Radar Observations of Intense Orographic Precipitation Associated with Typhoon Xangsane (2000)
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FEBRUARY 2008 YUANDC HENG 497 Radar Observations of Intense Orographic Precipitation Associated with Typhoon Xangsane (2000) 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 moun- tain 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 down- stream 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 © 2008 American Meteorological Society Unauthenticated | Downloaded 10/01/21 09:42 AM UTC 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 ra- in 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