The Influence of Terrain on the Tropical Rainfall Potential

JUNE 2009 L I U 785

The Inﬂuence of Terrain on the Tropical Rainfall Potential Technique in Taiwan

CHUNG-CHIH LIU Teaching Center of Natural Science, Minghsin University of Science and Technology, Hsin-Fong, Hsin-Chu County, Taiwan

(Manuscript received 25 February 2008, in ﬁnal form 5 October 2008)

ABSTRACT

The tropical rainfall potential (TRaP) technique is a simple concept that serves as a useful tool for forecasters in predicting the accumulated rainfall from typhoons. This research seeks to propose an algorithm for improving the accuracy of the results retrieved from the TRaP technique by taking into account the terrain’s inﬂuence on a typhoon’s rainfall pattern over Taiwan. The climatological percentages of the accumulated rainfall in Taiwan for six different typhoon tracks were established via statistical methods. By using the rainfall percentages with the original TRaP technique, results showed that the original TRaP technique could retrieve a graphical representation of the accumulated rainfall from typhoons for both oceans and ﬂat land areas. When factoring in Taiwan’s terrain, the accuracy in a typhoon’s accumulated rainfall estimation is seen to further improve.

1. Introduction vations, and despite the use of numerical weather prediction models, accurate forecasts are still difficult to Typhoons are considered to be extreme weather make. Fortunately, more advanced sensors and better events that pose a serious threat to the island of Taiwan. space–time resolution information available from sat- The livelihood of the people living on the island and the ellites have substantially helped researchers to better general economy are often adversely affected by the assess a typhoon’s rainfall potential, especially for those heavy rainfall and strong winds. Therefore, making ac- storms that approach land but are still out of the coastal curate predictions of the track, rainfall, and wind inten- radar’s range. In the past, satellite remote sensing only sity of typhoons has always been a key focus for scientists utilized the infrared and visible channels in subjectively and weather forecasters. Thanks to the development of and qualitatively determining the intensity of the pre- more in-depth observations, especially in the satellite- cipitation and convection. Later, the addition of micro- monitoring segment (Soden et al. 2001) and those of wave data further improved the quantitative estimation dropwindsondes (Burpee et al. 1996; Aberson and of a typhoon’s rainfall. Therefore, many research centers Franklin 1999; Wu et al. 2005), typhoon forecasting has have focused on utilizing satellite microwave data in improved significantly. However, due to multiscale in- performing a more precise forecast. teractions with a typhoon’s development, movement, This explains the creation of the tropical rainfall po- and structure, it is still an ongoing challenge to accu- tential (TRaP) technique. The development of the rately predict its evolution. This is made more difficult TRaP method has quite a long history (Kidder et al. by the high and complicated mesoscale topography of 2005). Initially, a ‘‘rule of thumb’’ that makes use of the Taiwan’s Central Mountain Range (CMR), which can inverse relationship with a cyclone’s propagation speed create significant mesoscale variations in the rainfall, was used to estimate the tropical cyclones rainfall. How- wind, and pressure distributions of typhoons. ever, this rule did not take into account a cyclone’s size Most typhoons initially form and develop over the and rainfall rate. Spayd and Scofield (1984) then used vast Pacific Ocean. Due to a lack of traditional obser- infrared images to determine the tropical rainfall potential, which is related to the cyclone’s propagation speed and rainfall rate. In more recent years, a tech- Corresponding author address: Chung-Chih Liu, Teaching Center nique called the areal TRaP was proposed (Kidder et al. of Natural Science, Minghsin University of Science and Techno- logy, No. 1, Hsin-Hsing Rd., Hsin-Fong, Hsin-Chu County 30401, 2001a,b). In essence, microwave satellite data are used to Taiwan. graphically illustrate the accumulated rainfall. Ferraro et al. E-mail: [email protected] (2005) used an objective validation package to validate

DOI: 10.1175/2008WAF2222135.1

Ó 2009 American Meteorological Society Unauthenticated | Downloaded 10/03/21 11:57 AM UTC 786 WEATHER AND FORECASTING VOLUME 24 the areal TRaP method by calculating commonly used statistical variables. They found that the satellite-based TRaP technique outperformed the numerical model and demonstrated a high level of accuracy. Taiwan’s CMR influences the rainfall distribution during typhoon landfalls, which applies to both moun- tainous and flat areas. Most of Taiwan’s cities are located in the flat areas, which is also where most of the popu- lation is concentrated. Thus, the goal of this paper is to depict the climatological percentage of the accumulated rainfall in Taiwan’s flat areas for various typhoon tracks via Taiwan’s Central Weather Bureau (CWB) rain gauge data. Another important task is employing the climatological percentage and TRaP technique in an effort to improve the accuracy of the estimated accumulated rainfall amounts for typhoons that pass over Taiwan or make landfall. The structure of this paper is as follows. A brief re- view of Taiwan’s terrain effect on typhoons is explained in section 2. The data collection, processing, and estab- lishment of the climatological percentage of the accumulated rainfall in Taiwan’s flat areas are illustrated in section 3. The methodology for the typhoon rainfall estimation, and the correction of the original TRaP technique results, are described in section 4. The results FIG. 1. The terrain of Taiwan. The black dots depict the locations and a discussion are presented in section 5. Finally, a of rain gauge stations used in this study. summary of this study is given in section 6.

a typhoon’s center can either continuously or discontin- 2. Terrain effect on typhoons in the vicinity uously cross Taiwan’s CMR (Wang 1980; Chang 1982; of Taiwan Yeh and Elsberry 1993a,b; Lin et al. 1999; and Wu and Taiwan’s terrain is extremely complex and unique, Kuo 1999). Weaker typhoons are likely to move past due to its steepness (an average elevation of 3000 m and Taiwan’s terrain in a discontinuous track. Such typhoons a dimension of 300 km 3 100 km) and near north–south are modified significantly by Taiwan’s CMR. Stronger orientation (as shown in Fig. 1). The fact that Taiwan is typhoons are only slightly altered by Taiwan’s CMR, surrounded by ocean and often situated in the path of as they follow a continuous track when approaching typhoons creates a very strong potential for interaction Taiwan. Two or more secondary lows frequently form between Taiwan’s CMR and a typhoon’s rainfall, struc- over the lee (west) side of the CMR during discontinuous- ture, wind intensity, and circulation. For orographic track typhoon cases. Lin et al. (2002, 2005) proposed rainfall, the typhoon precipitation is considered to be some control parameters, which are related to the height related to topography-enhanced moisture flux (Lin et al. and north–south orientation of the CMR, in determining 2001; Wu et al. 2002). Taiwan’s orographic influence on the continuity and deflection of typhoon tracks across a typhoon track has been studied extensively for many mountain range. The control parameters were also ap- years. Brand and Blelloch (1974) found that westward- plied to some typhoon simulations, which were proven to moving typhoons tend to move cyclonically around the be effective (Yang and Ching 2005; Lin et al. 2006). northern side of the CMR. The intensity decreases by an Moreover, Yeh and Elsberry (1993a,b) also showed that average of over 40% when a storm reaches the island. the degree of typhoon track deflection and of track Wang (1980) analyzed the track, intensity, propagation continuity across Taiwan depend on the typhoon’s posi- speed, and evolution of 53 typhoons that approached tion relative to the CMR. Generally, typhoons ap- Taiwan during 1946–75. A conceptual model was pro- proaching the northern part of the CMR tend to follow a posed to explain that the typhoon center tends to move continuous track around the northern end of the island. north when it approaches Taiwan and south after Those approaching the central and southern parts of passing the CMR. Some studies have also revealed that Taiwan’s topography tend to have a discontinuous track.

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FIG. 2. The experimental area (black rectangle). A typhoon’s movement was divided into six tracks (black arrows).

3. Data not all of the typhoons can be observed by the SSM/I. By considering only rainfall from typhoon clouds, 20 ty- Special Sensor Microwave Imager (SSM/I) micro- phoon cases in the period 1992–2004 were chosen for in- wave data were used in this research to estimate a ty- clusion in this research. Typhoons that were accompanied phoon’s rainfall. The SSM/I instrument is located on by other weather systems, such as frontal systems, board Defense Meteorological Satellite Program (DMSP) sun-synchronous satellites, which orbit the earth at a height of 833 km at an inclination angle of 98.88 TABLE 1. Typhoons used for validation in this research during (Hollinger et al. 1990). Generally, any area on the earth 1992–2004. can be observed by the SSM/I sensors in less than 72 h, Track excluding two circular areas within a radius of 280 km category Typhoon name (yr/elapsed time) from the South and North Poles. The SSM/I senses ra- I Aere (2004/28 h) diation at four frequencies (19, 22, 37, and 85 GHz), and II Herb (1996/21 h), Polly (1992/54 h), each channel has two polarization components (vertical Toraji (2001/52 h) and horizontal), except for the 22-GHz channel, which III Omar (1992/29 h), Caitlin (1994/22 h), Tim (1994/22 h), Otto (1998/37 h), has only has one polarization component (vertical). Billis (2000/21 h), Morakot (2003/41 h), Generally, rain begins to fall in Taiwan once IV Kent (1995/8 h), Maggie (1999/11 h), the center of the typhoon enters the area covering Dujuan (2003/16 h), Campasu (2004/3 h) 21.08–26.58N and 118.58–123.08E (hereafter referred to V Seth (1994/12 h), Yanni (1998/32 h), as this study’s experimental area; see Fig. 2). Due to Cimaron (2001/12 h), Conson (2004/10 h) VI Deanna (1995/25 h), Rachel (1999/24 h) limitations in the scanning range of the satellite sensors,

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TABLE 2. The station numbers in the six distinct regions used in this study.

Region Station No. Northern 466880, 466900, 466920, 466940, 467570, 46757 Central 467480, 467490, 467770 Southern 467410, 467420, 467440, 467590, 467780, 467790 Northeastern 467060, 467080 Eastern 466990 Southeastern 467540, 467610, 467660

moved near northern Taiwan, but did not make landfall. The second track included typhoons that originated in the Pacific and made landfall in Taiwan north of 23.58N. The third track was made up of Pacific typhoons that swept Taiwan south of 23.58N. The fourth track had Pacific typhoons that passed south of Taiwan but that did not make landfall. The fifth track included typhoons that passed near eastern Taiwan from lower latitudes and did not make landfall. The sixth track was for typhoons that made landfall over southern Taiwan from the South China Sea. Taking into account the complexity and unique layout of Taiwan (Fig. 1), along with each typhoon’s inherent cyclonic circulation, the island was divided into six distinct regions: north, central, south, northeast, east, and southeast (Fig. 3). The main purpose of this categorization was to understand the distribution of the accumulated rainfall amounts from the aforementioned six

FIG. 3. Taiwan was divided into six areas, which included the different typhoon tracks. The six areas are also regions northern, central, southern, northeastern, eastern, and southeastern regions.

TABLE 3. Typhoons used in this research to depict the climatological southwest air current systems, or the northeastern percentage of the accumulated rainfall in Taiwan. monsoon, were not included. The exclusion also helped Track reduce the complexity in analyzing the relationship category Typhoon name (yr/elapsed time) between the rainfall and Taiwan’s terrain. The typhoon I Fran (1970/35 h), Winnie (1972/10 h), cases that were used to validate the algorithm proposed in Betty (1972/13 h), Maury (1981/19 h), this research are listed in Table 1. The elapsed time period Nelson (1985/28 h), Ellie (1991/46 h) (showninTable1)wasdefinedastheperiodwhenthe II Agnes (1971/29 h), Bess (1971/21 h), typhoon’s center was located within the experimental area. Nina (1975/21 h),Billie (1976/21 h), Provided by Taiwan’s Central Weather Bureau, tipping- Della (1978/22 h), Norris (1980/27 h), Freda (1984/15 h), Yancy (1990/54 h), bucket rain gauge data from 1966 to 2004 were used in Gladys (1994/23 h), Amber (1997/33 h) this study. The unit of measurement was millimeters. III Nadine (1971/22 h), Betty (1975/26 h), When the rainfall amount exceeded 0.3 mm, the bucket Dot (1982/24 h), Dot (1990/21 h) would empty and a computer would immediately record IV Elsie (1975/32 h), Hope (1979/18 h), the data. The rainfall is also automatically recorded Gordon (1979/19 h), Ida (1980/32 h), Percy (1980/25 h), Wynne (1984/20 h), after each minute. Val (1985/30 h), Lynn (1987/66 h), Amy (1991/17 h) As the direction of the typhoon’s track is believed to V Ora (1978/25 h), Irma (1981/12 h), be correlated to the way the accumulated rainfall is Brenda (1985/14 h), Zeb (1998/22 h), distributed in Taiwan, the typhoon direction was di- Xangsane (2000/19 h) vided into six tracks (as shown in Fig. 2). The first track VI Elsie (1966/28 h), Judy (1966/25 h), Ike (1981/29 h), Susan (1988/22 h) featured typhoons that originated from the Pacific and

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FIG. 4. The average rainfall distribution for six different tracks when a typhoon’s center was inside the experimental area. (Thirty-eight typhoons were chosen from 1966 to 2000.) that local weather forecasters are most interested in un- tral, southern, northeastern, eastern, and southeastern derstanding. The average accumulated rainfall of several Taiwan), along with the total accumulated rainfall in rain gauge stations (Table 2) during the elapsed time Taiwan (the sum of the average accumulated rainfall in period represented the accumulated rainfall for each re- each region), were calculated. Their ratios were used to gion (northern, central, southern, northeastern, eastern, delineate the rainfall distribution when the typhoon and southeastern Taiwan). The sum of the average passed over Taiwan or made landfall (Fig. 4). Generally rainfall amount for the six different regions is defined speaking, the distributions of the accumulated rainfall as the total accumulated rainfall over Taiwan when a amounts over Taiwan for the six different typhoon typhoon’s center is located within the experimental area. tracks were very different but were consistent with the Thirty-eight typhoons (Table 3), which followed one rainfall distribution concept model. It is worth noting of the aforementioned typhoon tracks between 1966 that although the numbers and densities of the auto- and 2000 (not including the ones shown in Table 1), were matic rain gauge stations are much larger than those selected for this research. The Joint Typhoon Warning of the manually observed rain gauge stations in Taiwan, Center’s (JTWC) best-track data were used in acquiring the former, which were disposed in 1987, did not cover the position of the typhoon’s center every 15 min by an the entire island until 1998. Therefore, there it is diffi- interpolation technique. Once the typhoon’s center cult to sample enough typhoon cases for both calibra- entered the experimental area, calculation of the re- tion and verification through the automatic rain gauge corded rainfall by each rain gauge station would begin data. However, this study conducted a statistical anal- and continue until the center left the experimental area. ysis of the accumulated rainfall distributions of all the For each respective typhoon track, the average accu- typhoon landfall cases from 1998 to 2004 (Tables 1 and mulated rainfall amount in each region (northern, cen- 3). Figure 5 shows the correlation between the rainfall

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FIG. 5. The correlation between the rainfall distribution obtained from the automatic (height of rain gauge station lower than 200 m) and manually observed rain gauge data for all the typhoon landfall cases from 1998 to 2004 (Tables 1 and 3).

FIG. 6. Flow chart of the algorithm correcting the original TRaP technique results for a typhoon’s accumulated rainfall estimation.

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21 FIG. 7. The rainfall rate distribution (mm h ) of Typhoon Aere (2004) at 0955 UTC 24 Aug 2004.

FIG. 8. The accumulated rainfall distribution (mm) when the center of Typhoon Aere (2004) was located within the experimental area (21.08–26.58N, 118.58–123.08E) by using the original TRaP technique.

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FIG. 9. The accumulated rainfall from the CWB’s rain gauge data, and the original and corrected TRaP technique results in Taiwan’s local areas for typhoon track I. distribution obtained from the automatic and manually observed rain gauge datasets. The strong correlation FIG. 10. The best typhoon tracks sorted out for typhoon track II demonstrates that the two respective rain gauge data- during 1992–2004. sets are statistically equivalent. Thus, it should be fea- sible to use the manually observed rain gauge data in phoon’s center entered the experimental area (tstart) and this study. the time it departed (tdepart). Since R has units of millimeters per hour, the unit of TRaP is in millimeters. [See Kidder et al. (2005) for a more detailed explana- 4. Methodology tion.] The calculation is easy to perform; thus, the distri- Many algorithms have been proposed for estimating bution of a typhoon’s accumulated rainfall over Taiwan’s the rainfall of storms via satellite microwave data. A third-generation operational SSM/I rainfall retrieval algorithm was developed by the National Oceanic and Atmospheric Administration/National Environmental Satellite, Data, and Information Service (NOAA/NESDIS) is still in use. To allow weather forecast centers to ef- fectively apply the results of this paper, the SSM/I rainfall retrieval algorithm used by NESDIS was also employed. This is due to its versatile rainfall estimations from an 85-GHz scattering standpoint over land, and a combination of the 85-GHz scattering and 19-/37-GHz emission approaches over the ocean (Ferraro and Marks 1995; Ferraro 1997). Following Kidder et al. (2005), the accumulated rainfall was calculated as follows: ð tdepart TRaPðxj, yjÞ 5 Rðxj, yjÞdt, tstart where R(xj, yj) is the rainfall in the pixel that will be over point (xj, yj) at time t, between the times when the ty- FIG. 11. As in Fig. 9 but for typhoon track II.

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21 FIG. 12. The rainfall rate distribution (mm h ) of Typhoon Toraji (2001) at 2152 UTC 28 Jul 2001. regions can be obtained. However, due to the island’s in order to evaluate the algorithm proposed by this re- complex terrain, the accumulated rainfall calculated by search. For track I, only one typhoon (Aere, 2004) was the original TRaP technique needs to be corrected. This selected; Aere originated at 25.48N, 123.38E. Typhoon is done by ﬁrst drawing an accumulated rainfall graph Aere entered the experimental area at 0955 UTC on 24 using the SSM/I data and the TRaP technique. The August 2004, with an elapsed time of 28 h. The rainfall TRaP-retrieved accumulated rainfall amounts at the lo- rate distribution of Typhoon Aere (2004) at 0955 UTC cations of the 21 rain gauge stations are then selected. In is shown in Fig. 7. Since Typhoon Aere only moved past addition, the average of the TRaP-retrieved accumulated the oceans of northern Taiwan, and since the rainfall rainfall for the stations in northern (central, southern, areas was mainly concentrated around the typhoon’s northeastern, eastern, and southeastern) Taiwan is ac- northern part, the original TRaP technique showed the quired. Then, the average of the TRaP-retrieved accu- main accumulated rainfall areas to be concentrated near mulated rainfall for all six regions is summed and viewed the region north of 238N (shown Fig. 8). Only light as the total TRaP-retrieved accumulated rainfall over rainfall occurrences were observed in southern Taiwan. Taiwan. Second, a typhoon track is categorized. The total The accumulated rainfall amount from the tipping- TRaP-retrieved accumulated rainfall over Taiwan is bucket rain gauge, as well as those from the original and multiplied by the ratios of the six regions for the cate- corrected TRaP technique results, are shown in Fig. 9. It gorized typhoon track (Fig. 5) in retrieving the corrected is shown that the correction of the original TRaP tech- TRaP accumulated rainfall for each region. A ﬂow chart nique can improve the overall accuracy of the accumu- illustrating the correction of the original TRaP technique lated rainfall estimation in Taiwan’s local areas. The results is presented in Fig. 6. root-mean-square error (RMSE) decreased by 18.3 mm. Three independent typhoons were selected for track II (Fig. 10 shows their best tracks). The accumulated 5. Discussion rainfall amount from the tipping-bucket rain gauge, as Twenty independent typhoons from between 1992 and well as the original and corrected TRaP techniques, are 2004 were chosen for inclusion in this study (Table 1) shown in Fig. 11. Although the original TRaP technique

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FIG. 13. The accumulated rainfall distribution (mm) when the center of Typhoon Toraji (2001) was located within the experimental area (21.08–26.58N, 118.58–123.08E) by using the original TRaP technique. shows better accuracy for northern Taiwan, the oppo- the original and corrected TRaP techniques, are shown site is true for the remaining ﬁve regions. The average in Fig. 15. Excluding the northern region, the correction RMSE decreased by 60.3 mm. The tracks in Fig. 10 once again improved the accuracy of the accumulated show all of the typhoons moving in a direction that puts rainfall; the RMSE fell by 26.9 mm. The tracks in Fig. 14 them close to the northern half of Taiwan’s eastern shows all the typhoons moving in a direction near the coastline. Due to the terrain effects and the typhoon’s southern half of Taiwan’s eastern coastline. Due to the cyclonic circulation, the accumulated rainfall should be interaction with Taiwan’s CMR, a higher rainfall amount less in southern and southeastern Taiwan than in other should be expected in the eastern and southeast parts of regions. Figure 12 shows the rainfall rate estimation Taiwan. The corrected results (Fig. 15) do indeed cor- distribution of Typhoon Toraji (2001) at 2152 UTC on respond to this inference. Figure 16 shows the estimated 28 July 2001. The heavy rainfall area was located near rainfall rate distribution of Typhoon Tim (1994) at 0909 the southern section of the typhoon. Based on the as- UTC on 9 July 1994. The concentrated rainfall areas are sumption that there is little variation in the typhoon’s located near the southern section of the typhoon. Based structure, the accumulated rainfall was derived and seen on the assumption that there is little variation in the to be concentrated near the central, south, and south- typhoon’s structure, the accumulated rainfall around eastern areas (Fig. 13). By considering the terrain ef- Taiwan was derived and was found to be concentrated fects, the accumulated rainfall was reduced from 302.4 near the southern and southeastern areas (Fig. 17). to 54.9 mm for southern Taiwan, and from 292.6 to 49.0 After correcting the original TRaP technique results, mm for southeastern Taiwan. This demonstrates that the accumulated rainfall increased from 38.1 to 140.3 the terrain plays a very important role in the distribu- mm in eastern Taiwan, from 106.2 to 137.5 mm in tion of the accumulated rainfall. southeastern Taiwan, from 67.0 to 9.6 mm in central Six typhoons were picked for track III (Fig. 14 shows Taiwan, and from 117.0 to 47.1 mm in southern Taiwan. their best tracks). The accumulated rainfall amount Again, the results show the terrain greatly inﬂuencing from the tipping-bucket rain gauge, as well as those from the accumulated rainfall distribution. Figures 18–20,

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FIG. 15. As in Fig. 9 but for typhoon track III.

ratio of the number of correct forecasts to the number of observations. The FAR is the ratio of the number of FIG. 14. As in Fig. 10 but for typhoon track III. false alarms to the number of forecasts. The POD and FAR can range from zero to one: POD 5 1(i.e.,FAR5 0) indicates that all of the observations were correctly respectively, depict the best tracks for the selected predicted. As seen from Fig. 22, over 90% of the rainfall typhoons—designated as track IV, V, and VI. The cor- is detected by the original TRaP while 100% is found rected approach reduced the RMSE by 3.0 mm on av- for the corrected TRaP. The TRaP technique performs erage (ﬁgure omitted). The accumulated rainfall amount well in dichotomous (yes–no) forecasts, though the from the tipping-bucket rain gauge, and those from the corrected TRaP outperformed the original TRaP, original and corrected TRaP techniques for all the ty- demonstrating a high level of accuracy. The results show phoons in Table 1, are shown in Fig. 21. Meanwhile, that the corrections proposed by this study to the TRaP some other forecast evaluation parameters, such as the technique results deliver more accurate accumulated bias score (BS), threat score (TS), equitable threat score rainfall estimations when typhoons passed over Taiwan (ETS), probability of detection (POD), and false alarm or made landfall. The algorithm used in this research rate (FAR), were also used in this study (shown in Fig. 22). was processed in an automated manner with no human The BS measures the ratio of the frequency of forecast intervention. This enables the method to be continu- rainfall events to the frequency of observed rainfall ously used throughout a typhoon’s life cycle, and not events, ranging from zero to inﬁnity (the perfect score 5 1). merely limited to within 24–36 h before landfall. The The BS values are 1.06 and 0.95, respectively, for the technique can serve as an important supplementary tool corrected and original TRaP results in this study. The for weather forecast centers. TS measures the ratio of the forecast rainfall events that were correctly predicted, ranging from zero to one 6. Conclusions (TS 5 0 indicates no skill; the perfect score 5 1). The TS is 0.86 for the original TRaP and 0.94 for the corrected Of all the natural disasters occurring in Taiwan, ty- TRaP. In other words, the latter provides a better phoons are considered to be the most serious. Projecting forecast of the rainfall events. Another statistical pa- the amount and location of a typhoon’s heavy rainfall rameter that is widely used in assessing the forecasting has always been a challenge to forecasters. Kidder et al. skill and rainfall validation is the ETS. The ETS ranges (2005) pointed out that the TRaP method, with its from 21/3 to 1 (ETS 5 0 indicates no skill; the perfect simple concept and graphical representation of the score 5 1). There is a slight difference between the technique, can be quite useful to forecasters. However, original and corrected TRaP results. The POD is the the terrain of Taiwan alters the rainfall distribution of

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21 FIG. 16. The rainfall rate distribution (mm h ) of Typhoon Tim (1994) at 0909 UTC 9 Jul 1994.

FIG. 17. The accumulated rainfall distribution (mm) when the center of Typhoon Tim (1994) was located within the experimental area (21.08–26.58N, 118.58–123.08E) by using the original TRaP technique.

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FIG. 18. As in Fig. 10 but for typhoon track IV. FIG. 20. As in Fig. 10 but for typhoon track VI.

typhoons. Following the original TRaP technique, the rainfall estimation algorithm, the typhoon’s structure, and its cloud shape were considered to be adequate. The present research focuses on how the terrain inﬂuences a typhoon’s accumulated rainfall in Taiwan. During the calculations, JTWC’s best typhoon tracks were used to simplify this complex problem. When the correction of the original TRaP technique results becomes even more acceptable, it can subsequently be applied by relevant weather centers. Forecasters can also factor in several probable typhoon tracks in obtaining different scenarios for how the accumulated rainfall may be distributed over a region. This is considered to be particularly important to government agencies that handle natural disasters. Although the original TRaP technique can produce maps for delineating a typhoon’s accumulated rainfall for oceans or ﬂat land, after considering Taiwan’s terrain effects, the corrections made to the method produce even more accurate results. However, it is believed that further improvements can still be achieved in future studies. This includes producing an even more com- prehensive typhoon categorization by taking into account more typhoon tracks (only six tracks were used in our present research), along with consideration of the radius size and wind speed. In addition, the change in a typhoon’s structure and shape should also be consid- FIG. 19. As in Fig. 10 but for typhoon track V. ered during the calculation. Finally, the inclusion of a

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SAA system for the SSM/I data. This work was sup- ported in part by grants from NSC (96-2625-Z-159-002 and 96-2745-M-159-001).

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