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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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Shu-Jeng Lin and Kun-Hsuan Chou

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This study examines the characteristics of tropical cyclone (TC) lightning distribution and its relationship with TC intensity and environmental vertical wind shear (VWS) over the western North Pacific. It uses data from the World Wide Lightning Location Network and operational global analysis data from National Centers for Environmental Prediction Final Analysis for 230 TCs during 2005–17. The spatial distribution of TC lightning frequency and normalized lightning rate demonstrates that the VWS dominates the azimuthal distribution of the lightning. The flashes are active in the downshear-left side of the inner core and the downshear-right side of the outer region. TC lightning distribution for various VWS strengths and TC intensities are further investigated. As VWS increases, the flashes of lightning become more asymmetric and exhibit a higher proportion at the outer region of the downshear side. Moreover, the same features occur as TC intensity decreases. A series of composite analyses indicated that stronger TCs with weaker VWS exhibit a more compact and symmetric lightning distribution, whereas weaker TCs with stronger VWS have a more asymmetric lightning distribution. Furthermore, the TC lightning distribution and its association with TC intensity changes are also examined for three lead times. Results show that among the composite analyses of five TC intensity changes, the lightning distribution for rapid intensification type exhibits more inner-core lightning and is more axisymmetric than the distributions for other categories. These features result from favorable environmental conditions comprising greater upper-level divergence, sea surface temperature, maximum potential intensity, and weaker vertical wind shear.

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Shu-Jeng Lin and Kun-Hsuan Chou

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This study investigates the size changes of tropical cyclones (TCs) traversing the Philippines based on a 37-yr statistical analysis. TC size is defined by the radius of 30-kt (≈15.4 m s−1) wind speed (R30) from the best track data of the Japan Meteorological Agency. A total of 71 TCs passed the Philippines during 1979–2015. The numbers of size increase (SI; 36) and size decrease (SD; 34) cases are very similar; however, the last 15 years have seen more SI cases (17) than SD cases (11). SI and SD cases mostly occur along northerly and southerly paths, respectively, after TCs pass the Philippines. Before landfall, SI cases have small initial sizes and weak intensities, but SD cases have larger initial sizes and stronger intensities. After landfall, most SI cases are intensifying storms, and most SD cases are nonintensifying storms. Composite analyses of vertical wind shear, absolute angular momentum flux, relative humidity, and sea surface temperature between SI and SD cases are compared. All of these values are larger in SI cases than in SD cases. Furthermore, the interdecadal difference in the ratio of the numbers of SI to SD cases reveals an unusually high number of SI cases during 2001–15. The synoptic patterns between 1979–2000 and 2001–15 are analyzed. The high SI ratio in the latter period is related to strong southwesterly wind in the south of the South China Sea that raised relative humidity, warmed the sea surface, and increased import of angular momentum flux.

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Chun-Chieh Wu, Treng-Shi Huang, and Kun-Hsuan Chou

Abstract

Potential vorticity diagnosis is applied to study the factors contributing to the decrease of the forward motion of Typhoon Sinlaku (2002) and is used to evaluate the reason why the operational Aviation (AVN) model failed to predict this slowdown and had a southward track bias as Sinlaku approached the area offshore northeastern Taiwan. The analysis indicates that the initial deceleration was mainly associated with the retreat of the Pacific subtropical high (SH) under the influence of the deepening midlatitude trough (TR). The upper-level cold-core low (CCL) played only a minor role in impeding Sinlaku from moving northward, while the continental high (CH) over mainland China strongly steered Sinlaku westward. Because the steering effect from the above four systems (SH, TR, CCL, and CH) tend to cancel one another out, the subtle interaction therein makes it difficult to make a precise track forecast. The analyses of the AVN model forecasts show that overestimating the CH and underestimating the SH are the main causes for the forecast bias. It is believed that results from this work can provide a useful method to quantitatively determine the key factors affecting the typhoon track. Such an analysis should be able to guide future observing and modeling strategies to improve the forecasts of typhoons.

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Chun-Chieh Wu, Shin-Gan Chen, Jan-Huey Chen, Kun-Hsuan Chou, and Po-Hsiung Lin

Abstract

Targeted observation is one of the most important research and forecasting issues for improving tropical cyclone predictability. A new parameter [i.e., the adjoint-derived sensitivity steering vector (ADSSV)] has been proposed and adopted as one of the targeted observing strategies in the Dropwindsonde Observations for Typhoon Surveillance near the Taiwan Region (DOTSTAR). The ADSSV identifies the sensitive areas at the observing time to the steering flow at the verifying time through the adjoint calculation. In this study, the ADSSV is calculated from the nonlinear forecast model of the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) and its adjoint to interpret the dynamical processes in the interaction between Typhoon Shanshan (2006) and the midlatitude trough. The ADSSV results imply that high-sensitivity regions affecting the motion of Typhoon Shanshan are located at the edge of the subtropical high and the 500-hPa midlatitude trough over northern central China. These ADSSV signals are in very good agreement with the quantitative evaluation based on the potential vorticity (PV) diagnosis. The vertical structure of the ADSSV is also shown for more physical insights into the typhoon–trough interaction. The maximum ADSSV occurs at 800–500 hPa to the southeast of Shanshan (associated with the subtropical high), while distinct ADSSV signals are located upstream of the storm center at about 500–300 hPa (associated with the mid- to upper-tropospheric midlatitude trough). Overall, it is demonstrated that the ADSSV features can well capture the signal of the large-scale trough feature affecting the motion of Shanshan, which can also be well validated from the PV analysis.

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Chun-Chieh Wu, Jan-Huey Chen, Po-Hsiung Lin, and Kun-Hsuan Chou

Abstract

Since 2003, a field program has been conducted under the name of Dropwindsonde Observations for Typhoon Surveillance near the Taiwan Region (DOTSTAR). As the name DOTSTAR suggests, targeted observation is one of its key objectives. The prerequisite for designing the observing strategy is to identify the sensitive areas, which would exert great influence on the results of numerical forecast or the extent of the forecast error.

In addition to various sensitivity products already adopted in DOTSTAR, a new way to identify the sensitive area for the targeted observation of tropical cyclones based on the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (NCAR) Mesoscale Model (MM5) is proposed in this paper. By appropriately defining the response functions to represent the steering flow at the verifying time, a simple vector, adjoint-derived sensitivity steering vector (ADSSV), has been designed to demonstrate the sensitivity locations and the critical direction of typhoon steering flow at the observing time. Typhoons Meari and Mindulle of 2004 have been selected to show the use of ADSSV. In general, unique sensitive areas 36 h after the observing time are obtained.

The proposed ADSSV method is also used to demonstrate the signal of the binary interaction between Typhoons Fungwong and Fengshen (2002). The ADSSV is implemented and examined in the field project, DOTSTAR, in 2005 as well as in the surveillance mission for Atlantic hurricanes conducted by the Hurricane Research Division. Further analysis of the results will be vital to validate the use of ADSSV.

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Chung-Chuan Yang, Chun-Chieh Wu, Kun-Hsuan Chou, and Chia-Ying Lee

Abstract

A cyclonic loop was observed in the track of Typhoon Fungwong (2002) when it was about 765 n mi from Supertyphoon Fengshen (2002). It is shown that Fungwong’s special path is associated with the circulation of Fengshen, and such an association is regarded as an indication of binary interaction. In this paper, the binary interaction between Fengshen and Fungwong is studied based on the potential vorticity diagnosis. The impacts of large-scale flow fields on their motions are also investigated. Furthermore, the sensitivity of the storm characteristics to the binary interaction is demonstrated by the mesoscale numerical model simulations with different sizes and intensities for the initial bogused storms. Results of the study show that before Fungwong and Fengshen interacted with each other, their motions were governed by the large-scale environmental flow, that is, mainly associated with the subtropical high. During this binary interaction, Fungwong’s looping is partly attributed to Fengshen’s steering flow. This pattern shows up first as a case of one-way interaction in the early period, and then develops into a mutual interaction during the later stages. The numerical experiments show the sensitivity of the storm size and intensity to the binary interaction, implicating that a good representation of the initial storm vortex is important for the prediction of binary storms. Further analyses also indicate the influence of the monsoon trough and subtropical high systems on the binary interaction. These results provide some new insights into the motions of nearby typhoons embedded in the monsoon circulation.

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Chun-Chieh Wu, Hsiu-Ju Cheng, Yuqing Wang, and Kun-Hsuan Chou

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An interesting eyewall evolution occurred in Typhoon Zeb (1998) when it devastated Luzon. The eyewall of Zeb contracted before landfall and broke down and weakened after landfall; then a much larger new eyewall formed and strengthened as it left Luzon and reentered the ocean. The fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) with four nested domains was used to perform numerical experiments to understand the effects of terrain and land surface variation on the observed eyewall evolution. Results show that the presence of Luzon plays a critical role in the observed eyewall evolution. Quite different from the conventional concentric eyewall replacement, the eyewall replacement that occurred in Typhoon Zeb was triggered by the mesoscale landmass and terrain variation with a horizontal scale similar to the core of the typhoon. In Typhoon Zeb, the original eyewall contracted and broke down because of enhanced surface friction after landfall. The outer eyewall was triggered by convective rainbands near the western coastal region of Luzon and formed as a result of axisymmetrization well after the dissipation of the inner eyewall convection.

Several sensitivity experiments were conducted to elucidate the roles of both condensation heating and planetary boundary layer processes in the evolution of the typhoon eyewall. It is found that although condensational heating is the key to the maintenance of the annular potential vorticity (PV) structure, surface friction plays dual roles. Although friction is a sink to PV and thus dissipates PV in the eyewall, it helps keep the PV annulus narrow by enhancing the stretching deformation in the lower troposphere when condensational heating is present. In the absence of condensational heating, however, surface friction enhances the inward PV mixing by boundary layer frictional inflow and thus destroys the PV annulus.

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Chun-Chieh Wu, Shin-Gan Chen, Jan-Huey Chen, Kun-Hsuan Chou, and Po-Hsiung Lin
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Shin-Gan Chen, Chun-Chieh Wu, Jan-Huey Chen, and Kun-Hsuan Chou

Abstract

The adjoint-derived sensitivity steering vector (ADSSV) has been proposed and applied as a guidance for targeted observation in the field programs for improving tropical cyclone predictability, such as The Observing System Research and Predictability Experiment (THORPEX) Pacific Asian Regional Campaign (T-PARC). The ADSSV identifies sensitive areas at the observing time to the steering flow at the verifying time through adjoint calculation. In addition, the ability of the ADSSV to represent signals of influence from synoptic systems such as the midlatitude trough and the subtropical high prior to the recurvature of Typhoon Shanshan (2006) has also been demonstrated.

In this study, the impact of initial perturbations associated with the high or low ADSSV sensitivity on model simulations is investigated by systematically perturbing initial vorticity fields in the case of Shanshan. Results show that experiments with the perturbed initial conditions located in the high ADSSV area (i.e., the midlatitude trough and the subtropical high) lead to more track deflection relative to the unperturbed control run than experiments with perturbations in the low sensitivity area. The evolutions of the deep-layer-mean steering flow and the direction of the ADSSV are compared to provide conceptual interpretation and validation on the physical meaning of the ADSSV. Concerning the results associated with the perturbed regions in high sensitivity regions, the variation of the steering flow within the verifying area due to the initial perturbations is generally consistent with that of the direction of the ADSSV. In addition, the bifurcation between the ADSSV and the steering change becomes larger with the increased integration time. However, the result for the perturbed region in the low-sensitivity region indicates that the steering change does not have good agreement with the ADSSV. The large initial perturbations to the low-sensitivity region may interact with the trough to the north due to the nonlinearity, which may not be accounted for in the ADSSV. Furthermore, the effect of perturbations specifically within the sensitive vertical layers is investigated to validate the vertical structure of the ADSSV. The structure of kinetic energy shows that the perturbation associated with the trough (subtropical high) specifically in the mid-to-upper (mid-to-lower) troposphere evolves similarly to that in the deep-layer troposphere, leading to comparable track changes. A sensitivity test in which perturbations are locally introduced in a higher-sensitivity area is conducted to examine the different impact as compared to that perturbed with the broader synoptic feature.

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