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Tong Zhu and Da-Lin Zhang

Abstract

In this study, the effects of various cloud microphysics processes on the hurricane intensity, precipitation, and inner-core structures are examined with a series of 5-day explicit simulations of Hurricane Bonnie (1998), using the results presented in Part I as a control run. It is found that varying cloud microphysics processes produces little sensitivity in hurricane track, except for very weak and shallow storms, but it produces pronounced departures in hurricane intensity and inner-core structures.

Specifically, removing ice microphysics produces the weakest (15-hPa underdeepening) and shallowest storm with widespread cloud water but little rainwater in the upper troposphere. Removing graupel from the control run generates a weaker hurricane with a wider area of precipitation and more cloud coverage in the eyewall due to the enhanced horizontal advection of hydrometeors relative to the vertical fallouts (or increased water loading). Turning off the evaporation of cloud water and rainwater leads to the most rapid deepening storm (i.e., 90 hPa in 48 h) with the smallest radius but a wider eyewall and the strongest eyewall updrafts. The second strongest storm, but with the most amount of rainfall, is obtained when the melting effect is ignored. It is found that the cooling due to melting is more pronounced in the eyewall where more frozen hydrometeors, especially graupel, are available, whereas the evaporative cooling occurs more markedly when the storm environment is more unsaturated.

It is shown that stronger storms tend to show more compact eyewalls with heavier precipitation and more symmetric structures in the warm-cored eye and in the eyewall. It is also shown that although the eyewall replacement scenarios occur as the simulated storms move into weak-sheared environments, the associated inner-core structural changes, timing, and location differ markedly, depending on the hurricane intensity. That is, the eyewall convection in weak storms tends to diminish shortly after being encircled by an outer rainband, whereas both the cloud band and the inner eyewall in strong storms tend to merge to form a new eyewall with a larger radius. The results indicate the importance of the Bergeron processes, including the growth and rapid fallout of graupel in the eyewall, and the latent heat of fusion in determining the intensity and inner-core structures of hurricanes, and the vulnerability of weak storms to the influence of large-scale sheared flows in terms of track, inner-core structures, and intensity changes.

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Wei Zhong and Da-Lin Zhang

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In this study, the linearized, f-plane, shallow-water equations are discretized into a matrix eigenvalue problem to examine the full spectrum of free waves on barotropic (monopolar and hollow) vortices. A typical wave spectrum for weak vortices shows a continuous range between zero and an advective frequency associated with vortex Rossby waves (VRWs) and two discrete ranges at both sides associated with inertio-gravity waves (IGWs). However, when the vortex intensity reaches a critical value, higher-frequency waves will be “red shifted” into the continuous spectrum, while low-frequency waves will be “violet shifted” into the discrete spectrum, leading to the emergence of mixed vortex Rossby–inertio-gravity waves (VRIGWs).

Results show significant (little) radial wavelike structures of perturbation variables for IGWs (VRWs) with greater (much smaller) divergence than vorticity and the hybrid IGW–VRW radial structures with equal amplitudes of vorticity and divergence for mixed VRIGWs. In addition, VRWs only occur within a critical radius at which the perturbation azimuthal velocity is discontinuous. As the azimuthal wavenumber increases, lower-frequency waves tend to exhibit more mixed-wave characteristics, whereas higher-frequency waves will be more of the IGW type. Two-dimensional wave solutions show rapid outward energy dispersion of IGWs and slower dispersion of VRWs and mixed VRIGWs in the core region. These solutions are shown to resemble the previous analytical solutions, except for certain structural differences caused by the critical radius. It is concluded that mixed VRIGWs should be common in the eyewall and spiral rainbands of intense tropical cyclones. Some different wave behaviors associated with the monopolar and hollow vortices are also discussed.

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Wallace Hogsett and Da-Lin Zhang

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In this study, the life cycles of a series of four major mesoscale convective systems (MCSs) during genesis and their roles in transforming a vertically tilted vortex associated with a westerly wind burst (hereafter the WWB vortex) into Typhoon Chanchu (2006) are examined using 11-day cloud-resolving simulations presented in Part I. It is found that the tilted WWB vortex at early stages is characterized by an elevated cold-core layer (about 200 hPa thick) below and a weak warm column above with large vertical wind shear across the layer, which extends over a horizontal distance of about 450 km between the vortex’s 400- and 900-hPa centers. During the final two days of the genesis process, the upper-level warm column increases in depth and intensity as a result of the absorption of convectively generated vortices (CGVs), including a mesoscale convective vortex (MCV), causing more rapid amplification of cyclonic vorticity in the middle than the lower troposphere. The commencement of sustained intensification of Chanchu occurs when the upper-level warm column is vertically aligned with the surface-based warm-core vortex.

Results show that four unique MCSs develop in succession on the downtilt-right side of the WWB vortex. The first MCS develops as a squall line with trailing stratiform precipitation and an MCV; subsequent MCSs include a convective cluster whose shape changes from an inverted U to a question mark and finally a spiral rainband as the WWB vortex decreases its vertical tilt. Strong cold pools are favored behind the leading connective lines during the earlier tilted-vortex stages due primarily to dry intrusion by the midlevel sheared flows, whereas few cold downdrafts occur at later stages as the WWB vortex becomes more upright and sufficiently moist. The authors conclude that the roles of the MCSs during genesis are to (a) generate cyclonic vorticity and then store it mostly in the midtroposphere, after merging CGVs within the WWB vortex; (b) moisten the low- and midlevels; (c) enhance the northward displacement of the WWB vortex; and (d) reduce the vertical tilt of the WWB vortex.

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Hua Chen and Da-Lin Zhang

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Previous studies have focused mostly on the roles of environmental factors in the rapid intensification (RI) of tropical cyclones (TCs) because of the lack of high-resolution data in inner-core regions. In this study, the RI of TCs is examined by analyzing the relationship between an upper-level warm core, convective bursts (CBs), sea surface temperature (SST), and surface pressure falls from 72-h cloud-permitting predictions of Hurricane Wilma (2005) with the finest grid size of 1 km. Results show that both the upper-level inertial stability increases and static stability decreases sharply 2–3 h prior to RI, and that the formation of an upper-level warm core, from the subsidence of stratospheric air associated with the detrainment of CBs, coincides with the onset of RI. It is found that the development of CBs precedes RI, but most subsidence warming radiates away by gravity waves and storm-relative flows. In contrast, many fewer CBs occur during RI, but more subsidence warming contributes to the balanced upper-level cyclonic circulation in the warm-core (as intense as 20°C) region. Furthermore, considerable CB activity can still take place in the outer eyewall as the storm weakens during its eyewall replacement. A sensitivity simulation, in which SSTs are reduced by 1°C, shows pronounced reductions in the upper-level warm-core intensity and CB activity. It is concluded that significant CB activity in the inner-core regions is an important ingredient in generating the upper-level warm core that is hydrostatically more efficient for the RI of TCs, given all of the other favorable environmental conditions.

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Xingbao Wang and Da-Lin Zhang

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Because of the lack of three-dimensional (3D) high-resolution data and the existence of highly nonelliptic flows, few studies have been conducted to investigate the inner-core quasi-balanced characteristics of hurricanes. In this study, a potential vorticity (PV) inversion system is developed, which includes the nonconservative processes of friction, diabatic heating, and water loading. It requires hurricane flows to be statically and inertially stable but allows for the presence of small negative PV. To facilitate the PV inversion with the nonlinear balance (NLB) equation, hurricane flows are decomposed into an axisymmetric, gradient-balanced reference state and asymmetric perturbations. Meanwhile, the nonellipticity of the NLB equation is circumvented by multiplying a small parameter ε and combining it with the PV equation, which effectively reduces the influence of anticyclonic vorticity. A quasi-balanced ω equation in pseudoheight coordinates is derived, which includes the effects of friction and diabatic heating as well as differential vorticity advection and the Laplacians of thermal advection by both nondivergent and divergent winds.

This quasi-balanced PV–ω inversion system is tested with an explicit simulation of Hurricane Andrew (1992) with the finest grid size of 6 km. It is shown that (a) the PV–ω inversion system could recover almost all typical features in a hurricane, and (b) a sizeable portion of the 3D hurricane flows are quasi-balanced, such as the intense rotational winds, organized eyewall updrafts and subsidence in the eye, cyclonic inflow in the boundary layer, and upper-level anticyclonic outflow. It is found, however, that the boundary layer cyclonic inflow and upper-level anticyclonic outflow also contain significant unbalanced components. In particular, a low-level outflow jet near the top of the boundary layer is found to be highly unbalanced (and supergradient). These findings are supported by both locally calculated momentum budgets and globally inverted winds. The results indicate that this PV inversion system could be utilized as a tool to separate the unbalanced from quasi-balanced flows for studies of balanced dynamics and propagating inertial gravity waves in hurricane vortices.

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Wallace Hogsett and Da-Lin Zhang

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Although previous studies have shown the relationship between the Madden–Julian oscillation (MJO) and tropical cyclogenesis (TCG), many scale-interactive processes leading to TCG still remain mysterious. In this study, the larger-scale flow structures and evolution during the pregenesis, genesis, and intensification of Typhoon Chanchu (2006) near the equator are analyzed using NCEP’s final analysis, satellite observations, and 11-day nested numerical simulations with the Advanced Research Weather Research and Forecast model (ARW-WRF). Results show that the model could reproduce the structures and evolution of a synoptic westerly wind burst (WWB) associated with the MJO during the genesis of Chanchu, including the eastward progression of a WWB from the Indian Ocean into the Pacific Ocean, the modulation of the associated quasi-symmetric vortices, the initial slow spinup of a northern (pre-Chanchu) disturbance at the northeastern periphery of the WWB, and its general track and intensification.

It is found that the MJO, likely together with a convectively coupled Kelvin wave, provides the necessary low-level convergence and rotation for the development of the pre-Chanchu disturbance, particularly through the eastward-propagating WWB. The incipient vortex evolves slowly westward, like a mixed Rossby–gravity wave, on the northern flank of the WWB, exhibits a vertically westward-tilted circulation structure, and eventually moves northward off of the equator. Results show that the interaction of the tilted vortex with moist easterly flows assists in the downtilt-right (i.e., to the right of the upward tilt) organization of deep convection, which in turn forces the tilted vortex to move toward the area of ongoing deep convection, thereby helping to partly decrease the vertical tilt with time. It is shown that despite several days of continuous convective overturning, sustained surface intensification does not commence until the vortex becomes upright in the vertical. A conceptual model is finally presented, relating the decreasing vortex tilt to convective development, storm movement, TCG, and surface intensification.

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Da-Lin Zhang and Richard Harvey

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Considerable progress has been made in the past decades on understanding the life cycle of rapidly deepening winter cyclones. However, little attention has been paid to the role that mesoscale convective systems (MCSs) play during extratropical cyclogenesis within weak baroclinic environments. In this study, the impact of an MCS on the subsequent surface cyclogenesis is investigated by extending the previously documented 21-h simulation of the 10–11 June 1985 PRE-STORM squall line to 36 hours. The model reproduces the meteorological events from the initiation to the dissipation of the squall system and then to the formation of a surface cyclone and the amplification of midlevel baroclinic waves, as verified against all available observations.

It is found that the squall line is initiated ahead of a weak surface cold front with the aid of baroclinic forcing. Once initiated, however, the prefrontal squall system is primarily driven by the interaction of convectively generated circulations with a conditionally unstable environment. As it rapidly intensifies and accelerates east-ward, the squall system amplifies a midlevel short wave by warming the upper troposphere and cooling the lower troposphere, and then forces it to move with the system. On the other hand, the movement of a low to midlevel thermal wave is primarily determined by adiabatic processes. Thus, the convective system tends to enhance the larger-scale baroclinicity and increase the phase lag between the pressure and thermal waves such that the baroclinic environment becomes more favorable for the subsequent surface cyclogenesis.

The role of moist convection in the surface cyclogenesis is examined by comparing simulations with and without the convective system. It is found that, in the absence of moist convection, the model also produces a surface cyclone, but with much weaker intensity, much smaller horizontal extent, and much slower displacement. The relationships of convectively generated mesovortices and wake lows to the surface cyclogenesis are also examined.

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Wallace Hogsett and Da-Lin Zhang

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Despite considerable research on tropical cyclones (TCs), few studies have been performed to examine inner-core energy conversions because of the lack of high-resolution data. In this study, the TC energetic characteristics in relation to intensity and structural changes under different sheared environments are investigated using a 5-day cloud-resolving simulation of Hurricane Bonnie (1998). Results show that in the presence of intense vertical shear Bonnie undergoes high-frequency fluctuations in intensity and energy conversions (at a time scale of 3 h) during the partial eyewall stage. The fluctuations are closely related to the life cycle of individual convective elements that propagate cyclonically around the downshear portion of the eyewall. The energy conversions are shown to be maximized in the vicinity of the radius of maximum wind (RMW), thus affecting strongly TC intensity. On average, about 2% of latent energy can be converted to kinetic energy to increase TC intensity. After the vertical shear subsides below a threshold, intensity fluctuations become small as convective elements reorganize into an axisymmetric eyewall in which energy conversions are more evenly distributed.

Fourier decomposition is conducted to separate the wavenumber-0, -1, and -2 components of inner-core energetics. Whereas wavenumber-1 perturbations dominate the partial eyewall stage, the propagation of wavenumber-2 perturbations is shown to be closely related to individual convective elements during both the partial eyewall and axisymmetric stages. The wavenumber-2 perturbations can be traced as they move around the eyewall in the form of vortex–Rossby waves, and they play a role in determining the large intensity fluctuations during the partial eyewall stage and the formation of an outer eyewall to replace the partial inner eyewall at the later stage.

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Chanh Q. Kieu and Da-Lin Zhang

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This comment presents some concerns with the study of Stern et al. and their misinterpretation of the contraction of the radius of the maximum wind (RMW) in tropical cyclones. It is shown that their geometrical RMW contraction model provides little dynamical understanding of the RMW contraction during tropical cyclone intensification, and it differs fundamentally from the RMW contraction model of Willoughby et al. that was derived from the directional derivative concept. Moreover, it is demonstrated that Stern et al. were mistaken in commenting on the derivation of the governing equation for the RMW contraction in Kieu.

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Jung Hoon Shin and Da-Lin Zhang

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This study examines the relative roles of moist frontogenesis and tropopause undulation in determining the intensity, size, and structural changes of Hurricane Sandy using a high-resolution cloud-resolving model. A 138-h simulation reproduces Sandy’s four distinct development stages: (i) rapid intensification, (ii) weakening, (iii) steady maximum surface wind but with large continued sea level pressure (SLP) falls, and (iv) reintensification. Results show typical correlations between intensity changes, sea surface temperature, and vertical wind shear during the first two stages. The large SLP falls during the last two stages are mostly caused by Sandy’s northward movement into lower-tropopause regions associated with an eastward-propagating midlatitude trough, where the associated lower-stratospheric warm air wraps into the storm and its surrounding areas. The steady maximum surface wind occurs because of the widespread SLP falls with weak gradients lacking significant inward advection of absolute angular momentum (AAM). Meanwhile, three spiral frontogenetic zones and associated rainbands develop internally in the outer northeastern quadrant during the last three stages when Sandy’s southeasterly warm current converges with an easterly cold current associated with an eastern Canadian high. Cyclonic inward advection of AAM along each frontal rainband accounts for the continued expansion of the tropical storm–force wind and structural changes, while deep convection in the eyewall and merging of the final two surviving frontogenetic zones generate a spiraling jet in Sandy’s northwestern quadrant, leading to its reintensification prior to landfall. The authors conclude that a series of moist frontogenesis plays a more important role than the lower-stratospheric warmth in determining Sandy’s size, intensity, and structural changes.

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