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Two 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 systems intensified as they moved toward the southwestern coast. During the daytime, the cold pool remained over southwestern Taiwan without the development of onshore/upslope flow. Furthermore, with a south-southwesterly flow aloft parallel to terrain contours, orographic lifting aloft was absent and preexisting rain cells offshore diminished after they moved inland. Over northern Taiwan on the lee side, a sea breeze/onshore flow developed in the afternoon hours, resulting in heavy thundershowers. These results demonstrate the importance of diurnal and local effects on determining the location and timing of the occurrences of localized heavy precipitation during the early summer rainy season over Taiwan. [ABSTRACT FROM AUTHOR]

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]

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[SUP-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-induced westerlies immediately behind the convective line. From the numerical experiment with synoptic-scale forcing but without the island topography over Taiwan, a large-scale cloud band is simulated within the Taiwan Strait without the localized rainfall maximum along the northwestern coast. The coastal rainfall maximum is simulated when the influences of the orographic effects are included. The results from the radar analyses and numerical simulations indicate that the convergence off the northwestern coast is a combination of the synoptic-scale forcing, orographic effects, and the feedback of the convection. The offshore convergence is important for the production of the rainfall maximum along the northwestern coast of Taiwan. [ABSTRACT FROM AUTHOR]