Orographic Effects on a Squall Line System over Taiwan

Abstract

A north?south-oriented, multicellular squall line during the Taiwan Area Mesoscale Experiment?s (TAMEX) intensive observation period 2 was studied, using dual-Doppler radar data, as it moved over the island of Taiwan. Over the open ocean, it moved eastward at a constant speed (?15.5 m s?1). As the squall line encountered Taiwan Island?s complex mountainous areas, the leading edge moved slower in the mountain ridge areas than over the valleys. As a result, its orientation became approximately parallel to the terrain 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 result, the westward tilt of the rising motion at the front became less significant. Part of the system-relative rear-to-front flow entered the rising branch of the storm?s circulation as a result of orographic lifting, as illustrated 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 lowest levels. Furthermore, it is unlikely that the low-level cold pool behind 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 orographic lifting ahead of or near the mountain peaks. These changes occurred within a short time (?40 min) after the squall line encountered the mountainous terrain. The environmental low-level wind ahead of the squall line exhibited a weak southerly flow west of the mountain ridge areas and a weak southwesterly flow in the valley areas because of island blocking. As a result, the low-level 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 front decreased rapidly in both the former and the latter areas when the squall line encountered mountainous terrain. This was attributed to a rapid decrease in the depth of the low-level inflow layer and less moisture availability 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 farther over the mountainous terrain.