Characteristics of low-level jets over northern Taiwan in mei-yu season and their relationship to heavy rain events

The present study investigates the characteristics of low-level jets (LLJs) (greater than or equal to12.5 m s(-1)) below 600 hPa over northern Taiwan in the mei-yu season and their relationship to heavy rainfall events ( greater than or equal to50 mm in 24 h) through the use of 12-h sounding data, weather maps at 850 and 700 hPa, and hourly rainfall data at six surface stations during the period of May-June 1985-94. All LLJs are classified based on their height, appearance (single jet or double jet), and movement (migratory and nonmigratory). The frequency, vertical structure, and spatial and temporal distribution of LLJs relative to the onset of heavy precipitation are discussed. Results on the general characteristics of LLJs suggest that they occurred about 15% of the time in northern Taiwan. with a top speed below 40 m s(-1). The level of maximum wind appeared mostly between 850 and 700 hPa, with highest frequency at 825-850 hPa. A single jet was observed more often (76%) than a double jet (24%), while in the latter case a barrier jet usually existed at 900-925 hPa as the lower branch. Migratory and nonmigratory LLJs each constituted about half of all cases, and there existed no apparent relationship between their appearance and movement. Migratory LLJs tended to be larger in size, stronger over a thicker layer. more persistent, and were much more closely linked to heavy rainfall than nonmigratory jets. They often formed over southern China between 20degrees and 30degreesN and moved toward Taiwan presumably along with the mei-yu frontal system. Before and near the onset of the more severe heavy rain events (greater than or equal to100 mm in 24 h) in northern Taiwan, there was a 94% chance that an LLJ would be present over an adjacent region at 850 hPa, and 88% at 700 hPa, in agreement with earlier studies. Occurrence frequencies of LLJs for less severe events (50-100 mm in 24 h) were considerably lower, and the difference in accumulative rainfall amount was seemingly also affected by the morphology of the LLJs, including their strength, depth, elevation of maximum wind, persistence, proximity to northern Taiwan, source region of moisture, and their relative timing of arrival before rainfall. During the data period, about 40% of all migratory LLJs at 850 or 700 hPa passing over northern Taiwan were associated with heavy rainfall within the next 24 h. The figure, however, was much lower compared to earlier studies. and some possible reasons are offered to account for this deficit. Before and near the onset of the more severe heavy rain events (greater than or equal to100 min in 24 h) in northern Taiwan, there was a 94% chance that an LLJ would be present over an adjacent region at 850 hPa, and 88% at 700 hPa, in agreement with earlier studies. Occurrence frequencies of LLJs for less severe events (50-100 min in 24 h) were considerably lower, and the difference in accumulative rainfall amount was seemingly also affected by the morphology of the LLJs, including their strength, depth, elevation of maximum wind, persistence, proximity to northern Taiwan, source region of moisture, and their relative timing of arrival before rainfall. During the data period, about 40% of all migratory LLJs at 850 or 700 hPa passing over northern Taiwan were associated with heavy rainfall within the next 24 h. The figure, however, was much lower compared to earlier studies. and some possible reasons are offered to account for this deficit.