Radar Observations of Intense Orographic Precipitation Associated with Typhoon Xangsane (2000)

FEBRUARY 2008 YUANDC HENG 497

CHENG-KU YU AND LIN-WEN CHENG Department of Atmospheric Sciences, Chinese Culture University, Taipei, Taiwan

(Manuscript received 22 December 2006, in final form 22 May 2007)

ABSTRACT

With measurements from two ground-based Doppler radars located in northern Taiwan, this study documents the detailed aspects of intense orographic precipitation associated with Typhoon Xangsane (2000) as it moved northward immediately off the eastern coast of Taiwan, bringing strong low-level northeasterly to north-northeasterly winds impinging on the mountainous northern coast. With relatively good, persistent coverage of radar echoes on both inland and upstream regions, this particular event provides a unique description of the orographic precipitation and its relationship with orographic geometry, strong upstream oncoming flow, and the precipitation inherently associated with typhoon circulations. In this case, the heaviest precipitation was observed to occur primarily over two coastal mountain barriers: Mount Da-Tun (DT) and the Nangang-Keelung Range (NKR). Barrier DT is an approximately 3D mountain barrier, and the NKR, adjacent to the southeast of DT, is a relatively lower, narrower 2D mountain range. In particular, the distinct distribution and intensity of precipitation over the two barriers were observed. Analyses of vertical cross sections passing through the major regions of heavy precipitation over DT and NKR indicate the region of low-level heavy precipitation tended to shift downstream as the low-level oncoming flow intensified, and the precipitation exhibited a deeper, wider extent and stronger intensity at stronger oncoming flow regimes. However, changes in the location of maximum precipitation over DT (NKR) were confined mainly to regions over windward slopes (near and downstream of the mountain crest). The degree of downstream shift of low-level heaviest precipitation with respect to different magnitudes of oncoming flow was relatively limited (ϳ8 km) over NKR, as compared with a larger downstream shift of ϳ15–17 km over DT. This contrasting aspect can be understood as a consequence of a longer “lifting section” and relatively lower fall speed of hydrometeors over the windward slope of DT. In addition, the precipitation inherently associated with the typhoon circulations was found to be an important con- tributor to the observed variations in precipitation intensity over DT and NKR. Stronger background typhoon precipitation and a shorter downstream shift of precipitation (i.e., a quasi-stationary precipitation feature) over NKR may explain the fact of larger precipitation accumulation observed over this narrower, lower barrier.

1. Introduction mining the intensity and location of precipitation (Smith 1979; Blumen 1990; Colle 2004). Particularly, Orographic precipitation can occur in a wide variety with analyses of detailed Doppler radar measurements of synoptic conditions. However, our knowledge on this collected over mountains from recent field experi- subject is obtained mainly from previous studies of mid- ments, a number of observational and modeling studies latitude fall season and wintertime precipitation sys- have further improved our understanding on how the tems as they encounter topography. Two primary fac- orographically influenced circulations modulate the tors—the dynamical interaction between the airflow precipitation associated with midlatitude synoptic dis- and topography and its associated microphysical pro- turbances (e.g., Yu and Smull 2000; Yu and Bond 2002; cesses—have long been recognized as crucial in deter- Neiman et al. 2004; Lin et al. 2005b; Yu et al. 2007). In contrast to orographic precipitation associated with midlatitude weather disturbances, which has pre- Corresponding author address: Professor Cheng-Ku Yu, De- partment of Atmospheric Sciences, Chinese Culture University, viously been largely explored, we have very little 55 Hwa-Kang Road, Yang-Ming-Shan, Taipei 111, Taiwan. knowledge about orographic precipitation occurring in E-mail: [email protected] the tropical cyclone environment. Fundamentally, the

DOI: 10.1175/2007MWR2129.1

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MWR3562 498 MONTHLY WEATHER REVIEW VOLUME 136 thermodynamical characteristics and flow regime asso- tions and automatic rain gauges for 58 typhoons affect- ciated with tropical cyclones are distinctly different ing Taiwan during 1989–2001. Their results showed that from those of midlatitude weather systems (e.g., synop- a large average typhoon rainfall occurred generally tic fronts and cyclones). Regions in the vicinity of tropi- over the mountainous regions and that the averaged cal cyclones are usually characterized by abundant rainfall amount considerably increased with surface- moisture and extremely high winds at low levels. This station elevation. Nevertheless, owing to the relatively ϭ implies that a relatively large Froude number (Fr coarse conventional observations (primarily from sur- U/NH; U is the upstream wind speed, N is the Brunt– face stations), these previous studies provide only a Väisälä frequency, and H is the mountain height) flow gross view of the relationship between the typhoon’s regime can easily happen, which particularly favors precipitation and topography, and in particular, their orographic lifting and contributes to precipitation en- associated physical processes are poorly documented. hancement over the windward slopes (e.g., Hamuro et With analysis of Doppler radar measurements col- al. 1969; Lin et al. 2001; Wu et al. 2002). In addition, a lected over Taiwan and other mountainous geographi- tropical cyclone is an approximately circular, intense cal regions, a few observational studies have provided vortex, and as it approaches a mountain barrier, the detailed but limited aspects of orographic precipitation upstream oncoming flow in terms of its wind speed and occurring in the vicinity of typhoons. For example, Lee incident angle is expected to change considerably with and Tsai (1995) used a ground-based Doppler radar time. Multiple flow regimes usually occur during the located in northern Taiwan to investigate the charac- approach of a tropical cyclone and substantially com- teristics of rainbands associated with four landfalling plicate the dynamical interaction of tropical cyclone cir- typhoons. Their results indicated that, in addition to culations with topography (Shieh et al. 1996; Lin et al. relatively fast-moving typhoon rainbands, occurrence 2005a). Furthermore, the precipitation generated by of strong orographic precipitation was evident in all of orographic forcing and usually embedded within highly their analyzed typhoon events and its embedded rain- variable precipitation and/or rainbands inherently asso- bands tended to be quasi-stationary and persistent over ciated with tropical cyclones makes this scientific sub- the mountain slopes. Stationary characteristics of oro- ject highly challenging to investigate. graphically produced precipitation were also noted by As tropical cyclones approach or move across topog- Chang (2000) for another landfalling typhoon event in raphy, continuous torrential rains can frequently result Taiwan. Geerts et al. (2000) used airborne Doppler rain severe flooding (e.g., Hope 1975; Wu and Kuo 1999). dar to investigate Hurricane Georges making landfall Observations of landfalling tropical cyclones occurring on the mountainous island of Hispaniola. Their analy- over different geographical locations have shown a gen- ses suggest that the orographic lifting of boundary layer erally strong relationship between the total rainfall and air would play an important role in contributing to the local distribution of topography (Brunt 1968; Hamuro occurrence of deep convection within the hurricane’s et al. 1969; Parrish et al. 1982). For tropical cyclones eye and the enhancement of radar reflectivity at low affecting Taiwan, the importance of orography on levels. However, although these radar-related studies modulating mesoscale rainfall distributions has been have described some interesting features of orographic well known. For example, Chang et al. (1993) used sur- precipitation associated with landfalling tropical face observations from 82 typhoons occurring during a storms, none of them have explicitly addressed the 20-yr period to document the effects of Taiwan’s terrain physical connection between the precipitation distribu- on the surface features of typhoons. Their analyses tion and intensity, topographic features, and incident showed that the location of the typhoon circulation cen- flow associated with tropical cyclone circulations. ter relative to topography was crucial in controlling pre- Owing to the increasing capability of mesoscale mod- cipitation patterns over Taiwan. Wang (1980, 1989) els in realistically producing the detailed mesoscale comprehensively examined various observational as- structures associated with tropical cyclones, numerical pects of typhoons as they interacted with Taiwan to- simulations have been quite helpful in providing some pography and found that the precipitation distributions insight into the modifications of tropical cyclones by associated with typhoons were strongly modulated by topography. Most of the modeling work has focused on topography, especially with an obvious trend linked to the track deflection and change in intensity as a tropical the typhoon track. Based on Wang’s results, a reason- cyclone approaches or moves over topography, not on able rainfall forecast over Taiwan could be made when the orographically modified precipitation near tropical the typhoon track is predicted well. Lee et al. (2006) cyclones. Lin et al. (2002) used a nonhydrostatic me- analyzed rainfall data from conventional surface sta- soscale model to investigate the orographic rain accom-

Unauthenticated | Downloaded 10/01/21 09:42 AM UTC FEBRUARY 2008 YUANDC HENG 499 panying Typhoon Bilis (2000) as it passed Taiwan to- 2. Data and methodology pography. Their analyses indicate that the rainfall distribution over the mountainous regions was strongly The primary datasets used in this study were pro- controlled by the mechanism of orographic lifting vided by the Central Weather Bureau of Taiwan op- rather than by the original rainbands associated with erational S-band (10 cm) Doppler radar (WSR-88D) on the typhoon. Recent numerical work by Wu et al. Wu-Fen-San (WFS) and the Civil Aeronautics Admin- (2002), who simulated the landfalling of Typhoon Herb istration (CAA) operational C-band (5 cm) Doppler (1996) with a special emphasis on its associated rainfall radar located at Taoyuan International Airport. Loca- distribution and intensity, suggest that the forced up- tions of the two radar sites are indicated in Fig. 1. Both slope lifting associated with the interaction of the ty- radars provide volumetric distributions of reflectivity phoon’s circulations and Taiwan topography substan- and radial velocity with a temporal interval of ϳ6 min tially increased the total rainfall amounts over moun- (for WFS radar) and ϳ30 min (for CAA radar) be- tains. It is clear that our knowledge obtained from the tween each volume. Details on characteristics of both numerical studies is largely hampered by inherent un- WFS and CAA radars are summarized in Table 1. The certainties and complexities associated with parameter- WFS radar is located just ϳ10 km inland from the ized processes over topography, high sensitivities of northern coast and has longer wavelength, which can model and terrain horizontal resolution on simulated provide better data coverage and less attenuation than orographic rainfall (e.g., Wu et al. 2002; Colle et al. the CAA radar. Because the lowest plan-position indi- 2000), and the difficulties in model validation due to the cator (PPI) scan (0.4° elevation) of the WFS radar fre- lack of adequate observations over mountainous re- quently contains contamination due to mountain clutter gions. and/or blockage of the radar beam, the low-level pre- The primary objective of this study is to explore our cipitation patterns presented in this paper are derived knowledge of orographic precipitation occurring in the from the radar reflectivities from the 1.4° elevation PPI typhoon environment by analyzing Doppler radar mea- scan of the WFS radar. For this second scanning eleva- surements collected when Typhoon Xangsane affected tion, the contaminated echoes are not present over Taiwan on 31 October–1 November 2000. Xangsane northern Taiwan (i.e., the major study region); how- was one of the most unforgettable tropical cyclones in- vading Taiwan. In addition to Nari (2001) and Toraji ever, they still can be found near the southern portion (2001), severe flooding, landslides, and debris flow as- of Snow Mountain Range (Fig. 1) where terrain height sociated with this typhoon event caused the most seri- is generally above 2000 m (mean sea level; MSL). In ous loss of human life (64 deaths and 25 missing) in the addition to radar data, other data sources used in this past 30 yr. Moreover, a Singapore Airlines commercial study, including routine surface and sounding observa- aircraft crashed at Taoyuan International Airport while tions, are indicated and summarized in Fig. 1. attempting to take off in the stormy weather during the The dual-Doppler synthesis from the WFS and CAA influence of Xangsane, and 82 passengers were killed radars derived from multiple-view reflectivity and ra- by this unfortunate accident. Scientifically, because this dial velocity data (Ray et al. 1980) is also applied to particular event has relatively good, persistent coverage retrieve the 3D wind field off the northern coast of of radar echoes on both inland and upstream regions, it Taiwan, upstream of the barriers. The inset box in Fig. provides a unique description of the orographic precipi- 1 marks the synthesis domain extending from the north- tation and its physical relationship with orographic ge- ern coastline of Taiwan to ϳ15 km offshore. This de- ometry, strong upstream oncoming flow, and the pre- rived offshore kinematic information allows us to in- cipitation inherently associated with typhoon circula- vestigate the possible physical link between the up- tions. In this study, observations from two ground- stream oncoming flow and orographic precipitation. based Doppler radars located in northern Taiwan were Owing to the inherent limitation of synthesized geom- used to document the detailed aspects of the precipita- etry between the two radars, we did not attempt to tion distribution as Xangsane moved northward imme- perform wind synthesis over Taiwan landmass in this diately off the eastern coast of Taiwan, bringing strong, study. A significant portion of northern Taiwan is close low-level northeasterly to north-northeasterly winds to the baseline zone of the two radars, where the cross- impinging on the mountainous northern coast. How beam angles are too large or small, and it would cause these observed spatial and temporal variations of pre- substantial uncertainties and errors in the synthesized cipitation relate to the topographic features, upstream winds. airflow, and typhoon precipitation was the particular The National Center for Atmospheric Research focus of this study. (NCAR) “SOLO” software (Nettleton et al. 1993) is

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FIG. 1. Topographic features of northern Taiwan. Terrain height (m MSL) is indicated by shading. Locations of Snow Mountain Range, DT, NKR, and YTR are marked. Locations of the WFS Doppler radar and the CAA Doppler radar at Taoyuan International Airport are denoted by triangles. Locations of surface observing stations, automatic meteorological observing stations, automatic rain gauge, and sounding station are denoted by squares, hollow circles, solid circles, and asterisk, respectively. The surface observing station at Keelung (KE) and the sounding station at Banciao (BA) are also marked. The inset box immediately off the northern coast of Taiwan denotes the dual-Doppler synthesis domain (45 ϫ 15 km2) adopted in this study. used to unfold the radial velocities and to remove sea multiple reflectivity data are available, the maximum clutter and unreasonable or incorrect values of radar observed reflectivity value is retained to mitigate the reflectivity and radial velocity data. The NCAR effects of attenuation. Synthesis of the gridded radial “REORDER” software (Oye et al. 1995) is used to velocities into horizontal wind fields is performed using interpolate reflectivities and radial velocities from raw NCAR Custom Editing and Display of Reduced Infor- PPI scans to Cartesian coordinates with horizontal grid mation in Cartesian Space (CEDRIC; Mohr and Miller spacing of 1 km and vertical grid spacing of 500 m, over 1983). Vertical air motions are obtained through varia- a volume of 45 ϫ 15 km2 in the horizontal (box in Fig. tional adjustment of the anelastic continuity equation 1) and 10 km in the vertical, with the lowest analysis with boundary conditions of zero vertical motions near level located at 500 m (MSL). At grid points where the surface and at echo top. Owing to a relatively higher

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TABLE 1. WFS and CAA radar characteristics. the typhoon precipitation. These inherent uncertainties of the lower (upper) boundary condition may represent Parameter CAA WFS a significant source of error in deducing vertical air Lon 121.20°E 121.77°E velocity. In view of this, only dual-Doppler synthesized Lat 25.07°N 25.07°N horizontal winds are analyzed and presented in this pa- Alt 27 m 766 m Wavelength 5.31 cm 10 cm per. In this study, there are a total of 21 sets of synthe- Peak power 250 kW 750 kW sized winds derived from 2000 UTC 31 October to 0600 Frequency 5.60–5.65 GHz 2.7–3.0 GHz UTC 1 November with a time interval of ϳ30 min be- Antenna gain 43 dB 45.6 dB tween each wind field set. Beamwidth 0.85° 0.95° Pulse repetition 900/1200 Hz 318–1304 Hz frequency 3. Case overview Unambiguous Ϯ48 m sϪ1 Ϯ26.55 m sϪ1 velocity Ϯ30.95 m sϪ1 Typhoon Xangsane (2000) initially moved westward Max range 120 km 150–230 km after its formation over the tropical western Pacific ␮ ␮ Pulse duration 0.5 s 1.6 s Ocean on 26 October 2000 (Fig. 2). It started to head Min detectable signal Ϫ114 dBm Ϫ133 dBm Range resolution 1 km 1 km (reflectivity) north immediately west of the Philippines on 30 Octo- 250 m (radial ber 2000 and kept its northward journey over the velocity) Bashai Strait over the next two days. Xangsane did not Rotation rate 2.4 rpm 1.76–2.3 rpm make landfall on the landmass of Taiwan, but passed Elevations (°) 0.5, 0.9, 1.3, 2.4, 0.4, 1.4, 2.3, 3.3, nearshore, immediately off the eastern coast during 31 3.5, 4.5, 6.0, 8.0, 4.2, 6.0, 9.8, 10.0, 15.0 14.5, 19.5 October to 1 November 2000. The synoptic surface analysis at 0000 UTC 1 Novem- ber 2000 (Fig. 3) shows a continental anticyclone cen- altitude of the WFS radar site (ϳ766 m MSL), the de- tered over northern China near 35°N, 110°E and a tailed wind information near the surface cannot be ad- northeast–southwest-oriented stationary front located equately sampled by the radar. Moreover, the highest northeast of the low pressure associated with Typhoon elevation of radar beams is sometimes unable to reach Xangsane. During this time, wind fields in the vicinity the actual cloud top of deep convection associated with of Taiwan were dominated by the typhoon cyclonic cir-

FIG. 2. Track of Typhoon Xangsane (2000). Typhoon center is indicated every 6 h from 0000 UTC 26 Oct to 1800 UTC 1 Nov 2000. Filled (open) typhoon symbols indicate that the maximum wind speed of the typhoon is greater (smaller) than ϳ33 m sϪ1 (adapted from the Central Weather Bureau of Taiwan).

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FIG. 3. The Central Weather Bureau surface analysis at 0000 UTC 1 Nov 2000. Full wind barbs correspond to 5 msϪ1; half barbs correspond to 2.5 m sϪ1. culations whose pressure center was just located imme- coast of Taiwan. Northerly winds, characterized by in- diately adjacent to the eastern coast of northern Tai- tense approaching radial velocities (greater than 30 wan. Strong north-northeasterlies (Ͼ20 m sϪ1) pre- msϪ1), prevailed immediately upstream of the Taiwan vailed over northern Taiwan. An elongated zone of landmass (Fig. 4b). enhanced pressure gradient oriented approximately Some important evolving aspects of the upstream on- northeast–southwest between the continental high and coming flow associated with the typhoon circulations the typhoon’s depression was also evident. As Xang- can be clearly depicted by a sequence of dual-Doppler sane progressed further northeastward into the East observations from the synthesis domain in Fig. 1. At a China Sea at a later time, low-level prevailing winds given analysis level, winds at each grid point within the over the Taiwan area were still generally from the north synthesis domain are averaged to obtain a mean value and were mainly influenced by the arrival of large-scale to represent the upstream oncoming flow at that alti- anticyclonic circulations associated with the approach- tude. The standard deviations of the mean winds cal- ing continental high. culated at different analysis levels are found to be gen- During the passage of Xangsane, heavy rain occurred erally small (Ͻ4msϪ1) during the study period, sug- and appeared to be persistent over northern Taiwan. gesting a relatively horizontal uniform nature of the Figure 4 shows the 0329 UTC 1 November PPI display upstream oncoming flow over regions immediately off of radar reflectivity and radial velocity at 1.4° elevation the northern coast of Taiwan. The time–height cross from the WFS radar to illustrate the major precipitation section of the mean dual-Doppler-derived winds re- and airflow patterns in the vicinity of northern Taiwan. veals a general veering of low-level winds with height During this time, Xangsane’s center with near-zero re- and an anticlockwise transition of winds from relatively flectivities was located ϳ65 km offshore east of the weak easterlies to strong north-northeasterlies during WFS radar, and the primary precipitation associated the approach of Xangsane (Fig. 5). The low-level north- with the typhoon circulation was confined to regions erly flow off the northern coast reached its maximum north and west of the typhoon center (Fig. 4a). The intensity (ϳ40 m sϪ1) at around 0200 UTC 1 Novem- highly asymmetric distribution of typhoon precipitation ber, and afterward decreased continuously as Xangsane was mainly due to the influence of Taiwan topography. moved further northeastward away from northern Tai- The typhoon eyewall, with maximum reflectivities wan (cf. Fig. 2). Note that midtropospheric winds be- reaching above 40–45 dBZ, were also evident. The came northwesterly and/or westerly after 0400 UTC 1 outer typhoon precipitation bands typically moved on- November, consistent with the influence of the typhoon shore and made landfall on the northern and/or eastern circulation, whose center had moved to regions ϳ100

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FIG. 5. Time–height cross section of Doppler-derived winds averaged over the dual-Doppler synthesis domain (box in Fig. 1) during the influence of Xangsane (from 1500 UTC 31 Oct to 0600 UTC 1 Nov 2000). Wind flags correspond to 25 m sϪ1; full wind barbs correspond to 5 m sϪ1; half barbs correspond to 2.5 m sϪ1.

km northeast of Taiwan. In distinct contrast, northerly winds still prevailed in the low troposphere during this later period and were primarily influenced by the arrival of the continental anticyclonic circulation, as described earlier. As shown in Fig. 4a, there was obvious evidence of orographically enhanced precipitation (maximum reflectivity Ͼ40–45 dBZ) immediately inland along the northern coast of Taiwan, namely adjacent to and over the windward slopes of Mount Da-Tun (DT) and near the northern end of Snow Mountain Range (SMR). This zone of enhanced precipitation appeared to be quasi-stationary, distinct from the generally fast- moving characteristics of typhoon rainbands. This evolving aspect can be clearly seen from a sequence of low-level radar reflectivities averaged within the elongated boxes (locations in Fig. 4a) encompassing the two FIG. 4. Low-level PPI scan (1.4° elevation) from the WFS radar mountain barriers (i.e., the DT and the northern por- (location marked by ϩ) at 0329 UTC 1 Nov 2000. (a) Radar reflectivity (dBZ, shading); (b) radial velocity (m sϪ1) with a con- tion of SMR) and oriented approximately parallel to tour interval of 10 m sϪ1. Terrain height in (a) is indicated by the oncoming north-northeasterly flow, as shown in Fig. contours at 300, 600, and 1800 m while terrain height in (b) is 6. Before about 2000 UTC 31 October, northern Tai- indicated by shading (key at top of panel). In (a), two inset boxes wan was mainly influenced by the passage of outer ty- (A and B) encompassing the DT and the northern portion of phoon rainbands, with heavy precipitation extending SMR indicate the area for calculating the mean radar reflectivities shown in Fig. 6. from inland to regions well offshore. There was no obvious evidence of orographically enhanced precipitation during this early period. With time, Xangsane ap- proached northern Taiwan, and the low-level oncoming

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FIG. 7. Ten-hour accumulated rainfall (mm) observed by rain gauge over northern Taiwan from 2000 UTC 31 Oct to 0600 UTC 1 Nov 2000. Terrain height is indicated by contours with an interval of 300 m (MSL).

hanced immediately inland; in particular, the heaviest precipitation (Ͼ40 dBZ) occurred over the windward slopes of DT and SMR and tended to persist for several hours. The enhanced orographic precipitation started to weaken in association with the decrease in the oncoming northerly flow after 0300 UTC as Xangsane moved farther away from the northern coast of Taiwan. Persistent and intense precipitation over the two mountain barriers caused a significant amount of accumulated rainfall. The rainfall map (Fig. 7), which was produced objectively using rain gauge data with the interpolation algorithm of Cressman (1959), shows the most prominent rainfall nearby DT and the northern portion of SMR. More than 400 mm of rainfall over a 10-h period from 2000 UTC 31 October to 0600 UTC 1 No- vember was observed over these mountainous regions. The vertical thermodynamic characteristics over northern Taiwan can be revealed by a sounding taken FIG. 6. Temporal variation of precipitation in the vicinity of at Banciao (location in Fig. 1). It should be noted that northern Taiwan during 1500 UTC 31 Oct to 0600 UTC 1 Nov ϳ 2000. The low-level PPI scan (1.4° elevation) of radar reflectivity the Banciao sounding is located on the lee of DT, 30 from the WFS radar within the elongated boxes (shown in Fig. 4) km from the northern coast of Taiwan, and it would be was averaged in a direction normal to the orientation of the box probably less representative of thermodynamic condi- and plotted as a function of time and alongbox distance. Results tions upstream of the DT and SMR. A modified sound- for box A and B are shown in (a) and (b), respectively. The right ing profile can be obtained to check this possibility if panel of each figure shows the corresponding mean terrain height along the box. Low-level mean Doppler-derived winds (averaged the Banciao sounding data below 1 km (i.e., close to the below 1 km over the dual-Doppler synthesis domain indicated in height of DT) is replaced by a linear interpolation using Fig. 1) are also indicated (full wind barbs correspond to 5 m sϪ1; surface measurements at Keelung, located at the north- Ϫ1 half barbs correspond to 2.5 m s ). ern coastline (location in Fig. 1). It was found that this sounding profile (not shown) exhibited thermodynamic winds over this region became northeasterly after 2000 features highly similar to those seen in the Banciao UTC and started to have an appreciable flow compo- sounding (cf. Fig. 8). Hence, the influence of leeside nent perpendicular to the northern slopes of coastal effects on thermodynamics did not appear significant at mountain barriers. Precipitation was significantly en- Banciao. The thermodynamic profile shown in Fig. 8a

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ticularly at low levels, were much weaker, presumably because of the influence of relatively large surface fric- tion over land. The heights of coastal topography over northern Tai- wan (including DT and the northern portion of SMR) are generally less than ϳ1 km (MSL; cf. Fig. 1). The dry Brunt–Väisälä frequency (N) calculated below 1 km is equal to 1.1 ϫ 10Ϫ2 sϪ1. Given a saturated condition in the lower troposphere (Fig. 8), the saturated Brunt–

Väisälä frequency (Nm) derived by Durran and Klemp (1982) would be more suitable to approximate static stability for this case. Its value is calculated to be equal to 3.9 ϫ 10Ϫ3 sϪ1. With these values, a critical magnitude of upstream wind speed for a Froude number equal to 1 in the dry and saturated conditions is found to be ϳ11 and ϳ4msϪ1, respectively. Given low-level oncoming northerly winds associated with Xangsane are generally stronger than 20 m sϪ1 during the occurrence of intense orographic precipitation (cf. Figs. 5 and 6), a Froude number greater than 1 would be easily achieved for this case. In this flow regime, oncoming winds are expected to climb over mountains, instead of flowing around (Smith 1979). On the other hand, given the saturated and convectively neutral environment in the lower troposphere (Fig. 8), the buoyant force on air parcels being displaced by orographic lifting would be rather small, implying the insignificance of mountain- wave motions (Blumen 1990). The pattern and intensity of vertical motions over mountains would be primarily determined by the forced lifting produced as oncoming winds flow over their underlying mountain slopes. The possible importance of topographically forced vertical motions on the observed orographic precipitation will be discussed further in section 4.

4. Characteristics of precipitation distribution In northern Taiwan, most of surface observing sta- FIG. 8. Skew T–logp plot. (a) The Banciao sounding (location in tions are located in lowland regions, and the distribu- Fig. 1) taken at 0000 UTC 1 Nov 2000 and (b) the corresponding tion of rain gauge stations is rather sparse, particularly vertical profiles of potential temperature and equivalent potential temperature. In (a), full wind barbs correspond to 5 m sϪ1; half over higher-terrain regions (cf. Fig. 1). The high tem- barbs correspond to 2.5 m sϪ1. poral and spatial resolution of measurements from the low-level PPI scan (1.4° elevation) of the WFS radar reflectivity are analyzed to provide a detailed view of reveals saturated conditions throughout the low– trends and extremes in precipitation intensity over both midtroposphere. Nearly neutral convective instability upstream and mountainous regions. As described in was present in the lowest 2.5 km, with stable convective section 3, the occurrence of the prominent orographic instability aloft (Fig. 8b). Low-level winds veered with precipitation was confined to between 2000 UTC 31 height from northerly near the surface to easterly near October and 0600 UTC 1 November, and hence subse- the midtroposphere. These wind patterns were basi- quent analyses of radar observations will focus only on cally similar to those derived from dual-Doppler syn- this 10-h period. thesis off the northern coast of Taiwan at around 0000 Figure 9a shows the horizontal distribution of accu- UTC 1 November (cf. Fig. 5), but their intensities, par- mulated radar reflectivity during this interval. General

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FIG. 9. (a) Horizontal distribution of cumulative reflectivity (contours with 150-dBZ interval) derived from the low-level PPI scan (1.4° elevation) of the WFS radar during the 10-h period from 2000 UTC 31 Oct to 0600 UTC 1 Nov 2000. Terrain height (m MSL) is indicated by shading. (b) As in (a) except showing the frequency distribution of heavy precipitation (Ͼ40 dBZ) with contour interval of 10%. Line segments D1, D2, and N mark location of vertical cross sections shown in Figs. 11–13.

Unauthenticated | Downloaded 10/01/21 09:42 AM UTC FEBRUARY 2008 YUANDC HENG 507 precipitation patterns over land seen in Fig. 9a were sistent. There was also some evidence of precipitation similar to those indicated by surface rain gauge (cf. Fig. enhancement upstream of the coastal barriers; particu- 7) but with more detailed aspects resolved. There was a larly, a rapid onshore increase in either reflectivity ac- general onshore increase in the accumulated reflectiv- cumulation or frequency, starting from ϳ5–10 km off- ity. Two local accumulated reflectivity maxima (ϳ4000 shore, was located along the northern coastline. How- dBZ) were observed over the northern slopes (i.e., the ever, precipitation enhancement was not obvious windward side) of DT, but with a sharp decrease on the farther offshore, where much lower values (Ͻ30%) and lee. A localized precipitation minimum located in the relatively uniform frequency distributions were ob- low foothills immediately southwest of the barrier was served (Fig. 9b). Note that slightly larger frequency val- also evident. As clearly seen in Fig. 6a, a localized re- ues (ϳ10%–20%) found in offshore regions northeast gion of relatively weak radar echoes were present on of Taiwan were related to the passage of rainbands the lee slope of DT and appeared to persist for ϳ10 h associated with the inner core of the typhoon (cf. Fig. during the occurrence of strong oncoming northerly 4a). Given an unblocked flow regime for the present flow. The horizontal extent of this observed feature was case, as described in section 3, upstream blocking by approximately comparable to the half-width of the DT topography would probably not be a primary cause of barrier (cf. Figs. 6a and 9a). Its appearance is consistent the precipitation enhancement near the coast. Instead, with the influences of topography; namely, the prevail- the boundary layer convergence due to the differential ing north-northeasterly winds flow over the windward surface roughness between land and ocean (e.g., slopes of DT and descend on its leeside to favor the Roeloffzen et al. 1986) would be more likely to provide evaporation of hydrometeors. Several previous studies a zone of upward vertical motions along the coast, con- from other geographical locations have reported the tributing to the observed signatures. Pronounced dis- frequent occurrence of so-called rain shadow on the lee continuities in low-level winds due to the change in of an isolated, 3D mesoscale barrier (e.g., Mass and surface roughness at the coastline have been well docu- Ferber 1990). The previously documented rain shadow mented by previous modeling and observational studies is usually characterized by relatively dry, precipitation- of landfalling tropical cyclones (Powell 1982; Tuleya et free features. In contrast, considerable reflectivity ac- al. 1984). The coastal frictional convergence has also cumulation was still present near the local minimum of been observed to play a role in forming intense convec- precipitation on the lee of DT, presumably due to con- tive cells near the eyewall of landfalling Hurricane An- tribution from the background typhoon precipitation. drew (Willoughby and Black 1996). Another accumulated reflectivity maximum with val- An interesting but uncertain issue regards whether ues (Ͼ4300 dBZ) even greater than those seen over DT the distributions of orographically enhanced precipita- was found along the first, northern hill of SMR, called tion shown in Fig. 9 have some physical connection to the Nangang-Keelung Range (NKR; Fig. 1). In contrast the topographically forced vertical motions. To provide to the precipitation patterns over DT, where peak re- some insight on this, the topographically forced vertical flectivity values were confined to the windward slopes, motions are estimated and then compared with the pre- accumulated reflectivity with the maximum value of cipitation patterns shown in Fig. 9. Considering the 4300 dBZ spillover into the lee of NKR was clearly horizontal winds flowing over a mountain, the topo- evident. Gradually reduced precipitation was observed, graphically forced vertical velocity is proportional to however, over higher mountains and slopes further the steepness of the mountain slope along the wind south. direction of oncoming flow and can be approximated by To check whether the patterns of reflectivity accu- the expression mulation shown in Fig. 9a can reasonably represent the Ѩh͑x, y͒ Ѩh͑x, y͒ realistic distribution and intensity of precipitation, the ͑ ͒ ϭ ͑ ͒ ϩ ␷͑ ͒ Wterrain x, y, t u h, t h, t , frequency distribution of strong reflectivity (Ͼ40 dBZ) Ѩx Ѩy during the 10-h period is also calculated for comparison ͑1͒ and shown in Fig. 9b. The frequency distributions were highly similar to the accumulated reflectivity pattern where h is the terrain height; and u and ␷ represent the shown in Fig. 9a. Two localized maxima with frequen- east–west and south–north flow components of up- cies of 60%–70% were coincident with the peaks of stream oncoming winds, respectively. Equation (1) has accumulated reflectivity value (cf. Fig. 9a). The largest been used in many previous studies to evaluate the rela- frequency of heavy precipitation was found over NKR tive importance of orographic lifting and other convec- and reached above 80%, suggesting precipitation over tive forcings associated with synoptic and/or mesoscale this mountain range to be more intense and stably per- systems (e.g., Lin et al. 2001; Neiman et al. 2002; Wu et

Unauthenticated | Downloaded 10/01/21 09:42 AM UTC 508 MONTHLY WEATHER REVIEW VOLUME 136 al. 2002; Georgis et al. 2003). To obtain more represen- 5. Relation of precipitation to low-level upstream tative magnitudes for Wterrain at different slope heights oncoming flow and time periods, u and ␷ in (1) are not a constant value but a function of the terrain height h and time t.Inour In this section, we investigate how the precipitation calculations, u and ␷ values at different heights and time structure and intensity over DT and NKR change as the periods are obtained from the Doppler-derived wind intensities of the low-level oncoming flow change with profile shown in Fig. 5. Note that the patterns of topo- the approach of typhoon circulations. In pursuing the graphically forced vertical velocity for each calculated investigation of these important aspects, several repre- time period are slightly different because of the tem- sentative vertical cross sections of radar reflectivity poral variation of upstream oncoming flow. For the across the major regions of intense orographic precipi- convenience of comparison with Fig. 5, the frequency tation obtained from the different intensities of the on- values of topographically forced vertical velocity are coming northeasterly-to-northerly flow are analyzed. ϳ then computed during the 10-h interval of interest. For DT, a wider, approximately 3D barrier ( 10 km in Figure 10 shows the frequency distribution of topo- half-width), two vertical sections (D1 and D2 indicated in Fig. 9b) are selected to pass through the cores of graphically forced vertical velocity (Wterrain) greater than 1 m sϪ1. Consistent with prevailing north- maximum reflectivity accumulation and frequency and northeasterly oncoming flow, the regions of larger fre- be oriented roughly parallel to the oncoming flow. For ϳ quency values were generally found over the northern NKR, a narrower, approximately 2D barrier ( 5kmin slopes of mountains (Fig. 10a). Two primary areas of half-width), 16 vertical cross sections (N indicated in Ͼ Fig. 9b) running through the zone of strongest reflec- maximum frequency of Wterrain ( 90%) are located tivity accumulation and frequency and normal to the over the windward slopes of DT. These frequency 1 maxima did not exactly coincide with the regions of barrier are selected. Given a roughly 2D precipitation maximum precipitation accumulation and frequency pattern over NKR, a representative vertical section is (Fig. 10b), but the local maxima of precipitation fre- then obtained by averaging these different, respective quency tends to be located near and immediately down- vertical sections. stream of these local frequency maxima of W . The temporal interval of the WFS radar between terrain ϳ When considering the realistic advection effect of water each volumetric data is 6 min, which gives a set of droplets by ambient flow, the majority of precipitation about 100 vertical cross sections for each selected sec- ϳ particles reaching the ground in a position downwind of tion (i.e., D1, D2, and N) during the study period ( 10 major orographic lifting are reasonable (e.g., Blumen h). Because the temporal resolution of upstream on- 1990), although residence time scale for hydrometeor coming flow retrieved from the dual-Doppler synthesis ϳ growth aloft remains uncertain for this case. The zone (cf. Fig. 5) is 30 min, these Doppler-derived winds are of the most enhanced precipitation over NKR generally first interpolated into a 6-min interval and the oncom- coincided very well with the maximum frequency of ing flow component parallel to the orientation of selected cross sections is then calculated. Radar reflectivi- Wterrain (Fig. 10b), but still with a trend of shifting slightly ties along each selected section (i.e., D1, D2, and N) downstream of these frequency maxima of Wterrain. observed from different time periods are averaged Given a nearly neutral convective instability at low lev- Ϫ1 els, as shown in Fig. 8, the above results suggest that within an arbitrary interval (5 m s used herein) of condensation due to orographic lifting (instead of re- low-level mean oncoming wind component computed ϳ lease of convective instability) may have played an im- below the maximum height of mountains [ 1km portant role in contributing to the precipitation en- (MSL)] over the study region. This averaging proce- hancement over the mountain slopes (Houze 1993). dure may largely mitigate the relatively rapid temporal There was no obvious evidence of the local maximum variety of precipitation due to the passage of the indi- vidual typhoon’s rainbands so as to amplify the precipi- frequencies of Wterrain over the more distant southern mountains of SMR [e.g., Ying-Tzu Range (YTR) tation variations exclusively relevant to the changes in marked in Fig. 1] associated with the local maxima of upstream oncoming flow. precipitation frequency. Low correlation between the Mean vertical structures of precipitation along D1 degree of orographic lifting and the precipitation inten- and D2 at their corresponding intervals of low-level sity over these secondary, downwind mountain ranges implies the significance of other factors (such as the 1 The reason why such orientation is chosen herein is that, for leeside drying and blocking of low-level moisture sup- an approximately 2D barrier, the direction perpendicular to the ply by NKR) influencing precipitation development in orientation of the mountain barrier is more representative of dy- these regions. namical importance.

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Ϫ FIG. 10. (a) Frequency distribution of topographically forced vertical velocities (Ͼ1ms 1) during the 10-h period from 2000 UTC 31 Oct to 0600 UTC 1 Nov 2000, with a contour interval of 30%. Shading indicates terrain height (m MSL). (b) As in (a), except that shading indicates the frequency distribution of heavy precipitation (Ͼ40 dBZ).

Unauthenticated | Downloaded 10/01/21 09:42 AM UTC 510 MONTHLY WEATHER REVIEW VOLUME 136 oncoming flow component are shown in Figs. 11 and 12, precipitation can be obtained if reflectivities from the respectively. It is obvious that the trends of precipita- lowest analysis level [750 m (MSL)] along D1, D2, and tion change with increasing oncoming flow seen from N are plotted as a function of low-level oncoming flow, these two vertical cross sections are quite similar. As as shown in Fig. 14. Note that no averaging procedure the oncoming flow increased, the region of low-level was applied in the reflectivity field shown in Fig. 14 and heavy precipitation (Ͼ40 dBZ) tended to have an ob- thus a more continuous aspect regarding how the low- vious shift toward the mountain peak from regions ei- level precipitation evolves with the change in the inten- ther over the lower mountain slope (Fig. 11a) or slightly sity of oncoming flow can be seen. Over DT, a promi- upstream of the mountain (Fig. 12a). Horizontal extent nent downstream shift of heavy precipitation as the on- of 40-dBZ reflectivity was rather limited (only Ͻ5km coming flow intensified was evident (Figs. 14a,b). This in width) at weaker oncoming flow regimes (Figs. 11a,b variation was also accompanied by a pronounced wid- and 12a,b). However, a much wider region (ϳ10 km) of ening of the major precipitation region; the horizontal Ͼ heavy precipitation (Ͼ40 dBZ) could be seen when the extent of heavy precipitation ( 40 dBZ) was generally Ϫ ϳ oncoming flow was greater than 20 m s 1 (Figs. 11c–e increased from 3 km at a relatively weak oncoming Ͻ Ϫ1 ϳ and 12c–e). Reflectivity trough or minimum on the lee flow regime ( 15 m s ) to at least 10 km at a stron- Ͼ Ϫ1 of the barrier, as similarly seen in Fig. 9a, was more ger oncoming flow regime ( 20 m s ). Over NKR, evident at these stronger oncoming flow regimes, con- similar features of precipitation variation were found sistent with the influence of the increasing leeside dry- (Fig. 14c), but with relatively small changes in the lo- ing effect. The value of low-level observed strongest cation of the low-level precipitation maximum (i.e., heavy lines in Fig. 14). reflectivity was also generally increased at stronger on- Figure 15 shows the distance (denoted by D )ofthe coming flow regimes. Vertical extent of heavy precipi- p reflectivity maximum relative to the mountain peak tation (Ͼ40 dBZ) was confined to the lowest 2 km along the vertical cross sections as a function of low- (MSL) but it became deeper (with the 40-dBZ contour level oncoming flow.2 It is clear that, over DT, the po- reaching above 2 km) at stronger oncoming flow re- Ϫ sition of the low-level reflectivity maximum changed gimes (Ͼ25 m s 1). This deepening precipitation fea- prominently with increasing oncoming flow, from a Dp ture would be probably related to the deepening layer Ϫ Ϫ of Ϫ15 km at ϳ13 m s 1 to a D of ϳ0kmat32ms 1. of terrain-induced rising motions over the windward p The variety of precipitation locations along D1 and D2 slope as the low-level oncoming flow intensifies (e.g., exhibited a rather similar trend and were confined to Colle 2004). regions over the windward slopes, upstream of the Similar to DT, there was evidence of precipitation mountain peak. The position of the reflectivity maxi- enhancement over NKR when the oncoming flow in- mum relative to the mountain peak along N was situ- tensified, and the most enhanced precipitation was also ated in a region near and downstream of the mountain confined to the lowest levels (Fig. 13). The vertical ex- crest, entirely different from the curves describing the tent of heavy precipitation was generally deeper at changes in precipitation location over DT. Particularly, stronger oncoming flow regimes, with the 40-dBZ con- a lesser distance of downstream shift (Յϳ8 km) was Ն Ϫ tour extending to a height of 4 km (Figs. 13c–f). In evident as the oncoming flow intensified from 8 m s 1 contrast to the precipitation structures observed over to ϳ30 m sϪ1. This observed behavior, distinct from Ͼ DT, the coverage of low-level heavy precipitation ( 40 that over DT, is schematically illustrated in Fig. 16 and dBZ) over NKR was much wider, particularly not only qualitatively interpreted below. over the windward slope but also on the lee side (Fig. Despite the complexity of microphysical processes 13). This spillover characteristic was evident even in the contributing to the hydrometeor growth over topogra- relatively weak oncoming flow regimes (Figs. 13a,b). phy, the distance of downstream shift of precipitation The location of the low-level strongest reflectivity, as particles (⌬x) due to changes in upstream oncoming the oncoming flow increased, moved slightly down- flow component normal to the barrier (⌬u) would be stream from the mountain crest at X Ӎ 4 km (Fig. 13b) primarily related to the differential advection effect of to the lee at X Ӎ 8 km (Figs. 13d,e). Note that the hydrometeors by ambient wind (e.g., Sinclair et al. degree of the downstream shift of low-level heaviest precipitation due to the changes in upstream oncoming flow was relatively limited, compared to a larger down- 2 The upper-level [i.e., above 1 km (MSL)] oncoming flow ap- stream shift observed over DT (cf. Figs. 11 and 12). pears to have a minor relationship to Dp, as suggested by a promi- Some further aspects regarding the relationship be- nently decreasing correlation coefficient with height. Hence, our tween the upstream oncoming flow and the low-level discussions are focused only on the low-level oncoming flow.

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FIG. 11. Mean vertical structures of reflectivity (dBZ, shading) along D1 in Fig. 9b, obtained from different intervals of low-level oncoming flow component along the section. Shown is the oncoming flow component at (a) 10–15, (b) 15–20, (c) 20–25, (d) 25–30, and (e) 30–35 msϪ1. For clarity, regions of reflectivity greater than 40 dBZ are also contoured with a 1-dBZ interval. Shading and arrow in lower portion of each panel indicate topography and mountain peak along the section, respectively. The windward (i.e., northern) side is on the left of each panel.

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FIG. 12. As in Fig. 11, but along D2 in Fig. 9b.

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1997) and can be simply approximated by the expres- It has been generally recognized that the orographic sion geometry has a strong influence on the airflow, thermodynamic structures, and microphysical processes, h ⌬x ϭ t ϫ ⌬u ϭ ͯ ͯ ϫ ⌬u, ͑2͒ hence further altering precipitation intensity and distri- r ͑w ϩ ␷ ͒ air t bution in the vicinity of mountains (e.g., Jiang and

where tr is the residence time of hydrometeors above Smith 2003; Colle 2004; Colle and Zeng 2004; Roe and the ground, h is the altitude for hydrometeors starting Baker 2006). For the present case, the heights of both ϳ ϳ their descent to the ground, wair is the vertical motions DT ( 1 km MSL) and NKR ( 700 m MSL) are much ␷ of air, and t is the terminal fall speed of hydrometeors. lower than the melting level (0°C level corresponding Because there was no prominent difference in ⌬u (ϳ20 to a height of ϳ5.5 km; cf. Fig. 8a), which implies the Ϫ ms 1) between DT and NKR (Fig. 15), their difference dominance of warm-rain processes on the low-level in the degree of downstream shift should be mainly af- precipitation enhancement over the two barriers. Be- ϩ ␷ fected by h and the fall speed of hydrometeors (wair t). sides, given that the slope steepness for DT and NKR Hydrometeors formed over DT would experience a has no significant difference, a longer “lifting section” longer “lifting section” (Blumen 1990), since DT has for DT would suggest the occurrence of more conden- about two times the width of NKR, and potentially, sation over its windward slope. It is also difficult to they would have more significant vertical displacement reasonably explain our observations showing relatively Ͼ (i.e., hD hN in Fig. 16). Note that the magnitudes of weak radar echoes (compared to NKR) over this bar- topographically forced vertical velocity calculated over rier. Although the mountain geometry cannot be con- the two barriers are indeed comparable (not shown). clusively ruled out as a potential factor in influencing As shown in Figs. 11–13, the intensities of observed precipitation intensity over DT and NKR, a crucial reflectivities over NKR were generally stronger than physical link between them did not appear. 3 those over DT, implying a larger terminal fall speed of Instead, the typhoon precipitation would probably hydrometeors over NKR. In addition, given a large Fr have a more direct impact in this respect. As shown in regime and a saturated, nearly neutral environment for Figs. 4a and 11–13, orographic precipitation over NKR this case as described in section 3, upstream air should and DT were embedded within the larger-scale cover- easily flow over topography with prevailing ascending age of precipitation associated with Xangsane. In real (descending) motions over the windward (lee) slope. observations like the present case, however, it is almost Therefore, a relatively small (large) fall speed of hy- impossible to separate the typhoon’s precipitation from drometeors over the windward (lee) slope of DT the precipitation/cloud generated by orographically (NKR) can be anticipated (Fig. 16). According to (2), forced lifting because the two kinds of precipitation are these characteristics described above imply a larger expected to be mixed together and interact microphysi- downstream shift of hydrometeors over DT than NKR cally with each other. Nevertheless, an alternative way (i.e., ⌬x Ͼ⌬x in Fig. 16), which are consistent with D N to evaluate the possible influences of typhoon precipi- our radar observations showing more (less) changes in tation on the observed orographic precipitation can be precipitation location over the windward (lee) slope of performed with calculating mean reflectivities averaged DT (NKR; cf. Fig. 15). within the four boxes indicated in Fig. 17. Boxes A and B shown in Fig. 17a encompass the major area of oro- 6. Difference in precipitation intensity between graphically enhanced precipitation over DT and NKR DT and NKR (cf. Fig. 9), respectively, while reflectivity averaged Consistent with greater precipitation accumulation over boxes C and D represents mean precipitation in- and frequency over NKR than DT (cf. Fig. 9), the re- tensity upstream of DT and NKR, respectively. The flectivity structures displayed in Figs. 11–14 indicate reason box C (D) was selected to be located northeast that precipitation intensity over NKR was obviously of DT (NKR) is because it approximately encompasses stronger than that over DT. There are two possible the main path of typhoon rainbands influencing DT factors, both of which would be probably related to the (NKR). If the inherent evolution of typhoon precipita- observed difference in the precipitation intensity be- tion and/or rainbands as it moves from box C (D) to A tween DT and NKR: the orographic geometry and the (B), compared to the modification of precipitation by influence of ambient typhoon precipitation. topography, is assumed to be relatively minor, the mean reflectivity calculated over box C and D can be a reasonable estimate, at least in an ensemble sense, for 3 This observational aspect will be discussed further in section 6. the strength of background precipitation associated

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FIG. 14. Display of the lowest-level (750 m MSL) reflectivity (dBZ, shading) of the WFS radar as a function of oncoming flow component along (a) D1, (b) D2, and (c) N in Fig. 9b. Thick line in each panel denotes the location of strongest radar reflectivity found at the given oncoming flow components. Shad- ing and arrow in the lower portion of each panel indicates topography and mountain peak along the section, respectively.

with typhoon circulations. As a consequence, the dif- and to isolate the signals more related to background ferential reflectivity values among these select boxes typhoon precipitation, the differential reflectivities are may give some clues as to the influence of typhoon computed only when their associated oncoming flow precipitation on the observed intensity of orographic components have the same magnitudes, rather than at precipitation. It is important to note that the intensity the same observing time. of orographic precipitation observed over DT and Figure 17b shows the differential values in the mean NKR evidently increases with increasing low-level on- reflectivity (dBZ) between boxes A and B, which are coming flow (cf. Figs. 11–13). To preclude this effect plotted as a function of height and the intensity of up-

←

FIG. 13. As in Fig. 11, but along N in Fig. 9b, showing the oncoming flow component at (a) 5–10, (b) 10–15, (c) 15–20, (d) 20–25, (e) 25–30, and (f) 30–35 m sϪ1. Note that the blank region near the right end of each panel (from X ϭϳ14 to X ϭϳ20 km) was largely influenced by the interactions between topography over the southern slopes of the northern portion of SMR (cf. Fig. 9b) and landfalling typhoon rainbands coming from the eastern coast of Taiwan. Because radar echoes in this region were much less relevant to the orographic effects associated with NKR discussed in the text, for clear illustration they have been precluded in the analysis.

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tivity factor (Z;mm6 mϪ3) between selected boxes in Fig. 17a are also calculated and shown in Fig. 18. Unlike the radar reflectivity (dBZ), which is in log scale, the magnitude of reflectivity factor is linearly proportional to the radar receiving power and thus its differential values would allow for a more clear and correct depic- tion on the observed difference of intensity in radar signals between the boxes in Fig. 17a. The differential values of reflectivity factor between boxes A and B (Fig. 18a) exhibit a clear pattern similar to those between boxes C and D (Fig. 18b). The correlation coefficient is calculated to be ϳ0.8 below 4 km (MSL) and reduced to ϳ0.6 at higher altitudes. Larger correlation coefficient at lower altitudes may imply a more direct and significant impact of background typhoon precipitation on the intensity of low-level precipitation devel- oping over DT and NKR. Figure 19 further shows the mean values of the differential reflectivity factor averaged below 1 km (MSL) from Figs. 18a,b. The differential reflectivity factor between DT and NKR (dashed FIG. 15. Distance (D , km) between the location of the lowest- p curve) exhibits an obvious trend of variations highly level (750 m MSL) reflectivity maximum and the mountain peak along the vertical cross section (D1, D2, and N in Fig. 9b) as a corresponding to the fluctuations in their associated dif- function of the low-level oncoming flow component. The negative ferential background reflectivity factor (solid curve).

(positive) value of Dp denotes distance upstream (downstream) of These analysis results strongly suggest the influences of the mountain peak. Vertical line marks location of mountain typhoon precipitation to be an important factor in peak. modifying the precipitation intensity over mountains for this case. stream oncoming flow. At low levels, the precipitation Analyses presented above also provide an important intensity over NKR is generally stronger than that over implication. Assuming that the degree of precipitation DT, with positive differential reflectivities of ϳ0–20 enhancement (relative to the background value) over dBZ below 2 km (MSL). Positive differential reflectivi- DT and NKR is comparable, the differential values of ties even with larger values also prevailed aloft, except reflectivity factor between DT and NKR would be ap- for relatively small regions having negative values seen proximately identical to those of their background pre- near ϳ5 km (MSL) when the upstream oncoming flow cipitation. Obviously, this is not the case. As shown in lies between 20 and 24 m sϪ1. These results basically Fig. 19, the dashed curve tends to be suited generally reflect the stronger precipitation intensity over NKR well above the solid curve over different oncoming flow than DT, as indicated by the mean vertical structures of regimes. Similar tendencies can be found at higher al- mountainous precipitation shown in Figs. 11–13. titudes (cf. Fig. 18). Ideally, the difference between the Similar to Fig. 17b, the differential values in the mean two curves shown in Fig. 19 (abbreviated by DIFF) can reflectivity between boxes C and D are shown in Fig. represent the differential amount of precipitation en- 17c. For most of the observations, the intensity of low- hancement between NKR and DT. This relationship level background typhoon precipitation over NKR is can be more easily realized with the help of the follow- stronger than over DT (i.e., the positive differential ing mathematical expression: values). This result is not surprising because the loca- ϭ ͑ Ϫ ͒ Ϫ ͑ Ϫ ͒ tion of NKR is closer to Xangsane’s center and thus is DIFF ZB ZA ZD ZC frequently influenced by the passage of more intense I II typhoon rainbands. As evident in Fig. 6, the low-level ϭ ͑ Ϫ ͒ Ϫ ͑ Ϫ ͒ ͑ ͒ ZB ZD ZA ZC , 3 offshore reflectivities upstream of SMR were generally III IV stronger than those upstream of DT during intense orographic precipitation. Although the values of the dif- where I represents the differential value of reflectivity ferential reflectivity shown in Figs. 17b,c are not exactly factor between boxes A and B (cf. Fig. 18a), II repre- the same, their patterns appear roughly similar. sents the differential value of reflectivity factor be- For comparison, the differences in the mean reflec- tween boxes C and D (cf. Fig. 18b), III represents the

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FIG. 16. Schematic diagram illustrating the downstream shift of hydrometeors over (a) DT and (b) NKR due to changes in upstream oncoming flow. Shading denotes the main region of heavy precipitation with darker shading representing stronger precipitation intensity. Solid (dashed) arrows indicate the trajectory of hydrometeors in the weak (strong) oncoming flow condition. Open arrows denote airflow patterns over mountains. The

hD and hN represent the altitude for hydrometeors starting their descent to the ground over DT and NKR, ⌬ ⌬ respectively. The “ xD” and “ xN” represent the distance of downstream shift of hydrometeors over DT and NKR, respectively. net amount of precipitation enhancement over NKR, However, the lack of microphysical observations in this and IV represents the net amount of precipitation en- case, unfortunately, prevents a close inspection on the hancement over DT. According to (3) and Fig. 19, it is relevant processes described above. In the future, the clear that the degree of precipitation enhancement over detailed kinematic and microphysical measurements NKR was persistently more prominent than DT (i.e., over mountains plus the high-resolution mesoscale or III Ͼ IV; DIFF Ͼ 0), regardless of different oncoming cloud models will be required to investigate this issue, flow magnitudes. This observed characteristic would be and particularly to clarify the relative importance of the probably attributed to a stronger background typhoon mountain geometry and the background typhoon pre- precipitation over NKR that substantially favors the cipitation on the precipitation enhancement over to- collision and coalescence processes over this barrier. pography.

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FIG. 17. (a) The location of four boxes (A–D) used to calculate their differential reflectivity (see details in text). Shading indicates topographic features over northern Taiwan. (b) The differences in mean reflectivity (dBZ; contours) calculated over A and B (B Ϫ A) as a function of height and low-level oncoming flow. (c) The differences in mean reflectivity (dBZ, contours) calculated over C and D (D Ϫ C) as a function of height and low-level oncoming flow. In (b) and (c), regions of positive differential reflectivity are shaded.

7. Conclusions as it moved northward immediately off the eastern coast of Taiwan and brought strong low-level north- With measurements from two ground-based Doppler easterly to north-northeasterly winds impinging on the radars located in northern Taiwan, this study has docu- mountainous northern coast. The heaviest precipitation mented the detailed aspects of the intense orographic occurred primarily over two mountain barriers: Mount precipitation associated with Typhoon Xangsane (2000) Da-Tun and the Nangang-Keelung Range. The DT is

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ized minimum of precipitation was located at low foothills southwest of the barrier. The precipitation patterns observed over NKR are different from those over DT. A precipitation maximum with even larger accumulated reflectivity values was observed near and downwind of the mountain crest (i.e., the spillover characteristic), followed by gradually reduced precipitation on the farther lee. Analyses also indicate that local maxima of precipitation frequency over the two barriers were located near and immediately downstream of the frequency maxima of calculated topographically forced vertical motions. Analyses of vertical cross sections passing through the maximum of accumulated reflectivity over DT and NKR show that the most pronounced precipitation ex- isted in the lowest analysis level (750 m MSL). Changes in precipitation location, structure, and intensity over DT and NKR with evolving upstream oncoming flow were also evident. The region of low-level heavy precipitation tended to shift downstream as the low-level oncoming flow intensified, and the precipitation exhibited a deeper, wider extent and stronger intensity at stronger oncoming flow regimes. However, the changes in the location of major heavy precipitation over DT (NKR) were confined primarily to regions over windward slopes (near and downstream of the mountain crest). The degree of downstream shift of low-level heaviest precipitation with respect to different magnitudes of oncoming winds was relatively limited (ϳ8 km) over NKR, in comparison with a larger downstream shift (ϳ15–17 km) over DT. As elaborated in section 5, this contrasting characteristic of precipitation between DT and NKR can be understood as a consequence of a longer “lifting section” and relatively lower fall speed FIG. 18. As in Figs. 17b,c but showing the differences in mean of hydrometeors over the windward slope of DT. 6 Ϫ3 reflectivity factor (Z;mm m ): (a) over A and B, and (b) over In addition to the significance of the upstream on- C and D. coming flow on modulating the orographic precipitation over DT and NKR, the influence of precipitation an approximately 3D mountain barrier with peak inherently associated with the typhoon circulations was mountain heights of ϳ1000 m and a half-width of found to be an important factor in contributing to the ϳ10 km, and the NKR (i.e., the northernmost moun- observed variations of intensity of orographic precipi- tains of Snow Mountain Range), adjacent to the south- tation. The differences in mean precipitation intensity east of DT, is a relatively lower and narrower mountain calculated over DT and NKR had an obvious trend range (a half-width of ϳ5 km) oriented roughly south- closely related to the differential intensity of their as- west–northeast. Distinct aspects of precipitation, in sociated background typhoon precipitation. Stronger terms of its distribution, intensity, and relation to up- background typhoon precipitation and a shorter downstream oncoming flow over the two mountain barriers, stream shift of precipitation (i.e., a quasi-stationary pre- are observed. cipitation feature) over NKR may explain the fact of The patterns of accumulated reflectivity over DT larger precipitation accumulation observed over this were characterized by two maxima located over the narrower, lower barrier. northern (i.e., windward) slopes and by a sharp de- This study has shown that the complicated interac- crease near the mountain peak and on the lee. A local- tions between typhoon precipitation, orography, and

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Ϫ FIG. 19. Differential reflectivity factors (Z ϫ 103 mm6 m 3) averaged in the lowest 1 km (MSL) as a function of low-level oncoming flow. Dashed (solid) curve denotes results calculated over A and B (C and D) shown in Fig. 17a.

the precipitation exclusively produced by orographic This study is supported by the National Science Council forcings appear to be crucial in determining the distri- of Taiwan under Grants NSC 95-2111-M-034-001 and bution and intensity of the observed orographic pre- NSC 96-2111-M-034-001-MY3. cipitation. Additional detailed kinematic and microphysical observations over mountains, plus the high- REFERENCES resolution mesoscale or cloud models, will be required Blumen, W., Ed., 1990: Atmospheric Processes over Complex Ter- to investigate these processes and to clarify the relative rain. Meteor. Monogr., No. 45, Amer. Meteor. Soc., 323 pp. importance of the mountain geometry and the ambient Brunt, A. T., 1968: Space-time relations of cyclone rainfall in the northeast Australian region. Civil Eng. Trans. Inst. Eng. Aus- typhoon precipitation on the precipitation enhance- tralia, April issue, 40–46. [Available from Inst. of Eng. Aus- ment over topography. tralia, 11 National Circuit, Barton A.C.T. 2600, Australia.] Chang, C.-P., T.-C. Yeh, and J. M. Chen, 1993: Effects of terrain Acknowledgments. Wu-Fen-San radar and Taoyuan on the surface structure of typhoons over Taiwan. Mon. Wea. International Airport radar data used in this study were Rev., 121, 734–752. Chang, P.-L., 2000: Analysis of evolution of landfalling typhoon provided by the Central Weather Bureau and Civil circulations: A case study of Typhoon Herb (1996). Ph.D. Aeronautics Administration of Taiwan, respectively. dissertation, National Taiwan University, 158 pp. We thank Dr. Nolan Atkins for his review of the manu- Colle, B. A., 2004: Sensitivity of orographic precipitation to script and appreciate Dr. Ming-Jen Yang and anony- changing ambient conditions and terrain geometries: An ide- mous reviewers for providing detailed, helpful com- alized modeling perspective. J. Atmos. Sci., 61, 588–606. ——, and Y. Zeng, 2004: Bulk microphysical sensitivities within ments that improved the manuscript. We also thank the MM5 for orographic precipitation. Part II: Impact of bar- Mrs. Candace Gudmundson for help with the editing rier width and freezing level. Mon. Wea. Rev., 132, 2802– and Shao-Yu Chang for drawing the schematic figure. 2815.

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