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

Chuan-Chi Tu1, *, Yi-Leng Chen1, Ching-Sen Chen2, Pay-Liam Lin2, Po-Hsiung Lin3
1 Department of Meteorology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii
2 Institute of Atmospheric Physics, National Central University, Chung-Li, Taiwan
3 Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan
* Current affiliation: Institute of Atmospheric Physics, National Central University, Chung-Li, Taiwan.



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.
Tu, C.-C., Y.-L. Chen, C.-S. Chen, P.-L. Lin, and P.-H. Lin, 2014: A comparison of two heavy rainfall events during the Terrain-Influenced Monsoon Rainfall Experiment (TiMREX) 2008. Mon. Wea. Rev., 142, 2436–2463.


A Comparison_1.gifFig. 1. Rainfall accumulation (mm) during (a) 0000–0800 LT 31 May, (b) 0800–1800 LT 31 May, (c) 0000–0800 LT 16 Jun, and (d) 0800–1800 LT 16 Jun. (e) Terrain height for Taiwan (m). Banciao (25°N, 121.43°E), Penghu (23.56°N, 119.63°E), Tainan (Yongkang; 23.04°N, 120.23°E), Kaohisung (22.57°N, 120.31°E), Hengchun (22.01°N, 120.74°E), and South Ship (21.46°N, 118.36°E) are marked as B, P, T, K, H, and S, respectively. NCAR S-Pol radar is located at SPOL. Red circles mark rawinsonde sites. Purple plus signs mark Doppler radar sites.