationary relative to the cyclone center. This band was composed of convective cells that developed southeast of the center within a synoptic-scale convergence zone and propagated along the band toward the northeast quadrant. The speed of the downwind-propagating cells was greater than that of the cyclone, which resulted in a convective cluster northeast of the center. Only the downwind portion of the band consisted of convection with stratiform rain, whereas the upwind and middle portions contained active convective cells without stratiform rain.The band was located between the synoptic-scale flows of a weakly sheared, gradient-balanced westerly and a strongly sheared, nongradient-balanced prevailing southerly caused by the complex terrain of the Bay of Bengal's southeast region. Low-level convergence along the band was dominated by cross-band flow from both sides of the band and was confined below 3 km. As the cyclone moved northward, the convergence zone resulted in the extension of band length up to ;800 km. The southerly at the eastern side of the center gradually accelerated and was directed toward the center by a strong pressure gradient force. The flow accumulated a substantial amount of water vapor from the sea in addition to the increased moisture in the lower troposphere, resulting in further intensification of the convective cells.

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Section: Differences From Other Tcsmentioning

confidence: 99%

Numerical Simulation of Cyclone Sidr Using a Cloud-Resolving Model: Characteristics and Formation Process of an Outer Rainband

Akter

Tsuboki

2012

Monthly Weather Review

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“…Peng et al (1999) indicated that an out-of-phase condition between the asymmetric surface entropy flux and boundary layer moisture convergence could explain the intensity reduction. Dengler and Keyser (2000) found that, in their three-layer model, it was the penetration of stable dry air from midlevels into the regions of boundary layer convergence that reduced the intensity of the storm embedded in uniform environmental flows. Recently Wu and Braun (2004) suggested that the inhibiting effect of environmental flow is closely associated with the resulting eddy momentum flux, which tends to decelerate tangential and radial winds in both inflow and outflow layers.…”

Section: B Dynamical Controlmentioning

confidence: 99%

Environmental Dynamical Control of Tropical Cyclone Intensity—An Observational Study

Zeng

Wang

2007

Monthly Weather Review

133

118

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The effects of two environmental dynamical factors, namely, the transitional speed and vertical wind shear, on tropical cyclone (TC) intensification, intensity, and lifetime peak intensity were analyzed based on observations in the western North Pacific during 1981–2003. In general, both the fast translation and strong vertical shear are negative to both TC intensification and the lifetime peak intensity. Both the very intense TCs and the TCs with rapid intensification rate are found only to occur in a narrow range of translational speeds between 3 and 8 m s−1, and in relatively weak vertical shear. The overwhelming majority of western North Pacific TCs reach their lifetime peak intensity just prior to recurvature where their environmental steering flow and vertical shear are both weak. The results show that few TCs intensified when they moved faster than 15 m s−1, or when their large-scale environmental vertical shear is larger than 20 m s−1. The intensification rate of TCs is found to increase with decreasing vertical shear while the majority of the weakening storms experience relatively strong vertical shear. Overall, strong vertical shear prohibits rapid intensification and most likely results in the weakening of TCs, similar to the fast storm translation. Based on the statistical analysis, a new empirical maximum potential intensity (MPI) has been developed, which includes the combined negative effect of translational speed and vertical shear as the environmental dynamical control in addition to the positive contribution of SST and the outflow temperature as the thermodynamic control. The new empirical MPI can not only provide more accurate estimation of TC maximum intensity but also better explain the observed behavior of the TC maximum intensity and help explain the thermodynamic and environmental dynamical controls of TC intensity. Implications of the new empirical MPI are discussed.

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“…Previous studies have focused mainly on the effect of a specified large-scale environmental flow and suggested that asymmetric structure generated by environmental flow be a dynamical limiting factor to TC intensity at mature stage (Peng et al 1999;Wu and Cheng 1999;Dengler and Keyser 2000;Frank and Ritchie 2001). However, a clear determination of the mechanism by which the asymmetric structure affects TC intensity remains elusive.…”

Section: Introductionmentioning

confidence: 99%

The Effect of Internally Generated Inner-Core Asymmetries on Tropical Cyclone Potential Intensity*

Yang

Wang

2007

Journal of the Atmospheric Sciences

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In a quiescent environment on an f plane, the internal dynamic processes of a tropical cyclone (TC) can generate axially asymmetric circulations (asymmetries) in its inner-core region. The present study investigates how these inner-core asymmetries affect TC intensity. For this purpose, a three-dimensional (3D) TC model and its axisymmetric (2D) version were used. Both have identical model vertical structure and use the same set of parameters and the same initial conditions. The differences between the two model runs are considered to be due to mainly the effects of the TC asymmetries. The results show that the presence of asymmetries in the 3D run reduces the TC final intensity by about 15% compared with the 2D run, suggesting that the TC asymmetry is a limiting factor to the potential intensity (PI).In the 2D run without asymmetries, the convective heating in the eyewall generates an annular tower of high potential vorticity (PV) with relatively low PV in the eye. The eyewall tilts outward with height significantly. Underneath the tilted eyewall the downdrafts induced by evaporation of rain and melting of snow and graupel make the subcloud-layer inflow dry and cool, which lowers the boundary layer equivalent potential temperature ( e ), thus increasing the entropy difference between the air and sea in the vicinity of the radius of maximum wind (RMW). The increased air-sea entropy deficit leads to more energy input into TC from the underlying ocean and thus a greater final intensity. On the other hand, in the 3D run, the model-resolved asymmetric eddies, which are characterized by the vortex Rossby waves in the mid-lower troposphere, play important roles in modifying the symmetric structure of the TC. Potential vorticity and e budgets indicate that significant inward PV mixing from the eyewall into the eye results in a less-tilted eyewall, which in turn limits the drying and cooling effects of downdrafts in the subcloud layer and reduces the air-sea entropy deficit under the eyewall, thereby reducing the TC intensity. The angular momentum budget analysis shows that the asymmetric eddies tend to reduce the strength of the primary circulation in the vicinity of the RMW. This eddy contribution to the azimuthal mean angular momentum budget is larger than the parameterized horizontal diffusion contribution in the 3D run, suggesting an overall diffusive effect of the asymmetric eddies on the symmetric circulation.

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Intensification of tropical-cyclone-like vortices in uniform zonal background-flows

Cited by 4 publications

References 0 publications

Numerical Simulation of Cyclone Sidr Using a Cloud-Resolving Model: Characteristics and Formation Process of an Outer Rainband

Numerical Simulation of Cyclone Sidr Using a Cloud-Resolving Model: Characteristics and Formation Process of an Outer Rainband

Environmental Dynamical Control of Tropical Cyclone Intensity—An Observational Study

The Effect of Internally Generated Inner-Core Asymmetries on Tropical Cyclone Potential Intensity*

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