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This study explores the mechanisms responsible for valley precipitation enhancement over Da-Tun Mountain under the prevailing northeasterly monsoonal flow. Da-Tun Mountain, located adjacent to the northern coast of Taiwan, is a small-scale (15 km), concave-like terrain feature with two ridge arms and a funnel-shaped valley. A typical valley precipitation enhancement event that occurred on 13 December 2018 was chosen for detailed analyses. Upstream conditions were characterized by the absence of convective available potential energy with a large-Froude-number (>1) flow regime. Observational and modeling results indicate a consistent, important signature of flow splitting due to partial blocking as the low-level northeasterly flow encountered the ridge arms. Fine-scale structures of airflow and precipitation evident from the simulations further reveal that the deflected flows over the two ridge arms interacted with each other to produce lateral convergence and enhanced precipitation inside the valley. The smaller-scale splitting flows tended to occur over the ridge arms as upstream moist Froude number decreased from relatively higher (5–11) to lower (3–5) values due to the temporal change in moist static stability. Quantitative diagnoses of vertical velocities performed over the region of primary precipitation support that upward motions associated with lateral convergence greatly overwhelmed the upslope-forced lifting over the valley region during the valley precipitation enhancement periods. However, vertical motions over the ridge arms with steeper slopes were dominantly contributed by the upslope forcing, but their intensities were also modulated by the flow-splitting-induced divergence. Significance Statement Many mountain ranges around the world exhibit a concave-like terrain feature with various spatial scales and orientations. Orographic modulations of rainfall by concave mountains are of great importance to local weather, as torrential rainfall has been frequently reported over these ridges in different geographical locations. This study aims to advance our knowledge of precipitation mechanisms over Da-Tun Mountain, a small-scale concave topography located in northern Taiwan. Observational and modeling analyses reveal evidence of splitting flows over different ridge arms of this mountain barrier. These smaller-scale splitting flows and their interactions play important roles in modulating the intensity of upslope-forced lifting and contributing to valley precipitation enhancement. These identified processes are anticipated to be commonly active over highly three-dimensional, concave-like topography.

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 to 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 (16°–20°N, 122°–127°E), 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.

On 23 July 2014, a commercial aircraft (GE222) crashed near the Ma-Gong Airport on Penghu Island off the southwestern coast of Taiwan as it struggled to land in the stormy weather that was caused by the outer tropical cyclone rainbands (OTCRs) of Typhoon Matmo. This study aims to document the detailed aspects of airflow and precipitation of OTCRs through high-resolution radar and surface observations and to identify how these observed structures contribute to aviation weather hazards. Analyses indicate that the weather at the airport was significantly influenced by the passage of three OTCRs (R1, R2, and R3), and these rainbands share common characteristics of squall-line-like airflow and precipitation structures. As GE222 descended to approach the runway and flew immediately behind and roughly parallel to the leading edge of R3, the aircraft encountered the heaviest precipitation of the rainband and the prominent crosswind that was a manifestation of the rear-to-front flow generated locally by the rainband. The heavy rain–induced poor visibility and the occurrence of strong crosswinds were primary weather hazards affecting this flight event. Momentum budget analyses suggest that the frontward pressure gradient force provided by the near-surface, convectively generated mesohigh played a major role in driving the low-level rear-to-front flow inside the band. The results from the present study imply that closely monitoring convective activities in the outer regions of tropical cyclones and their potential transformation into squall-line-like storms is crucial to complement the routine aviation alert of severe weather under the influence of tropical cyclones.

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.

This study used radar and surface observations to track a long-lasting outer tropical cyclone rainband (TCR) of Typhoon Jangmi (2008) over a considerable period of time (~10 h) from its formative to mature stage. Detailed analyses of these unique observations indicate that the TCR was initiated on the eastern side of the typhoon at a radial distance of ~190 km as it detached from the upwind segment of a stratiform rainband located close to the inner-core boundary. The outer rainband, as it propagated cyclonically outward, underwent a prominent convective transformation from generally stratiform precipitation during the earlier period to highly organized, convective precipitation during its mature stage. The transformation was accompanied by a clear trend of surface kinematics and thermodynamics toward squall-line-like features. The observed intensification of the rainband was not simply related to the spatial variation of the ambient CAPE or potential instability; instead, the dynamical interaction between the prerainband vertical shear and cold pools, with progression toward increasingly optimal conditions over time, provides a reasonable explanation for the temporal alternation of the precipitation intensity. The increasing intensity of cold pools was suggested to play an essential role in the convective transformation for the rainband. The propagation characteristics of the studied TCR were distinctly different from those of wave disturbances frequently documented within the cores of tropical cyclones; however, they were consistent with the theoretically predicted propagation of convectively generated cold pools. The convective transformation, as documented in the present case, is anticipated to be one of the fundamental processes determining the evolving and structural nature of outer TCRs.

Four types (2D-video disdrometer: 2DVD; precipitation occurrence sensor system: POSS; micro-rain radar: MRR; and Joss–Waldvogel disdrometer: JWD) of sixteen instruments were collocated within a square area of 400 m2 from 16 April to 8 May 2008 for intercomparison of drop size distribution (DSD) of rain. This unique dataset was used to study the inherent measurement uncertainty due to the diversity of the measuring principles and sampling sizes of the four types of instruments. The DSD intercomparison shows generally good agreement among them, except that the POSS and MRR had higher concentrations of small raindrops (5.2 mm). The measurement uncertainty ( σ ) was obtained quantitatively after considering the zero or non-zero measurement error covariance between two instruments of the same type. The results indicate the measurement uncertainties were found to be neither independent nor identical among the same type of instruments. The MRR is relatively accurate (lower σ ) due to large sampling volumes and accurate measurement of the Doppler power spectrum. The JWD is the least accurate due to the small sampling volumes. The σ decreases rapidly with increasing time-averaging window. The 2DVD shows the best accuracy of R in longer averaging time, but this is not true for Z due to the small sampling volume. The MRR outperformed other instruments for Z for entire averaging time due to its measuring principle.

This study uses high-resolution radar and surface observations to investigate the finescale structural evolution of airflow and precipitation over complex terrain in the Pyeongchang area, South Korea. The Taebaek Mountain range (TMR) runs parallel to the northeastern coast of South Korea, with a perpendicular ridge known as the Pyeongchang branch (PCB). The objective of this study was to identify the mechanisms of wintertime precipitation over these topographic features during the passage of a low pressure system (LPS) through the southern Korean Peninsula. The analysis indicates that intense precipitation occurred over the southwestern and northeastern sides of the TMR during stage I but only over the northeastern side during stage II. The prevailing southwesterly winds were dominated by warm advection associated with the LPS over the PCB during stage I. These prevailing southwesterly winds locally enhanced precipitation on the southwestern end of the PCB; multiple influences of mountain waves, airflow convergence, and drifted particles are possible factors for causing precipitation on the northeastern side of the TMR. During stage II, the prevailing winds changed from easterlies to northeasterlies offshore from Gangneung. The easterly winds decelerated and were deflected locally along the mountainous coast, and this blocked zone interacted with the oncoming flow to trigger a precipitation band. Consequently, the northeasterly winds helped push the precipitation band toward the coast, causing heavy precipitation in Gangneung. The observational evidence presented shows that the interaction of temporally changing winds accompanying the movement of an LPS over topography is a critical factor for determining the distribution and intensity of precipitation.

An orographic/nonorographic rainfall classification scheme has been introduced for the operational algorithm of the Global Satellite Mapping of Precipitation (GSMaP) for passive microwave radiometers. However, problems of overestimations and false alarms of heavy orographic rainfall remain unresolved. This is because the current scheme selected lower constant thresholds of orographic rainfall conditions for global application and used values of orographically forced upward motion w derived from near-surface atmospheric data. This study improves the conceptual model of the warm-rain process for considering the strength of the upstream flow of the low-level troposphere. Under a weak upstream current, rain reaches the foothills of the windward mountain slope because of sufficient time for condensation and precipitation enhancement by the topography. Conversely, under a strong upstream current, precipitation enhancement occurs nearer to the mountain peak. This is because the upstream current flows so quickly that there is insufficient time for enhancement of precipitation over the foothills of the windward mountain slope. After implementing a variable threshold for w that depends on the mean horizontal low-level wind, the area of orographic enhancement of rain was detected reasonably well in cases of both strong and weak winds. To improve the accuracy of estimates of orographic rainfall, an adjustment to the rain estimation was introduced using a lower-frequency channel. The biases of the rainfall estimate for the adjusted scheme from the Tropical Rainfall Measuring Mission Precipitation Radar were improved for the cases considered here as well as for the Asian region of heavy orographic rainfall over land.

This study utilized ground-based Doppler radar observations to document the detailed structures of an outer tropical cyclone rainband (TCR) of Typhoon Longwang (2005) as the rainband passed over the topography of northern Taiwan on 1 October 2005. The TCR was oriented northeast-southwest and was roughly parallel to the mountain barrier with a higher (lower) terrain height in its southern (northern) landfalling segment. During the pre-landfall stage, the TCR was characterized by convective precipitation on its inner side, where the front-to-rear flow met with the rear-to-front flow to lift air upward with overturning-updraft-like signatures. Significant (relatively minor) changes in the TCR’s structures were evident in the southern (northern) landfalling segment during the landfalling stage. In particular, the southern landfalling segment exhibited a much stronger front-to-rear flow that passed through the region of the rainband, resulting in the absence of low-level organized convergence and a relatively disorganized nature of precipitation. This structural change was shown to be closely related to a significant decrease in the cross-barrier flow due to the orographic blocking that occurred as easterly ambient winds encountered local high mountains in the vicinity of the rainband segment. As the TCR propagated over the western side of the major topography, the low-level rear-to-front flow reappeared due to the collapse of low-level orographic blocking, and the convergence between the front-to-rear flow and the rear-to-front flow tended to rebuild a well-organized convective circulation along the rainband. Results from this case study provide insight into processes that contribute to rapid structural changes in a landfalling TCR over mountainous regions.

This study explored the impact of coastal radar observability on the forecast of the track and rainfall of Typhoon Morakot (2009) using a WRF-based ensemble Kalman filter (EnKF) data assimilation (DA) system. The results showed that the performance of radar EnKF DA was quite sensitive to the number of radars being assimilated and the DA timing relative to the landfall of the tropical cyclone (TC). It was found that assimilating radial velocity (Vr) data from all the four operational radars during the 6 h immediately before TC landfall was quite important for the track and rainfall forecasts after the TC made landfall. The TC track forecast error could be decreased by about 43% and the 24-h rainfall forecast skill could be almost tripled. Assimilating Vr data from a single radar outperformed the experiment without DA, though with less improvement compared to the multiple-radar DA experiment. Different forecast performances were obtained by assimilating different radars, which was closely related to the first-time wind analysis increment, the location of moisture transport, the quasi-stationary rainband, and the local convergence line. However, only assimilating Vr data when the TC was farther away from making landfall might worsen TC track and rainfall forecasts. Besides, this work also demonstrated that Vr data from multiple radars, instead of a single radar, should be used for verification to obtain a more reliable assessment of the EnKF performance.

Tropical cyclone rainbands (TCRs) are not only one of the most striking and persistent features of tropical cyclones (TCs) but also one of the major causes for extreme floods as TCs approach or encounter the land area. TCRs have been traditionally considered as manifestations of atmospheric waves initiated near the eyewall or close to the TC center. However, the limited evidence from recent studies showed the possibility for TCRs to develop squall-line-like characteristics in the outer region of TCs. In this study, the degree of the prevalence for this similarity is explored by radar and surface observations from a large set of 50 outer TCRs associated with 22 TCs as they approached Taiwan. The results indicate that around 58% of outer TCRs are similar to squall lines. These outer TCRs are generally characterized by convective precipitation, an obvious convergence zone between the band-relative rear-to-front flow and front-to-rear flow at low levels, either frontward or rearward tilting updrafts, and a surface cold pool signature. The frequent similarity between the outer TCRs and squall lines documented provides important insights into the formation of organized, heavy precipitation associated with TCs.

This study utilizes data compiled over 21 years (1993–2013) from the Central Weather Bureau of Taiwan to investigate the statistical characteristics of typhoon-induced rainfall for 53 typhoons that have impacted Taiwan. In this work the data are grouped into two datasets: one includes 21 selected conventional weather stations (referred to as Con-ST), and the other contains all the available rain gauges (250–500 gauges, mostly automatic ones; referred to as All-ST). The primary aim of this study is to understand the potential impacts of the different gauge distributions between All-ST and Con-ST on the statistical characteristics of typhoon-induced rainfall. The analyses indicate that although the average rainfall amount calculated with Con-ST is statistically similar to that with All-ST, the former cannot identify the precipitation extremes and rainfall distribution appropriately, especially in mountainous areas. Because very few conventional stations are located over the mountainous regions, the cumulative frequency obtained solely from Con-ST is not representative. As compared to the results from All-ST, the extreme rainfall assessed from Con-ST is, on average, underestimated by 23%–44% for typhoons approaching different portions of Taiwan. The uneven distribution of Con-ST, with only three stations located in the mountains higher than 1000 m, is likely to cause significant biases in the interpretation of rainfall patterns. This study illustrates the importance of the increase in the number of available stations in assessing the long-term rainfall characteristic of typhoon-associated heavy rainfall in Taiwan.

Acquisition of accurate raindrop fall speed measurements outdoors in natural rain by means of moderate-cost and easy-to-use devices represents a long-standing and challenging issue in the meteorological community. Feasibility experiments were conducted to evaluate the indoor accuracy of fall speed measurements made with a high-speed camera and to evaluate its capability for outdoor applications. An indoor experiment operating in calm conditions showed that the high-speed imaging technique can provide fall speed measurements with a mean error of 4.1–9.7 % compared to Gunn and Kinzer's empirical fall-speed–size relationship for typical sizes of rain and drizzle drops. Results obtained using the same apparatus outside in summer afternoon showers indicated larger positive and negative velocity deviations compared to the indoor measurements. These observed deviations suggest that ambient flow and turbulence play a role in modifying drop fall speeds which can be quantified with future outdoor high-speed camera measurements. Because the fall speed measurements, as presented in this article, are analyzed on the basis of tracking individual, specific raindrops, sampling uncertainties commonly found in the widely adopted optical disdrometers can be significantly mitigated.

In this article, five typhoon cases observed by quasi-concurrent satellite-based synthetic aperture radar SAR and Moderate Resolution Imaging Spectroradiometer MODIS were studied with a feature-tracking technique. The rainband features around typhoon eyes were first delineated using wavelet analysis, and then the wind speeds were estimated by feature tracking using quasi-concurrent multi-sensor images. It was found that the feature tracking-estimated wind speeds are reasonable compared with the maximum wind speed reported by the Joint Typhoon Warning Center JTWC, which accounts for the radial dependence of wind speed using the Rankine combined vortex approximation. In a specific case, with the aid of Doppler radar observations near the northern coast of Taiwan, wind speed estimation based on the multi-sensor also shows consistent results. This study demonstrates that the local wind distribution of cyclonic winds around typhoon eyes at different radial distances from the typhoon centres may be derived from rainband feature tracking using quasi-concurrent multi-sensor images. This technique may offer useful wind information for typhoon simulations and forecasting.

Dilution, chemical precipitation, biological uptake and inflow inputs are important processes regulating stream water chemistry but quantitative evaluations of their relative contributions are rare. We examined stream water chemistry and discharge in two streams, one heavily affected by volcanic activities, one much less so, and their confluence to evaluate the importance of these processes, with special emphasis on dilution and precipitation, on reducing solute concentrations in northern Taiwan. We found that differences in the relative importance of the four processes led to three scenarios of upstream-downstream patterns of solute concentrations and fluxes. In Scenario I, solute concentration was greater but flux was smaller in the upstream than the downstream, likely due to dilution through the input of lateral inflows with low solute concentrations. In Scenario II, both solute concentration and flux were greater in the downstream, likely because the inflow water contained high solute concentrations. In Scenario III, both solute concentration and flux were greater in the upstream, likely because chemical precipitation dominated the upstream-downstream pattern. Dilution accounts for 63.7 ± 1.1% of the reduction of concentrations of all solutes in the confluence due to the input of relatively “clean” water from the less contaminated stream. Net precipitation effect was solute-specific and was positive only for Fe and PO43−. It reduced concentration of Fe by 79 ± 0.07% and PO43− by 49 ± 0.25%, likely due to decreases of solubility with increases in pH. Frequent mild rainfall is more effective than more extreme rainfall on lowering solute concentrations because the dilution effects of even large rainfall are short lived. The projected increases in drought events will reduce dilution effects when the need for diverting water from “clean” streams for anthropogenic uses is greater, making water resource management more challenging.

Tropical cyclone rainbands (TCRs) are not only one of the most striking and persistent features of tropical cyclones (TCs) but also one of the major causes for extreme floods as TCs approach or encounter the land area. TCRs have been traditionally considered as manifestations of atmospheric waves initiated near the eyewall or close to the TC center. However, the limited evidence from recent studies showed the possibility for TCRs to develop squall-line-like characteristics in the outer region of TCs. In this study, the degree of the prevalence for this similarity is explored by radar and surface observations from a large set of 50 outer TCRs associated with 22 TCs as they approached Taiwan. The results indicate that around 58% of outer TCRs are similar to squall lines. These outer TCRs are generally characterized by convective precipitation, an obvious convergence zone between the band-relative rear-to-front flow and front-to-rear flow at low levels, either frontward or rearward tilting updrafts, and a surface cold pool signature. The frequent similarity between the outer TCRs and squall lines documented provides important insights into the formation of organized, heavy precipitation associated with TCs.

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.

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.

This study uses ground-based dual-Doppler radar and surface observations to document the structural and surface features of the arc-shaped radar echoes (ASREs) evident along an outer rainband of Typhoon Longwang as it approached northern Taiwan on 1 October 2005. The particular aim of this study is to explore the possible distinction between the present case, previously documented tropical cyclone rainbands (TCRs), and squall lines. The dual-Doppler-derived fields show that the leading precipitation of the studied ASREs exhibited a convective nature with a sharp horizontal gradient of reflectivity and a significant vertical extent. The regions behind the leading convection were characterized by band-relative rear-to-front flow at low levels and were associated with a broader area of stratiform precipitation. The deep layer of front-to-rear flow extending from the surface to the upper troposphere was generally present ahead of the ASREs. This flow appears to be lifted upward at and immediately ahead of the leading edge of the low-level rear-to-front flow to form rearward-tilting updrafts. These airflow patterns are similar to those of the convective region of squall lines but differ fundamentally from those of previously documented TCRs that were located closer to the inner core of cyclones. The detailed analyses of surface fluctuations during the passage of one of the studied ASREs further show an abrupt pressure rise (2 mb), a temperature drop (4°C), and a pronounced deceleration of inflow air coincident with the leading heavy precipitation. The evaluation presented suggests that the convectively generated cold pool may be important in influencing the structures and propagation of the studied ASREs.

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.

This study uses temporally high-resolution surface observations, Doppler radar, and micro rain radar to document the finescale features of the two landfalling rainbands associated with Typhoon Longwang (2005) as they passed over northern Taiwan. The present case allows a unique opportunity to investigate well-defined, convectively active tropical cyclone rainbands over land. In particular, the surface pressure fluctuations observed during the passage of the two rainbands and their possible causes are explored. The rainbands were predominantly convective in nature, with embedded stratiform precipitation outside their inner/outer edge. Analyses of surface observations show similar surface pressure fluctuations during the rainband’s passage. Low (high) pressure with relatively strong (weak) cross-band flow and warmer (colder) temperature was located inside the outer (inner) edge. Maximum (minimum) pressure perturbations were observed to be ∼1.5 (∼−1) mb, with smaller magnitudes (

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.

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.

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.

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).

This study uses airborne Doppler radar and flight-level measurements from the Coastal Observations and Simulations with Topography experiment to examine the detailed mesoscale structure of an oceanic cold front upstream of Vancouver Island on 13 December 1993. These aircraft observations show that there were dramatic differences in frontal structure and movement between nearshore and offshore regions, presumably due to the effects of blocking by the terrain. The aircraft observations are considered in two parts, since the behavior of the front evolved over the flight period. During the early and middle portion of the flight, the low-level flow east of the front was out of the south-southeast in the nearshore region, rather than southerly as found farther offshore. The nearshore segment of the front was oriented south–north and appeared to be stationary. The zone of orographic influence was found to extend to a region ∼20 km offshore and confined primarily to the lowest 1.5 km (MSL). In distinct c...

Airborne Doppler radar and flight-level measurements were used to document the three-dimensional thermodynamic structure of a convective line that developed off the southeastern coast of Taiwan on 16 June 1987 during the Taiwan Area Mesoscale Experiment. During the period of aircraft observation the convective line appeared to be quasi-stationary in a region ;90 km away from the coast. Calculations of perturbation pressure and buoyancy retrieved from the Doppler-radar-derived wind fields show that a high pressure perturbation associated with negative buoyancy existed in the region of heavy precipitation in the lowest level. In low levels, an elongated zone of low pressure was documented along the line, which was closely related to the distributions of buoyancy. These retrieved features are basically consistent with those calculated from in situ observations. In mid- to upper levels, the high (low) pressure perturbation lay around the updraft cores on the upshear (downshear) of environmental shear. The quantitative diagnosis for the perturbation pressure indicates that the positive buoyancy associated with the in-cloud latent heat release as well as evaporative cooling occurring near cloud base and subcloud regions is crucial to determine the low-level perturbation pressure distributions. However, dynamic forcing contributing to the pressure perturbation cannot be ignored in middle levels, because of updraft‐ shear interaction due to the presence of pronounced cross-line wind shear and corresponding updrafts reaching maximum values near midtroposphere. In distinct contrast to squall lines, which are rapidly moving, the near-surface temperature gradient across this quasi-stationary line was very weak, and at low levels no gust front, rear inflow, or dynamically induced high pressure was evident near the leading edge of the line. Analyses presented here suggest that the persistent, stationary convergence, in association with the deceleration of low-level southeasterly flow as it approached the nearshore blocked flow, played an important role in maintaining and organizing the deep convection in this case.

This study uses airborne Doppler radar observations to describe the mesoscale structure and evolution of a cold frontal system as it made landfall on the mountainous coast of Oregon and northern California on 1 December 1995 during the Coastal Observations and Simulations with Topography experiment. This section of coastline constitutes a steep, approximately two-dimensional north‐south-oriented orographic barrier. The front exhibited a northeast‐southwest orientation and thus intersected the axis of high terrain at an acute angle. The alongbarrier pressure gradient and low-level winds increased with time along the coastal zone and reached a maximum as the front made landfall. Stably stratified prefrontal flow was strongly blocked by the orography, resulting in a confluent transition from pervasive southwesterly winds offshore to a narrow zone of accelerated southsouthwesterly flow near the coast, where wind speeds approached 30 m s 21 at a height of 750 m above mean sea level. Postfrontal flow was much less affected by the topography, probably because of its weaker static stability. Upstream blocking by the steep coastal terrain also evidently led to modifications of precipitation in the vicinity of the front, including the rapid genesis of a narrow cold-frontal rainband (NCFR) and nearshore enhancement of two prefrontal precipitation bands. This evolution of the NCFR is interpreted in conjunction with changes in prefrontal vertical wind shear, which favored more upright convective ascent as the front neared shore and encountered accelerated along-barrier flow adjacent to the steep terrain. In addition, a statistical examination of observed radar reflectivity patterns shows that the intensity of frontal precipitation systematically decreased with upstream distance away from the orographic barrier.

The formative stage of a long-lived mesoscale cyclonic vortex was captured by the NOAA P-3 aircraft as it investigated a developing mesoscale convective system (MCS) near the southeastern coast of Taiwan on 16 June 1987 during the Taiwan Area Mesoscale Experiment. The supporting environment of the mesovortex was characterized by an exceptionally moist atmosphere and moderate ambient vertical shear through a deep layer from the near surface to ∼6 km, with much weaker shear and winds aloft. In addition, a pronounced low-level mesoscale shear/convergence zone, which resulted from the interaction of southeasterly flow with northeasterly flow confined to the near-coast region, existed in the vicinity of the observed mesovortex. Composite three-dimensional wind fields derived via pseudo-dual-Doppler synthesis show the vortex had a horizontal diameter expanding from ∼40 km to ∼70 km in the lower to midtroposphere, respectively, and exhibited considerable tilt through this layer. Contrary to previously d...

area of 620 km 2 ) located in eastern Taiwan were analyzed to fill the gaps in our knowledge concerning the linkage between typhoon track, rainfall patterns, and flood peak time. This study used spatially high-resolution radar-derived rainfall estimates from 38 storm events (� 2800 h) to investigate this linkage. The effect of spatial rainfall patterns on the timing of flood peak for the selected events was examined with the aid of a diffusive wave model. The results show that the typhoon rainfall was spatially aggregated and that the relative variations in the rainfall became smaller at higher rainfall rates. The maximum hourly rainfall was approximately twice the areal mean rainfall. Three major rainfall types were identified statistically, and different typhoon tracks appeared to have preferable rainfall types. This finding is presumably due to the interaction of the typhoon circulation and precipitation with the mountainous landscape. Flood lead times were derived for the different rainfall types, and it was found that differences in their lead times could be as large as � 3 h over the studied mesoscale watershed. It is recommended that this empirical approach be incorporated into flood forecasting and warning systems.