Night Time Heavy Rains in Northern Taiwan Associated with Meiyu Front

Mesoscale convective systems with linear structure produced heavy rain-flash flood in northern Taiwan at night of 20-21 May 2014 and 1-2 June 2017. New cells formed at the coastal region and propagated eastward into the urban area of Taipei basin. It is found the path and the depth of the cold air played crucial role in developing and organizing training features of MCSs along the surface front.

Flash flood storm developed over the coastal area at the beginning. The coastal environment provided favorable conditions for convection development even without pronounced strong wind along the coast. On 20-21 May, very shallow cold air (< 600m) moved through Keelong river valley formed a semi-persistent cold air dome at the center of the Taipei basin. Convections developed on the perimeter of the cold dome and organized into a quasi-stationary MCS on the top of the cold dome.

Later, the cold dome moved southwestward, new convections developed at west of the original position and propagated eastward to merge with storm within the basin. New convective line formed at the southern rim of the earlier rainband and then propagated northward to merge with the older line. This mesoscale pattern persisted for a few hours and produced heavy rain in the basin.

On 1-2 June 2017, pronounced coastal low-level jet was observed and the cold air depth from north reached about 2 km height. Severe heavy rainfall occurred just along the coast and it was co-located with the southward movement of the cold air. It is suggested that the movement of cold air and the depth of the cold air are important to determine the location and possibly the intensity of heavy rain.

Numerical model predicted the storm rainfall well at the beginning period while the rainfall was associated with the approaching surface front. However, the rainfall prediction was poor while the storm development was dominated over the cold air in the basin. We suspect that because of the shallowness of the cold air, high resolution in the vertical domain may be critical for the prediction of heavy rain at night.