Analysis of a Shallow Front during the Taiwan Area Mesoscale Experiment

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Abstract
A relatively dry surface front during IOP-9 of the Taiwan Area Mesoscale Experiment (1600 UTC 14 June–1700 UTC 15 June) was analyzed. This surface front possessed appreciable baroclinity over southern China due to the southeastward intrusion of the polar air. As the cold air advanced the surface front over southern China moved southeastward and crossed Taiwan. Windshift was observed approximately 12 hours prior to the arrival of the cold air. Detailed analysis of the small-scale frontal features were made based on high resolution P-3 aircraft observations. East of Taiwan the cold air boundary was rather diffuse with weak thermodynamic contrasts due to air mass modifications as the polar air traveled from northern China to the subtropics. The leading edge of the cold air resembled a density current with the following features: a vortex circulation with rising motion along the leading edge and sinking motion behind, low-level inflow from the rear with return flow above and a wavelike pattern at the top of the cold air. However, with calm winds in the environment the well-defined updraft at the nose was absent. In addition, the slope of the cold air boundary was rather gentle. A weak mesolow formed over the, southwestern plain of Taiwan after the passage of the windshift line. It intensified after the arrival of the cold continental air. The shallow northeasterlies were blocked by the mountains leaving the warm, moist air over the southwestern plain, The relatively low pressure in this region resulted from increasing pressure elsewhere due to the passage of the windshift line followed by the arrival of the cold continental air.

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