Orographic effects on a squall line system over Taiwan

A north-south-oriented, multicellular squall line during the Taiwan Area Me soscale Experiment's (TAMEX) intensive observation period 2 was studied, us ing dual-Doppler radar data, as it moved over the island of Taiwan. Over th e open ocean, it moved eastward at a constant speed (similar to 15.5 m s(-1 )). As the squall line encountered Taiwan Island's complex mountainous areas, t he leading edge moved slower in the mountain ridge areas than over the vall eys. As a result, its orientation became approximately parallel to the terr ain 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 re sult, the westward tilt of the rising motion at the front became less signi ficant. Part of the system-relative rear-to-front flow entered the rising b ranch of the storm's circulation as a result of orographic lifting, as illu strated 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 l owest levels. Furthermore, it is unlikely that the low-level cold pool behi nd 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 oro graphic lifting ahead of or near the mountain peaks. These changes occurred within a short time (similar to 40 min) after the squall line encountered the mountainous terrain. The environmental low-level wind ahead of the squall line exhibited a weak southerly how west of the mountain ridge areas and a weak southwesterly flo w in the valley areas because of island blocking. As a result, the low-leve l system-relative front-to-rear inflow was weaker in the valley areas than that in ridge areas. Nevertheless, the low-level moisture inflow from the f ront decreased rapidly in both the former and the latter areas when the squ all line encountered mountainous terrain. This was attributed to a rapid de crease in the depth of the low-level inflow layer and less moisture availab ility in the higher terrain areas. The drier midlevel air entered the storm from the west with the echo tops decreasing as the squall line moved farth er over the mountainous terrain.