The Rainfall Characteristics of Taiwan

Transcription

1 JULY 2003 CHEN AND CHEN 1323 The Rainfall Characteristics of Taiwan CHING-SEN CHEN Institute of Atmospheric Physics, National Central University, Chung-Li, Taiwan YI-LENG CHEN Department of Meteorology, University of Hawaii, Honolulu, Hawaii (Manuscript received 26 March 2002, in final form 10 December 2002) ABSTRACT The rainfall regimes of Taiwan are investigated using the 18-yr European Centre for Medium-Range Weather Forecasts (ECMWF) data ( ), the available 38-yr daily rainfall data from 25 conventional surface stations around Taiwan ( ), and the 5-yr hourly rainfall data ( ) from 249 high-spatial-resolution Automatic Rainfall and Meteorological Telemetry System (ARMTS) stations. Rainfall over the island is usually generated either by transient disturbances embedded in the prevailing monsoon flow or local rainshowers related to terrain or local winds. With the change in the direction of the prevailing winds between the warm and cold seasons as well as a variety of transient subsynoptic disturbances occurring in different seasons (e.g., winter monsoon cold surges, springtime cold fronts, mei-yu fronts in the early summer, typhoons in summer months, and cold fronts in fall) and the presence of the Central Mountain Range, the regional rainfall climate over the island shows large spatial and temporal variabilities. Nevertheless, despite the presence of these transient disturbances, it is shown that the horizontal distributions of climatological rainfall patterns for different rainfall regimes are strongly dependent on the direction of the low-level prevailing flow with much higher rainfall on the windward side. Furthermore, the seasonal variations in rainfall amount and type (light precipitation versus convective precipitation) are also dependent on the thermodynamic stratification and the availability of moisture. 1. Introduction Taiwan is located in the subtropics off the southeastern coast of China. Its climate is strongly affected by the East Asian monsoons. The presence of the Central Mountain Range (CMR) causes large spatial variations in the island climate throughout the year. The island terrain is dominated by the almost north south-orientated CMR, which has an average height of about 2 km and peaks of nearly 4 km (Fig. 1). The climate diversity over Taiwan is most evident in terms of the striking spatial variations in rainfall. It is apparent that to understand the regional climate over Taiwan, it is necessary to study the climate systems from the planetary scale down to the island scale because of the presence of mountainous terrain. Studies of island-scale regional climate will have a variety of important agricultural, hydrological, and meteorological applications. In this study, the effects of the annual march of the atmospheric circulations over East Asia on the rainfall characteristics Corresponding author address: Dr. Ching-Sen Chen, Institute of Atmospheric Physics, National Central University, Chung-Li 320, Taiwan. tchencs@atm.ncu.edu.tw over the island of Taiwan will be investigated from the available observational data. Precipitation in Taiwan is influenced by the northeasterly monsoon (September April) during the cold season and the southwesterly monsoon (May August) during the warm season (Tao and Chen 1987; Boyle and Chen 1987; Chen et al. 1999). Analyzing 39-yr of rainfall data from nine stations, Wang et al. (1984) categorized the rainfall regimes in Taiwan into the following five categories: winter (December February), the spring transition (March and April), the mei-yu season (mid- May to mid-june), the typhoon season (mid-july to August), and the autumn rainfall regime (September November). Nevertheless, due to the lack of adequate meteorological data, the linkage between the seasonal large-scale circulations and the various rainfall regimes, especially the impact of the CMR on the spatial distribution of rainfall for each regime, has not been discussed extensively. To accomplish this objective, we first describe the annual progression of the airflow and thermodynamic properties over Taiwan. This serves as the necessary background information for the study of rainfall regimes in Taiwan. Then, we identify and discuss various rainfall regimes for different parts of the island based on the archived rainfall data from 25 con American Meteorological Society

2 1324 MONTHLY WEATHER REVIEW VOLUME 131 FIG. 1. Taiwan topography with contours (solid lines) representing 500 and 1500 m. There are 274 rainfall stations composing 25 conventional stations (plus symbols) and 249 Automatic Rainfall and Meteorological Telemetry System stations (diamond symbols). ventional surface stations. In addition, the recent deployment of the dense Automatic Rainfall and Meteorological Telemetry System (ARMTS; Chen et al. 1999) allows us to study the horizontal distributions of various rainfall regimes under different prevailing flows during the annual cycle. Within each season, there are transient subsynoptic disturbances embedded within the prevailing flow. Except during the warm season, when afternoon local rainshowers are frequent, most of the rainfall over Taiwan occurs during the passage of these transient subsynoptic disturbances. An overview of these transient disturbances is given in section 2. Data used and analysis procedures are given in section 3, followed in sections 4 8 by a discussion of the evolution of the mean circulation patterns and rainfall regimes during the annual cycle. Finally, a summary and conclusions are given in the last section. 2. An overview of transient disturbances over the Taiwan area In winter, cold surges with a sharp temperature drop are accompanied by strong northeasterly winds over eastern China, Taiwan, and the South China Sea (Murakami 1979; Lau and Lau 1984; Chen et al. 2002; Yen and Chen 2002). Yeh and Chen (1984) show that during , the total number of cold fronts passing Taipei (northern Taiwan) for these years increased from 25 in November to more than 40 for each month of December, January, and February. The precipitation during the winter months occurs mainly over the windward northeastern and eastern coastal areas, with much less rainfall over the western side of the island (Central Weather Bureau 1990; Chen and Huang 1999). With the arrival of springtime cold fronts (Yeh and Chen 1984), thunderstorm activities commence in March over Taiwan. The number of thunderstorm days reaches the annual peak during the summer months (Central Weather Bureau 1990). Some of the springtime cold fronts affecting Taiwan in March and April originate in southern China (Yeh and Chen 1984). During the mei-yu season over Taiwan (15 May 15 June) (Chen 1992), the mei-yu frontal systems from southern China and the convective systems embedded within the southwesterly monsoon flow frequently bring in heavy precipitation to the island (Li et al. 1997; Yeh and Chen 1998, 2002; Teng et al. 2000). The orographic effects of the CMR on the spatial distribution of precipitation are quite significant (Y.-L. Chen 2000). Yeh and Chen (1998) show that during the 1987 mei-yu season, maximum rainfall occurred over the western windward sloping areas in central and southern Taiwan. In addition to orographic lifting, orographic blocking (Akaeda et al. 1995; Li et al. 1997; Yeh and Chen 2002) and thermally driven circulations during the diurnal heating cycle (Chen and Li 1995) are also important in the production of localized heavy precipitation (Chen et al. 1991; Johnson and Bresch 1991) when synoptic conditions are favorable. Yeh and Chen (1998) show that during the Taiwan Area Mesoscale Experiment (TA- MEX) afternoon rainshowers contribute substantially to the total rainfall over the western windward slopes of Taiwan. In summer, tropical storms frequently affect Taiwan and result in another major rainfall peak during the annual cycle for most stations over the island (Wang et al. 1984; Central Weather Bureau 1991; Shieh et al. 1998). From the analysis of 100 yr of typhoon data, Shieh et al. (1998) show that, on average, there are 3.6 typhoons per year, causing heavy rainfall over Taiwan as they move across the area. For example, during the passage of Typhoon Herb on 31 July 1996, the maximum rainfall over the western slopes of the CMR reaches as high as 1094 mm day 1. The convective systems originating from the northern South China Sea and southern Taiwan Strait also frequently move toward Taiwan under the prevailing southwesterly monsoon flow, resulting in heavy precipitation over western Taiwan (Tao et al. 2000). Local afternoon rainshowers also contribute substantially to rainfall over the windward slopes and mountainous areas in summer (Yang 2000).

3 JULY 2003 CHEN AND CHEN 1325 FIG. 2. (a) The evolution of vertical distribution of half-monthly mean moist static energy (kj kg 1 ) from 1980 to 1997 over Taiwan area. (b) The evolution of vertical integration of halfmonthly mean precipitable water (kg m 2 ) from 1980 to 1997 over Taiwan area.

4 1326 MONTHLY WEATHER REVIEW VOLUME 131 FIG. 3. The mean flow at the 850-hPa level in half-month periods during (a) 1 15 Sep, (b) Sep, (c) Oct, and (d) 1 15 Nov from 1980 to Winds (m s 1 ) with one pennant, full barb, and half barb represent 25, 5, and 2.5 m s 1, respectively. Geopotential height interval is 10 gpm. In September, one of the favorable typhoon paths extends from the west Pacific to northern Taiwan and then curves to the eastern China coast and Korea under the influence of midlatitude troughs (Central Weather Bureau 1990). Rainfall in autumn could be related to either late-season typhoon circulations (Chen and Wang 2000) or convective activities embedded within the northeasterly monsoon flow during the passage of a midlatitude cold front (Lin and Chen 2000). Chen and Wang (2000) suggest that about 39% of the total rainfall over Taiwan in autumn during is related to typhoon circulations. 3. Data analyses The mean airflow and thermodynamic stratification (moist static energy and precipitable water) are calculated based on the European Centre for Medium-Range Weather Forecasts (ECMWF)/Tropical Ocean Global Atmosphere (TOGA) data during and the ECMWF/World Meteorological Organization (WMO) data during The twice-daily data (0000 and 1200 UTC) are averaged for every half-month period. The areal-averaged moist static energy and precipitable water over Taiwan are taken as the averaged values from four data points. 25 N, 120 E; 22.5 N, 120 E; 25 N, E; and 22.5 N, E. In Taiwan, there are 25 routine surface stations (Fig. 1) that have long-term rainfall records archived. This dataset allows us to study the seasonal rainfall variations for different parts of the island. The 38-yr daily rainfall data ( ) from these 25 conventional stations are averaged for each half-month period from every station. For most stations, daily rainfall data are available for the 38-yr analysis period. For Suao (24.60 N, E), Tungchita (23.26 N, E), Chiayi (23.50 N, E), and Wuchi (24.26 N, E), the analysis periods are , , , and , respectively. The daily rainfall frequencies for different rainfall rates (1, 50, 100, and 150 mm day 1 ) for each half-month period are also calculated for each station. The rainfall regimes can be identified from long-term rainfall records. The hourly rainfall data from 249 rainfall stations of the ARMTS (Chen et al. 1999) and 25 conventional surface stations (Fig. 1) are used in describing horizontal distributions of precipitation for different rainfall regimes during

5 JULY 2003 CHEN AND CHEN 1327 FIG. 4. The mean flow at the 850-hPa level in half-month periods during (a) 1 15 Dec, (b) 1 15 Jan, and (c) 1 14 Feb from 1980 to Winds (m s 1 ) with one pennant, full barb, and half barb represent 25, 5, and 2.5 m s 1, respectively. Geopotential height interval is 10 gpm. 4. Winter Except along the northern and northeastern coastal areas, winter months (December, January, February) are the driest months for most of Taiwan. During this period, the winter monsoon cold surges are the most prominent transient disturbances that affect Taiwan (Boyle and Chen 1987; Chen et al. 2002). Nevertheless, with a stable (or weakly potentially unstable) stratification (Fig. 2a) and low moisture content (Fig. 2b), most of these cold surge events bring in stable precipitation along the northern and northeastern windward coasts with overcast conditions (Chen and Huang 1999). a. Evolution of the mean circulation patterns In late September, at the 850-hPa level, a semipermanent midlatitude East Asian trough develops off the East China coast (Fig. 3). The ridge axis behind the East Asian trough axis shifts southeastward as the season progresses and reaches Taiwan by early December (Fig. 4a). With the southeastward extension of the ridge axis from central China, the northeasterly flow at the 850- hpa level over Taiwan reaches its maximum strength ( 15ms 1 ) in late October (Fig. 3c). In early January, the ridge axis develops into a semipermanent anticyclone center east of Taiwan (24 N, 123 E) (Fig. 4b) with relatively weak mean winds over Taiwan. At the hpa level, the northeasterly monsoon flow over Taiwan commences in late September (Fig. 5a). It reaches the maximum intensity by late November (Fig. 5b) during which the 1000-hPa wind speed reaches as high as 25 ms 1 within the Taiwan Strait. In late January, the west Pacific subtropical high axis at the 850-hPa level extends westward and merges with the weakening anticyclone east of Taiwan. As a result, southwesterly flow commences along the southeastern China coast in early February (Fig. 4c). As will be shown later, the change in the mean low-level wind direction during January over Taiwan affects the rainfall distributions. After early February, the southwesterly flow at the 850-hPa level prevails over the Taiwan Strait (Fig. 4c) and covers the entire island of Taiwan in March, whereas at the 1000-hPa level, the northeasterly monsoon flow continues to weaken (Figs. 5c e). Furthermore, the semipermanent lee trough forms on the lee side of the Tibetan Plateau (Fig. 4c) foretelling the ending of the winter season and the coming of the spring season. In the upper levels, the westerly jet axis reaches its southernmost position in January (Fig. 6) when the monsoon cold surges are most frequent (Yeh and Chen 1984). b. Rainfall characteristics For most stations along the northern, northeastern, and eastern coasts of Taiwan, the daily rainfall accu-

6 1328 MONTHLY WEATHER REVIEW VOLUME 131 FIG. 5. The mean flow at the 1000-hPa level in half-month periods during (a) Sep, (b) Nov, (c) 1 15 Dec, (d) 1 14 Feb, and (e) 1 15 Mar from 1980 to Winds (m s 1 ) with one pennant, full barb, and half barb represent 25, 5, and 2.5 m s 1, respectively. Geopotential height interval is 10 gpm. mulation decreases rapidly from late October to December (Fig. 7). The decrease in rainfall along the windward coasts during this period is related to the southeastward extension of the ridge axis from central China to the Taiwan area (Figs. 3c and 4a) and the decrease in both the moisture content (Fig. 2b) and stability (Fig. 2a). Furthermore, tropical storms affecting Taiwan during the summer and early autumn are infrequent during the winter months. Note that the moist static energy at the 1000-hPa level decreases from 345 kj kg 1 in October to less than 330 kj kg 1 in December. With the decrease in instability (Fig. 2a) during October December, the rainfall gradually transforms from heavy convectivetype precipitation in the warm season to light, stable precipitation (Fig. 8). The occurrences of higher rainfall rates decrease. Nevertheless, with the increase in the frontal activities from autumn to winter months (Yeh and Chen 1984), the rainfall frequencies along the north-

7 JULY 2003 CHEN AND CHEN 1329 FIG. 6. The mean flow at the 200-hPa level in half-month periods during Jan from 1980 to Winds (m s 1 ) with one pennant, full barb, and half barb represent 50, 10, and 5ms 1, respectively. Geopotential height interval is 60 gpm. ern and northeastern coasts for lower rainfall rates increase (Figs. 8a and 8c). Note that for stations along the northern coast, the frequencies of the daily rainfall rate greater than 1 mm day 1 exceed 60% (e.g., station Anpu; Fig. 8a). In December, with the stable (or neutral) (Fig. 2a) northeasterly flow over Taiwan, the rainfall distribution over the island is dominated by stable orographic precipitation along the windward coastal regions (Fig. 9a). It is dry over the leeside western plain and the western slopes of the Central Mountain Range. During the winter months, precipitation along the northern and northeastern coasts occurs primarily during and after monsoon cold surges (Chen and Huang 1999) with a strong northeasterly flow in the lowest levels (Chen et al. 2002). In February, the rainfall along the northeastern coast is much less than in December, whereas precipitation over northwestern Taiwan, as well as on the western slopes of the Central Mountain Range north of central Taiwan, increases (Fig. 9b). The change in horizontal rainfall distribution is related to the evolution in the low-level mean flow patterns that occurs in January (Fig. 4). Note that over the northern Taiwan Strait, the mean flow at the 850-hPa level shifts from northeasterly flow to southwesterly flow because of the westward extension of the west Pacific subtropical high (Fig. 4c). Furthermore, in addition to monsoon cold surges, early springtime cold fronts also bring in rainfall to both the eastern and western sides of the island (Fig. 9b) as compared to midwinter (Fig. 9a). FIG. 7. The daily rainfall rate (mm day 1 ) in half-month periods from 1961 to 1998 at (a) Anpu (N); (b) Hualien (E), and (c) Ilan (NE) (see Fig. 1 for locations). 5. Spring transition From early March to mid-may is the spring transition period. During this period, the 850-hPa level southwesterly flow over Taiwan gradually strengthens and brings in warm, moist maritime air from the south (Figs. 2 and 10) as the season progresses. The lee trough over southwestern China on the lee side of the Tibetan Plateau is well developed. Some of the springtime cold fronts that originated on the lee side of the plateau move southeastward and affect Taiwan. Thunderstorm activity becomes possible during the spring transition, especially in the afternoon hours, and reaches the annual peak during the summer months (Central Weather Bureau 1990). Thunderstorms during this period are frequently related to the passage of springtime cold fronts (Yeh and Chen 1984). a. Evolution of the median circulation patterns In early March, the west Pacific subtropical high becomes well established (Fig. 10a). The 850-hPa ridge

8 1330 MONTHLY WEATHER REVIEW VOLUME 131 FIG. 8. The rainfall frequencies for various threshold values (1, 50, 100, and 150 mm day 1 ) at (a) Anpu (N), (b) Hualien (E), and (c) Ilan (NE). axis is located approximately at 20 N over the west Pacific and extends southwestward to Indochina. The southwesterly flow at the 850-hPa level prevails along the southeastern coast of China. From early March to mid-may, the west Pacific subtropical high retreats eastward (Fig. 10b). During the same period, the East Asian trough in the midlatitudes shifts northwestward. Monsoon cold surges are infrequent as compared to the winter months. In early May, the southwesterly flow prevails from northern Indochina to southeastern China and to the ocean southeast of southern Japan. The southwesterly wind belt is located along the northwestern flank of the west Pacific subtropical high. The establishment of the southwesterly flow over southeastern China brings spring rains to the region, especially during the passage of transient springtime subtropical cold fronts (Tao 1980). Over Taiwan, the southwesterly flow at the 850-hPa level commences over the northern Taiwan Strait in February and early March (Figs. 4c and 10a). With the eastward retreat of the west Pacific subtropical high, the FIG. 9. The daily mean rainfall rate (mm day 1 ) during (a) Dec and (b) Feb, derived from ARMTS and the 25 conventional stations.

9 JULY 2003 CHEN AND CHEN 1331 FIG. 11. The daily mean rainfall rate (mm day 1 ) during Apr derived from 1994 to 1998 ARMTS and the 25 conventional stations. FIG. 10. The mean flow at the 850-hPa level in half-month periods during (a) 1 15 Mar and (b) 1 15 May from 1980 to Winds (m s 1 ) with one pennant, full barb, and half barb represent 25, 5, and 2.5 m s 1, respectively. Geopotential height interval is 10 gpm. southwesterly flow gradually extends eastward and covers the entire island by late March (not shown). During the spring transition, the northeasterly flow at the hpa level over Taiwan continues to weaken (Figs. 5c e). Both the mean moist static energy in the lower troposphere (Fig. 2a) and the precipitable water (Fig. 2b) increase as the season progresses. At the upper levels, the westerly jet continues to weaken as it shifts northward (Wang and Cheng 1982; Central Weather Bureau 1991). b. Rainfall characteristics The rainfall frequencies for light rain rates along the northeastern and northern coasts decrease from more than 60% in February to less than 40% in late April (e.g., station Anpu; Fig. 8a) as the winter monsoon cold surges become infrequent. Furthermore, with the establishment of the southwesterly flow at the 850-hPa level (Figs. 4c and 10a) and the weakening of the northeasterly monsoon flow in the lowest levels (Figs. 5c e) over Taiwan, the orographic lifting becomes less significant there. During the spring transition, rainfall is more evenly distributed over the island (Fig. 11) as compared with the winter months, which exhibit a pronounced maximum along the northern and northeastern coasts (Fig. 9a). Rainfall along eastern and southeastern Taiwan is also higher than during the winter months because of the passage of springtime cold fronts over the island. The commencement of the thunderstorm activity over Taiwan associated with the springtime cold fronts (Central Weather Bureau 1990) attests to the fact that the rainfall gradually changes from stable precipitation to convective-type precipitation during the spring transition. With the presence of the southwesterly flow at the 850-hPa level (Fig. 10a) and the relatively cold, dry northeasterly flow in the lowest levels (Fig. 5e), rainfall on the western slopes is higher than in coastal regions. Despite the presence of the southwesterly flow at the 850-hPa level, the daily rainfall accumulation for most regions of Taiwan is still relatively low ( 8 mm day 1 )

10 1332 MONTHLY WEATHER REVIEW VOLUME 131 FIG. 12. The mean flow at the 850-hPa level in half-month periods during (a) May, (b) 1 15 Jun, and (c) Jun from 1980 to Winds (m s 1 ) with one pennant, full barb, and half barb represent 25, 5, and 2.5 m s 1, respectively. Geopotential height interval is 10 gpm. (Fig. 11). Note that prior to the mei-yu season, Taiwan is under the influence of the west Pacific subtropical high (Fig. 10a) with a weakly potentially unstable atmosphere (Fig. 2a). Chen and Hui (1992) studied a springtime cold front during TAMEX (13 14 May 1987) just before the onset of the mei-yu season over Taiwan. They found that the prefrontal soundings over Taiwan and the northern South China Sea were characterized by a low-level inversion with extremely warm air aloft caused by the large-scale subsidence associated with the subtropical high. The conditions were not favorable for the development of localized heavy rainfall. 6. Mei-yu season The early summer rainy season (mei-yu) is the dominant rainy season for the entire island. It occurs approximately during 15 May 15 June (Chen 1992). Widespread precipitation frequently occurs during the passage of a mei-yu front (Chen 1993). In addition, local rainshowers due to terrain and local winds also contribute substantially to the rainfall over the western windward slopes of the CMR (Yeh and Chen 1998). a. Evolution of the mean circulation patterns In early May, the South China Sea, the Philippines, and Taiwan are under the influence of the west Pacific subtropical high (Fig. 10b). The 850-hPa ridge axis is located over the Bashi Channel between the northern Philippines and southern Taiwan, extending westward to the northern South China Sea. With the continued eastward retreat of the west Pacific subtropical high, the ridge axis moves out of the Bashi Channel during the onset of the mei-yu season in May (Fig. 12a). As a result, the entire region of Indochina, the South China Sea, the northern Philippines, and Taiwan is under the influence of the prevailing southwest monsoon flow. Climatologically, the onset of the mei-yu season over Taiwan coincides with the occurrence of the upper-level south Asian anticyclone over northern Indochina, as found by Chen (1993) during TAMEX 1987 (Fig. 13a). The south Asian anticyclone moves northward and intensifies as the season progresses (Fig. 13b). In early June, at the climax of the mei-yu season over Taiwan, the axis of the southwesterly monsoon flow extends from Indochina to the northern South China Sea, and to the East China Sea (Fig. 12b). A pronounced lee trough is also present on the lee side of the Tibetan Plateau, which is the birthplace for most of the transient subsynoptic-scale disturbances (e.g., mei-yu fronts) (Chen and Hui 1990, 1992; Chen et al. 1994) that frequently bring in heavy rainfall over the Taiwan area. These subsynoptic-scale disturbances move eastward and intensify during the passage of an upper-level trough (Chen and Chen 1995).

11 JULY 2003 CHEN AND CHEN 1333 subtropical high shifts northward after mid-june. East of Taiwan, the ridge axis is around 23 N. The midlatitude semipermanent East Asian trough also weakens and moves northward as the season progresses. In conjunction with these changes, the southwesterly monsoon flow extends from the South China Sea to the Yangtze River valley in central China and toward Japan (Fig. 12c) leading to the onset of the mei-yu season over central China (Tao 1980) and the Baiu over Japan (Murakami 1958). FIG. 13. The mean flow at the 200-hPa level in half-month periods during (a) May and (b) Jun from 1980 to Winds (m s 1 ) with one pennant, full barb, and half barb represent 50, 10, and5ms 1, respectively. Geopotential height interval is 60 gpm. After mid-june, the Tibetan high dominates the upperlevel circulations over southern China. The westerly jet shifts north of the Tibetan Plateau. Taiwan is on the southeastern flank of the Tibetan high. The presence of upper-level northeasterly flow over Taiwan (Fig. 13b) foretells the ending of the early summer rainy season over Taiwan. It prevents the upper-level baroclinic disturbances embedded within the large-scale westerly flow from reaching Taiwan (Chen 1993). At the 850-hPa level, the ridge axis of the west Pacific b. Rainfall characteristics Most stations over the western plain (Fig. 14) and the western slopes of the Central Mountain Range (Fig. 15) recorded the largest bimonthly daily rainfall during the peak of the mei-yu season (1 15 June). These stations also exhibit a distinct peak in the daily rainfall occurrence during the same period (Figs. 16 and 17). Chen (1993) attributed the higher rainfall during the second half of the mei-yu season (1 15 June) than during the first half (16 31 May) to a warmer southwesterly monsoon flow with a much higher moisture content as the season progresses. For mountainous stations (e.g., Jihyuehtan, Alishan, and Yushan), the frequencies of the daily rainfall rate greater than 1 mm day 1 are about 70% (Fig. 17). A large portion of the rainfall (30% 50%) there occurs in the afternoon hours during nonfrontal periods as a result of orographic lifting enhanced by daytime anabatic winds (Yeh and Chen 1998). These stations are on the windward slopes under the potentially unstable southwesterly monsoon flow. Stations along the northern, northeastern, and eastern coasts of the island exhibit a secondary annual rainfall peak during 15 May 15 June in addition to the primary autumn rainfall maximum (Fig. 7). The rainfall occurrences for these stations have a decreasing trend from late winter to early summer, superimposed by a small peak during the mei-yu season (Fig. 8). Most of the rainfall there during the early summer rainy season occurs after the passage of a mei-yu front under the postfrontal northeasterly flow (Yeh and Chen 1998). The horizontal rainfall distribution of averaged daily rainfall accumulation for May June (Fig. 18) resembles the rainfall distribution during TAMEX 1987 compiled by Yeh and Chen (1998). It exhibits rainfall maxima over the western windward slopes (Fig. 18). In addition, a rainfall maximum is also observed over southwestern Taiwan. Secondary rainfall maxima ( 16 mm day 1 ) are found along the northern coast of Taiwan and the northeastern slopes south of Ilan (Fig. 1). 7. Summer rainfall regime In addition to the mei-yu season, late summer (from mid-july to late August) is the other major rainy season for most areas of Taiwan except along the northern and northeastern coasts. During this period, tropical storms

12 1334 MONTHLY WEATHER REVIEW VOLUME 131 FIG. 14. The daily rainfall rate (mm day 1 ) in half-month periods from 1961 to 1998 at (a) Taiepi (N), (b) Taichung (CW), (c) Tainan (SW), (d) Kaohsiung (SW), and (e) Hengchun (S) (See Fig. 1 for locations). frequently propagate from the west Pacific and affect Taiwan (Shieh et al. 1998). The tropical convective systems embedded within the southwesterly monsoon flow (Tao et al. 2000) frequently bring in localized heavy rainfall as they move inland. a. Evolution of the mean circulation patterns The late summer rainy season over Taiwan starts in late July after the development of the late summer monsoon trough, located east of the Philippines (Fig. 19a). The peak of the late summer rainy season occurs during August as the late summer monsoon trough reaches its northernmost position and affects Taiwan (Figs. 3a, 19b, and 19c). In the upper levels, the Tibetan high reaches its maximum intensity in early August (Wang and Cheng 1982; Central Weather Bureau 1991). b. Rainfall characteristics The atmospheric stratification over Taiwan is potentially unstable during the summer months (Fig. 2a) under the prevailing southwesterly monsoon flow, which brings in warm, moist maritime air from the south. In August, the low-level moist static energy exceeds 355 kj kg 1 and reaches its annual maximum (Fig. 2a). The precipitable water over Taiwan also reaches an annual maximum in August (Fig. 2b). The summer rainfall is mainly convective in nature. Most stations over western Taiwan (Figs. 14 and 16) and the western windward slopes of the CMR (Figs. 15 and 17) exhibit a pronounced peak in rainfall and a maximum daily rainfall frequency in early August. In addition to tropical storms and mesoscale disturbances, with a pronounced diurnal heating cycle during the summer months, afternoon convective rainshowers are frequent along the western windward slopes of the CMR (Yang 2000). For stations on the western windward slopes (Fig. 17), the occurrences of daily rainfall rate greater than 1 mm day 1 are greater than or are about 60%. Along the northern and northeastern leeside coasts, the rainfall occurrences have a pronounced minimum in either late July or early August (Fig. 8).

13 JULY 2003 CHEN AND CHEN 1335 easterly winds impinge on the southeastern slopes of the CMR (Shieh et al. 1998). In August, the maximum frequency of westward propagating typhoon track shifts northward corresponding to the northward shift of the summer monsoon trough in August (Figs. 19b and 19c). Tropical storms that move across central or northern Taiwan from the east also frequently bring in abundant rainfall over northern and western Taiwan (Shieh et al. 1998). The horizontal rainfall distribution during mid-july to August for (Fig. 22) shows pronounced rainfall maxima on the western windward slopes and in the southwestern lowlands. In addition, in contrast to the mei-yu season, a rainfall maximum also occurs on the eastern slopes over northern Taiwan. The rainfall on the eastern slopes is caused by tropical storms coming in from the west Pacific (Shieh et al. 1998), as well as the local afternoon rainshowers. 8. Autumn rainfall regime It is well known that the mei-yu and summer seasons are the two main rainy seasons in southern China and Taiwan (Wang et al. 1984). Nevertheless, because of the presence of steep terrain, most stations along the northern and northeastern coasts exhibit an autumn rainfall maximum during the annual cycle. Rainfall during this period is mainly related to late-season typhoons (Chen and Wang 2000) or transient disturbances embedded in the northeasterly monsoon flow during the passage of cold fronts (Y.-Y. Chen 2000; Lin and Chen 2000). FIG. 15. The daily rainfall rate (mm day 1 ) in half-month periods from 1961 to 1998 for stations on the western slopes of the CMR: (a) Alishan (M), (b) Yushan (M), and (c) Jihyuehtan (M) (see Fig. 1 for locations). For stations in southeastern Taiwan (Fig. 20), the summer rainfall peak occurs in late July instead of early August (Figs. 14 and 15). Note that for stations in southeastern Taiwan, the occurrences of daily rainfall rate greater than 1 mm day 1 (Fig. 21) are lower in late July even though the averaged rainfall is higher. The shift in the summer rainfall peak from early August to late July along the southeastern coast of Taiwan is related to tropical storm activity. In late July, the monsoon trough extends from the region east of the northern Philippines through the Bashi Channel south of Taiwan (Fig. 19a). The maximum frequency of westward propagating typhoon tracks is over the Bashi Channel (Central Weather Bureau 1990). During the passage of tropical storms south of Taiwan, strong easterly winds associated with storm circulation cover southern Taiwan. These storms frequently bring in abundant rainfall as strong a. Evolution of the mean circulation patterns The transition from the southwesterly monsoon to the northeasterly monsoon over Taiwan occurs in early September, during which time the late summer monsoon trough retreats southward from its northernmost position in late August (Figs. 19c and 3a). Concurrently, the 850- hpa ridge axis of the west Pacific subtropical high also begins to retreat southeastward from its northernmost position ( 33 N; Fig. 19c) resulting in easterly winds over Taiwan (Fig. 3a). In late September, an 850-hPa semipermanent high pressure cell forms over central China (36 N, 115 E) west of the deepening semipermanent East Asian trough (Fig. 3b). Over Taiwan, the northeasterly monsoon flow at 850 hpa commences in late September when Taiwan is along the southeastern flank of the strengthening semipermanent anticyclone over central China (Fig. 3b). As discussed in section 4a, because of the continued strengthening and subsequent southeastward movement of the ridge axis of this anticyclone, the northeasterly flow at the 850-hPa level reaches its peak intensity in October (Fig. 3c). At the 1000-hPa level, weakening of the northeasterly flow starts in early December (Figs. 5b and 5c). In early September, the upper-level Tibetan high weakens (Fig. 23) as the midlatitude westerly shifts

14 1336 MONTHLY WEATHER REVIEW VOLUME 131 FIG. 16. The rainfall frequencies for various threshold values (1, 50, 100, and 150 mm day 1 ) at (a) Taiepi (N), (b) Taichung (CW), (c) Tainan (SW), (d) Kaohsiung (SW), and (e) Hengchun (S). southward and strengthens. During the onset of the lowlevel norteasterly monsoon flow (Fig. 3b), the upperlevel northeasterly flow over Taiwan is replaced by westerly flow (Fig. 23b). The presence of upper-level westerlies over southern China and Taiwan after the onset of the low-level northeasterly monsoon flow in mid-september allows baroclinic disturbances in the westerlies to affect Taiwan. These disturbances frequently produce heavy rainfall along the northeastern coast of Taiwan (Y.-Y. Chen 2000). b. Rainfall characteristics Time series analyses of twice-monthly climatological rainfall for stations along the northern, northeastern, and eastern coasts of the island exhibit a pronounced autumn rainfall maximum (Fig. 7). In contrast to other parts of the island that exhibit summer wet/winter dry conditions, the highest rainfall rate during the year for this area occurs during 16 September 31 October (Fig. 7) after the onset of the northeasterly monsoon flow (Fig. 5a) and the transition from northeasterly to westerly flow in the upper levels (Fig. 23). This is the only period throughout the year that this area is on the windward side with potentially unstable stratification. The climatological rainfall pattern during 15 September 31 October shows that orographic effects are important in determining the horizontal rainfall distribution (Fig. 24). The daily mean rainfall rate exceeds 32 mm day 1 along the northern and northeastern windward slopes. The western coastal plain on the lee side is dry. The low-level moist static energy exceeds 355 kj kg 1 during the summer months. It decreases rapidly in early September as the low-level southwesterly monsoon flow diminishes. Despite the decrease in moist static energy and precipitable water at low levels after August, the mean thermodynamic stratification remains potentially unstable in October (Fig. 2a). During 16 September 31 October, most of the heavy rainfall events ( 100 mm day 1 ) along the northern and northeastern coasts are related to mesoscale convective systems (Lin and Chen 2000) or cold fronts (Y.-Y. Chen 2000) embedded in the northeasterly monsoon flow or late-season tropical storms. Stratiform precipitation with low rainfall rates ( 2.5 mm day 1 ) (Fig. 8) and overcast conditions are frequent in either the postfrontal stable flow or weakly

15 JULY 2003 CHEN AND CHEN 1337 FIG. 18. The daily mean rainfall rate (mm day 21 ) during mid-may mid-jun derived from 1994 to 1998 ARMTS and the 25 conventional stations. FIG. 17. The rainfall frequencies for various threshold values (1, 50, 100, and 150 mm day 21 ) at (a) Alishan (M), (b) Yushan (M), and (c) Jihyuehtan (M). potentially unstable flow in late October because of persistent orographic lifting (Fig. 8). 9. Summary and conclusions The subtropical climate over the island of Taiwan is strongly affected by East Asian monsoons and transient disturbances embedded in the prevailing monsoon flow. It is shown that the horizontal distributions of climatological rainfall patterns for different rainfall regimes are strongly dependent on the direction of the low-level prevailing flow with much higher rainfall on the windward side of the Central Mountain Range (CMR). Furthermore, the seasonal variations in rainfall amount and type (light precipitation versus convective precipitation) are also dependent on the thermodynamic stratification and the availability of moisture. In early December, the ridge axis of the semipermanent, low-level high pressure cell in China extends over Taiwan, resulting in a weak northeasterly flow at the 850-hPa level. Light rain associated with cold surges is frequently observed over the windward slopes and coastal areas of northeastern Taiwan under a relatively dry, stable environment. The rainfall frequencies of light precipitation over northeastern Taiwan increase from the autumn to the winter season as the cold surges become more frequent in the winter months. In early February, the west Pacific subtropical high extends westward bringing in low-level southwesterly flow over the southeastern coasts of China and northwestern Taiwan. As a result, precipitation increases along the northwestern coast of the island and along the northwestern slopes of the CMR, whereas rainfall along the northern and northeastern coasts decreases. During the spring transition, the low-level high pressure cell over China weakens. Concurrently, the 850hPa ridge axis of the strengthening west Pacific subtropical high retreats eastward. As a result, southwesterly flow at the 850-hPa level prevails over the entire island with increasing precipitable water; the relatively dry, stable atmospheric stratification transforms into weakly potentially unstable conditions over Taiwan. The spring rains associated with the passage of springtime cold fronts commence over northwestern Taiwan in late February and early March, spreading over the entire island during late March in conjunction with the eastward retreat of the west Pacific subtropical high. Rainfall along the eastern slopes of the CMR also increases in the weakly potentially unstable atmosphere. The rainfall rate over the island is not high as compared to the

16 1338 MONTHLY WEATHER REVIEW VOLUME 131 FIG. 19. The mean flow at the 850-hPa level in half-month periods during (a) Jul, (b) 1 15 Aug, and (c) Aug from 1980 to Winds (m s 1 ) with one pennant, full barb, and half barb represent 25, 5, and 2.5 m s 1, respectively. Geopotential height interval is 10 gpm. FIG. 20. The daily rainfall rate (mm day 1 ) in half-month periods from 1961 to 1998 at (a) Taitung (SE) and (b) Tawu (SE) (see Fig. 1 for locations). FIG. 21. The rainfall frequencies for various threshold values (1, 50, 100, and 150 mm day 1 ) at (a) Taitung (SE) and (b) Tawu (SE).

17 JULY 2003 CHEN AND CHEN 1339 FIG. 22. The daily mean rainfall rate (mm day 1 ) during mid-jul Aug derived from 1994 to 1998 ARMTS and the 25 conventional stations. summer months. Rainfall rates over the northern and northeastern coasts decrease from winter values. During the summer monsoon, there are two primary islandwide rainy seasons over Taiwan: the mei-yu season (mid-may mid-june) and the late summer rainfall season (mid-july August). During the mei-yu season, mei-yu fronts that typically originated on the lee side of the Tibetan Plateau are the main subsynoptic disturbances that affect Taiwan. During late summer (mid- July August), the late summer monsoon trough is located over Taiwan. In the upper level, the Tibetan high dominates the upper-level circulation patterns preventing the baroclinic disturbances embedded in the largescale westerly flow from reaching the Taiwan area. Tropical storms are the main subsynoptic disturbances that bring in significant rainfall over the island in late summer. The warm, moist southwesterly monsoon flow is potentially unstable (especially during July August) with abundant moisture. Local rainshowers frequently develop along the western windward slopes during the afternoon hours. In addition, mesoscale convective systems embedded in the southwestern monsoon flow frequently drift inland, bringing in substantial rainfall over land. Most stations that are well exposed to the southwesterly monsoon flow on the windward side exhibit double rainfall peaks (the mei-yu season and in late summer) during the annual cycle. For both rainfall regimes, the horizontal rainfall distributions show pronounced rainfall maxima on the windward slopes. During the mei-yu season, the northern coast of Taiwan and FIG. 23. The mean flow at the 200-hPa level in half-month periods during (a) 1 15 and (b) Sep from 1980 to Winds (m s 1 ) with one pennant, full barb, and half barb represent 50, 10, and 5ms 1, respectively. Geopotential height interval is 60 gpm. the northeastern slopes south of Ilan (Fig. 1) also exhibit appreciable rainfall. Rainfall in these areas occurs mainly under the influence of postfrontal northeasterly flow after the passage of a mei-yu front. The transition from the southwesterly to the northeasterly monsoon flow occurs in early September. During the transition, a semipermanent high pressure cell develops over central China at the 850-hPa level. Concurrently, the west Pacific subtropical high retreats southeastward. The atmosphere remains potentially un-

18 1340 MONTHLY WEATHER REVIEW VOLUME 131 stable over Taiwan after the onset of the northeasterly monsoon. As a result of the change in low-level wind direction, the horizontal distribution of the climatological rainfall patterns also changes dramatically. Immediately after the onset of the northeasterly monsoon flow, stations that are well exposed to this flow are wet, whereas stations that are on the leeside slopes or located on the leeside plains are dry. For stations along the northern and northeastern windward coasts, the heaviest rainfall period during the annual cycle occurs immediately after the onset of the northeasterly monsoon flow. This is the only period throughout the year that these stations are on the windward side with potentially unstable thermodynamic stratification. During the transition, the upper-level northeasterly flow over Taiwan is replaced by westerly flow as the Tibetan high weakens and moves southward allowing baroclinic disturbances embedded in the westerlies to affect Taiwan. In autumn, late-season typhoons and cold fronts are the two primary subsynoptic disturbances that bring in appreciable rainfall. Furthermore, convective systems embedded within the northeasterly monsoon flow also bring in rainfall as they move onshore. Despite the increase in frontal activity from October to the winter months, the rainfall along the northern and northeastern coasts decreases rapidly after October. The precipitation type also changes from convective-type rainfall into light rainfall. There are two main factors that account for the rapid decrease in rainfall after October: weakening of the northeasterly flow at the 850-hPa level as the ridge axis of the semipermanent high pressure cell extends southeastward toward Taiwan, and the changes in thermodynamic stratification from potentially unstable to stable (or neutral) conditions with decreasing precipitable water. Acknowledgments. This work is supported by the National Science Council of the Republic of China under Grant NSC M We would like to thank the Central Weather Bureau, Taipei, Taiwan, and the Data Center of the Institute of Atmospheric Physics, National Central University, Chung-Li, Taiwan, for providing the data; D. Henderson for editing the text; and Wanchin Chen and Annie Kuo for preparing the figures. This work is also supported in part by the National Science Foundation under Grant ATM to the University of Hawaii. REFERENCES FIG. 24. The daily mean rainfall rate (mm day 1 ) during mid-sep Oct derived from 1994 to 1998 ARMTS and the 25 conventional stations. Akaeda, K., J. Reisner, and D. Parsons, 1995: The role of mesoscale and topographically induced circulations initiating a flash flood observed during the TAMEX project. Mon. Wea. Rev., 123, Boyle, J. S., and G. T. J. Chen, 1987: Synoptic aspects of the wintertime East Asian monsoon. Monsoon Meteorology, C. P. Chang and T. N. Krishnamurti, Eds., Oxford University Press, Central Weather Bureau, 1990: Climatic Atlas of Taiwan, Republic of China. Vol. I, 96 pp. [Available from Central Weather Bureau, 64 Kung-Yung Rd., Taipei, Taiwan.], 1991: Climatic Atlas of Taiwan, Republic of China. Vol. II, 128 pp. [Available from Central Weather Bureau, 64 Kung-Yung Rd., Taipei, Taiwan.] Chen, C.-S., and J.-M. Huang, 1999: A numerical study of precipitation characteristics over Taiwan Island during the winter season. Meteor. Atmos. Phys., 70, , W. S. Chen, and Z.-S. Deng, 1991: A study of a mountaingenerated precipitation system in northern Taiwan during TA- MEX IOP 8. Mon. Wea. Rev., 119, Chen, G. T. J., 1992: Mesoscale features observed in the Taiwan meiyu season. J. Meteor. Soc. Japan, 70, Chen, J.-M., and F.-J. Wang, 2000: The long-term variability of rainfall of Taiwan: The quasi 20-year oscillation of autumn rainfall (in Chinese with English abstract). Atmos. Sci., 28, Chen, T.-C., M.-C. Yen, J.-C. Hsieh, and R. W. Arritt, 1999: Diurnal and seasonal variations of the rainfall measured by the automatic rainfall and meteorological telemetry system in Taiwan. Bull. Amer. Meteor. Soc., 80, ,, W.-R. Huang, and W. A. Gallus Jr., 2002: An East Asian cold surge: Case study. Mon. Wea. Rev., 130, Chen, X. A., and Y.-L. Chen, 1995: Development of low-level jets (LLJs) during TAMEX. Mon. Wea. Rev., 123, Chen, Y.-L., 1993: Some synoptic-scale aspects of the surface fronts over southern China during TAMEX. Mon. Wea. Rev., 121, , 2000: Effects of island induced airflow on rainfall distributions during the Mei-Yu season over Taiwan. Proc. Workshop on Numerical Simulations of Precipitation in Taiwan Area, Chung-Li, Taiwan, National Central University, 5 11., and N. B.-F. Hui, 1990: Analysis of a shallow front during the Taiwan Area Mesoscale Experiment. Mon. Wea. Rev., 118, , and, 1992: Analysis of a relatively dry front during the Taiwan Area Mesoscale Experiment. Mon. Wea. Rev., 120, , and J. Li, 1995: Characteristic of surface airflow and pressure

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