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Cheng-Ku Yu and Lin-Wen Cheng

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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 (∼8 km) over NKR, as compared with a larger downstream shift of ∼15–17 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 contributor 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.

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Lin-Wen Cheng and Cheng-Ku Yu

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This study uses a dense rain gauge network, radar observations, and an upslope model to document the detailed aspects of precipitation over Da-Tun Mountain (DT) of northern Taiwan under prevailing northeasterly monsoonal flow. DT is one of the most concentrated areas of heavy rainfall in Taiwan, with a size of ~15 km and a terrain peak of ~1 km (MSL), and it constitutes a concave ridge with two windward ridge arms that encompass a funnel-shaped valley. Twenty-one rainfall events identified over DT from January 2011 to February 2015 are chosen for analysis. More than half of the studied cases exhibit two local maxima of rainfall over the ridge arms, and asymmetric characteristics between these two maxima are evident. The other frequent rainfall pattern, observed as upstream winds are more from the east-northeast, is characterized by a local maximum of precipitation inside the valley. Analyses from the upslope model confirm that the occurrence of the windward maxima of rainfall is primarily caused by upslope lifting, and their asymmetric characteristics are closely related to the difference in the azimuthal variations of slope steepness between the two ridge arms. The observed characteristics of rainfall intensities inside the valley, however, cannot be well described by the upslope model. It is found that the lateral flow confluence induced by the deflected flows over the ridge arms may play an essential role in intensifying upslope-forced precipitation within the valley. This effect emerges as upstream winds are roughly parallel to the central axis of funneling regions located between the ridge arms.

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Cheng-Ku Yu and Lin-Wen Cheng

Abstract

Using a combination of Doppler radar observations and rain gauge data, this study documents detailed aspects of the orographic precipitation associated with Typhoon Morakot (2009). Rainfall distribution over underlying topographical features and possible physical mechanisms responsible for the observed orographic enhancement are explored. The study region constitutes an approximately two-dimensional, south–north-oriented orographic barrier with higher, wider (lower, narrower) terrain features in its northern (southern) portions (i.e., the northern and southern barriers). Upstream conditions were characterized by abundant typhoon background precipitation embedded within strong, nearly saturated westerly to west-southwesterly oncoming flow. The observations show that a wide area of topographically enhanced precipitation and the rainfall maxima were confined to the windward slopes of the northern barrier, whereas the strongest rainfall tended to occur near and/or slightly downstream of the mountain crest of the southern barrier. Quantitative analysis indicates that upslope lifting may explain the observed precipitation enhancement over the northern barrier; however, this mechanism was found to be less relevant to precipitation enhancement for the southern barrier. The characteristics of the enhanced precipitation observed over the southern barrier are, instead, consistent with the theoretical prediction of the seeder–feeder process. In this context, the degree of orographic enhancement was shown to be proportional to the intensity of the typhoon background precipitation multiplied by the oncoming wind speed. The results suggest that for the tropical cyclone environment, understanding and predicting rainfall over narrow, low mountain ranges is particularly challenging because it involves complex dynamical and microphysical processes.

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Cheng-Ku Yu and Lin-Wen Cheng

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This study used ground-based dual-Doppler observations to explore an understanding of kinematic characteristics of the southwesterly flow associated with the southwest (SW) and ordinary (OR) typhoons immediately off the southwestern coast of Taiwan. The SW (OR) typhoon stated herein is referred to as a typhoon with (without) an obvious combination of its outer circulations and the summer southwesterly monsoon active over the South China Sea. Six typhoon events [Mindulle (2004), Kalmaegi (2008), Morakot (2009), Talim (2005), Jangmi (2008), and Fungwong (2008)] were chosen for analysis; the first (latter) three listed belong to the family of the SW (OR) typhoons. The vertical profiles generated from hourly synthesized winds for these typhoons indicate that intense orographic rainfall tended to occur during the prevalence of the west-southwesterly (WSW) flow that was more perpendicular to the south–north orientation of the topography in southern Taiwan. A unique, consistent feature of the WSW flow associated with the SW typhoon was its persistently increasing intensity with decreasing height in the low to midtroposphere, in contrast to a minor vertical variation in the intensity of the WSW flow for the OR typhoon. A relatively large (small) ratio of the radial and tangential velocities was evident for the SW (OR) typhoon, and the mean inflow angle of the SW typhoon was significantly larger than the typical near-surface inflow angle of previously documented hurricanes over the open ocean. In addition to the typhoon background precipitation, the observed characteristics of the SW- and OR-typhoon-induced WSW flow were shown to be closely related to the degree of orographic enhancement of precipitation.

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Cheng-Ku Yu and Che-Yu Lin

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In this study, observations from the C-band Doppler radar on Green Island, which is located off the southeastern coast of Taiwan, during 1998–2004 were analyzed to investigate the statistical characteristics of the convective lines occurring off the mountainous coast of southeastern Taiwan, with emphasis on their formative location and timing. A total of 211 cases of the lines were identified during the study period. It is shown that the lines were clearly a year-round phenomenon and five cases occurred per month on average. Statistical analyses for all identified cases reveal that the formative area of the lines was extended substantially from nearshore regions to at least ∼100 km offshore, with the region of the most frequent formation primarily confined to an elongated zone located ∼30 km off the coast. Along-coast variations of line formation were also evident and were shown to be closely related to the nearshore terrain features. The lines tended to form more frequently during the nighttime hours than the daytime hours, with a formative peak between 2000 and 2200 LST. Minimum formation was found near noon between 1200 and 1400 LST; however, considerable cases still could be found during the late morning and late afternoon hours. More than 70% of the lines (∼150 cases) had a duration of less than 4 h, and the mean duration for all lines was calculated to be ∼3.5 h. In addition, this study also documented statistical differences in the formative and flow characteristics between the nearshore and offshore lines, which are distinguished by offshore distances of formative location that are less or greater than 40 km, respectively. Of particular note, the analyses presented strongly suggest that the physical mechanisms contributing to the initiation of the nearshore and offshore lines are fundamentally different.

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Che-Yu Lin and Cheng-Ku Yu

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This study used Doppler radar data, surface observations, and National Centers for Environmental Prediction reanalysis data to explore the statistical characteristics of Taiwan rainbands (TRs) that formed in the outer region of tropical cyclones (TCs). A comprehensive examination of the available radar measurements from 2002–2017 identified a total of 103 TRs from 44 TC events and showed that approximately 47% of all TCs influencing Taiwan could develop TRs. The spatial distribution of TR formation exhibited a substantial offshore extent, with the highest frequency observed ~25–100 km offshore. The TRs tended to be initiated when the northwestward-moving typhoons passed over the oceanic area northeast of Luzon Island (122°–127° E and 16°–20° N), the Philippines. This track characteristic brought stronger easterly onshore flow to the eastern coast of Taiwan and favored the development of a pronounced coastal pressure ridge. In particular, the offshore convergence caused by upstream deceleration of the onshore flow due to orographic blocking was found to be a primary contributor to the initiation of the TRs. The strength of the observed coastal pressure ridge and its high correlation with the intensity of environmental onshore flow associated with outer circulations of TCs were consistent with the theoretical prediction of pressure distributions generated as incident flow interacted dynamically with the Taiwan topography. Results from the study suggest that the typhoon location relative to the Taiwan landmass is a critical factor determining TR initiation.

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Cheng-Ku Yu and Che-Yu Lin

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Taiwan rainbands (TRs), defined here as convective lines, which form off the mountainous eastern coast of Taiwan under weakly synoptically forced weather conditions, are a well-known mesoscale phenomenon, but their formative processes remain the subject of debate. This study uses surface and radar observations within the coastal zone of eastern Taiwan and NCEP reanalysis data to document a long-lived TR with a lifetime of ~36 h during 1–3 March 2003 to advance the current general understanding of mechanisms responsible for the TR’s formation and maintenance. Detailed analyses indicate that the rainband was initiated by convergence that was produced as low-level environmental northeasterly/easterly onshore flow encountered topographically blocked northerlies that developed nearshore. The northerly blocked flow was observed to weaken and subsequently dissipate because of changing synoptic pressure patterns that caused prevailing southeasterlies/southerlies at low levels. However, colder nearshore air that resulted from the combined effects of orographic blocking, the evaporation of the TR’s precipitation, and radiative cooling over coastal land continued to persist and acted to provide a continuing source of lifting for the subsequent maintenance of moist convection. Temporal variations in the precipitation intensity of the studied TR were also shown to be consistent with the theoretical prediction of the interaction between the cold pool and ambient vertical shear. This study suggests that multiple precipitation mechanisms, which involve interactions of diurnally, topographically, and convectively generated circulations along the mountainous coast, may operate and contribute to the longevity of a TR event under suitable circumstances, such as the rapidly evolving synoptic flow observed in the present case.

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Cheng-Ku Yu and Ying Chen

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With radar measurements and temporally high-resolution surface observations, this study investigates surface fluctuations associated with tropical cyclone rainbands (TCRs) observed in the vicinity of Taiwan during 2000–08. A total of 263 TCRs identified from 37 typhoon events during the study period were analyzed to show the mean and common nature of perturbations of various meteorological variables associated with the passage of TCRs.

The main patterns of surface thermodynamic fluctuations, as revealed from the composite analysis of all identified TCRs, include a persistent decrease in temperature, dewpoint temperature, and equivalent potential temperature θe from the outer to inner edge of the rainband. A wavelike variation of pressure perturbations associated with the rainband was evident, with a minimum coincident with the outer edge and a maximum located inside the inner edge. The kinematics of the rainband was characterized by an obvious decrease in cross-band wind component, relatively minor variations in along-band wind component, and the wind veering. Quantitative analyses indicate that the majority of the TCRs (~80%–90%) exhibited variations in surface temperature, pressure, wind speed, and wind direction less than 2°C, 1.5 mb, 5 m s−1, and 20°, respectively. However, a clear trend of the magnitude of TCR thermodynamic fluctuations increasing with the radial distance from the tropical cyclone center was observed.

The TCRs identified in this study were also classified into the outer and inner rainbands, which are distinguished by a radial distance of 3 times the radius of maximum wind. The composite and magnitude analyses of their surface fluctuations indicate that the outer rainbands had a higher potential than the inner rainbands to reduce the near-surface θe values. This observed characteristic is likely related to more pronounced evaporative cooling taking place in drier subcloud regions and the downward transport of low-θe air aloft by more vigorous convective downdrafts for the outer rainband. Fundamentally different features of surface pressure fluctuations and mean frictional vertical velocity and relative vorticity between the outer and inner rainbands were also documented. These results reflect a possibly different origin. Nevertheless, there was no dramatic difference in the pattern of kinematic fluctuations between the outer and inner rainbands, and their mean magnitudes were also found to be statistically identical, which suggests that there is not an entirely clear distinction of surface characteristics for these two types of rainbands.

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Cheng-Ku Yu and Ben Jong-Dao Jou

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This study documents offshore convective lines along the southeastern coast of Taiwan, a frequent but poorly understood mesoscale phenomenon that influences coastal weather during the Taiwan mei-yu season. Doppler radar and surface observations were gathered from a specially chosen period (11–15 May 1998) when the offshore convective lines were active off the southeastern coast of Taiwan. These observations were used to show the basic character, structure, and possible formative processes of offshore convective lines. The synoptic environment accompanying these events was found to be relatively undisturbed and featured uniformly prevailing southerly/south-southeasterly winds in the boundary layer with southwesterlies/westerlies aloft. Examination of radar data during the study period indicates that the lines generally occurred ∼10–30 km offshore and were characterized by an elongated narrow zone (∼5–10 km wide) of heavy precipitation. The lines were oriented roughly parallel to the coastline and generally did not move significantly. The intensity of the radar reflectivity associated with the lines exhibited a marked diurnal variation and was closely related to the coastal offshore flow developing at night.

Detailed analyses of an event on 14–15 May 1998 further show the important physical link between the offshore flow and the development of the line. The offshore line was found to be located near and immediately ahead of the seaward extent of the offshore flow. Particularly, a very narrow zone (∼2 km) of low-level heavy precipitation (40–45 dBZ) coincided with regions of strong updrafts and convergence, where the prevailing southerly onshore flow encountered the cool offshore flow nearshore. This offshore flow–induced convergence, given a stable thermodynamic condition in the lowest ∼1 km in the inflow region, was a crucial low-level forcing that provided lifting to trigger moist deep convection in this case. The line’s precipitation tilt eastward was confined primarily to the warmer inflow side rather than feeding the offshore flow to the west of the line. No consistent upshear tilt of updrafts throughout the storm layer was observed, which is consistent with the presence of a strong westerly shear in the line’s environment. Both of these observations explain a relatively strong (weak) modification of low-level onshore (offshore) flow by precipitation. Additionally, a combination of surface and Doppler radar observations indicates that the leading edge of the offshore flow moved seaward very slowly at 0.7 m s−1 and possessed a frontal character with notable discontinuities in near-surface wind and temperature (instead of pressure and dewpoint temperature).

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Cheng-Ku Yu and Ying-Hsun Hsieh

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Convective lines frequently occurring off the mountainous coast of southeastern Taiwan under weakly synoptically forced weather conditions are one of the most well-known mesoscale phenomena in Taiwan. These lines usually develop close to the coast but frequently can also be observed well offshore. While nearshore lines are better understood, the formative processes of the offshore lines remain unclear. The main objective of this study is to use various available observations (e.g., Doppler radar, surface and sounding observations, QuikSCAT, and NCEP data) collected over southeastern Taiwan to investigate a specially chosen case on 3 January 2004, in an attempt to identify possible mechanisms leading to the formation of the offshore lines. The studied line was formed ∼50–60 km away from the southeastern coast of Taiwan and oriented south-southwest to north-northeast and approximately parallel to the coast. Detailed analyses of the event indicated that the formation of the line did not appear directly relevant to coastal land/sea breezes or offshore flow. Instead, orographic forcing was observed to play a crucial role in the initiation of the line. Particularly, an orographically forced northerly flow due to upstream blocking was evident to the west of the line. The convergence generated as the prevailing northeasterly onshore flow encountered the nearshore blocked flow was found to be an important low-level forcing conducive to the initiation of moist convection associated with the line. In addition, the persistence of the coastally blocked flow and the degree of ambient convective available potential energy (CAPE) were also shown to be closely related to the maintenance and overall intensity of the observed line’s convection. The orographically induced convective forcing identified in this study is distinctly different from the conceptual model of line formation proposed by previous studies of the nearshore lines. It is thus strongly suggested that multiple mechanisms may exist upstream of coastal mountains in southeastern Taiwan, which act to initiate the lines with diverse formative locations relative to the coastline.

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