Six cases of prefrontal squall lines were observed over the Taiwan Strait and western Taiwan during the 1987 Taiwan Area Mesoscale Experiment (TAMEX). The mean propagation speed was 10 m s−1, and the mean life span was 11.4 h for the six squall lines. All the lines occurred ahead of the Mei-Yu front and moved away from the front with time. The mean environmental conditions associated with the squall lines were analyzed by compositing the six cases. The environmental conditions observed during the TAMEX squall lines were found with characteristics between tropical and midlatitude squall lines. The steering level was near 7 km during the mature stage. A low-level jet at 3–4 km was present, with strong vertical shear in the presquall environment below 700 hPa. The squall lines were oriented 45° to the shear in the 1–3-km layer, like midlatitude cases. The CAPE, however, is similar to the tropical squall lines. The inflow ahead of the squall lines was deeper and stronger below 400 hPa, and the CAPE was higher during the mature stage as compared to the intensifying stage. The squall-line collapse is correlated with decreasing CAPE and low-level inflow ahead of the lines.

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