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Chun-Chieh Wu

Abstract

Numerical integrations using the Geophysical Fluid Dynamics Laboratory (GFDL) hurricane model were performed to study the evolution of Typhoon Gladys (1994) and its interaction with the Taiwan terrain. Consistent with most previous studies, the Taiwan topography results in the deceleration of Gladys’s translation speed and southward deviation as it approaches Taiwan. On the other hand, Gladys accelerates northwestward while passing Taiwan, which is likely to be related to the moist processes, and differs from the track pattern in the dry model of Lin et al. Although the GFDL hurricane model forecast underestimates Gladys’s intensity, the model can capture the evolution of Gladys’s intensity, especially its weakening during landfall, which is primarily due to the cutoff of the water vapor supply in the boundary layer as Gladys approached the Taiwan terrain. Other mesoscale phenomena, including the pattern of heavy precipitation and the formation of secondary lows, are well simulated by the model, though their locations are somewhat different from those observed. Detailed analyses indicate that the surface low pressure center to the east of the Central Mountain Range (CMR) is induced by the downslope adiabatic warming (foehn) associated with the circulation of Gladys. The quasi-stationary secondary low to the west of the CMR is mainly induced by the environmental easterly flow over the CMR, while the downslope adiabatic warming associated with the circulation of Gladys acts to enhance it as Gladys is close to Taiwan. The potential vorticity budget analysis indicates that the condensational heating plays a major role in the potential vorticity evolution around the storm, while the surface frictional dissipation of the potential vorticity becomes more significant as Gladys is over the Taiwan terrain. Finally, the experiment with a larger and stronger initial typhoon vortex indicates that different initial specification of a typhoon vortex can result in a different track pattern and thus leads to a totally different typhoon–topography interaction, suggesting the importance of typhoon initialization for storm prediction near Taiwan.

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Kun-Hsuan Chou
and
Chun-Chieh Wu

Abstract

Issues concerning the initialization and simulation of tropical cyclones by integrating both dropwindsonde data and a bogused vortex into a mesoscale model have been studied. A method is proposed to combine dropwindsonde data with a bogused vortex for tropical cyclone initialization and to improve track and intensity prediction. A clear positive impact of this proposed method on both the tropical cyclone track and intensity forecasts in a mesoscale model is demonstrated in three cases of typhoons, including Meari (2004), Conson (2004), and Megi (2004). The effectiveness of the proposed method in improving the track and intensity forecasts is also demonstrated in the evaluation of all 10 cases of Dropwindsonde Observations for Typhoon Surveillance near the Taiwan Region (DOTSTAR) missions in 2004. This method provides a useful and practical means to improve operational tropical cyclone prediction with dropwindsonde observations.

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Guo-Ji Jian
and
Chun-Chieh Wu

Abstract

A series of numerical simulations are conducted using the advanced research version of the Weather Research and Forecasting model with a 4-km fine mesh to examine the physical processes responsible for the significant track deflection and looping motion before the landfall of Supertyphoon Haitang (2005) in Taiwan, which poses a unique scientific and forecasting issue. In the control experiment, a low-level northerly jet induced by the channeling effect forms in the western quadrant of the approaching storm, where the inner-core circulation is constrained by the presence of Taiwan’s terrain. Because of the channeling effect, the strongest winds of the storm are shifted to the western portion of the eyewall. The northerly advection flow (averaged asymmetric winds within 100-km radius) results in a sharp southward turn of the westward-moving storm. The time series of the advection flow shows that the advection wind vectors rotate cyclonically in time and well match the motion of the simulated vortex during the looping process. A sensitivity study of lowering the Taiwan terrain elevations to 70% or 40% of those in the control experiment reduces the southward track deflection and loop amplitude. The experiment with the reduced elevation to 10% of the control experiment does not show a looping track and thus demonstrates the key role of the terrain-induced channeling effect. Experiments applying different values of the structure parameter α illustrate that increasing the strength, size, and translation speed of the initial storm results in a smaller interaction with Taiwan’s terrain and a smaller average steering flow caused by the asymmetric circulation, which leads to a proportionally smaller southward track deflection without making a loop.

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Ting-Chen Chen
and
Chun-Chieh Wu

Abstract

The goal of this work is to improve understanding of the mechanisms leading to a heavy rainfall event under the combined influences of the outer circulation of Typhoon Megi (2010), the Asian monsoon, and the topography of Taiwan. Megi is a case featuring high forecast uncertainty associated with its sudden recurvature, along with remote heavy rainfall over northeastern Taiwan (especially at Yilan) and its adjacent seas during 19–23 October 2010. An ensemble simulation is conducted, and characteristic ensemble members are separated into subgroups based on either track accuracy or rainfall forecast skill. Comparisons between different subgroups are made to investigate favorable processes for precipitation and how the differences between these subgroups affect the rainfall simulation.

Several mechanisms leading to this remote rainfall event are shown. The northward transport of water vapor by Megi’s outer circulation provides a moisture-laden environment over the coastal area of eastern Taiwan. Meanwhile, the outer circulation of Megi (with high ) encounters the northeasterly monsoon (with low ), and strong vertical motion is triggered through isentropic lifting in association with low-level frontogenesis over the ocean northeast of Yilan. Most importantly, the northeasterly flow advects the moisture inland to the steep mountains in south-southwestern Yilan, where strong orographic lifting further induces torrential rainfall. In addition, the analyses further attribute the uncertainty in simulating Megi’s remote rainfall to several factors, including variations of storm track, strength and extension of the northeasterly monsoon, and, above all, the impinging angle of the upstream flow on the topography.

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Chun-Chieh Wu
and
Hsiu-Ju Cheng

Abstract

The European Centre for Medium-Range Weather Forecasts Tropical Ocean–Global Atmosphere advanced analysis was used to study the mechanisms that affect the intensity of Typhoons Flo (1990) and Gene (1990). The outflow structure, eddy momentum flux convergence, and the mean vertical wind shear were examined.

The evolution of potential vorticity (PV) in the outflow layer showed low PV areas on top of both Typhoons Flo and Gene, and the low PV areas expanded as the typhoons intensified. The outflow pattern of the two typhoons was influenced by the upper-tropospheric environmental systems. The upper-level environmental features were shown to play a crucial role in the intensification of the two typhoons.

The tropical upper-tropospheric trough cell east of Flo provided the outflow channel for the typhoon. The enhanced outflow, the upper-level eddy flux convergence (EFC), the low vertical wind shear, and the warm sea surface temperature provided all favorable conditions for the development of Flo. On the other hand, the intensification of Gene was associated with its interaction with an upper-level midlatitude trough. The approach of the trough produced upper-level EFC of angular momentum outside 10° lat radius, and the EFC shifted inward with time. As the EFC shifted into the vicinity of the storm core, Gene started to intensify steadily until the midlatitude trough passed over.

The intensifying processes of the above cases indicate the importance of the upper-tropospheric systems to the intensity change of typhoons. The influence of upper-level environmental systems on the tropical cyclones is prominent in the low inertial stability outflow layer. However, results from the piecewise PV inversion of the upper-level environmental PV anomalies showed little evidence that the intensification of both typhoons were directly associated with the superposition of PV anomalies.

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Yi-Hsuan Lin
and
Chun-Chieh Wu

Abstract

Remote rainfall related to tropical cyclones (TCs) can be attributed to interaction between the northeasterly monsoon and TC circulation (hereafter monsoon mode), and topographic blocking and lifting effects (hereafter topographic mode). Typhoon Khanun (2017) is a case in point affected by both modes. The objective of this study is to understand the key factors leading to uncertainty in the TC-induced remote rainfall. Ensemble simulations are conducted, with the ensemble members related to the monsoon mode classified into subtypes based on the geographic location of the precipitation maxima. The results demonstrate that frontogenesis and terrain-induced uplifting are the main mechanisms leading to the heavy precipitation in northeastern Taiwan, while the orographic lifting and the interaction between the TC circulation and the topographically blocked northeasterlies result in the heavy rainfall in southeastern Taiwan. For the topographic mode, at a larger rainfall threshold, strong relation is found between the inflow angle of the TC circulation and the cumulative frequency of the rainfall, while at a smaller rainfall threshold, rainfall cumulative frequency is related to the ensemble track directions. Sensitivity experiments with TC-related moisture reduced (MR) and the terrain of Taiwan removed (TR) show that the average of the 3-day accumulated rainfall is reduced by 40% and more than 90% over the mountainous area in MR and TR, respectively. Overall, this study highlights the fact that multiple mechanisms contribute to remote rainfall processes in Khanun, particularly the orographic forcing, thus providing better insights into the predictability of TC remote rainfall.

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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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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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