Statistical Location and Timing of the Convective Lines off the Mountainous Coast of Southeastern Taiwan from Long-Term Radar Observations

Abstract

In this study, observations from the C-band Doppler radar on Green Island, which is located off the southeastern coast of Taiwan, during 1998?2004 were analyzed to investigate the statistical characteristics of the convective lines occurring off the mountainous coast of southeastern Taiwan, with emphasis on their formative location and timing. A total of 211 cases of the lines were identified during the study period. It is shown that the lines were clearly a year-round phenomenon and five cases occurred per month on average. Statistical analyses for all identified cases reveal that the formative area of the lines was extended substantially from nearshore regions to at least ?100 km offshore, with the region of the most frequent formation primarily confined to an elongated zone located ?30 km off the coast. Along-coast variations of line formation were also evident and were shown to be closely related to the nearshore terrain features. The lines tended to form more frequently during the nighttime hours than the daytime hours, with a formative peak between 2000 and 2200 LST. Minimum formation was found near noon between 1200 and 1400 LST; however, considerable cases still could be found during the late morning and late afternoon hours. More than 70% of the lines (?150 cases) had a duration of less than 4 h, and the mean duration for all lines was calculated to be ?3.5 h. In addition, this study also documented statistical differences in the formative and flow characteristics between the nearshore and offshore lines, which are distinguished by offshore distances of formative location that are less or greater than 40 km, respectively. Of particular note, the analyses presented strongly suggest that the physical mechanisms contributing to the initiation of the nearshore and offshore lines are fundamentally different.