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Yueh-Li Chen
and
Chun-Chieh Wu

Abstract

Two types of eye formation are proposed: clearing formation (CF) and banding formation (BF). The objectives are to identify the tropical cyclone (TC) characteristics associated with these two types of eye formation and to clarify how the environment, pattern of synoptic systems, and TC structure during initial and development stages affect the eye formation. The satellite imagery and best track data are used to classify and analyze the TCs named in the western North Pacific from 2007 to 2020. The results show that TCs with CF have a significantly higher intensity and intensification rate during the period of the first eye presence, smaller size after genesis and prior to eye formation, smaller eye size when the eye forms, and a more westward track. It is noted that CF and BF TCs tend to occur in autumn and summer, respectively. Meanwhile, the composite results using reanalysis data show that the CF TCs are generally characterized by easterly wave features with a drier environment, smaller initial size, and larger radial gradient of vorticity, while the BF TCs feature a monsoon-depression structure with a wetter environment, larger initial size, and flatter vorticity profile. A conceptual hypothesis is thus proposed, as compared to BF TCs, the smaller size and weaker outer wind in CF storms associated with the easterly wave disturbance are facilitated by inactive outer convection, leading to larger radial gradient of inertial stability. The low-level inflow can penetrate inward close to the center, resulting in more diabatic heating inside the radius of maximum wind with much higher heating efficiency, and also a higher intensification rate.

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Chun-Chieh Wu
and
Kerry A. Emanuel

Abstract

The validity of balance dynamics in the Tropics allows an exploration of the dynamics of hurricanes using the potential vorticity (PV) framework. Part I demonstrated the use of PV diagnostics in understanding the hurricane steering flow and also the interaction between the cyclone and its environment. To obtain a broader understanding of this PV methodology, two other observational case studies are performed (Tropical Storm Ana of 1991 and Hurricane Andrew of 1992) emphasizing the same methods of analysis.

The results are consistent with a previous finding that the hurricane advection flow, defined by inverting the entire PV distribution excluding the storm's own positive anomaly, is a good approximation to real cyclone movement, even though the original data cannot capture the actual hurricane strength. This study confirms that upper-level PV anomalies can play an important role in the motion of the storm. But their quantitative effect on the cyclone's motion depends strongly on the relative location of the vortex and the upper-air PV features. Due to the limitations of the data, the β effect or the mechanism proposed by Wu and Emanuel was not able to be supported or disproved.

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Chun-Chieh Wu
and
Kerry A. Emanuel

Abstract

Potential vorticity (PV) diagnostics are applied to evaluate the control by the large-scale environment of hurricane movement and, more importantly, to assess the storm's influence on its own track. As a first application of these diagnostics, an observational case study of Hurricane Bob (1991) is presented using the twice-daily National Meteorological Center Northern Hemisphere final analyses gridded datasets. Defining the seasonal climatology as the mean reference state, piecewise potential vorticity inversions are performed under the nonlinear balance condition. This allows one to determine the balanced flows associated with any individual perturbation of PV. By examining the balanced flows at the central position of the hurricane, one can identify the influence of each PV perturbation on hurricane movement. The hurricane advection flow is also defined as the balanced flow at the storm center associated with the whole PV distribution, excluding the positive PV anomaly of the hurricane itself.

The results from the observational study of Bob show that the hurricane advection flow is a good approximation to the real storm motion. The results also show that the balanced flows associated with the climatological mean PV and perturbation PV distribution in both the lower and upper troposphere are both important in contributing to Bob's movement. However, it is difficult to separate PV anomalies directly or indirectly attributable to the storm from ambient PV anomalies. Results from other cases will be presented in a companion paper.

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Chin-Hsuan Peng
and
Chun-Chieh Wu

Abstract

The rapid intensification (RI) of Typhoon Soudelor (2015) is simulated using a full-physics model. To investigate how the outer-core surface heat fluxes affect tropical cyclone (TC) structure and RI processes, a series of numerical experiments are performed by suppressing surface heat fluxes between various radii. It is found that a TC would become quite weaker when the surface heat fluxes are suppressed outside the radius of 60 or 90 km [the radius of maximum surface wind in the control experiment (CTRL) at the onset of RI is roughly 60 km]. However, interestingly, the TC would experience stronger RI when the surface heat fluxes are suppressed outside the radius of 150 km. For those sensitivity experiments with capped surface heat fluxes, the members with greater intensification rate show stronger inner-core mid- to upper-level updrafts and higher heating efficiency prior to the RI periods. Although the outer-core surface heat fluxes in these members are suppressed, the inner-core winds become stronger, extracting more ocean energy from the inner core. Greater outer-core low-level stability in these members results in aggregation of deep convection and subsequent generation and concentration of potential vorticity inside the inner core, thus confining the strongest winds therein. The abovementioned findings are also supported by partial-correlation analyses, which reveal the positive correlation between the inner-core convection and subsequent 6-h intensity change, and the competition between the inner-core and outer-core convections (i.e., eyewall and outer rainbands).

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Liguang Wu
,
Jia Liang
, and
Chun-Chieh Wu

Abstract

Typhoon Morakot made landfall on Taiwan with a record rainfall of 3031.5 mm during 6–13 August 2009. While previous studies have emphasized the influence of southwesterly winds associated with intraseasonal oscillations and monsoon surges on moisture supply, the interaction between Morakot and low-frequency monsoon flows and the resulting influence on the slow movement and asymmetric precipitation structure of the typhoon were examined observationally.

Embedded in multi-time-scale monsoonal flows, Morakot generally moved westward prior to its landfall on Taiwan and underwent a coalescence process first with a cyclonic gyre on the quasi-biweekly oscillation time scale and then with a cyclonic gyre on the Madden–Julian oscillation time scale. The coalescence enhanced the synoptic-scale southwesterly winds of Morakot and thus decreased its westward movement and turned the track northward, leading to an unusually long residence time in the vicinity of Taiwan. The resulting slow movement and collocation with the low-frequency gyres also maintained the major rainfall in southern Taiwan because the low-frequency flows played an important role in enhancing the winds on the southern side, especially during 6–9 August 2009. In addition to the lifting effect of the Taiwan terrain and the moisture supply associated with monsoon flows, the study suggests that the monsoonal influence maintained the major rainfall area in southern Taiwan through reducing the translation speed, shifting Morakot northward, and enhancing the low-frequency flows on the southern side of the typhoon. Since the enhanced low-frequency flows did not shift northward with the movement of Morakot, its primary rainfall expanded outward with time as the typhoon center moved northwestward after its landfall on Taiwan.

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Zhihua Zeng
,
Yuqing Wang
, and
Chun-Chieh Wu

Abstract

The effects of two environmental dynamical factors, namely, the transitional speed and vertical wind shear, on tropical cyclone (TC) intensification, intensity, and lifetime peak intensity were analyzed based on observations in the western North Pacific during 1981–2003. In general, both the fast translation and strong vertical shear are negative to both TC intensification and the lifetime peak intensity. Both the very intense TCs and the TCs with rapid intensification rate are found only to occur in a narrow range of translational speeds between 3 and 8 m s−1, and in relatively weak vertical shear. The overwhelming majority of western North Pacific TCs reach their lifetime peak intensity just prior to recurvature where their environmental steering flow and vertical shear are both weak. The results show that few TCs intensified when they moved faster than 15 m s−1, or when their large-scale environmental vertical shear is larger than 20 m s−1. The intensification rate of TCs is found to increase with decreasing vertical shear while the majority of the weakening storms experience relatively strong vertical shear. Overall, strong vertical shear prohibits rapid intensification and most likely results in the weakening of TCs, similar to the fast storm translation. Based on the statistical analysis, a new empirical maximum potential intensity (MPI) has been developed, which includes the combined negative effect of translational speed and vertical shear as the environmental dynamical control in addition to the positive contribution of SST and the outflow temperature as the thermodynamic control. The new empirical MPI can not only provide more accurate estimation of TC maximum intensity but also better explain the observed behavior of the TC maximum intensity and help explain the thermodynamic and environmental dynamical controls of TC intensity. Implications of the new empirical MPI are discussed.

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Ruifen Zhan
,
Yuqing Wang
, and
Chun-Chieh Wu

Abstract

The impact of the sea surface temperature anomaly (SSTA) in the East Indian Ocean (EIO) on the tropical cyclone (TC) frequency over the western North Pacific (WNP) and the involved physical mechanisms are examined using the International Pacific Research Center (IPRC) Regional Atmospheric Model (iRAM) driven by the reanalysis and the observed SSTs. The model reproduces generally quite realistic climatic features of the WNP TC activity, including the interannual variability of the WNP TC genesis frequency, the geographical distributions of TC genesis and frequency of occurrence. In particular, the model reproduces the observed statistical (negatively correlated) relationship between the WNP TC frequency and the EIO SSTA, as recently studied by Zhan et al.

The experiments with artificially imposed SSTA in the EIO in the year 2004 with normal EIO SST and WNP TC activity confirm that the EIO SSTA does affect the TC genesis frequency in the entire genesis region over the WNP by significantly modulating both the western Pacific summer monsoon and the equatorial Kelvin wave activity over the western Pacific, two major large-scale dynamical controls of TC genesis over the WNP. Additional sensitivity experiments are performed for two extreme years: one (1994) with the highest and one (1998) with the lowest TC annual frequencies in the studied period. The results reveal that after the EIO SSTAs in the two extreme years are removed, the TC frequency in 1998 is close to the climatological mean, while the excessive TCs in 1994 are still simulated. The model results suggest that the warm EIO might be a major factor contributing to the unusually few TCs formed over the WNP in 1998, but the cold EIO seemed to contribute little to the excessive WNP TCs in 1994.

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Hui Wang
,
Chun-Chieh Wu
, and
Yuqing Wang

Abstract

The secondary eyewall formation (SEF) in an idealized simulation of a tropical cyclone (TC) is examined from the perspective of both the balanced and unbalanced dynamics and through the tangential wind (Vt) budget analysis. It is found that the expansion of the azimuthal-mean Vt above the boundary layer occurs prior to the development of radial moisture convergence in the boundary layer. The Vt expansion results primarily from the inward angular momentum transport by the mid- to lower-tropospheric inflow induced by both convective and stratiform heating in the spiral rainbands. In response to the Vt broadening is the development of radial inflow convergence and the supergradient flow near the top of the inflow boundary layer. Results from the Vt budget analysis show that the combined effect of the mean advection and the surface friction is to spin down Vt in the boundary layer, while the eddy processes (eddy radial and vertical advection) contribute positively to the spinup of Vt in the SEF region in the boundary layer. Therefore, eddies play an important role in the spinup of Vt in the boundary layer during SEF. The balanced Sawyer–Eliassen solution can well capture the secondary circulation in the full-physics model simulation. The radial inflow diagnosed from the Sawyer–Eliassen equation is shown to spin up Vt and maintain the vortex above the boundary layer. However, the axisymmetric balanced dynamics cannot explain the spinup of Vt in the boundary layer, which results mainly from the eddy processes.

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Mengwen Wu
,
Chun-Chieh Wu
,
Tzu-Hsiung Yen
, and
Yali Luo

Abstract

This study investigates the statistical characteristics of extreme hourly precipitation over Taiwan during 2003–12 that exceeds the 5-, 10-, and 20-yr return values and 100 mm h−1. All the extreme precipitation records are classified into four types according to the synoptic situations under which they occur: tropical cyclones (TCs), fronts, weak-synoptic forcing, and vortex/shear line types. The TC type accounts for over three-quarters of the total records, while the front type and weak-synoptic forcing type are comparable (9%–13%). Extreme hourly precipitation is mostly caused by mei-yu fronts during May–mid-June and by TCs during July–October. The TC type tends to have a long duration time (>12 h) with a symmetrical evolution of hourly rainfall intensity, while the front type and weak-synoptic forcing type mainly occur over a short period (<6 h) with a slightly asymmetrical evolution pattern. The TC type is further divided into seven subtypes according to the location of the TC center relative to the island. When the TC center is over the island or near the coastline (distance <100 km), the spatial distribution of subtypes I–IV is largely determined by the interaction between the TC circulation and topography when a TC center is over the northwest, south, east, or northeast portion of Taiwan, respectively. When the TC center is far away (distance >100 km) from the island, the strength of the environmental southwesterly or northeasterly winds and the impingement of TC circulation on the east side of the Central Mountain Range are also key factors determining the spatial distribution of subtypes V–VII.

Open access
Jia Liang
,
Liguang Wu
,
Xuyang Ge
, and
Chun-Chieh Wu

Abstract

In the second part of this study, numerical experiments are conducted to investigate the influences of multi-time-scale monsoonal flows on the track change of Typhoon Morakot (2009). While the control simulation captures the slowing and northward deflections in the vicinity of Taiwan Island, the highly asymmetric rainfall structure, and the associated rainfall pattern, sensitivity experiments suggest that the westward movement prior to the landfall on Taiwan and the subsequent northward shifts in the vicinity of Taiwan were closely associated with the interaction between Morakot and multi-time-scale monsoonal flows.

Prior to the landfall on Taiwan, Morakot moved westward directly toward Taiwan because of a synoptic wave train–like pattern, which consisted of Goni over mainland China, Morakot, and a cyclone over the western North Pacific with an anticyclone to the west of Morakot. Numerical simulation suggests that strong northerly winds between Morakot and the anticyclone reduced the northward steering component associated with the low-frequency flow prior to the landfall. Numerical experiments confirm that the northward track shifts that occurred in the vicinity of Taiwan Island were a result of the coalescences of Morakot with a quasi-biweekly oscillation (QBW)-scale gyre prior to the landfall on Taiwan and a Madden–Julian oscillation (MJO)-scale gyre in the Taiwan Strait. The simulation of Morakot and the associated sensitivity experiments agree with the previous barotropic study that the interaction between tropical cyclones and low-frequency monsoon gyres can cause sudden changes in tropical cyclone tracks.

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