Search

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]

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 s[sup -1] ). 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 front-to-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... [ABSTRACT FROM AUTHOR]