Search Results

You are looking at 1 - 4 of 4 items for

  • Author or Editor: Tzu-Hsiung Yen x
  • All content x
Clear All Modify Search
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
Chun-Chieh Wu, Shin-Gan Chen, Shih-Chieh Lin, Tzu-Hsiung Yen, and Ting-Chen Chen

Abstract

Using special data from the field campaign of 2008 and an ensemble Kalman filter–based vortex initialization method, this study explores the impact of different track clusters categorized under the ensemble simulations of Typhoon Sinlaku (2008) on the associated precipitation. In particular, the distinct pattern of cumulative frequencies in the 28 members is identified to correspond to three types of track clusters. The simulation integrated from the initial ensemble mean slightly underestimates the maximum amount of the observed rainfall in central Taiwan by about 30%. The quantitative evaluation based on the equitable threat score indicates that members with tracks close to the best track produce more consistent rainfall distribution in northern Taiwan although their cumulative frequencies are underestimated. For members with southwestward-biased tracks, although the cumulative frequencies are closer to the observation, the simulated rainfall pattern is less consistent with the observation in northern Taiwan and the maximum rainfall amount is overestimated. The comparison of rainfall simulation during landfall between two representative members shows that the distinct differences in the rainfall amount and distribution are primarily associated with the track differences on the windward side of the mountain. With a finer horizontal grid resolution, the rainfall accumulation becomes greater as a result of the enhancement of updraft from the better-resolved topography, yet the cumulative frequency stays nearly unchanged. Based on ensemble simulations, this study highlights that the uncertainties in rainfall patterns and amounts can be assessed from ensemble track variations, thus providing better insights into the rainfall predictability associated with typhoons near Taiwan.

Full access
Chun-Chieh Wu, Tzu-Hsiung Yen, Ying-Hwa Kuo, and Wei Wang

Abstract

In this study, a series of numerical experiments are performed to examine the ability of a high-resolution mesoscale model to predict the track, intensity change, and detailed mesoscale precipitation distributions associated with Typhoon Herb (1996), which made landfall over Taiwan. The fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5), with a 2.2-km horizontal grid spacing, successfully simulates the mesoscale rainfall distribution associated with Herb, and the predicted maximum 24-h rainfall of 1199 mm accounts for about 70% of the observed amount of 1736 mm at Mount A-Li. It is shown that, with an accurate track simulation, the ability of the model to simulate successfully the observed rainfall is dependent on two factors: the model's horizontal grid spacing and its ability to describe the Taiwan terrain. The existence of the Central Mountain Range has only a minor impact on the storm track, but it plays a key role in substantially increasing the total rainfall amounts over Taiwan. The analysis presented here shows that the model and terrain resolutions play a nearly equivalent role in the heavy precipitation over Mount A-Li. The presence of maximum vertical motion and heating rate in the lower troposphere, above the upslope mountainous region, is a significant feature of forced lifting associated with the interaction of the typhoon's circulation and Taiwan's mountainous terrain. Overall, Typhoon Herb is a case in point to indicate the intimate relation between Taiwan's topography and the rainfall distribution associated with a typhoon at landfall.

Full access
Chun-Chieh Wu, Tzu-Hsiung Yen, Yi-Hsuan Huang, Cheng-Ku Yu, and Shin-Gan Chen

Abstract

This study utilizes data compiled over 21 years (1993–2013) from the Central Weather Bureau of Taiwan to investigate the statistical characteristics of typhoon-induced rainfall for 53 typhoons that have impacted Taiwan. In this work the data are grouped into two datasets: one includes 21 selected conventional weather stations (referred to as Con-ST), and the other contains all the available rain gauges (250–500 gauges, mostly automatic ones; referred to as All-ST). The primary aim of this study is to understand the potential impacts of the different gauge distributions between All-ST and Con-ST on the statistical characteristics of typhoon-induced rainfall. The analyses indicate that although the average rainfall amount calculated with Con-ST is statistically similar to that with All-ST, the former cannot identify the precipitation extremes and rainfall distribution appropriately, especially in mountainous areas. Because very few conventional stations are located over the mountainous regions, the cumulative frequency obtained solely from Con-ST is not representative. As compared to the results from All-ST, the extreme rainfall assessed from Con-ST is, on average, underestimated by 23%–44% for typhoons approaching different portions of Taiwan. The uneven distribution of Con-ST, with only three stations located in the mountains higher than 1000 m, is likely to cause significant biases in the interpretation of rainfall patterns. This study illustrates the importance of the increase in the number of available stations in assessing the long-term rainfall characteristic of typhoon-associated heavy rainfall in Taiwan.

Full access