Analysis of a Relatively Dry Front during the Taiwan Area Mesoscale Experiment

Abstract

The evolution of a relatively dry front during the early-summer rainy Season of Taiwan is analyzed. Because of the synoptic subsidence associated with a subtropical high pressure cell over the northern South China Sea, prefrontal soundings over the Taiwan area exhibited a shallow, warm, moist layer in the lowest levels, capped by an inversion with extremely dry air aloft. Over the Taiwan area, the southwest flow ahead of the surface front was more than 10 m s?1 at the 850-mb level. It interacted with the central mountain range, resulting in the windward ridge, leeside trough. Downstream of the blocked region, strong southwesterly winds (?1 5 m s?1) developed in the lowest levels along the northwest coast, where the flow deflected by the mountain barrier merged with the undetected southwest monsoon flow. The hilly terrain along the southeastern China coast retarded the cold air behind the surface front. The cold air was then ducted around the southeastern China coast. At the 850-mb level, a weak short-wave trough was embedded in the prefrontal monsoon flow. It moved off the southeastern China coast before cold northewterlies arrived at the surface. It deepened in the Ice side of the highlands along the southeastern China coast, with significant low-level warming and drying. Aircraft observations of the leading edge of the shallow front revealed that a warm, moist tongue was ahead of the wind-shift line, where the winds shifted from northwesterlies to northeasterlies. Behind the leading edge, the air had a uniform equivalent potential temperature below 700 m. The stable, cold air was found 50 km north of the leading edge, with a warm, moist tongue ahead of it. East of Taiwan, the shallow, cold air behind the front appeared to be warmer than its western counterpart, with a well-mixed layer below 650 m. Since the prefrontal soundings over the Taiwan area were dry with a level of free convection (LFC) above the 800-mb level, local lifting by a shallow front was apparently not sufficient to initiate deep convection leading to heavy precipitation.