Taiwan Area Mesoscale Experiment Research Articles

Single-Doppler Retrieval of the Three-Dimensional Wind in a Deep Convective System Based on an Optimal Moving Frame of Reference

A single-Doppler radar velocity retrieval method, based on an optimally determined moving frame of reference, is developed. This method enables the recovery of the complete three-dimensional wind field using reflectivity, and radial wind observations, detected by a single Doppler radar over multiple volume scans. This method first computes a set of optimal moving speeds (U, V, W), which can be functions of the height. It is considered optimal in the sense that when a reference frame is traveling at this speed, as much of the radar reflectivity, interpolated onto this frame can be conserved as possible. The perturbation velocities (u′, v′, w′) are then solved simultaneously, with respect to the moving speeds, by minimizing a cost function through the three-dimensional variational (3D-Var) approach. This cost function consists of a set of weak constraints, mainly the equation for radar reflectivity conservation, the continuity equation, the geometric relationship between the radar-observed radial wind and the Cartesian winds, and a smoothness filter. Finally, by adding the optimal moving speed to the perturbation velocities, a complete three-dimensional wind field is obtained.The performance of this method is tested under different scenarios, including cases with deep convections, embedded in an environment without mean wind, with constant mean wind, with vertical wind shear of the horizontal mean wind, and with differences in the radar observational time at each grid point within one volume scan are considered.This method is demonstrated to be feasible by applying it to recover the three-dimensional wind structure of a subtropical squall line from Doppler radar data, collected during the 1987 Taiwan Area Mesoscale Experiment (TAMEX).

Numerical Study of the Influence of Central Mountain Ranges in Taiwan on a Cold Front

The numerical simulations were performed to study the intrusion of a shallow cold front along the China coastal mountain range, and the deformation of the front by the Central Mountain Range (CMR) in Taiwan during the IOP-9 (1600 UTC June 14-1700 UTC June 15, 1987) of the Taiwan Area Mesoscale Experiment (TAMEX). The essential features of the observed front, such as the faster movement of the eastern part of the front than the western part, were well reproduced. The control and sensitivity simulations suggested that the frontal deformation in Taiwan was caused by the dynamical interaction between the front and the underlying topography. As the cold air approached northern Taiwan from the north, a relative high pressure with anticyclonic circulation built up on the windward side. More oncoming flow was diverted to the northeast of Taiwan than to the northwest and caused the difference of the frontal speed between the eastern and western parts of the front. The dynamics of the frontal movement along southeast China was similar to the dynamics of coastally trapped mesoscale ridges, or orographical jet. The momentum budget equation showed that the contribution of the nonlinear advection, ageostrophic forcing, and friction were equally important to the local change of the wind just behind the front. This might indicate that the dynamics of the front was more complicated than that described by the density current theory derived from an irrotational fluid. The invicid sensitivity simulation revealed a faster propagating front, and a stronger wind behind its leading edge in comparison with the control simulation. The front bears no dynamical resemblance to either an orographically trapped density current or a Kelvin wave, because the major forces in the momentum budget are the pressure gradient force, Coriolis force and advection terms.

A Three-Dimensional Variational Approach for Deriving the Thermodynamic Structure Using Doppler Wind Observations—An Application to a Subtropical Squall Line

A newly designed retrieval scheme based on three-dimensional variational analysis is used to extract the thermodynamic field of a weather system from Doppler wind measurements. As compared with the traditional retrieval method, with this formulation the proposed scheme is able to find a set of optimal solutions for the pressure and buoyancy perturbations that, in the least squares sense, will simultaneously satisfy three momentum equations and a simplified thermodynamic equation. Therefore, the products of the retrieval are the complete thermodynamic fields in three dimensions. To test the performance of this method in real cases, it is applied to the analysis of a subtropical squall line. The required wind data were synthesized by two C-band Doppler radars during the 1987 Taiwan Area Mesoscale Experiment (TAMEX). The emphasis of this study is devoted to an examination of the validity of the retrieved thermodynamic structure, especially along the vertical direction. The results indicate that the distributions of the retrieved thermodynamic parameters are consistent with the kinematic structure and can be reasonably explained by the conceptual model of a squall line. Evidence is collected that strongly supports the validity of the derived thermodynamic structure. Thus, the applicability of this new retrieval scheme is demonstrated.

On the Formation of Leeside Mesolows under Different Froude Number Flow Regime in TAMEX

During the Mei-Yu season, when the surface front approaches and pre-frontal southwesterly flow intensifies, shallow and short-lived mesolows often form to the southeast of Taiwan on the leeside of the Central Mountain Range (CMR). Although they develop under similar synoptic settings, cases observed during the Taiwan Area Mesoscale Experiment (TAMEX) in 1987 still exhibited some case-to-case variations, particularly in the upstream Froude number (Fr) and the amount of windward-side precipitation. The present study selects two cases with more contrasting mesoscale characteristics for detailed examination, and differences found in their flow structure and associated features are discussed and compared with theoretical aspects. Results indicate that as long as Fr exceeds a certain value, two possible scenarios associated with relatively higher and lower Fr values can lead to formation of leeside mesolow through slightly different combination of processes. The case with a larger Fr (>0.5) on June 78 had a flow structure relatively more in favor of the “flow-over” regime under stronger prevailing winds. The airflow produced a large amount of precipitation at the windward side then sunk at the leeside together with parcels from higher levels. The adiabatic warming led to formation of the mesolow, which had no corresponding mesovortex most of the time with a higher Rossby number (Ro≈2), but small and localized vortices independent from the low might appear at other favorable spots near the terrain. The second TAMEX case (June 2) had smaller Fr (

Case Study of the Leeside Mesolow and Mesocyclone in TAMEX

An observational study was performed on a leeside mesolow case that occurred 8 June 1987 during the Taiwan Area Mesoscale Experiment (TAMEX). The case developed as prevailing west-southwest winds strengthened and interacted with the terrain of Taiwan, with a Froude number (Fr = U/Nh) increasing from below 0.3 to over 0.5. A quasi-stationary mesolow formed to the southeast of Taiwan with no closed circulation through most of its 13-h life stages until passage of the Mei-yu front. A stationary and localized mesovortex also formed about 90 km southwest of the low center, but little adjustment was observed between the mesolow and the vortex. Results suggest that airflow at lower levels was blocked and moved around the terrain of the southern Central Mountain Range (CMR). This led to the formation of low-level jets (LLJs) both to the northwest and southeast of Taiwan. The latter branch provided shear vorticity in the background region of the vortex. Air parcels at higher levels, on the other hand, tended to climb over the mountain and caused precipitation on the windward slope, then subsided at the lee side. The subsidence produced the mesolow through adiabatic warming and drying, which was strongest between 1 and 2 km. Latent heat release at the windward side was estimated to contribute a maximum of about 55%–60% of the total warming. Eventually, as the Mei-yu front moved southward along the eastern coast of Taiwan, the mesolow merged with the front and transformed into a migratory mesocyclone along the front.

The Role of Offshore Convergence on Coastal Rainfall during TAMEX IOP 3

Abstract The role of offshore convergence on the coastal rainfall maximum along the northwestern coast of Taiwan is analyzed based on TAMEX (Taiwan Area Mesoscale Experiment) data and numerical experiments using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5). From a case study during TAMEX IOP 3, moderate rainfalls (∼40 mm h–1) are observed along the northwestern coast of Taiwan associated with the arrival of three rainbands in succession during 1000–1600 LST. These rainbands form over the Taiwan Strait with northeast–southwest or north–south orientation. They intensify off the northwestern coast. There is a tendency for the most intense echoes to align in a northeast–southwest orientation off the northwestern coast. An orographically enhanced convergence zone with a northeast–southwest orientation occurs in the area where the deflected southerly flow converges with the prevailing southwesterly flow that is modified by the storm...

An Investigation of the Moving-Frame Single-Doppler Wind Retrieval Technique Using Taiwan Area Mesoscale Experiment Low-Level Data

The moving-frame single-Doppler radar wind retrieval technique is investigated using field experimental data observed during the 1987 Taiwan Area Mesoscale Experiment intensive observation period 2. Emphasis is placed on studying the impact of the geometric position of the radar in relation to the quality of the retrievals. Experiments show that this technique is capable of recovering the missing cross-beam wind components, but their magnitude is always underestimated. As a consequence, when the unobserved tangential components are smaller than the observable radial components, the best results are produced. By contrast, if the unknown cross-beam winds considerably exceed the radial winds because of a disadvantageous relationship to the radar observation site, the retrievals become less reliable. However, even without a priori knowledge of the true flow structure, which is always the case in reality, this single-Doppler retrieval scheme demonstrates its ability to distinguish which component (radial or azimuthal) is greater. In other words, when the tangential component completely dominates the radial winds, this message can be correctly delivered through the retrieval algorithm to alert the users so that the radar data and these single-Doppler-retrieved results will be interpreted and used with caution.

Retrieved Thermodynamic Structure of a Subtropical, Orographically Influenced, Quasi-Stationary Convective Line

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.

Orographic Effects on a Squall Line System over Taiwan

A north‐south-oriented, multicellular squall line during the Taiwan Area Mesoscale Experiment’s (TAMEX) intensive observation period 2 was studied, using dual-Doppler radar data, as it moved over the island of Taiwan. Over the open ocean, it moved eastward at a constant speed (;15.5 m s21). As the squall line encountered Taiwan Island’s complex mountainous areas, the leading edge moved slower in the mountain ridge areas than over the valleys. As a result, its orientation became approximately parallel to the terrain contours. In the upper levels, the eastward movement of the squall line aloft was less affected by the terrain compared to the low levels. As a result, the westward tilt of the rising motion at the front became less significant. Part of the system-relative rear-to-front flow entered the rising branch of the storm’s circulation as a result of orographic lifting, as illustrated by the trajectory analysis, and was enhanced by latent heat release. The system-relative rear-to-front flow did not appear to descend to the lowest levels. Furthermore, it is unlikely that the low-level cold pool behind the leading edge would move upslope. When the squall line moved over the higher terrain areas, the midlevel system-relative westerly flow from the rear entered the storm’s updraft. The vertical motion pattern was dominated by orographic lifting and sinking. Low-level system-relative frontto-rear inflow was absent. Weak echo maxima were found, mainly associated with orographic lifting ahead of or near the mountain peaks. These changes occurred within a short time (;40 min) after the squall line encountered the mountainous terrain. The environmental low-level wind ahead of the squall line exhibited a weak southerly flow west of the mountain ridge areas and a weak southwesterly flow in the valley areas because of island blocking. As a result, the low-level system-relative front-to-rear inflow was weaker in the valley areas than that in ridge areas. Nevertheless, the low-level moisture inflow from the front decreased rapidly in both the former and the latter areas when the squall line encountered mountainous terrain. This was attributed to a rapid decrease in the depth of the low-level inflow layer and less moisture availability in the higher terrain areas. The drier midlevel air entered the storm from the west with the echo tops decreasing as the squall line moved farther over the mountainous terrain.

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...

Characteristics of Rainfall Distributions over Taiwan during the Taiwan Area Mesoscale Experiment (TAMEX)

Abstract Based on 808 daily rainfall stations, rainfall distributions over Taiwan during TAMEX (Taiwan Area Mesoscale Experiment) (10 May–27 June 1987) were analyzed. The total rainfall had a maximum on the windward slopes under the prevailing southwesterly monsoon flow, indicating that orographic lifting plays an important role in determining rainfall distribution. About 50% of the rainfall occurred as scattered orographic showers during nonfrontal periods. Along the northwestern coastal plain, about 80% of the rainfall was associated with the passage of surface fronts. Along the eastern coastal, mountainous region, which is on the lee side under the prevailing southwesterly monsoon flow, a large portion of the rainfall was produced by upper-level, cold-core lows and a midlevel vortex event. For these cases, low-level flow was southerly or southeasterly because Taiwan was on the western or southwestern periphery of the western Pacific subtropical high. In this situation, eastern Taiwan was on the windwar...

Barrier Jets during TAMEX

Abstract A barrier jet is frequently found along the northwestern coast of Taiwan in the prefrontal southwesterly flow regime during the Taiwan Area Mesoscale Experiment (TAMEX). It has a maximum wind speed of 14 m s−1 at approximately 1 km above the surface with a vertical wind shear approximately 10 × 10−3 s−1 below and 4 × 10−3 s−1 above that altitude. During TAMEX, the southwesterly monsoon flow strengthens over Taiwan when the low-level pressure trough/surface front moves toward the southeastern China coast. The barrier jet is a result of the stably stratified airflow past an island obstacle under a small–Froude number [<O(1)] flow regime. During the occurrence of a barrier jet, a windward pressure ridge is observed along the southwestern coast due to island blocking. In low levels, the incoming southwesterly flow decelerates off the southwestern coast and moves around the island. Along the western coast, the deflected airflow accelerates northward with a large cross-contour wind component down the p...

The Intensification of the Low-Level Jet during the Development of Mesoscale Convective Systems on a Mei-Yu Front

Abstract During the period of 21–25 June 1991, a mei-yu front, observed by the post–Taiwan Area Mesoscale Experiment, produced heavy precipitation along the western side of the Central Mountain Range of Taiwan. Several oceanic mesoscale convective systems were also generated in an area extending from Taiwan to Hong Kong. Numerical experiments using the Penn State–NCAR MM5 mesoscale model were used to understand the intensification of the low-level jet (LLJ). These processes include thermal wind adjustment and convective, inertial, and conditional symmetric instabilities. Three particular circulations are important in the development of the mei-yu front. First, there is a northward branch of the circulation that develops across the upper-level jet and is mainly caused by the thermal wind adjustment as air parcels enter an upper-level jet streak. The upper-level divergence associated with this branch of the circulation triggers convection. Second, the southward branch of the circulation, with its rising mot...

TAMEX IOP 9期間中に台湾の東岸沖で航空機により観測された梅雨前線先端部内の気流のメソスケール構造について

In order to investigate the mesoscale structures and appropriate mechanisms for their maintenance in the Mei-yu front leading edge during the TAMEX (Taiwan Area Mesoscale EXperiment) IOP 9 (15 June 1987) off the east coast of Taiwan, we measured the fine-scale in-situ data by different sensors and sensed radar data by two airborne radars mounted on the NOAA P-3 research aircraft traversing the frontal system at six different altitudes. Based upon the sudden increase of the turbulence intensities, the position of the frontal leading edge at each flight level was identified exactly and, after deducting the propagating distance of the frontal system, the composited vertical cross sections of the system in kinematics, dynamics and thermodynamics was feasible. The significant, findings are: the frontal edge was parallel to the isolines of horizontal component of wind and perturbed air density and the frontal edge demonstrated a well-defined density current, the variation of thermodynamic parameters was not obvious, but the pattern of potential temperature revealed a cold core in the lower level behind the frontal edge, which coincided with the position of the heaviest air density. The probable mechanisms for the propagation of the density current and the maintenance of the frontal system were proposed to be the intense horizontal pressure gradient force from rear to front in the cold core region and the inoderate convective instability at the head of the system as well as the kinetic energy transport from the mean flow.

A Numerical Study of the Low-Level Jet during TAMEX IOP 5*

Abstract The Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model Version 4 is used to simulate the cyclogenesis and the development of a low-level jet (LLJ) that occurred during the Taiwan Area Mesoscale Experiment intensive observing period 5 over southern China. Evaluation of the model results during a 36-h period indicates that the model successfully reproduces most principal features of this event, including cyclone path, intensification of the LLJ, distribution of precipitation, and the secondary circulation across the jet–front system. Sensitivity tests show that latent heat release is important for the deepening of the cyclone and the development of the LLJ, whereas the model results are not sensitive to boundary layer physics. The lee trough east of the Tibetan Plateau provides the initial low-level vorticity. The initial deepening of the lee cyclone and the development of the low-level southwesterly flow are caused by the vertical motion associated with the uppe...

A preliminary study of the formation of precipitation systems under undisturbed conditions during TAMEX

The effect of mountains on the occurrence of precipitation systems on Taiwan island is very significant, especially as mountain areas occupy about two-thirds of the land-mass. The mountains are, on average, about 3 km high. To investigate the formation of precipitation systems influenced by Pacific high pressure systems, we selected five cases (May 24, 25 and 26, June 19 and 20 in 1987) during a field program, TAMEX (Taiwan Area Mesoscale Experiment, Kuo and Chen, 1990). In all cases most of the rainfall took place in the afternoon when the level of free convection (LFC) was at about the 1 km height. If the average wind (below 3 km in height) was from the south (May 25 and 26), higher amounts of precipitation would be found along the sloped areas of western and eastern Taiwan. Rainfall also occurred in southern and northern Taiwan. If the average wind was from the southwest (May 24), the precipitation pattern was similar to that on May 25, except over the plains area in southwest and northeast Taiwan, where the amount was less. However, if the prevailing wind direction changed little with height and the average wind was from the south-southeast (June 19), higher rainfall amounts occurred from northwestern to central Taiwan. If the average wind was from the south and wind direction changed little with height (June 20), higher rainfall amounts took place in northern and central Taiwan. A nonhydrostatic model was used to simulate the formation of precipitation systems in all five cases. Simulation results indicated that the mixing ratio of rainwater could occur on the upstream side of a mountain slope and in the central mountain areas, where topographic lifting from the environmental wind and an upslope flow due to surface heating were evident. On the downstream side of the mountain, upward motion due to lee-side convergence and upslope motion from surface heating would also help rain form.

Analysis of a Heavy Rainfall Event during TAMEX

Abstract A heavy rainfall event during the Taiwan Area Mesoscale Experiment intensive observing period 13 has been studied using upper-air, surface mesonet, and dual-Doppler radar data. The heavy rainfall (≥231 mm day−1) occurred over northwestern Taiwan with the maximum rainfall along the northwestern coast and was caused by a long-lived, convective rainband in the prefrontal atmosphere. It occurred in an upper-level divergence region and along the axis of the maximum equivalent potential temperature at the 850-hPa level. As a Mei-Yu front advanced southeastward, the postfrontal cold air in the lowest levels was retarded by the hilly terrain along the southeastern China coast. As a result, a low-level wind-shift line associated with a pressure trough at the 850-hPa level moved over the Taiwan Strait before the arrival of the surface front. The westerly flow behind the trough interacted with a barrier jet along the northwestern coast of Taiwan. The barrier jet is caused by the interaction between the pref...

A study of a precipitation system in northeastern Taiwan during TAMEX IOP#10

Two-thirds of the land mass of Taiwan island is covered by mountains that affect precipitation systems over the island. To understand the influence of such terrain on a precipitation system was one of the objectives of TAMEX (Taiwan Area Mesoscale Experiment, Kuo and Chen, 1990). During the passage of these precipitation systems, Doppler radar readings as well as conventional data were collected. On 17 June, 1987 a precipitation system moving toward northeastern Taiwan dumped over 100 mm of rainfall per day near the mountain foothills, not far from the ocean. Over the lee side, the precipitation amount was less. The radar data results indicate that a series of cells formed about 10 km upstream of the coastal area and moved toward the mountains under the influence of an easterly wind. The zonal speed was about 4 to 8 ms−1. The time interval for the formation of these convective cells was about 40 minutes. They intensified near the coastal area, the foot hills and the mountain slope, but their intensity decreased on the lee-side. A two-dimensional, nonhydrostatic model with a terrain-following coordinate system was employed to study the influence of environmental wind patterns and terrain on the characteristics of a precipitation system. Simulation results indicate that a series of clouds associated with an updraft formed at the middle level, about 10 to 20 km east of the mountain foothills (near the coast line), under the influence of easterly winds in a very moist environment. Then, updrafts associated with cloud water travelled westward from the cloudy region, intensifying near the bottom of the mountains and in the coastal areas due to orographic lifting. Then, convective cells formed. As these cells continued moving westward and upward near the foothills as well as the upslope area near the mountain top, their intensity increased. But once they passed over the mountain top to the lee side, their intensity decreased. The time interval for the formation of cells was about 35 minutes and the size of the cells was about 5 to 8 km horizontally. The numerical results are qualitatively consistent with the observations. Sensitivity studies indicate that the magnitude of the wind speed influenced the formation of the cells. The low level wind profiles affected the movement of cells on the lee-side of the mountain, and the height of mountain also had an impact on the characteristics of the precipitation cells.

Development of Low-Level Jets during TAMEX

Detailed synoptic-scale map analysis and diagnosis for all six low-level jet (LLJ) cases that occurred during the Taiwan Area Mesoscale Experiment (TAMEX) were made to study the structure and the development of the LLJs. It was shown that the development of these LLJs was closely linked to the development of a lee trough to the cast of the Tibetan Plateau. Seasonal change had a profound influence on the secondary circulation across the jet-front system. Before the seasonal transition, the formation of a lee trough with closed isobars in low levels east of the Tibetan Plateau occurred as an upstream short-wave trough moved eastward with strengthening westerlies over the Tibetan Plateau in upper levels. After the development of the lee trough, the southwest flow appeared in the southeast quadrant of the Ice trough. As the upstream trough moved farther eastward, it merged with the lee trough, and the low-level southwesterly flow intensified. For most cases, a midlatitude deep trough was present to the north or northeast at this time; the northwesterlies behind the midlatitude trough brought in the cold air to the rear of the merged trough. The upper-level jet ahead of the merged trough strengthened. The secondary circulation across the jet-front system was characterized by a thermally direct circulation across the baroclinic zone, with warm air rising within the southwest monsoon flow, cold air sinking in the postfrontal northeasterlies, and a weak thermally indirect circulation to the south. After the seasonal transition, the major baroclinic zone shifted northward, and the upper-level south Asian high pressure cell moved over the Tibetan Plateau. The Tibetan high dominated the upper-level circulations with easterlies along the southern China coast, preventing baroclinic disturbances from moving into southern China. At 1200 UTC 22 June 1987, an orographically induced lee trough formed in the lee side of the Tibetan Plateau. An LLJ was observed during 0000 UTC 23 June-0000 UTC 25 June 1987 over the southern China coast as a southwest vortex developed within the lee trough. The formation of the vortex was driven by latent heat release in a barotropic environment. The secondary circulation associated with the LLJ was characterized by a rising motion northwest of the LLJ axis and a broad sinking motion to the south.

The Role of Mesoscale and Topographically Induced Circulations in Initiating a Flash Flood Observed during the TAMEX Project

Abstract This study investigates the evolution of a mesoscale convective system that formed during the Taiwan Area Mesoscale Experiment (TAMEX) on the 19 June 1987. With respect to the upstream flow, the convective system formed in the lee with rainfall totals from this system exceeding 100 mm. Any event that produces over 100 mm of rain in 24 h is thought to be capable of producing, flooding over the steep orography of Taiwan. Analysis of Doppler radar data showed that the convective cells during this heavy rainfall event repeatedly formed near a fixed location over the foothills and moved slowly (˜4 m s−1) northward. Although the radar reflectivities within these cells were relatively modest (35–40 dBZ), the repeated passage of slowly moving cells partly supplemented by relatively steady, stratiform rainfall during the later stages of the event resulted in the high precipitation totals. The heavy rain in this event resulted from a number of factors including 1) a moist, convectively unstable southerly f...