Ching-Sen Chen

The Rainfall Characteristics of Taiwan

Abstract The rainfall regimes of Taiwan are investigated using the 18-yr European Centre for Medium-Range Weather Forecasts (ECMWF) data (1980–97), the available 38-yr daily rainfall data from 25 conventional surface stations around Taiwan (1961–98), and the 5-yr hourly rainfall data (1994–98) from 249 high-spatial-resolution Automatic Rainfall and Meteorological Telemetry System (ARMTS) stations. Rainfall over the island is usually generated either by transient disturbances embedded in the prevailing monsoon flow or local rainshowers related to terrain or local winds. With the change in the direction of the prevailing winds between the warm and cold seasons as well as a variety of transient subsynoptic disturbances occurring in different seasons (e.g., winter monsoon cold surges, springtime cold fronts, mei-yu fronts in the early summer, typhoons in summer months, and cold fronts in fall) and the presence of the Central Mountain Range, the regional rainfall climate over the island shows large spatial and...

An intercomparison of model predicted wave breaking for the 11 January 1972 Boulder windstorm

Abstract Two-dimensional simulations of the 11 January 1972 Boulder, Colorado, windstorm, obtained from 11 diverse nonhydrostatic models, are intercompared with special emphasis on the turbulent breakdown of topographically forced gravity waves, as part of the preparation for the Mesoscale Alpine Programme field phase. The sounding used to initialize the models is more representative of the actual lower stratosphere than those applied in previous simulations. Upper-level breaking is predicted by all models in comparable horizontal locations and vertical layers, which suggests that gravity wave breaking may be quite predictable in some circumstances. Characteristics of the breaking include the following: pronounced turbulence in the 13–16-km and 18–20-km layers positioned beneath a critical level near 21-km, a well-defined upstream tilt with height, and enhancement of upper-level breaking superpositioned above the low-level hydraulic jump. Sensitivity experiments indicate that the structure of the wave bre...

Statistics of Heavy Rainfall Occurrences in Taiwan

Abstract The seasonal variations of heavy rainfall days over Taiwan are analyzed using 6-yr (1997–2002) hourly rainfall data from about 360 rainfall stations, including high-spatial-resolution Automatic Rainfall and Meteorological Telemetry System stations and 25 conventional stations. The seasonal variations and spatial variations of nontyphoon and typhoon heavy rainfall occurrences (i.e., the number of rainfall stations with rainfall rate >15 mm h−1 and daily accumulation >50 mm) are also analyzed. From mid-May to early October, with abundant moisture, potential instability, and the presence of mountainous terrain, nontyphoon heavy rainfall days are frequent (>60%), but only a few stations recorded extremely heavy rainfall (>130 mm day−1) during the passage of synoptic disturbances or the drifting of mesoscale convective systems inland. During the mei-yu season, especially in early June, these events are more widespread than in other seasons. The orographic effects are important in determining the spati...

A Comparison of Two Heavy Rainfall Events during the Terrain-Influenced Monsoon Rainfall Experiment (TiMREX) 2008

AbstractTwo contrasting localized heavy rainfall events during Taiwan’s early summer rainy season with the daily rainfall maximum along the windward mountain range and coast were studied and compared using a combination of observations and numerical simulations. Both events occurred under favorable large-scale settings including the existence of a moisture tongue from the tropics. For the 31 May case, heavy rainfall occurred in the afternoon hours over the southwestern windward slopes after a shallow surface front passed central Taiwan. The orographic lifting of the prevailing warm, moist, west-southwesterly flow aloft, combined with a sea breeze–upslope flow at the surface provided the localized lifting needed for the development of heavy precipitation. On 16 June before sunrise, pronounced orographic blocking of the warm, moist, south-southwesterly flow occurred because of the presence of relatively cold air at low levels as a result of nocturnal and rain evaporative cooling. As a result, convective sys...

Meteorology-hydrology study targets Typhoon Nari and Taipei flood

Typhoon Nari struck Taiwan on 16 September 2001, taking 92 lives. Analysis reveals that the storms heavy rains were due to warmer ocean temperatures, Naris unique track and slow-moving speed, and the terrain of Taiwan. Analysis further suggests that the heavy rains in Nari contained many small raindrops. The typhoon rains overwhelmed existing flood protection capacities downstream of the Chi-Lung River in a part of Taipei that has no regulatory reservoirs, resulting in major flooding. Preliminary findings underscore several key issues for future study, the goal of which will be to improve quantitative precipitation estimation/prediction, hydrologic modeling, and flood prediction.

A study of afternoon heavy rainfall in Taiwan during the mei-yu season

Abstract This study investigates how sounding characteristics affects afternoon rainshowers on days during which the rainfall amount exceeded 100 mm, as observed at least at 1 out of 232 to 326 automatic rainfall and meteorological telemetry system stations and 22 conventional surface stations in Taiwan during the mei-yu (monsoon rain) season between 1993 and 1997. In the first group of three out of five events (Group A), the West Pacific Subtropical High was just to the east of Taiwan, and a southerly wind beneath 700 hPa was observed with a speed ranging from 4 to 13 m/s. In the other two events (Group B), the ridge axis of the West Pacific Subtropical High was over southern Taiwan and the wind was southwesterly with a more uniform speed of below 700 hPa. In Group A, most of the precipitation was observed in low sloping and mountainous areas over northern and central Taiwan while in Group B, most of the precipitation occurred at elevations over 500 m in central Taiwan. Based on the sounding characteristics of these two groups, the Penn State/NCAR MM5 Mesoscale model was employed in 31 experiments to investigate the impact of wind direction, speed and relative humidity on the distribution of heavy precipitation during the mei-yu season. The predominant southerly or southwesterly wind converged in northern Taiwan over the lee side of the north–south-orientated central mountain range (CMR). Meanwhile, surface heating resulted in a northerly upslope wind in northern Taiwan. The lee side convergence and upslope wind accompanied by onshore flow from northern coastal areas, promoted simulated precipitation systems in northern Taiwan. In the simulation, an upslope wind over the windward side of western Taiwan, intensified precipitation systems over sloping areas. A larger wind component perpendicular to CMR was produced in predominantly southwesterly flow than in predominantly southerly flow. Subsequently, a larger wind shear was generated between the boundary layer and the free atmosphere over the western slope, resulting in a high entrainment to inhibit the upward motion and the development of precipitation systems. Consequently, the simulated rainfall accumulation over Taiwan in predominantly southwesterly wind was less than in predominantly southerly wind. The increase in the speed of the predominant wind also increased wind shear over the slope, reducing the vertical velocity and the intensity of the precipitation. Accordingly, less rainfall accumulated over the island in simulations with higher predominant wind speeds. These results are consistent with observation. Simulated rainfall fell over low land in high low-level relative humidity experiments, with a low level of free convection (LFC).

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

SummaryThe 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.

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

SummaryTwo-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.

A numerical study of airflow over Taiwan island

Two-thirds of the land mass of Taiwan island is covered by mountains that affect precipitation systems over the island. Here, we tried to understand the distribution of precipitation associated with the interaction of the prevailing wind (a northern and southern wind direction, respectively) and terrain, if the synoptic forcing was relatively small. For a northern flow under stable conditions, near surface airflow with low Froude number diverted around the Taiwan island. The relatively higher amount of rainfall occurred in the higher mountains on the upstream side, due to topographic lifting of the airflow at similar height. Along the eastern and western side of the island, rainfall occurred over the confluence area of low-level airflow. If the prevailing flow with convectively unstable air and a low Froude number was from the south, the flow was diverted near southern Taiwan and converged near northern Taiwan. Surface heating could induce an upslope flow and enhance the convergence that occurred in northern Taiwan. The upward motion associated with an upslope wind and the convergence air could help precipitation systems form along sloped areas.

Analysis and Simulations of a Heavy Rainfall Event over Northern Taiwan during 11–12 June 2012

AbstractDuring 11–12 June 2012, heavy precipitation occurred over the northwestern Taiwan coast (~435 mm) and within the Taipei basin (~477 mm). With the presence of a midlatitude omega-blocking pa...