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Juan Fang and Fuqing Zhang

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Through observational analysis and numerical simulations, this study examines the roles of the Madden–Julian oscillation (MJO) and tropical waves in the three-stage formation of Supertyphoon Megi (2010) including 1) convective bursts followed by vorticity aggregation, 2) vortex rearrangement during decaying convection, and 3) convective reinvigoration and vortex intensification. The MJO was responsible for preconditioning the large-scale circulation and low-level moisture favorable for convection during all stages, while the counterpropagating Kelvin and equatorial Rossby (ER) waves brought low-level convergence and cyclonic vorticity anomalies to enhance massive convection in the western tropical Pacific in stage 1. Convection strengthened the vorticity anomalies nearby, which subsequently developed into Megi’s embryo by the end of stage 1 through merging with the positive vorticity anomaly carried by a westward-propagating mixed Rossby–gravity and tropical depression (MRG–TD)-type wave. The ER- and MRG–TD-type waves might also contribute to Megi’s formation through increasing low-level southwesterlies to the southwest of the precursor during stages 2 and 3. These tropical waves also indirectly affect Megi’s genesis through modulating surroundings near the precursor. Without the MJO, the low-level vorticity anomaly to the near west of the precursor would intensify more effectively and develop into a tropical cyclone instead of the observed Megi. Removing the Kelvin or ER wave would enhance convection to the far west of Megi’s precursor, which was less favorable for low-level convergence in the region of the precursor, and thus the formation of Megi.

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Juan Fang and Fuqing Zhang

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As a follow-up to a previously published article on the initial development and genesis of Hurricane Dolly (2008), this study further examines the evolution of, and interactions among, multiscale vortices ranging from the system-scale main vortex (L > 150 km) to the intermediate-scale cloud clusters (50 km < L < 150 km) and individual vorticity-rich convective cells (L < 50 km). It is found that there are apparent self-similarities among these vortices at different scales, each of which may undergo several cycles of alternating accumulation and release of convective available potential energy. Enhanced surface fluxes below individual cyclonic vortices at each scale contribute to the sustainment and reinvigoration of moist convection that in turn contributes to the maintenance and upscale growth of these vortices.

Spectral analysis of horizontal divergence and relative vorticity further suggests that the cloud-cluster-scale and system-scale vortices are predominantly balanced while the individual convective vortices are largely unbalanced. The vorticity and energy produced by these individual vorticity-rich convective cells first saturate at convective scales that are subsequently transferred to larger scales. The sum of the diabatic heating released from these convective cells may be regarded as a persistent forcing on the quasi-balanced system-scale vortex. The secondary circulation induced by such forcing converges the cluster- and convective-scale vorticity anomalies into the storm center region. Convergence and projections of the smaller-scale vorticity to the larger scales eventually produce the spinup of the system-scale vortex. Meanwhile, convectively induced negative vorticity anomalies also converge toward the storm center, which are weaker and shorter lived, and thus are absorbed rather than expelled.

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Juan Fang and Fuqing Zhang

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Based on a successful cloud-resolving simulation with the Weather Research and Forecasting Model, this study examines key processes that led to the early development of Hurricane Dolly (2008). The initial development of Dolly consisted of three stages: (i) an initial burst of convection; (ii) stratiform development, dry intrusion, and thermodynamic recovery; and (iii) reinvigoration of moist convection and rapid intensification. Advanced diagnosis of the simulation—including the use of vorticity budget analysis, contour frequency analysis diagrams, and two-dimensional spectral decomposition and filtering—suggests that the genesis of Dolly is essentially a “bottom-up” process. The enhancement of the low-level vorticity is mainly ascribed to the stretching effect, which converges the ambient vorticity through stretching enhanced by moist convection. In the rapid intensification stage, smaller-scale positive vorticity anomalies resulting from moist convection are wrapped into the storm center area under the influence of background convergent flow. The convergence and accompanying aggregation of vorticity anomalies project the vorticity into larger scales and finally lead to the spinup of the system-scale vortex. On the other hand, although there is apparent stratiform development in the inner-core areas of incipient storm after the initial burst of convection, little evidence is found to support the genesis of Dolly through downward extension of the midlevel vorticity, a key process in the “top-down” thinking.

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Juan Fang and Fuqing Zhang

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Through cloud-resolving simulations, this study examines the effect of β on the evolution of tropical cyclones (TCs). It is found that the TC simulated on a β plane with variable Coriolis parameter f is weaker in intensity but larger in size and strength than the TC simulated on an f plane with constant f. Such differences result mainly from the effect of the β shear rather than from the variation of f due to the latitudinal change of the TC position, as illustrated in a three-stage conceptual model developed herein. The first stage begins with the establishment of the β shear and the emergence of asymmetries as the TC intensifies. The β shear peaks in value during the second stage that subsequently leads to the formation of an extensive stratiform region outside of the primary eyewall. The evaporative cooling associated with the stratiform precipitation acts to sharpen the low-level equivalent potential temperature gradient into a frontlike zone outside of the eyewall region, which leads to the burst of convection outside of the primary eyewall. The third stage is characterized by a weakening β shear and the corresponding TC vortex axisymmetrization and expansion. The convection on the inner edge of the stratiform region becomes more organized in the azimuthal direction and eventually causes the TC structure to evolve in a manner similar to the secondary eyewall formation and eyewall replacement usually observed in TCs. It is the active convection outside of the primary eyewall that contributes to a relatively weaker but larger TC on the β plane than that on the f plane.

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Juan Fang, Olivier Pauluis, and Fuqing Zhang

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An isentropic analysis technique is adopted in this study to investigate the intensification of Hurricane Edouard (2014) predicted by an experimental real-time convection-permitting hurricane analysis and forecast system. This technique separates the vertical mass transport in terms of equivalent potential temperature θ e for the rising air parcels at high entropy from the subsiding air at low entropy. It is found that as Edouard intensifies the vertical circulation becomes wider via the expansion of upward (downward) mass flux to higher (lower) θ e. In the early developing stages, the asymmetric convection dominates the vertical circulation and leads to a remarkable upward mass flux maximum center in the upper troposphere. When Edouard becomes intense, the axisymmetric convection becomes important to the upper-level vertical mass transport while the asymmetric convection still dominates the low-level vertical mass transport. Development of the warm core in the eye leads to double maxima along the θ e axis for both the isentropic-mean relative humidity and tangential velocity. The isentropic-mean properties such as the mid- to upper-level relative humidity, vertical velocity, and radial outflow decrease considerably while the mid- to upper-level vorticity enhances on the high-θ e side before the onset of rapid intensification. The isentropic analysis also reveals that as Edouard intensifies the eye characterized by warm and dry core first forms in the low to middle troposphere and then gradually expands upward. The abovementioned results indicate that the isentropic framework may have the advantages of binning common variables with θ e that could reflect the changes of the tropical cyclone structure in the inner-core region without a prior specification of the location of the storm center.

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Xiaomin Chen, Ming Xue, and Juan Fang

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The notable prelandfall rapid intensification (RI) of Typhoon Mujigae (2015) over abnormally warm water with moderate vertical wind shear (VWS) is investigated by performing a set of full-physics model simulations initialized with different sea surface temperatures (SSTs). While all experiments can reproduce RI, tropical cyclones (TCs) in cooler experiments initiate the RI 13 h later than those in warmer experiments. A comparison of structural changes preceding RI onset in two representative experiments with warmer and cooler SSTs (i.e., CTL and S1) indicates that both TCs undergo similar vertical alignment despite the moderate VWS. RI onset in CTL occurs ~8 h before the full vertical alignment, while that in S1 occurs ~5 h after. In both experiments precipitation becomes more symmetrically distributed around the vortex as vortex tilt decreases. In CTL, precipitation symmetricity is higher in the inner-core region, particularly for stratiform precipitation. All experiments indicate that RI onset occurs when the radius of maximum wind (RMW) contraction reaches a certain degree measured in terms of local Rossby number. The contraction occurs much earlier in CTL, leading to earlier RI. These results suggest that vertical alignment, albeit necessary, is not an effective RI indicator under different SSTs, while a more immediate cause of RI is the formation of a strong/compact inner core with high precipitation symmetry. Diagnoses using the Sawyer–Eliassen equation indicate that in CTL the enhanced microphysical diabatic heating of additional midlevel and deep convection along with surface friction contribute to stronger boundary layer inflow near/inside the RMW, facilitating earlier RMW contraction.

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Juan Fang, Olivier Pauluis, and Fuqing Zhang

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This study expands on a previous analysis of the intensification of Hurricane Edouard (2014) in the isentropic coordinates to further examine the thermodynamic processes that lead to the strengthening of the storm. Thermodynamic cycles are extracted using the methodology known as the Mean Airflow as Lagrangian Dynamics Approximation. The most intense thermodynamic cycle here is associated with the air rising within the hurricane eyewall. Its structure remains mostly steady during the early development of Edouard but evolves rapidly as the storm intensifies. Through intensification, the ascent shifts toward high values of entropy under the effect of enhanced surface heat fluxes and stronger surface winds, while reaching higher altitudes and lower temperatures. The near–rapid intensification onset of Edouard corresponds to an increase in the energy input into the cycle and an increase in the amount of kinetic energy generated. The external heating fluctuates considerably in the two low-level legs with a period of about 16–24 h, indicative of diurnal variation in the thermodynamic cycle. During the intensification of Edouard, the mechanical work production and the Carnot efficiency both increase dramatically, which can be attributed to the increase in energy transport and deepening of the thermodynamic cycle. In addition, there is a substantial increase of the mechanical work done during the horizontal expansion of air parcels near Earth’s surface, and a larger fraction of the kinetic energy generated is used to sustain and intensify the horizontal flow rather than to provide a vertical acceleration in the updrafts.

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Ke Peng, Richard Rotunno, George H. Bryan, and Juan Fang

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In a previous study, the authors showed that the intensification process of a numerically simulated axisymmetric tropical cyclone (TC) can be divided into two periods denoted by “phase I” and “phase II.” The intensification process in phase II can be qualitatively described by Emanuel’s intensification theory in which the angular momentum (M) and saturated entropy (s*) surfaces are congruent in the TC interior. During phase I, however, the M and s* surfaces evolve from nearly orthogonal to almost congruent, and thus, the intensifying simulated TC has a different physical character as compared to that found in phase II. The present work uses a numerical simulation to investigate the evolution of an axisymmetric TC during phase I. The present results show that sporadic, deep convective annular rings play an important role in the simulated axisymmetric TC evolution in phase I. The convergence in low-level radial (Ekman) inflow in the boundary layer of the TC vortex, together with the increase of near-surface s* produced by sea surface fluxes, leads to episodes of convective rings around the TC center. These convective rings transport larger values of s* and M from the lower troposphere upward to the tropopause; the locally large values of M associated with the convective rings cause a radially outward bias in the upper-level radial velocity and an inward bias in the low-level radial velocity. Through a repetition of this process, the pattern (i.e., phase II) gradually emerges. The role of internal gravity waves related to the episodes of convection and the TC intensification process during phase I is also discussed.

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Xiaomin Chen, Yuqing Wang, Juan Fang, and Ming Xue

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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.

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Xiaodong Tang, Zhe-Min Tan, Juan Fang, Y. Qiang Sun, and Fuqing Zhang

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The sensitivity of the secondary eyewall formation (SEF) of Hurricane Edouard (2014) to the diurnal solar insolation cycle is examined with convection-permitting simulations. A control run with a real diurnal radiation cycle and a sensitivity experiment without solar insolation are conducted. In the control run, there is an area of relatively weak convection between the outer rainbands and the primary eyewall, that is, a moat region. This area is highly sensitive to solar shortwave radiative heating, mostly in the mid- to upper levels in the daytime, which leads to a net stabilization effect and suppresses convective development. Moreover, the heated surface air weakens the wind-induced surface heat exchange (WISHE) feedback between the surface fluxes (that promote convection) and convective heating (that feeds into the secondary circulation and then the tangential wind). Consequently, a typical SEF with a clear moat follows. In the sensitivity experiment, in contrast, net radiative cooling leads to persistent active inner rainbands between the primary eyewall and outer rainbands, and these, along with the absence of the rapid filamentation zone, are detrimental to moat formation and thus to SEF. Sawyer–Eliassen diagnoses further suggest that the radiation-induced difference in diabatic heating is more important than the vortex wind structure for moat formation and SEF. These results suggest that the SEF is highly sensitive to solar insolation.

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