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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 temporal variabilities. Nevertheless, despite the presence of these transient disturbances, it is shown that the horizontal distributions of climatological rainfall patterns for different rainfall regimes are strongly dependent on the direction of the low-level prevailing flow with much higher rainfall on the windward side. Furthermore, the seasonal variations in rainfall amount and type (light precipitation versus convective precipitation) are also dependent on the thermodynamic stratification and the availability of moisture. [ABSTRACT FROM AUTHOR]

Orographic effects on the airflow over Taiwan under the prevailing southwesterly monsoon flow are studied using the fifth-generation Pennsylvania State University-National Center for Atmospheric Research (PSU-NCAR) Mesoscale Model (MM5). In particular, the orographically induced low-level strong winds along the northwestern coast of Taiwan are simulated in the absence of radiative forcing, thermal forcing from the surface, and synoptic forcing. The momentum balance for the low-level strong winds along the northwest coast of Taiwan shows that, above the mixed layer, the alongshore force balance is dominated by the inertial advection term and the pressure gradient force term. The significant Lagrangian flow acceleration of the northern branch of the deflected airflow along the northwest coast is mainly related to the pressure gradient force as the airflow moves down the orographically induced pressure ridge along the windward coast. Even though the alongshore pressure gradient force along the northwest coast is largest at the surface, the observed and simulated wind profiles show that the wind speed maximum in the vertical of the barrier jet occurs at a height of ∼ 800 m primarily due to frictional retardation in the lowest levels. Within the well-mixed layer, the alongshore force balance is a three-way balance between inertial advection, pressure gradient force, and frictional force. Normal to the northwest coast, the force balance is approximately geostrophic. In addition, the location and the strength of the orographically induced low-level strong winds along the northwest coast of Taiwan also depend on the impinging angle and the wind speed of the prevailing winds. [ABSTRACT FROM AUTHOR]

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 21 at approximately 1 km above the surface with a vertical wind shear approximately 10 3 10 23 s 21 below and 4 3 10 23 s 21 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 pressure gradient along the western coast, resulting in a barrier jet over northwestern Taiwan. The barrier jet is the strongest when the surface windward ridge--leeside trough pressure pattern is most significant and weakens after a surface front arrives. [ABSTRACT FROM AUTHOR]