Journal of the Meteorological Society of Japan, Vol. 83, No. 6, pp. 1001--1023, 2005 1001

Doppler Radar Analysis of Typhoon Otto (1998) —Characteristics of Eyewall and with and without the Influence of Taiwan Orography

Tai-Hwa HOR, Chih-Hsien WEI, Mou-Hsiang CHANG

Department of Applied Physics, Chung Cheng Institute of Technology, National Defense University, Taiwan, Republic of China

and

Che-Sheng CHENG

Chinese Air Force Weather Wing, Taiwan, Republic of China

(Manuscript received 27 October 2004, in final form 26 August 2005)

Abstract

By using the observational data collected by the C-band Doppler radar which was located at the Green Island off the southeast coast of Taiwan, as well as the offshore island airport and ground weather stations, this article focuses on the mesoscale analysis of inner and outer features of Typhoon Otto (1998), before and after affected by the Central Mountain Range (CMR) which exceeds 3000 m in elevation while the storm was approaching Taiwan in the northwestward movement. While the typhoon was over the open ocean and moved north-northwestward in speed of 15 km/h, its eyewall was not well organized. The rainbands, separated from the inner core region and located at the first and second quadrants relative to the moving direction of typhoon, were embedded with active con- vections. The vertical cross sections along the radial showed that the outer rainbands tilted outward and were more intense than the inner ones. As the typhoon system gradually propagated to the offshore area near the southeast coast of Taiwan, the semi-elliptic eyewall was built up at the second and third quad- rants. Moreover, the strength of the eyewall became more intense compared with the outer rainbands, and the maximum wind axis was quite parallel to the vertical orientation of radar reflectivity in the eyewall. After the detailed streamline analysis, it indicated that the eyewall was enhanced by the con- fluence between the westerly flow, triggered by the farther outer circulation of the storm around the Taiwan Island, and the northwesterly flow near the inner circulation of the storm itself. Also, the left quadrant in the lower portion (below 2 km in altitude) possessed stronger Doppler velocity than that in the right quadrant, and the upper portion (above 2.0 km) had the opposite mode. This reverse phenom- enon of Doppler wind in the lower portion of the typhoon became more pronounced while the storm was getting closer to the mountain. The estimated typhoon center below 1.5 km in altitude had a slower

Corresponding author: Tai-Hwa Hor, Department of Applied Physics, Chung Cheng Institute of Technology, National Defense University, 190 Sanyuan 1st Street, Dahsi, Taoyuan 33509, Tai- wan, Republic of China. E-mail: [email protected] ( 2005, Meterological Society of Japan 1002 Journal of the Meteorological Society of Japan Vol. 83, No. 6

propagating speed due to the orographical blocking and corner effects, and the storm entity suggested a distorted appearance in the lower portion.

1. Introduction vective structure (Barnes et al. 1983; Jorgensen 1984a). Rainbands had fewer vertically ori- Typhoons (severe tropical cyclones with max- ented cores of reflectivity, and fewer organized imum sustained surface wind speed greater updrafts than the eyewalls had. Radially out- than 17 m/s) can produce widespread damage ward from the eyewall in a vertical plane in and account for the loss of many lives. The Hurricane Alicia, the rainbands were charac- western North Pacific Ocean and the South terized by extensive horizontally homogeneous China Sea could expect about twenty seven ty- reflectively patterns, with bright bands of en- phoons a year. It has been observed that ty- hanced reflectivity at altitudes of 4.0 to 4.5 km, phoons are more nearly circularly symmetric just below the melting level (Jorgensen 1984b). than frontal cyclones since they involve no air Jorgensen (1984a) estimated that stratiform mass discontinuities. Fully mature typhoons precipitation in the rainbands of Hurricanes range in size from 100 km in diameter to well Frederic and Allen covered areas about 10 over 1600 km in diameter. The surface winds times larger than convective precipitation. Par- spiral inward cyclonically, becoming more rish et al. (1982) found that the strong horizon- nearly circular near the center. The winds do tal winds in the inner regions of hurricanes not converge toward a point but rather become advected individual cumulus cells at about the roughly tangential to a circle bounding the speed of the low level wind, counterclockwise of the storm. The spiraling lines of cumulus and about the storm center. The mesoscale rain- cumulonimbus with rain ceilings down to 70 m, bands, however, remained fairly stationary rel- separated by relatively clear bands, in which ative to the storm center in some hurricanes, the ceiling may be 3300 m or more. These spi- and rotated about the center in others. There ral bands wrap around the eye, which may it- are a lot of changes in mesoscale structures of self be cloudless (Huschke 1959). tropical storms during landfall. Powell (1982) The radar data were first collected in hurri- showed an abrupt discontinuity in wind speed canes (severe tropical cyclones with maximum and a change in wind direction at the Gulf sustained surface wind greater than 33 m/s) in coastline for the composite of - mid-1940s (Wexler 1947). Using ground-based eric (1979), such that the streamlines were ori- Doppler radar to collected dataset for tropical ented more toward the storm center over land. cyclones began in the late 1970s (Donaldson et The discontinuity was clearly a result of the al. 1978). Since then, Doppler radars have pro- change in surface drag that occurred at the vided significant observations on the mesoscale coast. In spite of the uncertainties about structures of tropical cyclones (Sakakibara et the pattern of surface convergence in the right al. 1985; Ishihara et al. 1986; Donaldson and front quadrant of Frederic, the coastline ap- Ruggiero 1986; Bluestein and Hazen 1989). In peared to enhance the initiation of major con- the eyewall region, Shea and Gray (1973) com- vective features. Burpee and Marks (1984) sug- posited the research aircraft observational data gested that the land-sea interface appeared to in hurricanes and found that inflow was con- have an important effect on the initiation of or- fined to the lowest 1500 m, maximum winds ganized convection in Frederic. Bluestein and occurred within the eyewall, and descent oc- Hazen (1989) made a dual-radar analysis of curred in the eye. Jorgensen (1984a, b) showed Hurricane Alicia (1983) over land. They found that the circulation in the eyewall was highly that the vertical cross sections of averaged ra- organized in a vertical plane along a radial dial and azimuthal wind components in the ab- through the hurricane center. Embedded with- sence of significant topographical features were in the two-dimensional eyewall were cores of similar to the analysis based upon data re- high reflectivity that were 2–5 km in diameter. corded aboard an aircraft prior to its landfall. Radar reflectivity observations showed that There were three prominent types of mesoscale hurricane rainbands had a stratiform and con- areas of precipitation: the central area, princi- December 2005 T.-H. HOR et al. 1003 pal rainband and outer band. The central area Typhoon Herb (1996) by using Doppler radar and principal rainband were relatively strati- data, and the rotation of the eye was suggested form, and the outer band was cellular. The by the axisymmetrization, vorticity redistribu- maximum radial wind speed in 25–30 m/s was tion, wave breaking and vortex merging pro- displayed in the principal rainband, located to cesses. The evolution and structure of eyewall the east and southeast of the storm’s center. circulation of the landfalling Typhoon Herb On the average based upon the 100-years (1996) was documented by Chang et al. (2002). (1897–1996) observational data, about 3.6 ty- They found that before landfall, the elliptic eye phoons per year invaded Taiwan. Their posi- had a long axis of 35–45 km and a short axis of tions, intensities and structures were influenced 25–35 km, while the eye rotated counterclock- significantly by the steep and high Central wise with a period of 140–150 minutes. More- Mountain Range (CMR), which averaged eleva- over, the tangential wind increased up to 70 m/s tion reaches 2000 m in a north-northeast- as it approached northern Taiwan, and the ra- south-southwest orientation with a width of dius of maximum wind at 2-km height was about 120 km and a length of about 300 km. about 35–45 km and tilted outward at about The size of CMR was comparable to the ty- 40–50 degrees, which were similar to the fea- phoon core region of damaging winds and tures of radar reflectivity. Lin et al. (1999) in- heavy precipitation (Shieh et al. 1998). The vestigated the orographic influence on a cyclone lack of meteorological data over the vast Pacific propagating from the east and impinging on Ocean, and the strong interaction between ty- the central portion of an idealized mountain phoon circulation and CMR, are two major similar to CMR. The major findings were that a factors that make the forecasting of typhoons northerly surface jet tended to form upstream in the vicinity of Taiwan highly challenging. of the mountain between the primary cyclone Therefore, numerical models become a crucial and the mountain due to blocking and channel- research vehicle to improve the knowledge. ing effects. Two pressure ridges and one trough However, increased observations are needed were produced and when the cyclone ap- for model initiation and verification (Wu and proached the mountain, the low-level vorticity Kuo 1999). Observational studies of typhoon and low pressure center decelerated and turned tracks around Taiwan indicated significant southward upstream of the mountain due to track changes, and occasional secondary low orographic blocking. However, several argu- developments on the opposite side of the ments are still unknown and need to be cleared mountain range. These track modifications and up by using observational data in higher tem- precipitation distribution in rugged terrain poral and spatial resolutions (radars, satellites, were a function of the direction of approach of aircrafts, etc.), especially for the northwestward the typhoon (Wu and Wang 1983). Chang moving typhoons off the southeast coast of (1982) executed a numerical model to realize Taiwan: (1) the mesoscale variations of circula- the effect of the terrain on typhoons crossing tion of the typhoon as well as the features of Taiwan, and showed that the mountain- related rainbands and eyewall with and without induced flow deflections were mainly confined the influence of high terrain, (2) the primary to the lower levels, and the upper-level center mechanisms for the change of the mesoscale experienced little change as it passed the circulation inside the storm system due to the mountain ridge. Also, the track deflection up- complicated mountainous orography of Taiwan. stream of the barrier depended on the intensity By using the observational data collected by of the approaching typhoon. Bender et al., the DWSR-92 Doppler weather radar (284 m (1987) suggested that the terrain-induced ASL) which was deployed at the Green Island steering flow modification was the main cause (hereafter, GRI, located at 22.67N and of the typhoon track deflection, and the track 121.48E, at about 36 km off the southeast deflection was much reduced when the vortex coast of Taiwan) in the late 1997, this study translation speed was increased from 5 m/s will focus on the mesoscale analysis of eyewall, (18 km/h) to 10 m/s (36 km/h). Kuo et al. (1999) inner and outer rainband features of Typhoon found that the elliptical eye rotated cyclonically Otto (1998) while it was approaching the with a period of approximately 144 minutes in southeast coast of Taiwan in the northwest- 1004 Journal of the Meteorological Society of Japan Vol. 83, No. 6 ward movement with speed of 20 km/h on the sible mechanisms on its deformation for north- third and fourth of August 1998, based on the westward moving typhoons while it was over SOLO and SPRINT (Sorted Position Radar IN- the open ocean and while it was affected by Terpolation) radar software tools. SOLO was CMR of Taiwan. developed at NCAR/RDP and is an editing 2. Synoptic-scale aspects of Typhoon software for radar data. SPRINT was pro- Otto off the southeast coast of Taiwan grammed by the NCAR/MMM and is a co- ordinates transformation software. Due to lack 2.1 History of aerial measurements as well as surface Based on the warning report announced by observations over the western North Pacific the Joint Typhoon Warning Center (JTWC), Ocean, it was quite difficult to provide a lot of the Typhoon Otto built up in the ocean out of real time observations on the mesoscale struc- eastern Philippines at 1200 UTC 2 August tures of typhoons. Therefore the unique inte- 1998 (Fig. 1). It moved from north-northwest- gration of the GRI Doppler radar data and the ward to northwestward out of the northeastern limited conventional observations over land be- tip of Philippines in speed of 15 km/h and came a key tool to support this study. Typhoon made landfall on the southeastern coast of Tai- Otto (1998) was the first typhoon reconnoitered wan at about 0420 UTC on the fourth of Au- by the GRI Doppler radar. Also, it was the first gust. The affected duration of storm over the northwestward-moving typhoon observed by island was about 5.3 hours. Then it propagated Doppler weather radar as well as the offshore over the Taiwan Strait and made landfall again airport stations (Green Island and Orchid Is- on the southeastern part of Mainland China land) for investigating the three dimensional in the early morning on the fifth of August mesoscale structures of typhoon over ocean and became weakening gradually. Although near the southeast coast of Taiwan. The radar data gave a detailed information to improve the accuracy of storm track forecast, to realize the possible precipitation and wind field dis- tributions as well as to illustrate the variations of eyewall and rainband structures before its landfall at about 0420 UTC 4 August 1998. The island weather station data stood for the ground truth and could be used to compare with the radar observations. Although, un- fortunately, just shortly after the last shot at 0200 UTC on 4 August, the radar dome was severely damaged by the intense eastward upslope wind more than 35 m/s (gusty wind @45 m/s) in magnitude, and no further radar data could be collected after the accident. How- ever, the storm system had been affected by the high and rugged orography significantly before its center made landfall. Therefore the GRI Doppler radar observations still supported the center location identification and the track pre- diction and gave official agencies to disseminate Fig. 1. Track map of Typhoon Otto (1998) the forecast results of wind field distribution reported by the Joint Typhoon Warning Center (JTWC) every 12 hours. The and precipitation intensity two more hours open circles represent the positions of earlier than previous cases for warning of flash typhoon center. The ‘‘g’’, ‘‘Q’’, ‘‘o’’, flood and mud-rock flow and evacuation of the ‘‘Y’’, ‘‘C’’ symbols stand for the loca- local communities and residents. Also, the in- tions of Green Island, Orchid Island, valuable radar data helped us to elucidate the Chengkung, Taidong and A-li-shan, re- exclusive variations of structures and the pos- spectively. December 2005 T.-H. HOR et al. 1005 the storm in its entire life just reached the in- rence made one dead and one missing. More- tensity of slight typhoon (maximum sustained over, the gross damage in agriculture, traffic wind between 17.2 m/s and 32.6 m/s) based and public utility was worth 10 million US dol- upon the warning report of the Central lars (Shyu 2001). Weather Bureau in Taiwan, but the high wind and torrential rainfall accompanying with the 2.2 Surface analysis storm still brought severe damage in this is- The surface weather chart at 0000 UTC 4 land. During the passage of this typhoon over August shows that Typhoon Otto propagated the Taiwan area, the maximum gusty wind of along the isobaric lines out of the southwest 68.2 m/s occurred at Orchid Island, and the part of a subtropical high (Fig. 2a). This in- heaviest total accumulated rainfall of 412.5 mm dicates that the tropical storm was steered in 3 days was located at A-li-shan. This occur- steadily by the outside flow field of a subtropi-

Fig. 2. Weather charts analyzed by Japan Meteorological Agency in the East Asian region at (a) surface at 0000 UTC 4 August, (b) 850 hPa at 0000 UTC 4 August, (c) 700 hPa at 0000 UTC 4 August, (d) 500 hPa at 0000 UTC 4 August 1998. 1006 Journal of the Meteorological Society of Japan Vol. 83, No. 6

Fig. 3. Meteorological factors of (a) temperature (solid line) and dew point (dashed line), (b) sea level pressure, (c) precipitation, (d) surface wind speed and direction measured by Green Island Airport weather station from 1700 UTC 3 August to 0500 UTC 4 August 1998. The rainfall rates between 1700 and 1900 UTC in (c) are much less than 5 mm/hr. The wind bar in (d) stands for the coming wind direction and the wind pennant represents the wind intensity. cal high. Clearly, the typhoon was located at perature and dew point temperature were 22.9 the subtropical region and was not affected by and 22.5 deg C, respectively, at 2248 UTC 3 the midlatitude circulations. August due to the diurnal variation. The mini- The Green Island was the most representa- mum pressure fall was 975 hPa at 0228 UTC 4 tive locus for observing the variations of the August and the wind direction with the maxi- meteorological parameters on the route of mum sustained speed of 35 m/s and gusty wind the typhoon. Due to the data collected by the of 50 m/s shifted from NW to SW 40 minutes AWOS (Automated Weather Observing Sys- later (between 0300 UTC and 0320 UTC). After tem) at the Green Island Airport (Fig. 3), it the passage of the storm center, the accumu- displayed that the measured minimum tem- lated rainfall rose up to 55 mm/hr abruptly. December 2005 T.-H. HOR et al. 1007

Fig. 4. Skew-T log-p diagram of (a) Green Island sounding (46780) and (b) Hualien sounding (46699) at 0000 UTC 4 August 1998. The wind bar stands for the coming wind direction and the wind pennant represents the wind intensity.

2.3 Upper level analysis 4b). The sounding was good for understanding the northern part of atmospheric circumstance The weather charts of 850, 700 and 500 hPa related to the typhoon system. The wind speed levels at 0000 UTC 4 August 1998 are shown increased from surface up to 400 hPa. The in- in Figs. 2b to 2d. During this period, the orien- tense wind field was concentrated between 500 tation of the axis of the subtropical high over and 400 hPa with intensity of 30 m/s. This in- the northwest Pacific Ocean was northwest- dicated that the strongest kinematic strength southeastward at 850 and 700 hPa. After as- of storm was lifted vertically from 700 hPa at sessment of sounding data, it indicated that the GRI to 400 hPa at Hualien probably due to the steering flow on the southwest side of the sub- mass accumulation by topographic blocking. tropical high directed to northwest with inten- The wind fields at the levels above 400 hPa sity of 5 m/s at 700 hPa level and 7.5 m/s at gradually turned to southeast, and the wind 850 hPa level while the storm was moving to speed decreased. that region (off the eastern coast of Taiwan). Before landfall of typhoon, the last Green Is- 2.4 Satellite analysis land sounding was collected at 0000 UTC 4 The visible satellite imagery (Fig. 5) provided August (Fig. 4a). The wind profile showed that an entire view for the cloud structure and the the maximum wind appeared at the 700 hPa location of the storm center. The typhoon and 925 hPa levels in speed of 25 m/s. The es- moved toward northwest in the speed of 15– timated convective available potential energy 20 km/h as it was close to the eastern coastal (CAPE) could reach 1600 m2/s2, although the line of Taiwan based upon the time series of temperature and dew point profiles had nearly satellite imageries. The combined visible and pseudo-moist adiabatic lapse rate above the infrared satellite pictures (not shown) showed 850 hPa level. This exhibited that the thermal that the circulation of typhoon was nearly sym- energy provided by the lower atmosphere was metric in the upper-levels, but it was difficult to able to maintain condition within the inner cir- resolve the inner structure of typhoon just from culation of typhoon while it was approaching the analysis of these imageries. Therefore, the the eastern Taiwan area. The Hualien sound- analysis of GRI Doppler radar dataset became ing station, which is about 100 km north- a unique and powerful approach to realize the northwest away from the Green Island, also evolution of mesoscale structures of typhoon recorded sounding data at the same time (Fig. and the track identification before its landfall. 1008 Journal of the Meteorological Society of Japan Vol. 83, No. 6

Table 1. The characteristics of DWSR- 92C Doppler radar at Green Island (284 m ASL in altitude) installed in November 1997. Doppler Non-Doppler Items mode mode Wavelength 5 cm Scanning range 120 km 480 km Angular resolution 1 deg Range resolution 0.5 km 2 km Sample size 64 samples 32 samples Ray width 2.31 deg/s 1.06 deg/s Nyquist velocity G15.65 m/s None Pulse length 0.8 ms 2.0 ms

Fig. 5. Visible imagery captured by GMS-5 satellite at 0300 UTC (1100 LST) 4 August 1998. than 120 km away from the radar site, the non- Doppler mode scanning strategy with range of 480 km and resolution of 2 km was executed only. Six elevation angles (including 0.4, 0.3, 3. Radar analysis of Typhoon Otto over 1.2, 2.4, 4.0, 6.6 degrees) was proposed in order the open Ocean to analyze the horizontal and vertical struc- Typhoon Otto was the first northwestward- tures of typhoon over open ocean. This long moving typhoon observed by Doppler weather range scanning mode was executed at the radar for investigating its three dimensional minute of 00, 20 and 40 for each hour. Sec- mesoscale structures over the open ocean and ondly, the Doppler mode scanning strategy was near the southeast coast of Taiwan. Such radar applied while typhoon moved near the vicinity data also offered sufficient information to im- of 120 km range, and it consisted of thirteen prove the accuracy of storm track estimation, to elevation angles (0.4, 1.0, 1.7, 2.6, 3.6, 4.8, 6.0, realize the possible precipitation and wind field 7.5, 9.5, 12.5, 16.5, 24.0 and 38.0 degrees) with distributions as well as to illustrate the varia- range of 120 km and resolution of 500 m. The tions of eyewall and rainband structures before scanning mode was executed at the 30th and its landfall at about 0420 UTC 4 August 1998. the 50th minute each hour. Since the unambiguous velocity of GRI radar 3.1 Description of the radar data is 15.65 m/s, the radar has a serious limitation The Doppler radar data used in the study for the intense Doppler wind speed measure- were collected by the CAF GRI DWSR-92C ments due to the folding problem. The editing Doppler weather radar, which was developed software named ‘‘SOLO’’ was much feasible to by Enterprise Electronic Corporation (EEC). achieve de-folding task in the present study. The elevation of radar site is 284 m ASL, where is the highest point of GRI and does not have 3.2 General structures of typhoon any blocking of radar beam in viewing angle. The positions of precipitation echoes corre- Therefore, it can provide an excellent monitor- sponding to rainbands associated with and ing tool for weather systems. There were two within the typhoon could be identified by four scanning strategies and two range resolutions quadrants according to the moving direction of being employed during the surveillance of Ty- the typhoon (see Fig. 6b). The time sequence of phoon Otto in 1998 (Table 1). Firstly, when PPIs at the 0.4 deg elevation angle of long the typhoon was located at a distance more range scanning (480 km) could provide an en- December 2005 T.-H. HOR et al. 1009 tire picture for the evolution of rainbands ap- peared over the open ocean. While the lowest elevation angle of 0.4 degrees was applied for scanning, the precipitation echoes located at distance between 100 km and 300 km away from radar site were at the height of 1 km to 3 km ASL. By 1320 UTC 3 August (Fig. 6a), the typhoon center was about 320 km away from the coast of Taiwan, and the precipitation echoes which were concentrated within the inner and outer areas of the first and second quadrants could be displayed clearly by GRI radar. At that time the core bands at range of 250 km from radar site could also be figured out in the scanning. The intensity of the inner core region including the typhoon center and the core bands was less than 35 dBZ in reflectivity, but it was hard to determine the appearance of eyewall due to the weaker echoes at the inner core region probably according to the attenuation problem. Also, the most intense portion of rainbands existed at the outer ring of the first quadrant with a spiral curvature. Several convective cells in linear orientation with maximum intensity up to 45 dBZ (Fig. 6a) were embedded inside the outer rainbands. The other rainbands, which were located at the area of the first and second quadrants, also had curved shape with intense echoes more than 35 dBZ involved. Therefore, the radar echoes could be classified as outer Fig. 6. The reflectivity analysis of PPI for rainbands, inner rainbands and core band. long range scanning (480 km) at 0.4 Their patterns seemed different from the model degrees elevation angle at (a) 1320 of the stationary band complex which was com- UTC, (b) 1400 UTC 3 August 1998. The posed of the principal band, the connecting inner circular range is 80 km, and the band and the secondary band (Willoughby et al. outermost ring is 480 km. The red spot 1984) since the principle band (the inner rain- stands for the position of typhoon cen- band) and the connecting band (joining the in- ter. The radar site is at the center of the circles. The red solid lines represent ner rainband to the core cloud) couldn’t recog- the locations of vertical cross sections nize well in this case probably due to the weak shown in Fig. 7. The blue dashed arrow intensity of the storm and the worse radar in Fig. 6b represents the moving direc- range resolution (@2 km) in the non-Doppler tion of the typhoon and I, II, III and IV mode. stand for the first, second, third and The linear convective cells in the outer rain- fourth quadrant of the system, respec- bands at the first quadrant were well organized tively. between 1320 UTC and 1400 UTC (Fig. 6b). This convective system propagated north- northwestward in speed of 45 km/h relative to the ground which was faster than the moving strength of rainbands located at the second and speed (@20 km/h) of the typhoon itself. It third quadrants did not vary dramatically dur- sustained more than 2 hours and began dis- ing the period, and the certain aspect of eyewall sipating after moving inland. In addition, the was still unavailable. 1010 Journal of the Meteorological Society of Japan Vol. 83, No. 6

3.3 Vertical structures of rainbands As mentioned previously, the long range scanning strategy (non-Doppler mode) is useful in surveillance of typhoon motion. However, it is rarely used to analyze the inner structures of typhoon while it was developing over the open ocean probably because of short sweeping levels. Fortunately, the scanning strategy for long range mode during this typhoon approach had constructed three dataset collections every hour and each dataset had included six eleva- tion angles (0.4, 0.3, 1.2, 2.4, 4.0, 6.6 degrees). Thus the dataset could be interpolated in Car- tesian coordinates and made the three dimen- sional structure of typhoon feasible. Based on the six elevation angle sweepings every 20 min- utes and the determination of the typhoon cen- ter by the weakest echoes within the inner core (typhoon center) region, the analysis and com- parison of the vertical structures in the inner Fig. 7. The vertical cross sections of re- core clouds and outer rainbands were fulfilled. flectivity of Typhoon Otto: (a) cross sec- At 1320 UTC 3 August, the vertical profile of tion A-B at 1320 UTC as shown in Fig. echoes at the cross section A-B (Fig. 7a where 6a, (b) cross section A-C at 1320 UTC as the typhoon center was about 320 km away shown in Fig. 6a, (c) cross section A0-B0 from the coast of Taiwan) demonstrated clearly at 1400 UTC as shown in Fig. 6b. The that two cells with reflectivity larger than typhoon center is located at the position 30 dBZ were developing at a distance of of R ¼ 0. 120 km and 220 km away from the typhoon center. The horizontal width for both cells in 30 dBZ spread over 20 km. The most intense top (reflectivity > 10 dBZ) could reach 10 km in cell was the farther one with maximum inten- altitude. The aspect of the outer one, which was sity up to 40 dBZ and the shaded area of located at 190 km away from the center and 10 dBZ extended upward over 12 km in height. composed of two cells, was more well-defined. In contrast to the intense convections at the The horizontal width was 10 km for each cell outer range, there was no prominent convection and the maximum intensity was larger than appearing at the inner core, and it wasn’t 35 dBZ. feasible to see the stratiform cloud there. Evi- The intensity of convective cells related to dently, the distribution of rainband echoes at the outer rainband at first quadrant still re- this azimuth was more intense outward. mained strengthening. The vertical cross sec- One might suspect whether the patterns of tion A0-B0 at 1400 UTC (Fig. 7c where the ty- convective cells were similar at the other azi- phoon center was about 300 km away from the muths. In order to get more clues, the vertical coast of Taiwan) was remarkable to recognize cross section A-C crossing the stronger echo at the vertical structure of rainband and showed the inner core is shown in Fig. 7b for compari- the developing convection. Consider the envel- son of the cells in strength between the core oped area of 30 dBZ in the outer rainband as cloud and the outer region. Along this azimuth well, its structure not only spread more than it is easy to identify six organized cells in 30 km in horizontal width, but also extended to 30 dBZ with intervals of 30 km to 45 km in 10 km deep in altitude with an apparent out- distance. The vertical orientation of cells ex- ward slant at about 34 degrees from the verti- hibited a slight slant outward. The echo top in cal. The maximum intensity of the cell went up 30 dBZ for each cell was quite similar, which to 45 dBZ at the present time. could extend to approximate 6 km, and cloud Consequently, the radar echoes in long range December 2005 T.-H. HOR et al. 1011 scanning mode displayed that the storm center involved the maximum positive and negative seemed to be kept from the influence of high radial velocities were primarily used to derive terrain. The radar echoes could be classified the position of typhoon centers via the scheme as outer rainbands, inner rainbands and core initiated by Wood and Brown (1992). And then band which were different from the model of the estimated centers were subjectively modi- the stationary band complex (Willoughby et al. fied referring to the geometric centers of area 1984). The eyewall was not well organized and enclosed by weaker echoes. However, the the rainbands around the first and second identification of centers for non-Doppler ob- quadrants separating from the inner core re- servations only referred to the geometric cen- gion were embedded with active convection. ters of area with minimum echoes. During this Nonetheless, the outer rainbands were more period, the direction of the storm movement intense than the core band and inner rain- changed from northwest to north-northwest in bands. The core cloud was in incomplete elliptic the maximum speed of 35 km/h. It seemed that appearance with a cyclonic circulation. The the vortex course shifted northerly and speeded vertical cross sections along the radial showed up when its inner rainband gradually ap- that the outer rainbands tilted outward at proached the coastal mountains (Figs. 8i@8l). about 34–44 degrees from the vertical with the And this situation was getting clearer while the maximum reflectivity larger than 45 dBZ and system itself stayed closer to the orography. the maximum altitude (reflectivity > 10 dBZ) Furthermore, the composite of these intensive more than 12 km, and they were more devel- sequences of CAPPIs could offer more signifi- oped than the core band with maximum re- cant information to realize the detailed insights flectivity of 35 dBZ and maximum altitude of the typhoon structure and to delineate the (reflectivity > 10 dBZ) less than 10 km. More- evolution of rainbands embedded inside the over, the core band did not possess a pro- typhoon. nounced slanting feature, and its minimum As the typhoon was approaching the sea distance with reflectivity of 10 dBZ was about shore closely since 2250 UTC 3 August 1998 20 km away from the typhoon center. (Fig. 8c where the storm center was about 110 km away from the Taiwan coastal line), the 4. Mesoscale behaviors of Typhoon Otto eyewall could be determined clearly. Its core under the influence of Taiwan cloud near the vortex center was getting or- orography ganized and strengthened at the second and 4.1 The insights of typhoon structure third quadrant during this period with a quasi Figure 8 shows the sequential structures of semi-elliptic feature, displaying that the more the typhoon at 2 km ASL in constant altitude pronounced core clouds concentrated at the plan position indicator (CAPPI) in reflectivity west side of the long axis of the semi-elliptic from 2230 UTC 3 August to 0130 UTC 4 Au- eye. It was difficult to identify the rotation of gust while the storm center was about 130 km eyewall due to the diversity of its envelopes, (at 2230 UTC) and 80 km (at 0130 UTC) away but it’s appropriate to learn that the long axis from the Taiwan coastal line and had been of the semi-elliptic eye was about 40–50 km in affected by the Taiwan orography. In order to length based upon the analysis of the Doppler identify the temporal variations of structure mode pictures (Figs. 8a, 8c, 8f, 8i and 8l) and inside the typhoon optimally at this stage, the the short axis was about 30–40 km as well. series of CAPPIs included both observations In addition, some convective cells more than collected from Doppler and non-Doppler modes, 30 dBZ distributed along the Taiwan coast line which were interpolated in the Cartesian coor- could be distinguished from the eyewall. Espe- dinates with horizontal resolutions of 1 km and cially, the convective cells concentrated at the 2 km, respectively. Therefore, the echo patterns outer region of the first quadrant were prop- in non-Doppler scanning mode were smoother agating westward and west-northwestward, than those in Doppler mode owing to the worse and had an angular difference larger than 10 data resolution. The time interval between two degrees with respect to the moving direction of sequential CAPPIs was confined to 10 to 30 the typhoon (@345 deg). Their mean propagat- minutes. The datasets in Doppler mode which ing speed was 60 km/h relative to the ground 1012 Journal of the Meteorological Society of Japan Vol. 83, No. 6

Fig. 8. The CAPPIs of reflectivity at the altitude of 2 km at (a) 2230 UTC, (b) 2240 UTC, (c) 2250 UTC, (d) 2300 UTC, (e) 2320 UTC, (f) 2330 UTC on 3 August, (g) 0000 UTC, (h) 0020 UTC (i) 0050 UTC, (j) 0100 UTC, (k) 0120 UTC and (l) 0130 UTC on 4 August 1998. The panels at minute of 00, 20 and 40 for each hour are non-Doppler observations, and the others are in Doppler mode. The domain size is 240 240 km2. The ‘‘þ’’ symbol stands for the radar site and the ‘‘’’ symbol repre- sents the typhoon center. The solid lines illustrated on Figs. 8i@8l represent the reference base of 260 degrees in azimuth. December 2005 T.-H. HOR et al. 1013

Fig. 8. Cont. The shaded arrows shown in Figs. 8c@8f designate the northernmost cells in the outer rainband and the arrows shown in Figs. 8i@8l stand for the inner rainband of the typhoon system. The dashed arrows in Figs. 8a and 8g represent the moving direction of the typhoon and I, II, III and IV stand for the first, second, third and fourth quadrant of the system, respectively. 1014 Journal of the Meteorological Society of Japan Vol. 83, No. 6

(@27 km/h relative to the storm motion), about the development of eyewall periodically. Con- two times larger than that (@33 km/h) of the clusively speaking, the rainbands at the second storm. The cells came from two sources: the and third quadrants were less intense than the northernmost cells migrated from the outer eyewall, which was opposite to the findings rainbands shown by a shaded arrow in Figs. over the open ocean. Also, the stationary band 8c@8f, and the other cells were initiated inside complex was not feasible to be distinguished, the inner rainbands. They had cellular features but several well-defined rainbands were or- in the beginning, and then became a linear ganized at the left side of the system. rainband entity parallel to the coastal moun- Furthermore, Fig. 9 elucidated the more de- tains in the northnortheast-southsouthwest tailed insights of the characteristics, including orientation. At that time the rainband was in- the wind fields as well as the reflectivity pat- tensified just off the sea shore and later on di- terns. The positive peak Doppler velocity in minished gradually after it moved inland. The speed of 43 m/s at 0050 UTC 4 August was convective echo line was unrelated to the dis- located inside the eyewall with a semi-elliptic tance between the Taiwan Island and the ty- shape in maximum reflectivity of more than phoon center. 40 dBZ, and the negative peak value of 49 m/s In addition, the outer rainbands in the sec- with weak echo was on the opposite side (see ond and third quadrants also shifted inward Fig. 9a). The Doppler velocity at 2 km altitude step by step and intensified the inner rain- corresponding to the position of Orchid Island bands and the eyewall. The phenomenon could (x ¼ 5 km, y ¼70 km) obtained at 0130 UTC be identified by the time sequences of the rain- on 4 August (refer to Fig. 9b) was about 20 m/s, bands A, B and C shown in Figs. 8i to 8l. Even- and if the west northwesterly wind direction, tually, the inner rainbands propagated inward and the tangential wind speed as well as the and merged with the eyewall, making it more radial wind speed were considered at that organized. The propagating speed of the bands point, the total wind could reach 30 m/s. The was about 30 km/h relative to ground. Due to sustained wind speed of 28 m/s observed at the the event occurred discontinuously, it made Orchid Island weather station (@324 m ASL) at

Fig. 9. The composite CAPPI of Doppler velocity (m/s) and reflectivity (dBZ) at the altitude of 2 km at (a) 0050 UTC and (b) 0130 UTC 4 August 1998. The shaded areas are the reflectivity intensity and the contours represent the radial velocity. The domain size is 240 240 km2. The ‘‘þ’’ symbol stands for the radar site and the ‘‘’’ symbol represents the typhoon center. The Orchid Island is located at the point (X ¼ 5 km, Y ¼70 km) designated by the ‘‘L’’ symbol. The dashed arrow represents the moving direction of the typhoon and I, II, III and IV stand for the first, second, third and fourth quadrant of the system, respectively. December 2005 T.-H. HOR et al. 1015

0200 UTC of 4 August suggested the consis- tude. Furthermore, the radius of maximum ra- tence. However, the 270 deg wind direction dial wind at 2-km height was about 20–25 km measured by the Orchid Island station, which and the vertical axis of maximum wind in the is located between two mountains in north- eyewall cloud tilted outward at about 20–30 south orientation, was quite different from that degrees, which was similar to the feature of estimated from the storm circulation due to radar reflectivity. In contrast, the convections surface friction and channel flow. in the inner and outer rainbands were weaker (about 30 dBZ in maximum) with the highest 4.2 The vertical structures of rainbands altitude of 9 km. Also, the upward extension As the typhoon was off the southeast coast of of these rainbands exhibited a more outward Taiwan, the vertical characteristics of rain- slantwise feature at about 50 degrees, implying bands inside the storm was apparently differ- that the intense outward flow occurred at the ent from those that existed over the open ocean. upper level due to the noticeable gradient of Based on the vertical echo structures of the wind field. Moreover, the behavior of rainbands system at the 0050 UTC, 0100 UTC, 0120 UTC propagating inward, and merging into the eye- and 0130 UTC on the fourth of August along wall cloud, seemed to play a great role to the 240–260 deg in azimuth (Fig. 10), the maintain the development of the core cloud strength of the eyewall (>10 dBZ in reflec- system. Although the spatial resolution in non- tivity) became more intense compared with the Doppler mode in Figs. 10b and 10c was worse outer rainbands and its height could reach than that in Doppler mode (Figs. 10a and 10d), more than 12 km (>10 dBZ). Also, the radii of it was still helpful to improve the recognition of the eyewall curvature (>10 dBZ and >40 dBZ) the structure and the movement of the rain- were getting smaller, with 11 km and 18 km in bands inside the typhoon. minimum, respectively. It means the structure In Figs. 10a and 10d which showed the com- of typhoon was more organized at the second posite of the Doppler velocity field as well as and third quadrants. The most intense convec- the reflectivity field inside the storm, both tion in the eyewall was more than 40 dBZ in fields were pretty in phase in the vicinity of the reflectivity which could reach 2.5 km in alti- eyewall. The gradient of Doppler velocity was

Fig. 10. The vertical cross sections of Typhoon Otto at (a) 0050 UTC 4 August at azimuthal angle of 260 degrees, (b) 0100 UTC 4 August at azimuthal angle of 255 degrees, (c) 0120 UTC 4 August at azimuthal angle of 250 degrees and (d) 0130 UTC 4 August 1998 at azimuthal angle of 240 degrees. The Doppler velocity is superimposed over reflectivity in (a) and (d). The typhoon center is located at the position of R ¼ 0. 1016 Journal of the Meteorological Society of Japan Vol. 83, No. 6 quite great, especially at the inward and up- passing the tip. ward sides. It implied that the intense vertical According to the statement in Section 4.2, it , as well as the horizontal wind suggests that the obstacle blocking at the aver- shear, existed over this region. age altitude of 2000 m in the southernmost After 2230 UTC of 3 August, the storm kept part of CMR was the key factor for the wind approaching the southeast coast of Taiwan deceleration at the first quadrant of the storm and its structure in Doppler velocity field was system in lower atmosphere when the storm expected to have distinctive changes in the propagated close to the Taiwan coastal line. vertical. At lower levels (less than 1.5 km in al- The strong difference of roughness over the titude) at 0050 UTC on 4 August, the Doppler land-sea surface (shown in Fig. 13) might be velocity fields showed that the maximum wind another factor. Also, the confluence of the speeds at the second quadrant (with 50 m/s at westerly flow triggered by the farther outer cir- the 1 km level shown in Fig. 11a and with culation of the storm around the island (refer to 45 m/s at the 1.5 km level shown in Fig. 11b) the streamline analysis in Fig. 12) and the were more intense than those at the first northwesterly flow from the inner circulation of quadrant (with 42 m/s and 44 m/s at the same the storm played a role to accelerate the air levels, respectively). However, the Doppler ve- speed at the second quadrant of the system in locity fields at the upper levels (higher than lower atmosphere (Figs. 11a@11c). However, 1.5 km) had the opposite situation (Figs. the orographically blocking effect didn’t have 11c@11f ). Table 2 gave more detailed descrip- obvious influence on the storm flow at the up- tions on these reverse phenomena in Doppler per levels where the storm could keep its origi- wind field between 2230 UTC 3 August and nal feature with the stronger wind field at the 0130 UTC 4 August. While the storm center right quadrant and the weaker one in the left was about 109 km away from the radar site quadrant. (about 149 km away from the southeast coast of The Doppler velocity analysis at the left top Taiwan), the reverse mode just occurred at the corner from Figs. 11a to 11c displayed the pos- levels lower than 1 km. Then, the reverse situ- sible change of the real wind direction below ation was getting more striking and up to the 2 km in altitude along the coastal line at 0050 altitudes of 1.5 km and 2 km while the system UTC 4 August 1998. The wind direction just off was located at about 94 km and 26 km away the coastal line was northeasterly. However, from the radar, respectively. the wind direction over inland turned to north- northwesterly probably due to the deflection of 5. Mechanisms of circulation variations the flow by the terrain blocking. This local The mechanisms for maintaining the devel- combination of these two wind fields sug- opment of inner rainbands and eyewall while gested a triggering of slight confluence which the typhoon was approaching the southeast managed a favorite environment for the devel- coast of Taiwan could be figured out reason- opment of convections over the sea shore region ably. The isotach patterns illustrated in the (x ¼ 0km@40 km, y ¼ 0km@50 km), result- stream line analysis at 0000 UTC and 0100 ing in the rainband enhancement over that UTC on 4 August 1998 (Fig. 12) offered the evi- area (see Figs. 8i@8j). However, based on the dence that this phenomenon was related to the streamline analysis (Fig. 12) and the radar confluence between the westerly flow in speed CAPPI pictures (Fig. 11), it was unable to rec- of 25 m/s triggered by the farther outer circu- ognize a similar northerly surface jet forming lation of the storm around the island, and the upstream of the mountain between the cyclone northwesterly flow in speed of 15 m/s near the and the mountain found by Lin et al. (1999). inner circulation of storm itself. This confluent Based upon the description in Section 4.1, approach hinted to enhance the wind intensity under the influence of Taiwan orography, the in the left flank of the storm. The rainbands at position of typhoon center at each time was de- the west side of the southernmost tip of Taiwan rived by the scheme initiated by Wood and also propagated along the streamlines, moving Brown (1992) and then the further modification southeastward in the beginning and then shift- was subjectively made by the geometric center ing to northeastward smoothly while they were of weaker echoes. Therefore, the direction of December 2005 T.-H. HOR et al. 1017

Fig. 11. The Doppler velocity analysis of CAPPIs at (a) 1 km, (b) 1.5 km, (c) 2 km, (d) 2.5 km, (e) 3 km and (f) 4 km in altitude at 0050 UTC 4 August 1998. The ‘‘þ’’ symbol stands for the radar site and the ‘‘’’ symbol represents the typhoon center. The domain size is bounded in 150 150 km2. The arrows appeared from (a) to (c) represent the estimated wind directions at the Taiwan coastal line. The pink dashed arrow in Fig. 11a represents the moving direction of the typhoon and I, II, III and IV stand for the first, second, third and fourth quadrant of the system, respectively. 1018 Journal of the Meteorological Society of Japan Vol. 83, No. 6

Table 2. The maximum positive and negative Doppler velocities measured by the Green Island weather radar near the inner core region of Typhoon Otto from 2230 UTC 3 August 1998 to 0130 UTC 4 August 1998. The shaded area represents the reverse phenomena of Doppler peak winds in the lower portion of typhoon. 2230 UTC 2250 UTC 2330 UTC 0050 UTC 0130 UTC Time Levels 3 AUG. 3 AUG. 3 AUG. 4 AUG. 4 AUG. 28.1 m/s 30.8 m/s 38.5 m/s 42.4 m/s 43.8 m/s 1.0 km þ32.0 m/s þ32.6 m/s þ41.5 m/s þ50.5 m/s þ48.9 m/s 45.6 m/s 40.8 m/s 41.5 m/s 44.3 m/s 45.4 m/s 1.5 km þ42.5 m/s þ42.5 m/s þ42.8 m/s þ45.0 m/s þ46.2 m/s 46.7 m/s 46.3 m/s 47.3 m/s 49.3 m/s 43.0 m/s 2.0 km þ42.9 m/s þ42.5 m/s þ42.1 m/s þ42.9 m/s þ44.1 m/s 45.8 m/s 46.7 m/s 48.6 m/s 52.5 m/s 43.7 m/s 2.5 km þ39.0 m/s þ41.4 m/s þ40.4 m/s þ41.1 m/s þ42.5 m/s 46.4 m/s 49.0 m/s 49.3 m/s 54.3 m/s 46.5 m/s 3.0 km þ36.0 m/s þ39.3 m/s þ39.7 m/s þ40.0 m/s þ42.0 m/s 45.4 m/s 48.3 m/s 50.1 m/s 56.3 m/s 45.1 m/s 4.0 km þ35.5 m/s þ35.1 m/s þ46.5 m/s þ38.3 m/s þ41.1 m/s Distance between typhoon center 109 km 94 km 86 km 59 km 26 km and radar site at 2 km level

the storm movement changed from northwest in altitude) where the storm could keep its to north-northwest in the maximum speed of original propagating speed. The estimated ty- 35 km/h. And this situation was getting more phoon centers for different levels at 0050 UTC obvious while the system itself stayed closer to on 4 August 1998 were shown in Fig. 11 and the orography (refer to Fig. 13). Again, this more pronounced features were displayed in phenomenon at the lower levels could be ex- the conceptual model (Fig. 14) which suggested plained by the combination of the westerly flow that the vortex entity possessed a distorted triggered by the farther outer circulation of the appearance in the lower portion. Generally storm around the island (the corner effect re- speaking, this conceptual model schematically ferred to the streamline analysis on the surface described that in the lower portion of the tropi- in Fig. 12) and the northwesterly flow from the cal storm off the coastal line it had the tilting inner circulation of the storm (the blocking ef- appearance from the vertical position and the fect shown in Fig. 11a at the altitude of 1 km). wind speed at the second and third quadrants According to Tsai and Wang (1991), the corner was greater than that at the first and fourth flow occurred on the north and south tips of quadrants and vise versa in the middle and CMR and enhanced the wind speed there while upper portions. The tilting phenomenon in the typhoons approached the southeastern coast of lower portion resulted from the topographic Taiwan in the northwest direction. Relatively blocking of CRM with average altitude of speaking, the orographical blocking and corner 2000 m. Also, the eyewall and rainbands in effects didn’t have obvious retarding on the the west side of the storm possessed more or- storm entity at the upper levels (above 2.5 km ganized features due to the confluent effect. December 2005 T.-H. HOR et al. 1019

6. Discussions and summary of tropical cyclones. Therefore, the unique inte- gration of the GRI Doppler radar data and the Due to lack of aerial measurements as well limited conventional observations over land be- as surface observations over the western North came a key tool to figure out the following ar- Pacific Ocean, it was quite difficult to provide a guments: (1) the mesoscale variations of circu- lot of observations on the mesoscale structures lation of the typhoon as well as the features of related rainbands and eyewall with and with- out the influence of high terrain, (2) the pri- mary mechanisms for the change of the meso- scale circulation inside the storm system due to the complicated mountainous orography of Taiwan. Typhoon Otto (1998) was the first typhoon reconnoitered by the GRI Doppler radar. Also, it was the first northwestward-moving typhoon before landfall observed by Doppler weather radar as well as the offshore island airport and ground weather stations for carefully inves- tigating the three dimensional mesoscale structures of typhoons in the Taiwan area. The observed radar data gave more detailed infor- mation to improve the accuracy of storm center identification and recognize the evolution of eyewall and rainbands while the storm was developing over the open ocean about 300 km away from the coastal line of Taiwan, and to realize the possible precipitation and wind field distributions as well as the variations of eye- wall and rainband structures before its landfall at about 0421 UTC 4 August 1998. The storm system had gradually propagated into the ra- dar scanning range in Doppler mode (120 km) since 2230 UTC on 3 August 1998 (Fig. 9). Actually, the typhoon itself had been affected by the high and rugged orography at that time. Therefore the GRI Doppler radar observations could support reliable and detailed information to nowcasting. The scanning strategy for non-Doppler long range mode (480 km) was considerately useful

Fig. 12. Surface flow pattern analysis for the Taiwan area at (a) 0000 UTC 4 Au- gust and (b) 0100 UTC 4 August 1998. The solid contours represent the iso- tachs and the dashed lines stand for the streamlines. The area enclosed by dot lines denotes the wake zone and the ar- row stands for the westerly flow coming from the farther outer circulation of the typhoon around the island. 1020 Journal of the Meteorological Society of Japan Vol. 83, No. 6

Fig. 14. The conceptual model of Ty- phoon Otto in the presence of Taiwan orography. The hollow arrow notation stands for the moving direction of the typhoon entity, the black arrows sym- bolize the flow circulations around the vortex, and the shaded arrows repre- sent the westerly flow triggered by the Fig. 13. The schematic diagram for the farther outer circulation of the storm temporal variation of typhoon centers around the island. The ‘‘’’ symbols and related rainbands at the altitude of represent the typhoon centers at differ- 2 km in the presence of Taiwan orog- ent levels. raphy. The ‘‘þ’’ symbol stands for the radar site and the ‘‘’’ symbol repre- sents the typhoon center. The solid straight and curved lines symbolize the from vertical cross sections demonstrated that rainbands related to the typhoon sys- the outer rainband was composed of active con- tem. The dashed line with arrow means vection with outward tilting at about 34–44 the outer circulation of typhoon illus- degrees from the vertical, and the boundary trated in the streamline analysis shown with reflectivity of 10 dBZ could extend upward in Fig. 12. The values for each isopleths more than 12 km in altitude. The inner rain- of height are 100, 500, 1000 and 1500 m ASL, respectively. band (or the principle band) and the core cloud were weaker in intensity and didn’t have a pronounced slanting feature, and its maximum altitude with reflectivity of 10 dBZ was less for the surveillance of a typhoon over the open than 10 km. Furthermore, the rainband pat- ocean. This kind of data set consisted of 6 level tern over the open ocean seemed different from sweepings (0.4, 0.3, 1.2, 2.4, 4.0 and 6.6 deg in the model of the stationary band complex since elevation) in a single volume scan at time the principle band and the connecting band interval of 20 minutes. Therefore, such inten- (joining the inner rainband to the core cloud) sive observations could give better spatial and couldn’t recognize well in this case probably temporal resolutions for the understanding of due to the weak intensity of the storm and the the three dimensional structures of a typhoon worse radar range resolution (@2 km) in the while it was far away from the influence of ter- non-Doppler mode. rain. In this case, the eyewall feature did not As the storm approached the Taiwan coastal appear well in the radar analysis, and the core line, the core cloud (the so-called eyewall) in cloud possessed a slight cyclonic circulation. the lower atmosphere showed an obvious semi- In addition, the outer rainband accompany- elliptic shape, but probably due to the weak- ing with well organized linear convection ness of the typhoon system and the coarse tem- propagated outward more obviously than the poral resolution of radar data, it didn’t retain a counter-clockwise rotation following the storm cyclonic rotation periodically as compared with movement, and vise versa for the motion of in- the findings from Kuo et al. (1999) and Chang ner rainband. Moreover, the results obtained et al. (2002). Based on the schematic diagram December 2005 T.-H. HOR et al. 1021 for the temporal variation of typhoon center shown in Fig. 13, the typhoon entity changed its moving direction from north-westward to north-northwestward in speed up to 33–36 m/s under the weak influence of steering flow (less than 7.5 m/s in the 850 hPa and 700 hPa) and seemed to be trapped by the impact of terrain blocking. In contrast, it didn’t follow a clock- wise island-circulation path similar to the sim- ulation of topographic effects on barotropic vor- tex motion without a mean flow (Kuo et al. 2001). It suggests that Typhoon Otto can’t be regarded as a barotropic vortex without a mean flow. Also, this north-northwestward ap- proaching, less-organized typhoon didn’t have an abrupt track deflection upstream of the bar- rier. This phenomenon was consistent with the simulation made by Chang (1982), showing that the track deflection upstream of the bar- rier depended on the intensity of the approach- ing typhoon. In the presence of Taiwan orography, there were two low-level centers (surface pressure < 1003 hPa) induced on the lee side of CMR Fig. 15. Mesoscale subjective surface which approaches 3997 m in the peak elevation analysis for the Taiwan area at 0000 UTC 4 August, 1998. (Referred from above mean sea level and the original low-level Shieh et al. 2003) center was blocked on the east side of the mountain with the minimum surface pressure of less than 978 hPa shown in the subjective mesoscale surface weather chart (Fig. 15). Also, wall was not well organized and the rainbands two pressure ridges and one trough similar to around the first and second quadrants separat- the findings by Lin et al. (1999) were produced. ing from the inner core region were embedded Basically, the following summaries can be with active convection. The outer rainbands made after the above discussions: were more intense than the core band and in- 1. The storm in its entire life just reached ner rainbands. The vertical cross section along the intensity of slight typhoon (maximum sus- the radial showed that the farther outer rain- tained wind between 17.2 m/s and 32.6 m/s) band tilting outward and extending upward according to the typhoon warning report made over 10 km with maximum reflectivity larger by the Central Weather Bureau of Taiwan. than 45 dBZ were better organized than the However, the measured maximum mean wind inner clouds with height less than 10 km and at Orchid Island was 45.5 m/s as well as the maximum reflectivity larger than 30 dBZ. The gusty wind speed could be up to 68.2 m/s which life duration of the outer rainbands at the first reached the strength of a super typhoon quadrant could last more than 2 hours usually. (>65 m/s). During the passage of this typhoon 3. As the typhoon system was approaching over the Taiwan area, the heaviest total accu- the Taiwan coastal line gradually, its moving mulated rainfall of 412.5 mm in 3 days was lo- direction changed to north-north-west and cated at A-li-shan. The above facts state that propagating speed went up to 33–36 km/h. The the wind field inside the tropical storm can’t be semi-elliptic eyewall was built up at the second regarded as a Rankine vortex, and the rainfall and third quadrants due to the low level con- rate is in diversity. fluence between the westerly flow triggered by 2. While the typhoon was over the open the farther outer circulation of the storm ocean, the radar echoes displayed that the eye- around the island and the northwesterly flow 1022 Journal of the Meteorological Society of Japan Vol. 83, No. 6 near the inner circulation of the storm itself. 1.5 km in altitude under the influence of high The confluent phenomenon was probably the terrain had a slower propagating speed due to primary mechanism to maintain the develop- the orographical blocking and corner effects, ment of eyewall and enhance the wind speed at and the storm entity suggested a distorted ap- the left flank of the storm in the lower atmo- pearance in the lower portion. sphere. Moreover, based on the vertical struc- Although it is not feasible to derive the ther- tures of the system, the strength of the eyewall modynamic structures of the tropical storms became more intense (more than 40 dBZ in re- just from the single Doppler radar data, how- flectivity) comparing with the outer rainbands ever, based on the conventional observations at and its height could reach more than 12 km. Orchid Island and Green Island as well as the Such characteristics were opposite to the fea- single Doppler surveillance, we can clearly tures without the influence of orography. The identify the storm as a windy type or a rainy linear rainband at the first quadrant propa- type one and realize its precise track before the gated outward and behaved a well organized storm hits Taiwan. Therefore, Green Island feature just off the coastal line seemingly due and Orchid Island can be regarded as two sig- to a localized confluence at that region (Figs. nificant outposts for surveilling the westward 11a@11c). moving typhoons. Due to this case study, it tells 4. Under the influence of Taiwan orography, us that the intensive spatial (more than 6 ele- the eyewall cloud possessed an obvious out- vation sweeps) and temporal (less than 20 ward slanting feature at about 20–30 degrees minutes in time interval) resolutions in long from the vertical which was much less than the range conventional mode while the storm is finding (@40–50 degrees) studied in Typhoon over the open ocean as well as the combination Herb by Chang et al. (2002), probably coming of Doppler and non-Doppler mode radar data as from the weaker magnitude in the vertical the system is near the southeast coast of Tai- shear of radial velocity in the Otto case. The wan are necessary for watching the typhoon semi-elliptic eye had a long axis of 40–50 km evolution and getting more information for its and a short axis of 30–40 km which were mesoscale three-dimensional structure, center longer than those found in the Herb case (35– location identification and possible wind and 45 km for long axis and 25–35 km for short precipitation distribution. More case studies, axis, respectively) seemly due to the weak in- especially the more intense tropical cyclones, tensity in the Otto storm. are necessary for better realization of the storm 5. The wind direction discontinuity and de- structures with and without the effect of orog- celeration of wind speed in the vicinity of raphy of Taiwan in the near future. the coastal line over the lower atmosphere Acknowledgements might be resulted from the complexity of ter- rain and the land-sea contrast. However, the The authors would like to thank Prof. Tai- orographical blocking effect played a more sig- Chi Chen at the National Central University nificant role than the change in surface drag for idea support and programming assistance occurring at the coast line in this case due to in the beginning of the study, Dr. Wen-Chau the steep slope and spatial scale of CMR. Lee at NCAR/ATD for the delivery and ques- Moreover, the left quadrant in the lower por- tion answers of NCAR SOLO software, and tion (below 2 km in altitude) possessed stronger Prof. Ben Jong-Dao Jou at the National Taiwan Doppler velocity than the right quadrant in the University for instructive comments. Also, we same layer, and the upper portion (above 2 km) are grateful to two anonymous reviewers and had the opposite mode while the typhoon was Dr. Hiroyuki Iwasaki, the editor in charge of about 30 km away from the radar site (about the Journal of the Meteorological Society of 70 km away from the southeast coast of Tai- Japan. The radar data are kindly provided by wan). The discontinuity of wind direction at Chinese Air Force Weather Wing (Taiwan). We coastal line appeared at the altitude below are indebted to the National Science Council of 2 km seemingly due to the obstacle blocking the Republic of China for the financial support and the sharp transition in land-sea roughness. through Grant NSC 93-2625-Z-014-001 and 6. The estimated typhoon center below NSC 94-2625-Z-014-001. December 2005 T.-H. HOR et al. 1023

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