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Zonghui Huo, Da-Lin Zhang, and John R. Gyakum

Abstract

The relative importance of various potential vorticity (PV) perturbations and their mutual interactions associated with the superstorm of 12–14 March 1993 are investigated by applying a piecewise PV inversion diagnostic system to a 36-h simulation of the storm. It is shown that the contributions from all PV anomalies to the surface development increase with time, although their relative significance varies during the rapid deepening stage. In general, the upper-level dry PV anomalies contribute the most to the rapid deepening of the storm, followed, in order, by the lower-level thermal anomaly and latent heat release.

Comparing the PV anomalies and their inverted circulations reveals that there exists a favorable phase tilt between the upper- and lower-level anomalies that allows lower- and upper-level mutual interactions, in which the circulations associated with the upper-level PV anomalies enhance the lower-level anomalies and vice versa. In addition to the vertical interactions, lateral interactions are also present among the upper-level PV anomalies and the background flow. It is also found that the background flow advection dominates the vortex–vortex and vortex–background flow interactions in the deepening of the storm. The vortex–vortex interactions of the two upper-level positive PV anomalies cause the negative tilt of the main upper-level trough during the rapid deepening period.

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Marco L. Carrera, John R. Gyakum, and Da-Lin Zhang

Abstract

Secondary cyclogenesis has been identified as a difficult forecast challenge. In this paper, the authors examine the dominant physical processes associated with the predictability of a case of explosive secondary marine cyclogenesis and provide a better understanding of the large variability in the recent model-intercomparison simulations of the case. A series of sensitivity experiments, involving changes to the model initial conditions and physical parameterizations, is performed using the Canadian Mesoscale Compressible Community Model with a grid size of 50 km.

It is found that errors in the model initial conditions tend to decay with time, and more rapidly so in “dry” simulations. The model fails to produce the secondary cyclogenesis in the absence of latent heating. Water vapor budget calculations from the control experiment show that the surface moisture flux from 6 to 12 h is the largest contributor of water vapor to the budget area in the vicinity of the cyclone center, and remains an important moisture supply throughout the integration period. During the first 12 h, these fluxes are crucial in inducing grid-scale diabatic heating and destabilizing the lower troposphere, thereby facilitating the subsequent rapid deepening of the storm. A secondary maximum in surface latent heat flux to the north and east of the primary maximum acts to force the cyclogenesis event to the south and east of a coastal circulation center. When the surface evaporation is not allowed, much less precipitation is produced and the secondary cyclone fails to develop. Calculations of the potential temperature on the dynamic tropopause (i.e., 2-PVU surface) in the absence of surface evaporation indicate a significantly damped thermal wave when compared with the control integration.

This result for a case of secondary cyclogenesis differs from those generally found for large-scale extratropical cyclogenesis where upper-level baroclinic forcings tend to dominate, and motivates the need for better physical parameterizations, including the condensation and boundary layer processes, in operational models. The authors speculate that the different treatment of condensation and boundary layer processes may have been partly responsible for the enhanced variability in the simulation of this case in a recently completed international mesoscale model intercomparison experiment.

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Chanh Q. Kieu, Hua Chen, and Da-Lin Zhang

Abstract

In this study, the dynamical constraints underlining the pressure–wind relationship (PWR) for intense tropical cyclones (TCs) are examined with the particular focus on the physical connections between the maximum surface wind (VMAX) and the minimum sea level pressure (PMIN). Use of the Rankine vortex demonstrates that the frictional forcing in the planetary boundary layer (PBL) could explain a sizeable portion of the linear contributions of VMAX to pressure drops. This contribution becomes increasingly important for intense TCs with small eye sizes, in which the radial inflows in the PBL could no longer be neglected. Furthermore, the inclusion of the tangential wind tendency can make an additional contribution to the pressure drops when coupled with the surface friction.

An examination of the double-eyewall configuration reveals that the formation of an outer eyewall or well-organized spiral rainbands complicates the PWR. An analysis of a cloud-resolving simulation of Hurricane Wilma (2005) shows that the outer eyewall could result in the continuous deepening of PMIN even with a constant VMAX. The results presented here suggest that (i) the TC size should be coupled with VMAX rather than being treated as an independent predictor as in the current PWRs, (ii) the TC intensity change should be at least coupled linearly with the radius of VMAX, and (iii) the radial wind in the PBL is of equal importance to the linear contribution of VMAX and its impact should be included in the PWR.

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Jinfang Yin, Da-Lin Zhang, Yali Luo, and Ruoyun Ma

Abstract

In this study, a nocturnal extreme rainfall event induced by the urban heat island (UHI) effects of the coastal city of Guangzhou in South China on 7 May 2017 is examined using observational analyses and 18-h cloud-permitting simulations with the finest grid size of 1.33 km and the bottom boundary conditions nudged. Results show that the model reproduces convective initiation on Guangzhou’s downstream side (i.e., Huashan), where a shallow thermal mesolow is located, the subsequent back-building of convective cells as a larger-scale warm-moist southerly flow interacts with convectively generated cold outflows, and their eastward drifting and reorganization into a localized extreme-rain-producing storm near Jiulong under the influences of local orography. In particular, the model produces the maximum hourly, 3- and 12-hourly rainfall amounts of 146, 315, and 551 mm, respectively, at nearly the right location compared to their corresponding observed extreme amounts of 184, 382, and 542 mm. In addition, the model reproduces an intense meso-γ-scale vortex associated with the extreme-rain-producing Jiulong storm, as also captured by Doppler radar, with organized updrafts along cold outflow boundaries over a semicircle. A comparison of sensitivity and control simulations indicates that despite the occurrence of heavier rainfall amounts without the UHI effects than those without orography, the UHI effects appear to account directly for the convective initiation and heavy rainfall near Huashan, and indirectly for the subsequent formation of the Jiulong storm, while orography plays an important role in blocking cold outflows and enhancing cool pool strength for the sustained back-building of convective cells over the semicircle, thereby magnifying rainfall production near Jiulong.

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Da-Lin Zhang, Yubao Liu, and M. K. Yau

Abstract

No abstract available.

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Zonghui Huo, Da-Lin Zhang, and John R. Gyakum

Abstract

In Part I of this series of papers, the static potential vorticity (PV) inversion diagnostics are performed to examine the lateral and vertical interactions between two upper-level (northern and southern) troughs and their possible influences on the development of the March 1993 superstorm. This study continues the investigation of the relative importance of two PV anomalies associated with the short-wave troughs and their interaction in the surface cyclogenesis by considering their individual roles as an initial-value problem.

The piecewise PV inversion technique is first used to isolate the upper-level disturbances with a balanced vortex for each. The vortex is then either removed or its intensity doubled by subtracting the balanced vortex from or adding to the control initial conditions. With the modified initial conditions, a mesoscale model is integrated for 36 h to examine how each of the upper-level disturbances contributes to the surface development.

It is found that the northern and southern troughs play different roles in the surface cyclogenesis. Without the northern trough, the eastward propagation of the southern trough slows down and the surface development is severely impeded. More rapid cyclogenesis occurs when the northern trough’s intensity is doubled. On the other hand, the deepening rate increases considerably when the southern trough is removed. Doubling the southern trough intensity slows the eastward progression of the northern trough and leads to a reduced deepening rate. In conclusion, the northern trough is crucial for the rapid development of the superstorm, whereas the southern trough is important only during the incipient stage of the cyclone. In particular, a much stronger surface cyclone could be spawned in the absence of the southern trough. This finding appears to contradict the previous work on the roles of trough mergers in extratropical cyclogenesis.

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Yubao Liu, Da-Lin Zhang, and M. K. Yau

Abstract

Despite considerable research, understanding of the temporal evolution of the inner-core structures of hurricanes is very limited owing to the lack of continuous high-resolution observational data of a storm. In this study, the results of a 72-h explicit simulation of Hurricane Andrew (1992) with a grid size of 6 km are examined to explore the inner-core axisymmetric and asymmetric structures of the storm during its rapid deepening stage. Based on the simulation, a conceptual model of the axisymmetric structures of the storm is proposed. Most of the proposed structures confirm previous observations. The main ingredients include a main inflow (outflow) in the boundary layer (upper troposphere) with little radial flow in between, a divergent slantwise ascent in the eyewall, a penetrative dry downdraft at the inner edge of the eyewall, and a general weak subsiding motion in the eye with typical warming/drying above an inversion located near an altitude of about 2–3 km. The storm deepens as the axes of these features contract.

It is found that the inversion divides the eye of the hurricane vertically into two parts, with a deep layer of warm/dry air above and a shallow pool of warm/moist air below. The air aloft descends at an average rate of 5 cm s−1 and has a residency time of several days. In contrast, the warm/moist pool consists of air from the main inflow and penetrative downdrafts, offset somewhat by the air streaming in a returning outflow into the eyewall in the lowest 2 km; it is subject to the influence of the upward heat and moisture fluxes over the underlying warm ocean. The warm/moist pool appears to play an important role in supplying high-θ e air for deep convective development in the eyewall. The penetrative downdraft is dry and originates from the return inflow in the upper troposphere, and it is driven by sublimative/evaporative cooling under the influence of the (asymmetric) radial inflow of dry/cold air in the midtroposphere. It penetrates to the bottom of the eye (azimuthally downshear with a width often greater than 100 km) in a radially narrow zone along the slantwise inner edge of the eyewall.

It is further shown that all the meteorological fields are highly asymmetric. Whereas the storm-scale flow features a source–sink couplet in the boundary layer and dual gyres aloft, the inner-core structures exhibit alternative radial inflow and outflow and a series of inhomogeneous updrafts and downdrafts. All the fields tilt more or less with height radially outward and azimuthally downshear. Furthermore, pronounced fluctuations of air motion are found in both the eye and the eyewall. Sometimes, a deep layer of upward motion appears at the center of the eye. All these features contribute to the trochoidal oscillation of the storm track and movement. The main steering appears to be located at the midtroposphere (∼4.5 km) and the deep-layer mean winds represent well the movement of the hurricane.

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Yubao Liu, Da-Lin Zhang, and M. K. Yau

Abstract

In this study, the inner-core structures of Hurricane Andrew (1992) are explicitly simulated using an improved version of the Penn State–NCAR nonhydrostatic, two-way interactive, movable, triply nested grid mesoscale model (MM5). A modified Betts–Miller cumulus parameterization scheme and an explicit microphysics scheme were used simultaneously to simulate the evolution of the larger-scale flows over the coarser-mesh domains. The intense storm itself is explicitly resolved over the finest-mesh domain using a grid size of 6 km and an explicit microphysics package containing prognostic equations for cloud water, ice, rainwater, snow, and graupel. The model is initialized with the National Centers for Environmental Prediction analysis enhanced by a modified moisture field. A model-generated tropical-storm-like vortex was also incorporated. A 72-h integration was made, which covers the stages from the storm’s initial deepening to a near–category 5 hurricane intensity and the landfall over Florida.

As verified against various observations and the best analysis, the model captures reasonably well the evolution and inner-core structures of the storm. In particular, the model reproduces the track, the explosive deepening rate (>1.5 hPa h−1), the minimum surface pressure of 919 hPa preceding landfall, the strong surface wind (>65 m s−1) near the shoreline, as well as the ring of maximum winds, the eye, the eyewall, the spiral rainbands, and other cloud features. Of particular significance is that many simulated kinematics, thermodynamics, and precipitation structures in the core regions compare favorably to previous observations of hurricanes.

The results suggest that it may be possible to predict reasonably the track, intensity, and inner-core structures of hurricanes from the tropical synoptic conditions if high grid resolution, realistic model physics, and proper initial vortices (depth, size, and intensity) in relation to their larger-scale conditions (e.g., SST, moisture content, and vertical shear in the lower troposphere) are incorporated.

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Hong-Bo Liu, Jing Yang, Da-Lin Zhang, and Bin Wang

Abstract

During the mei-yu season of the summer of 2003, the Yangtze and Huai River basin (YHRB) encountered anomalously heavy rainfall, and the northern YHRB (nYHRB) suffered a severe flood because of five continuous extreme rainfall events. A spectral analysis of daily rainfall data over YHRB reveals two dominant frequency modes: one peak on day 14 and the other on day 4 (i.e., the quasi-biweekly and synoptic-scale mode, respectively). Results indicate that the two scales of disturbances contributed southwesterly and northeasterly anomalies, respectively, to the mei-yu frontal convergence over the southern YHRB (sYHRB) at the peak wet phase. An analysis of bandpass-filtered circulations shows that the lower and upper regions of the troposphere were fully coupled at the quasi-biweekly scale, and a lower-level cyclonic anomaly over sYHRB was phase locked with an anticyclonic anomaly over the Philippines. At the synoptic scale, the strong northeasterly components of an anticyclonic anomaly with a deep cold and dry layer helped generate the heavy rainfall over sYHRB. Results also indicate the passages of five synoptic-scale disturbances during the nYHRB rainfall. Like the sYHRB rainfall, these disturbances originated from the periodical generations of cyclonic and anticyclonic anomalies at the downstream of the Tibetan Plateau. The nYHRB rainfalls were generated as these disturbances moved northeastward under the influence of monsoonal flows and higher-latitude eastward-propagating Rossby wave trains. It is concluded that the sYHRB heavy rainfall resulted from the superposition of quasi-biweekly and synoptic-scale disturbances, whereas the intermittent passages of five synoptic-scale disturbances led to the flooding rainfall over nYHRB.

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Chanh Kieu, Vijay Tallapragada, Da-Lin Zhang, and Zachary Moon

Abstract

This study examines the formation of a double warm-core (DWC) structure in intense tropical cyclones (TCs) that was captured in almost all supertyphoon cases during the 2012–14 real-time typhoon forecasts in the northwestern Pacific basin with the Hurricane Weather Research and Forecasting Model (HWRF). By using an idealized configuration of HWRF to focus on the intrinsic mechanism of the DWC formation, it is shown that the development of DWC in intense TCs is accompanied by a thin inflow layer above the typical upper outflow layer. The development of this thin inflow layer in the lower stratosphere (~100–75 hPa), which is associated with an inward pressure gradient force induced by cooling at the cloud top, signifies intricate interaction of TCs with the lower stratosphere as TCs become sufficiently intense, which has not been examined previously. Specifically, it is demonstrated that a higher-level inflow can advect potentially warm air from the lower stratosphere toward the inner-core region, thus forming an upper-level warm core that is separated from a midlevel one of tropospheric air. Such formation of the upper-level warm anomaly in intense TCs is linked to an episode of intensification at the later stage of TC development. While these results are produced by HWRF, the persistent DWC and UIL features in all HWRF simulations of intense TCs suggest that the lower stratosphere may have significant impacts on the inner-core structures of intense TCs beyond the current framework of TCs with a single warm core.

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