Formation and Maintenance of a Long-Lived Taiwan Rainband during 1–3 March 2003

Abstract

Abstract Taiwan rainbands (TRs), defined here as convective lines, which form off the mountainous eastern coast of Taiwan under weakly synoptically forced weather conditions, are a well-known mesoscale phenomenon, but their formative processes remain the subject of debate. This study uses surface and radar observations within the coastal zone of eastern Taiwan and NCEP reanalysis data to document a long-lived TR with a lifetime of ~36 h during 1–3 March 2003 to advance the current general understanding of mechanisms responsible for the TR’s formation and maintenance. Detailed analyses indicate that the rainband was initiated by convergence that was produced as low-level environmental northeasterly/easterly onshore flow encountered topographically blocked northerlies that developed nearshore. The northerly blocked flow was observed to weaken and subsequently dissipate because of changing synoptic pressure patterns that caused prevailing southeasterlies/southerlies at low levels. However, colder nearshore air that resulted from the combined effects of orographic blocking, the evaporation of the TR’s precipitation, and radiative cooling over coastal land continued to persist and acted to provide a continuing source of lifting for the subsequent maintenance of moist convection. Temporal variations in the precipitation intensity of the studied TR were also shown to be consistent with the theoretical prediction of the interaction between the cold pool and ambient vertical shear. This study suggests that multiple precipitation mechanisms, which involve interactions of diurnally, topographically, and convectively generated circulations along the mountainous coast, may operate and contribute to the longevity of a TR event under suitable circumstances, such as the rapidly evolving synoptic flow observed in the present case.