Formation of the Convective Lines off the Mountainous Coast of Southeastern Taiwan: A Case Study of 3 January 2004

Convective lines frequently occurring off the mountainous coast of southeastern Taiwan under weakly synoptically forced weather conditions are one of the most well-known mesoscale phenomena in Taiwan. These lines usually develop close to the coast but frequently can also be observed well offshore. While nearshore lines are better understood, the formative processes of the offshore lines remain unclear. The main objective of this study is to use various available observations (e.g., Doppler radar, surface and sounding observations, QuikSCAT, and NCEP data) collected over southeastern Taiwan to investigate a specially chosen case on 3 January 2004, in an attempt to identify possible mechanisms leading to the formation of the offshore lines. The studied line was formed; 50-60 km away from the southeastern coast of Taiwan and oriented south-southwest to north-northeast and approximately parallel to the coast. Detailed analyses of the event indicated that the formation of the line did not appear directly relevant to coastal land/sea breezes or offshore flow. Instead, orographic forcing was observed to play a crucial role in the initiation of the line. Particularly, an orographically forced northerly flow due to upstream blocking was evident to the west of the line. The convergence generated as the prevailing northeasterly onshore flow encountered the nearshore blocked flow was found to be an important low-level forcing conducive to the initiation of moist convection associated with the line. In addition, the persistence of the coastally blocked flow and the degree of ambient convective available potential energy (CAPE) were also shown to be closely related to the maintenance and overall intensity of the observed line's convection. The orographically induced convective forcing identified in this study is distinctly different from the conceptual model of line formation proposed by previous studies of the nearshore lines. It is thus strongly suggested that multiple mechanisms may exist upstream of coastal mountains in southeastern Taiwan, which act to initiate the lines with diverse formative locations relative to the coastline.