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

Abstract

In this study, the vertical force balance in the inner-core region is examined, through the analysis of vertical momentum budgets, using a high-resolution, explicit simulation of Hurricane Andrew (1992). Three-dimensional buoyancy- and dynamically induced perturbation pressures are then obtained to gain insight into the processes leading to the subsidence warming in the eye and the vertical lifting in the eyewall in the absence of positive buoyancy.

It is found from the force balance budgets that vertical acceleration in the eyewall is a small difference among the perturbation pressure gradient force (PGF), buoyancy, and water loading. The azimuthally averaged eyewall convection is found to be conditionally stable but slantwise unstable with little positive buoyancy. It is the PGF that is responsible for the upward acceleration of high-θ e air in the eyewall. It is found that the vertical motion and acceleration in the eyewall are highly asymmetric and closely related to the azimuthal distribution of radial flows in conjunction with large thermal and moisture contrasts across the eyewall. For example, the radially incoming air aloft is cool and dry and tends to suppress updrafts or induce downdrafts. On the other hand, the outgoing flows are positively buoyant and tend to ascend in the eyewall unless evaporative cooling dominates. It is also found that the water loading effect has to be included into the hydrostatic equation in estimating the pressure or height field in the eyewall.

The perturbation pressure inversions show that a large portion of surface perturbation pressures is caused by the moist-adiabatic warming in the eyewall and the subsidence warming in the eye. However, the associated buoyancy-induced PGF is mostly offset by the buoyancy force, and their net effect is similar in magnitude but opposite in sign to the dynamically induced PGF. Of importance is that the dynamically induced PGF points downward in the eye to account for the maintenance of the general descent. But it points upward in the outer portion of the eyewall, particularly in the north semicircle, to facilitate the lifting of high-θ e air in the lower troposphere. Furthermore, this dynamic force is dominated by the radial shear of tangential winds. Based on this finding, a new theoretical explanation, different from previously reported, is advanced for the relationship among the subsidence warming in the eye, and the rotation and vertical wind shear in the eyewall.

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

Abstract

Although considerable progress has been made in understanding the development of hurricanes, our knowledge of their three-dimensional structures of latent heat release and inner-core thermodynamics remains limited. In this study, the inner-core budgets of potential temperature (θ), moisture (q), and equivalent potential temperature (θe) are examined using a high-resolution (Δx = 6 km), nonhydrostatic, fully explicit simulation of Hurricane Andrew (1992) during its mature or intensifying stage.

It is found that the heat energy is dominated by latent heat release in the eyewall, sublimative–evaporative cooling near the eye–eyewall interface, and the upward surface fluxes of sensible and latent heat from the underlying warm ocean. The latent heating (θ) rates in the eyewall range from less than 10°C h–1 to greater than 100°C h–1, depending upon whether latent heat is released in radial inflow or outflow regions. The latent heating rates decrease inward in the inflow regions and become negative near the eye–eyewall interface. It is shown that the radial θ advective cooling in the inflow regions accounts for the initiation and maintenance of the penetrative downdrafts at the eye–eyewall interface that are enhanced by the sublimative-evaporative cooling. It is also shown that the vertical θ advection overcompensates the horizontal θ advection for the generation of the warm-cored eye, and the sum of latent heating and radial advective warming for the development of intense cooling in the eyewall. The moisture budgets show the dominant upward transport of moisture in the eyewall updrafts (and spiral rainbands), partly by the low-level outflow jet from the bottom eye regions, so that the eyewall remains nearly saturated.

The θe budgets reveal that θe could be considered as an approximately conserved variable in the eyewall above the boundary layer even in the presence of deposition–sublimation and freezing–melting. The development of higher-θe surfaces at the eye–eyewall interface is discussed in the context of deep convection, the θe gradient and the mass recycling across the eyewall. It is concluded that the simulated hurricane is thermodynamically maintained by the upward surface flux of higher-θe air from the underlying warm ocean, the descent of higher-θe air in the upper troposphere along the eye–eyewall interface, and the recycling of some warmed-eye air at the eye–eyewall interface.

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Xiaoling Jiang, Yali Luo, Da-Lin Zhang, and Mengwen Wu

Abstract

An extensive urban agglomeration has occurred over the Yangtze River delta (YRD) region of East China as a result of rapid urbanization since the middle 1990s. In this study, a 44-yr (i.e., 1975–2018) climatology of the summertime extreme hourly precipitation (EXHP; greater than the 90th percentile) over the YRD is analyzed, using historical land-use data, surface temperature, and hourly rain gauge observations, and then the relationship between rapid urbanization and EXHP changes is examined. Results show significant EXHP contrasts in diurnal variation and storm type roughly before and after middle July. That is, tropical cyclones (TCs) account for 16.4% of the total EXHP hours, 80.5% of which occur during the late summer, whereas non-TC EXHP accounts for 94.7% and 66.2% during the early and late summer, respectively. Increasing trends in occurrence frequency and amount of the non-TC and TC-induced EXHP are detected over the urban agglomeration. Statistically significant larger increasing trends in both the EXHP and surface temperature are observed at urban stations than those at the nearby rural stations. An analysis of 113 locally developed non-TC extreme rainfall events during 2011–18 summers also suggests the contribution of the urban heat island effects to the more occurrences of EXHP, especially over a band-shaped urban region where several major cities are distributed. This study reveals a significant correlation between rapid urbanization and increased EXHP during the past two decades over the YRD region. The results have important implications for understanding the impact of urbanization on EXHP changes in a warming climate.

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Hua Chen, Da-Lin Zhang, James Carton, and Robert Atlas

Abstract

In this study, a 72-h cloud-permitting numerical prediction of Hurricane Wilma (2005), covering its initial 18-h spinup, an 18-h rapid intensification (RI), and the subsequent 36-h weakening stage, is performed using the Weather Research Forecast Model (WRF) with the finest grid length of 1 km. The model prediction uses the initial and lateral boundary conditions, including the bogus vortex, that are identical to the Geophysical Fluid Dynamics Laboratory’s then-operational data, except for the time-independent sea surface temperature field. Results show that the WRF prediction compares favorably in many aspects to the best-track analysis, as well as satellite and reconnaissance flight-level observations. In particular, the model predicts an RI rate of more than 4 hPa h−1 for an 18-h period, with the minimum central pressure of less than 889 hPa. Of significance is that the model captures a sequence of important inner-core structural variations associated with Wilma’s intensity changes, namely, from a partial eyewall open to the west prior to RI to a full eyewall at the onset of RI, rapid eyewall contraction during the initial spinup, the formation of double eyewalls with a wide moat area in between during the most intense stage, and the subsequent eyewall replacement leading to the weakening of Wilma. In addition, the model reproduces the boundary layer growth up to 750 hPa with an intense inversion layer above in the eye. Recognizing that a single case does not provide a rigorous test of the model predictability due to the stochastic nature of deep convection, results presented herein suggest that it is possible to improve forecasts of hurricane intensity and intensity changes, and especially RI, if the inner-core structural changes and storm size could be reasonably predicted in an operational setting using high-resolution cloud-permitting models with realistic initial conditions and model physical parameterizations.

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

Abstract

The objectives of Part VI of this series of papers are to (a) simulate the finescale features of Hurricane Andrew (1992) using a cloud-resolving grid length of 2 km, (b) diagnose the formation of small-scale wind streaks, and (c) perform sensitivity experiments of varying surface fluxes on changes in storm inner-core structures and intensity.

As compared to observations and a previous 6-km model run, the results show that a higher-resolution explicit simulation could produce significant improvements in the structures and evolution of the inner-core eyewall and spiral rainbands, and in the organization of convection. The eyewall becomes much more compact and symmetric with its width decreased by half, and the radius of maximum wind is reduced by ∼10 to 20 km. A zone of deep and intense potential vorticity (PV) is formed at the edge of the eye. A ring of maximum PV is collocated in regions of maximum upward motion in the eyewall and interacts strongly with the eyewall convection. The convective cores in the eyewall are associated with small-scale wind streaks.

The formation of the wind streaks is diagnosed from an azimuthal momentum budget. The results reveal small-scale Lagrangian acceleration of the azimuthal flow. It is found that at the lowest model level of 40 m, the main contributor to the Lagrangian azimuthal wind tendency is the radial advection of angular momentum per unit radius. At an altitude of 1.24 km, vertical advection of the azimuthal wind, in addition to the radial advection of angular momentum per unit radius, plays important roles.

Results of a series of sensitivity tests, performed to examine the impact of several critical factors in the surface and boundary layer processes on the inner-core structures and the evolution of the hurricane intensity, are presented.

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Badrinath Nagarajan, M. K. Yau, and Da-Lin Zhang

Abstract

A 16-h numerical simulation of the growing and mature stages of the 15 December 1992 Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) mesoscale convective system (MCS) is performed to demonstrate the predictability of tropical MCSs when initial conditions and model physical processes are improved. The MCS began with two entities S1 and S2, which developed and eventually merged to form a large anvil cloud.

To obtain a realistic simulation of the MCS, the initial moisture field in the operational European Centre for Medium-Range Weather Forecasts (ECMWF) analysis is improved, based on previous findings. The deep column ascent and surface potential temperature dropoff (SPTD) are implemented into the initiation mechanism of the Kain–Fritsch cumulus parameterization scheme (KF CPS). Other refinements to the KF CPS include the introduction of the accretion process in the formation of convective rain and the detrainment of rain and ice particles at the cloud top.

With the improved initial conditions and model physics, the modeled MCS shows many features similar to the observations, including the evolution of the anvil cloud fraction, the three convective onsets at three different times during the growing stage, and the characteristics of two deep convective lines during the mature stage. A series of sensitivity tests indicates that the SPTD is largely responsible for the successful prediction of the life cycle of the MCS, while inclusion of the deep column ascent criterion yields a better timing for the onset of the mature stage.

The effects of modifying the initial moisture field are also investigated.

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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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Ming-Jen Yang, Da-Lin Zhang, and Hsiao-Ling Huang

Abstract

Although there have been many observational and modeling studies of tropical cyclones, understanding of their intensity and structural changes after landfall is rather limited. In this study, several 84-h cloud-resolving simulations of Typhoon Nari (2001), a typhoon that produced torrential rainfall of more than 1400 mm over Taiwan, are carried out using a quadruply nested–grid mesoscale model whose finest grid size was 2 km. It is shown that the model reproduces reasonably well Nari’s kinematic and precipitation features as well as structural changes, as verified against radar and rain gauge observations. These include the storm track, the contraction and sizes of the eye and eyewall, the spiral rainbands, the rapid pressure rise (∼1.67 hPa h−1) during landfall, and the nearly constant intensity after landfall. In addition, the model captures the horizontal rainfall distribution and some local rainfall maxima associated with Taiwan’s orography.

A series of sensitivity experiments are performed in which Taiwan’s topography is reduced to examine the topographic effects on Nari’s track, intensity, rainfall distribution, and amount. Results show that the impact of island terrain on Nari’s intensity is nearly linear, with stronger storm intensity but less rainfall in lower-terrain runs. In contrast, changing the terrain heights produces nonlinear tracks with circular shapes and variable movements associated with different degrees of blocking effects. Parameter and diagnostic analyses reveal that the nonlinear track dependence on terrain heights results from the complex interactions between the environmental steering flow, Nari’s intensity, and Taiwan’s topography, whereas the terrain-induced damping effects balance the intensifying effects of latent heat release associated with the torrential rainfall in maintaining the near-constant storm intensity after landfall.

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Badrinath Nagarajan, M. K. Yau, and Da-Lin Zhang

Abstract

In Part I, the authors presented a successful numerical simulation of the life cycle of a warm-pool mesoscale convective system (MCS) that occurred on 15 December 1992 during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment. In this study, the simulation results of Part I are diagnosed to investigate the organization of the MCS and the convective onsets that occurred during the growing and mature stages of the MCS.

During the life cycle of the MCS, four convective onsets occur in the presence of large-scale ascent, convective available potential energy (CAPE), and surface potential temperature drop-off (SPTD). It is found that the first convective onset is caused by the existence of upward motion, CAPE, and SPTD in the model initial conditions. The second convective onset is regulated by the favorable occurrence of SPTD. The third and fourth convective onsets arise from the development of upward motion associated with the westward propagation of the quasi-2- day wave. The four mesoscale precipitation features clustered together to form the MCS in response to the evolution of the vertical motion field.

The organization of the MCS is characterized by the presence of a midtropospheric mesovortex situated near the position of the first convective onset. Analysis of the relative vorticity (RV) budget indicates that the mesovortex originates and intensifies largely from vortex stretching induced by deep convective heating. A decrease in RV above (below) the mesovortex arises because of the combined effects of the tilting and horizontal advection terms (the tilting, stretching, and solenoidal terms). Our results suggest that the mesovortex played little role in the subsequent onsets (i.e., second, third, and fourth) and that other warm-pool MCSs occurring near the transequatorial flow are likely to be associated with mesovortices.

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Dian Wen, Ying Li, Da-Lin Zhang, Lin Xue, and Na Wei

Abstract

A statistical analysis of tropical upper-tropospheric trough (TUTT) cells over the western North Pacific Ocean (WNP) during 2006 to 2015 is performed using the NCEP Final reanalysis. A total of 369 TUTT-cell events or 6836 TUTT cells are identified, with a peak frequency in July. Most TUTT cells form to the east of 150°E and then move southwestward with a mean speed of 6.6 m s−1 and a mean life span of 4.4 days. About 75% of the TUTT cells have radii of <500 km with 200-hPa central heights of <1239.4 dam. In general, TUTT cells exhibit negative height anomalies above 450 hPa, with their peak amplitudes at 200 hPa, pronounced cold anomalies in the 650–200-hPa layer, and significant cyclonic vorticity in the 550–125-hPa layer. A comparison of the composite TUTT cells among the eastern, central, and western WNP areas shows the generation of an intense cold-cored vortex as a result of the southward penetration of a midlatitude trough into a climatological TUTT over the eastern WNP region. The TUTT cell with pronounced rotation is cut off from the midlatitude westerlies after moving to the central WNP region, where it enters its mature phase, under the influence of northeasterly flow. The TUTT cell weakens in rotation and shrinks in size, diminishing within the TUTT after arriving at the western WNP region. Results suggest that, although most TUTT cells may diminish before reaching the western WNP, their vertical influences may extend to the surface layer and last longer than their signals at 200 hPa.

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