Abstract
This study investigates the evolution of a mesoscale convective system that formed during the Taiwan Area Mesoscale Experiment (TAMEX) on the 19 June 1987. With respect to the upstream flow, the convective system formed in the lee with rainfall totals from this system exceeding 100 mm. Any event that produces over 100 mm of rain in 24 h is thought to be capable of producing, flooding over the steep orography of Taiwan. Analysis of Doppler radar data showed that the convective cells during this heavy rainfall event repeatedly formed near a fixed location over the foothills and moved slowly (˜4 m s−1) northward. Although the radar reflectivities within these cells were relatively modest (35–40 dBZ), the repeated passage of slowly moving cells partly supplemented by relatively steady, stratiform rainfall during the later stages of the event resulted in the high precipitation totals. The heavy rain in this event resulted from a number of factors including 1) a moist, convectively unstable southerly flow of tropical origin, 2) a shallow convergence zone on the western side of the island dividing flow with a northerly component from that with a southerly component, 3) a quasi-stationary area of storm formation, and 4) a mesoscale environment that produced convective systems with a favorable storm structure and movement. Due to the fact that the large-scale forcing—as evidenced by vertical ascent calculated using rawinsonde data—was small, the authors believe that the conceptual model of low-Froude number flow around the island of Taiwan in the presence of heating can be used to explain the local convergent region that initiated this convective event. A numerical simulation of flow around the island of Taiwan in the presence of surface heating predicted a persistent quasi-stationary area of convergence in the foothills near the location of the observed convection. In this study the hypothesis is discussed that this persistent, quasi-stationary area of convergence may have also played a role in maintaining this convective system. These results and the application of this conceptual model will he discussed within the more general context of forecasting flash floods in Taiwan and the United States.
