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Xingbao Wang and Da-Lin Zhang

Abstract

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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Da-Lin Zhang and Ning Bao

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The genesis of intense cyclonic vorticity in the boundary layer and the transformation of a low-level cold pool to a warm-core anomaly associated with the long-lived mesoscale convective systems (MCSs), which produced the July 1977 Johnstown flash flood and later developed into a tropical storm, are examined using a 90-h real-data simulation of the evolution from a continental MCS/vortex to an oceanic cyclone/storm system. It is shown that the midlevel vortex/trough at the end of the continental MCS's life cycle is characterized by a warm anomaly above and a cold anomaly below. The mesovortex, as it drifts toward the warm Gulf Stream water, plays an important role in initiating and organizing a new MCS and a cyclonic (shear) vorticity band at the southern periphery of the previously dissipated MCS. It is found from the vorticity budget that the vorticity band is amplified through stretching of absolute vorticity as it is wrapped around in a slantwise manner toward the cyclone center. Then, the associated shear vorticity is converted to curvature vorticity near the center, leading to the formation of a “comma-shaped” vortex and the rapid spinup of the surface cyclone to tropical storm intensity.

Thermodynamic budgets reveal that the vertical transfer of surface fluxes from the warm ocean and the convectively induced grid-scale transport are responsible for the development of a high-θe tongue, which is wrapped around in a fashion similar to the vorticity band, causing conditional instability and further organization of the convective storm. Because the genesis occurs at the southern periphery of the vortex/trough, the intensifying cyclonic circulation tends to advect the pertinent cold air in the north-to-northwesterly flow into the convective storm and the ambient warmer air into the cyclone center, thereby transforming the low-level cold anomaly to a warm-cored structure near the cyclone core. It is shown that the transformation and the evolution of the surface cyclone are mainly driven by the low-level vorticity concentrations.

It is found that many of the cyclogenesis scenarios in the present case are similar to those noted in previous tropical cyclogenesis studies and observed at the early stages of tropical cyclogenesis from MCSs during the Tropical Experiment in Mexico. Therefore, the results have significant implications with regard to tropical cyclogenesis from MCSs.

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Wallace Hogsett and Da-Lin Zhang

Abstract

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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Da-Lin Zhang and Ning Bao

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Recent observations have revealed that some mesoscale convective systems (MCSs) could undergo multiple cycles of convective development and dissipation, and, under certain environments, they appeared to be responsible for (barotropic) oceanic or tropical cyclogenesis. In this study, oceanic cyclogenesis, as induced by an MCS moving offshore and then driven by deep convection in a near-barotropic environment, is investigated by extending to 90 h the previously documented 18-h simulation of the MCSs that were responsible for the July 1977 Johnstown flash flood. It is demonstrated that the mesoscale model can reproduce very well much of the meso-β-scale structures and evolution of the long-lived MCS out to 90 h. These include the development and dissipation of the continental MCSs as well as the associated surface and tropospheric perturbations, the timing and location in the initiation of a new MCS after 36 h and in the genesis of a surface mesolow over the warm Gulf Stream water after 60-h integration, the track and the deepening of the surface cyclone into a “tropical storm,” the maintenance of a midlevel mesovortex/trough system, and the propagation of a large-scale cold front with respect to the surface cyclone.

It is found that the new MCS is triggered after the vortex/trough moved offshore and interacted with the land-ocean thermal contrasts during the afternoon hours. The oceanic cyclogenesis begins at 150–180 km to the south of the vortex, as the associated surface trough advances into the Gulf Stream and weakens. Then, the cyclone overpowers quickly the low-level portion of the vortex circulation and deepens 14 hPa in 24 h. A comparison with a dry sensitivity simulation shows that the cyclogenesis occurs entirely as a consequence of the convective forcing. Without it, an 84-h simulation produces only a surface trough with the minimum pressure being nearly the same as that left behind by the previous MCSs. It is shown that the vortex/trough provides persistent convergence at its southern periphery for the continued convective development, whereas the convectively enhanced low-level flow tends to (i) “pump” up sensible and latent heat fluxes from the warm ocean surface and (ii) transport the high-θe air in a slantwise fashion into the region having lower θe aloft, thereby causing further conditional instability, increased convection, and more rapid deepening. The interactions of the continental MCS/vortex and the oceanic cyclone/storm systems with their larger-scale environments are also discussed.

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Nannan Qin and Da-Lin Zhang

Abstract

Hurricane Patricia (2015) broke records in both peak intensity and rapid intensification (RI) rate over the eastern Pacific basin. All of the then-operational models predicted less than half of its extraordinary intensity and RI rate, leaving a challenge for numerical modeling studies. In this study, a successful 42-h simulation of Patricia is obtained using a quintuply nested-grid version of the Weather Research and Forecast (WRF) Model with the finest grid size of 333 m. Results show that the WRF Model, initialized with the Global Forecast System Final Analysis data only, could reproduce the track, peak intensity, and many inner-core features, as verified against various observations. In particular, its simulated maximum surface wind of 92 m s−1 is close to the observed 95 m s−1, capturing the unprecedented RI rate of 54 m s−1 (24 h)−1. In addition, the model reproduces an intense warm-cored eye, a small-sized eyewall with a radius of maximum wind of less than 10 km, and the distribution of narrow spiral rainbands. A series of sensitivity simulations is performed to help understand which model configurations are essential to reproducing the extraordinary intensity of the storm. Results reveal that Patricia’s extraordinary development and its many inner-core structures could be reasonably well simulated if ultrahigh horizontal resolution, appropriate model physics, and realistic initial vortex intensity are incorporated. It is concluded that the large-scale conditions (e.g., warm sea surface temperature, weak vertical wind shear, and the moist intertropical convergence zone) and convective organization play important roles in determining the predictability of Patricia’s extraordinary RI and peak intensity.

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Da-Lin Zhang and Eric Altshuler

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The effects of dissipative heating on hurricane intensity are examined using a 72-h explicit simulation of Hurricane Andrew (1992) with a state-of-the-art, three-dimensional, nonhydrostatic mesoscale (cloud resolving) model (i.e., MM5). It is found that the inclusion of dissipative heating increases the central pressure deficit of the storm by 5–7 hPa and its maximum surface wind by about 10% prior to landfall. It is shown that dissipative heating tends to warm the surface layer, causing a decrease (increase) in sensible heat flux at the sea surface (the top of the surface layer) that acts to cool the surface layer, although the net (sensible plus dissipative) heating rates are still 30%–40% greater than the sensible heating rates in the control simulation. Finally, the potential effects of energy transfer into the ocean, sea surface temperature changes within the inner core, and evaporation of sea spray, interacting with dissipative heating, on hurricane intensity are discussed.

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William Miller and Da-Lin Zhang

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Hurricane Joaquin (2015) took a climatologically unusual track southwestward into the Bahamas before recurving sharply out to sea. Several operational forecast models, including the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS), struggled to maintain the southwest motion in their early cycles and instead forecast the storm to turn west and then northwest, striking the U.S. coast. Early cycle GFS track errors are diagnosed using a tropical cyclone (TC) motion error budget equation and found to result from the model 1) not maintaining a sufficiently strong mid- to upper-level ridge northwest of Joaquin, and 2) generating a shallow vortex that did not interact strongly with upper-level northeasterly steering winds. High-resolution model simulations are used to test the sensitivity of Joaquin’s track forecast to both error sources. A control (CTL) simulation, initialized with an analysis generated from cycled hybrid data assimilation, successfully reproduces Joaquin’s observed rapid intensification and southwestward-looping track. A comparison of CTL with sensitivity runs from perturbed analyses confirms that a sufficiently strong mid- to upper-level ridge northwest of Joaquin and a vortex deep enough to interact with northeasterly flows associated with this ridge are both necessary for steering Joaquin southwestward. Contraction and vertical alignment of the CTL vortex during the early forecast period may have also helped draw the low-level vortex center southward. The results indicate that for TCs developing in vertically sheared environments, improved inner-core sampling by means of cloudy radiances and aircraft reconnaissance missions may help reduce track forecast errors by improving the model estimate of vortex depth.

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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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Rudi Xia and Da-Lin Zhang

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This study examines the synoptic- and mesoscale processes leading to the generation of three extreme rainfall episodes with hourly rates of greater than 100 mm h−1 over the southern, middle, and northern portions of the eastern foothills of Mt. Taihang in North China on 19–20 July 2016. The extreme rainfall episodes took place over the 200–600-m elevation zones in the southern and northern portions but also over the lower elevations in the middle portion of the target region, sequentially during late morning, early evening, and midnight hours. Echo training accounted for the development of a linear convective system in the southern region after the warm and moist air carried by a southeasterly low-level jet (LLJ) was lifted to condensation as moving across Mt. Yuntai. In contrast, two isolated circular-shaped convective clusters, with more robust convective cores in its leading segment, developed in the northern region through steep topographical lifting of moist northeasterly airflow, albeit conditionally less unstable. Extreme rainfall in the middle region developed from the convergence of a moist easterly LLJ with a northerly colder airflow associated with an extratropical cyclogenesis. Results reveal that the LLJs and associated moisture transport, the intensifying cyclone interacting with a southwest vortex and its subsequent northeastward movement, and the slope and orientation of local topography with respect to and the stability of the approaching airflows played different roles in determining the timing and location, the extreme rainfall rates, and convective organizations along the eastern foothills of Mt. Taihang.

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William Miller and Da-Lin Zhang

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When computing trajectories from model output, gridded winds are often temporally interpolated to a time step shorter than model output intervals to satisfy computational stability constraints. This study investigates whether trajectory accuracy may be improved for tropical cyclone (TC) applications by interpolating the model winds using advection correction (AC) instead of the traditional linear interpolation in time (LI) method. Originally developed for Doppler radar processing, AC algorithms interpolate data in a reference frame that moves with the pattern translation, or advective flow velocity. A previously developed trajectory AC implementation is modified here by extending it to three-dimensional (3D) flows, and the advective flows are defined in cylindrical rather than Cartesian coordinates. This AC algorithm is tested on two model-simulated TC cases, Hurricanes Joaquin (2015) and Wilma (2005). Several variations of the AC algorithm are compared to LI on a sample of 10 201 backward trajectories computed from the modeled 5-min output data, using reference trajectories computed from 1-min output to quantify position errors. Results show that AC of 3D wind vectors using advective flows defined as local gridpoint averages improves the accuracy of most trajectories, with more substantial improvements being found in the inner eyewall where the horizontal flows are dominated by rotating cyclonic wind perturbations. Furthermore, AC eliminates oscillations in vertical velocity along LI backward trajectories run through deep convective updrafts, leading to a ~2.5-km correction in parcel height after 20 min of integration.

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