Cheng-Ku Yu

Airborne Doppler Observations of a Landfalling Cold Front Upstream of Steep Coastal Orography

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.

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

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 moun...

Distribution and Mechanisms of Orographic Precipitation Associated with Typhoon Morakot (2009)

AbstractUsing 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 c...

Linking typhoon tracks and spatial rainfall patterns for improving flood lead time predictions over a mesoscale mountainous watershed

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.

Radar Observations of the Diurnally Forced Offshore Convective Lines along the Southeastern Coast of Taiwan

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).

Airborne Doppler Observations of a Cold Front in the Vicinity of Vancouver Island

Abstract 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...

Formative Stage of a Long-Lived Mesoscale Vortex Observed by Airborne Doppler Radar

Abstract 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...

Surface Fluctuations Associated with Tropical Cyclone Rainbands Observed near Taiwan during 2000–08

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 eventsduring the studyperiod 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 ue 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 windveering.Quantitative analysesindicatethatthemajorityoftheTCRs(;80%‐90%) exhibitedvariations in surface temperature, pressure, wind speed, and wind direction less than 28C, 1.5 mb, 5 m s 21 , and 208, respectively.However,acleartrendofthemagnitudeofTCRthermodynamicfluctuationsincreasingwiththe 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 distinguishedbyaradialdistanceof3timestheradiusofmaximumwind.Thecompositeandmagnitudeanalyses of their surface fluctuations indicate that the outer rainbands had a higher potential than the inner rainbands to reduce the near-surface ue values. This observed characteristic is likely related to more pronounced evaporative cooling taking place in drier subcloud regions and the downward transport of low-ue 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.

Surface Pressure Features of Landfalling Typhoon Rainbands and Their Possible Causes

Abstract 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 (<∼0.4 mb) outside th...

Multiple Precipitation Mechanisms over Mountains Observed by Airborne Doppler Radar during MAP IOP5

Abstract This study uses airborne Doppler radar measurements from the Special Observing Period of the Mesoscale Alpine Programme (MAP) to document the detailed airflow and precipitation structure over the mountainous regions near the border of northeastern Italy and Slovenia as a cold frontal system moved eastward and encountered the eastern Alps on 4 October 1999, during MAP IOP5. In contrast to previously documented MAP cases, the environmental conditions associated with this case are characterized by a deep layer of strong convective instability in the lower troposphere and by a cold, northeasterly continental flow coming down from the mountains (the so-called bora wind) along the southeastern Alps. Over the study region, there are two primary mountain barriers: the Julian Alps, oriented roughly west–east with a peak mountain height of ∼2500 m and a significant variation in terrain height along its length, and the other, the Dinaric Alps, a relatively lower mountain range oriented northwest–southeast, ...