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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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Kosuke Ito
and
Chun-Chieh Wu

Abstract

A new sensitivity analysis method is proposed for the ensemble prediction system in which a tropical cyclone (TC) position is taken as a metric. Sensitivity is defined as a slope of linear regression (or its approximation) between state variable and a scalar representing the TC position based on ensemble simulation. The experiment results illustrate important regions for ensemble TC track forecast. The typhoon-position-oriented sensitivity analysis (TyPOS) is applied to Typhoon Shanshan (2006) for the verification time of up to 48 h. The sensitivity field of the TC central latitude with respect to the vorticity field obtained from large-scale random initial perturbation is characterized by a horizontally tilted pattern centered at the initial TC position. These sensitivity signals are generally maximized in the middle troposphere and are far more significant than those with respect to the divergence field. The results are consistent with the sensitivity signals obtained from existing methods. The verification experiments indicate that the signals from TyPOS quantitatively reflect an ensemble-mean position change as a response to the initial perturbation. Another experiment with Typhoon Dolphin (2008) demonstrates the long-term analysis of forecast sensitivity up to 96 h. Several additional tests have also been carried out to investigate the dependency among ensemble members, the impacts of using different horizontal grid spacing, and the effectiveness of ensemble-Kalman-filter-based perturbations.

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Jae-Deok Lee
and
Chun-Chieh Wu

Abstract

High-resolution numerical experiments for Typhoon Megi (2010) in the western North Pacific are conducted using the Advanced Research version of the Weather Research and Forecasting (WRF) Model to understand the mechanisms of rapid intensification (RI). With a dynamically initialized vortex, sensitivity experiments are carried out focusing on the planetary boundary layer (PBL) and the following microphysics schemes: WRF single-moment 6-class (WSM6) and WRF double-moment 6-class (WDM6) microphysics with Yonsei University, Mellor–Yamada–Janjić (MYJ), and Mellor–Yamada–Nakanishi–Niino (MYNN) 2.5-level (MN2.5) and 3.0-level (MN3) PBL schemes. The largest differences are found between WSM6-MN3 and WDM6-MN3, and we therefore examine RI mechanisms based on the results of these experiments. Prior to RI, WDM6-MN3 shows a drier environment and stronger downdrafts in the lower troposphere than in WSM6-MN3. As a result, during the RI period, WSM6-MN3 (WDM6-MN3) significantly intensifies with the minimum sea level pressure decreasing by 51 (29) hPa and the maximum surface wind increasing by 28 (12) m s−1 in 24 h. In both experiments, the maximum values of surface heat fluxes, potential vorticity (PV), radial absolute angular momentum advection, inertial stability, supergradient wind, and convective bursts inside the radius of maximum winds are frequently observed at each vertex of polygonal eyewalls in the lower troposphere. In particular, WSM6-MN3 exhibits more convective cells inside the inner-core region, a more persistent and thicker polygonal eyewall in the lower troposphere, and a more robust vertical structure of hydrometeors and vertical velocity than WDM6-MN3. This study suggests that within the inner-core region, polygonal eyewalls like WSM6-MN3 provide favorable conditions for RI.

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Chun-Chieh Wu
and
Yoshio Kurihara

Abstract

The interaction between a hurricane and its environment is studied by analysing the generation and influence of potential vorticity (PV) from the Geophysical Fluid Dynamics Laboratory hurricane model analysis system. Two sets of numerical experiments are performed: one with and the other without a bogused hurricane vortex in the initial time, for cases of Hurricanes Bob (1991), Gilbert (1988), and Andrew (1992).

The PV budget analysis of Bob shows that the condensational heating within the vortex redistributes the PV, causing a PV sink in the upper part of the vortex and a PV source in the lower part. This tendency is compensated for largely, but not entirely, by the upward transport of high-PV air from the lower levels to the upper levels. The net effect contributes to the increase of the negative upper-level PV anomaly during the vortex intensification period. This result indicates that the diabatic heating effect plays a crucial role in the evolution of the PV field in hurricanes. It also suggests the importance of accurate representation of the heating profile in hurricane models.

It is shown that the negative upper-level PV anomaly is spread out by the upper-level outflow and the large-scale background flow. The impact of the spread of the negative upper PV anomaly to the storm is quantitatively evaluated by computing the nonlinear balanced flow associated with the PV perturbation. Notable contribution to the steering of the storm from the upper-level PV anomaly is found. The result supports the theory advanced by Wu and Emanuel concerning the effect of the upper negative PV anomaly on hurricane motion. This study also indicates the need of enhanced observation and accurate analysis and prediction in the upper troposphere in order to improve hurricane track forecasting.

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Chun-Chieh Wu
and
Kerry A. Emanuel

Abstract

Most extant studies of tropical cyclone movement consider a barotropic vortex on a β plane. However, observations have shown that real tropical cyclones are strongly baroclinic, with broad anticyclones aloft. Also, the distribution of the large-scale potential vorticity gradient in the tropical atmosphere is very nonuniform. These properties may substantially influence the movement of such storms.

Note that the anticyclone above a hurricane will interact with the lower hurricane vortex and induce storm motion. Such interaction can be caused by both the direct effect of ambient vertical shear and the effect of vertical variation of the background potential vorticity gradient. In this paper, an attempt to isolate the effect of background vertical shear is made. The hurricane is represented in a two-layer quasigeostrophic model as a point source of mass and zero potential vorticity air in the upper layer, collocated with a point cyclone in the lower layer. The model is integrated by the method of contour dynamics and contour surgery.

The results show that Northern Hemisphere tropical cyclones should have a component of drift relative to the mean flow in a direction to the left of the background vertical shear. The effect of weak shear is also found to be at least as strong as the β effect, and the effect is maximized by a certain optimal ambient shear. The behavior of the model is sensitive to the thickness ratio of the two layers and is less sensitive to the ratio of the vortices' horizontal scale to the radius of deformation. Storms with stronger negative potential vorticity anomalies tend to exhibit more vortex drift.

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Chun-Chieh Wu
and
Kerry A. Emanuel

Abstract

Flow fields from the model of Wu and Emanuel are presented. It is demonstrated that the interaction of background shear with a uniform source of low potential vorticity air at the storm top can produce many of the observed characteristics of hurricane outflow, including outflow jets. Our model is compared to other extant models of hurricane outflow.

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Cheng-Hsiang Chih
and
Chun-Chieh Wu

Abstract

The statistical relationships between tropical cyclones (TCs) with rapid intensification (RI) and upper-ocean heat content (UOHC) and sea surface temperature (SST) from 1998 to 2016 in the western North Pacific are examined. RI is computed based on four best track datasets in the International Best Track Archive for Climate Stewardship (IBTrACS). The statistical analysis shows that the UOHC and SST are higher in the RI duration than in non-RI duration. However, TCs with high UOHC/SST do not necessarily experience RI. In addition, the UOHC and SST are lower in the storm inner-core region due to storm-induced ocean cooling, and the UOHC reduces more significantly than the SST along the passages of TCs in the lower-latitude regions. Moreover, most of the RI (non-RI) duration is associated with the higher (lower) UOHC, but this is not the case for the SST pattern. Meanwhile, the TC intensification rate during the RI period does not appear to be sensitive to the SST, but shows statistically significant differences in the UOHC. In addition, there is a statistically significant increasing trend in the UOHC underlying TCs from 1998 to 2016. It is also noted that the percentages of the TCs with RI show different polynomial and linear trends based on different calculations of the RI events and RI durations. Finally, it is shown that there is no statistically significant difference in the UOHC, SST, and the percentage of RI among the five categories of ENSO events (i.e., strong El Niño, weak El Niño, neutral, weak La Niña, and strong La Niña).

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Chun-Chieh Wu
and
Ying-Hwa Kuo

Of all the natural disasters occurring in Taiwan, tropical cyclones are the most serious. Over a 20-yr period, Taiwan was hit by an average of 3.7 typhoons per year. These storms can produce heavy rainfall and strong winds, leading to severe damage to agriculture and industry, and serious loss of human life. An outstanding example is Typhoon Herb, which made landfall in Taiwan on 31 July 1996. Typhoon Herb took 70 lives and caused an estimated $5 billion of damage to agriculture and property.

Accurate prediction of the track, intensity, precipitation, and strong winds for typhoons affecting Taiwan is not an easy task. The lack of meteorological data over the vast Pacific Ocean and the strong interaction between typhoon circulation and Taiwan's mesoscale Central Mountain range are two major factors that make the forecasting of typhoons in the vicinity of Taiwan highly challenging. Improved understanding of the dynamics of typhoon circulation and their interaction with the Taiwan terrain is needed for more accurate prediction. With this objective in mind, the National Science Council in Taiwan sponsored the Workshop on Typhoon Research in the Taiwan Area at Boulder, Colorado, on 17–18 May 1997. In this paper, the authors review the observational and numerical studies of typhoons affecting Taiwan, present some preliminary results from the study of Typhoon Herb, summarize the recommendations obtained from the workshop, and provide suggestions for future research.

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Yu-An Chen
and
Chun-Chieh Wu

Abstract

The interaction between Typhoon Nepartak (2016) and the upper-tropospheric cold low (UTCL) is simulated to better understand the impact of UTCL on the structural and intensity change of tropical cyclones (TCs). An experiment without UTCL is also performed to highlight the quantitative impacts of UTCL. Furthermore, idealized sensitivity experiments are carried out to further investigate the specific TC–UTCL configurations leading to different interactions. It is shown that a TC interacting with the UTCL is associated with a more axisymmetric inner-core structure and an earlier rapid intensification. Three plausible mechanisms related to the causality between a UTCL and the intensity change of TC are addressed. First, the lower energy expenditure on outflow expansion leads to higher net heat energy and intensification rate. Second, the external eddy forcing reinforces the secondary circulation and promotes further TC development. Ultimately, the shear-induced downward and radial ventilation of the low-entropy air is unexpectedly reduced despite the presence of UTCL, leading to stronger inner-core convections in the upshear quadrants. In general, the TC–UTCL interaction process of Nepartak is favorable for TC intensification owing to the additional positive effect and the reduced negative effect. In addition, results from sensitivity experiments indicate that the most favorable interaction would occur when the UTCL is located to the north or northwest of the TC at a stable and proper distance of about one Rossby radius of deformation of the UTCL.

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Chih-Chi Hu
and
Chun-Chieh Wu

Abstract

Ensemble sensitivity analysis based on convective-permitting ensemble simulations is used to understand the processes associated with tropical cyclone (TC) intensification under idealized conditions. Partial correlations between different variables and the future TC intensification rate, with the effect of intensity removed, are used to identify the sensitive factors. It is found that the equivalent potential temperature (θ e ) in the region from the radius of maximum wind (RMW) to 3 times the RMW below 2 km (hereafter, the sensitive region) has the largest correlation (over 0.7) with 2.5-h intensity change. It is found that higher θ e in the sensitive region is associated with not only a stronger updraft but also an inward shift of vertical motion in the mid- to upper eyewall. This suggests that higher θ e just outside the RMW is favorable to TC intensification not only because of the larger amount of the heating, but also due to the heating location that is closer to the center. Trajectory analysis shows that the parcels in the sensitive region are mainly from the boundary layer inflow and the midlevel inflow. It is found that when the outer rainband is active, the midlevel inflow becomes stronger and is able to bring more low-θ e air into the boundary layer, and the θ e radially inward to the rainband decreases. Verification experiments justify that higher θ e around the RMW to 3 times the RMW is favorable to TC intensification, while higher θ e away from 5 times the RMW is shown to be unfavorable for TC intensification.

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