Diagnosed Characteristics of Precipitation Systems over Taiwan during the May–June 1987 TAMEX

Abstract

Characteristics of Mei-Yu precipitating cloud systems over Taiwan during the May–June 1987 Taiwan Area Mesoscale Experiment (TAMEX) have been studied using sounding, surface precipitation, and radar data. Vertical motion has been computed over the island at 6-h intervals from 13 May to 15 June using a modification of the kinematic method that takes into account the mountainous lower boundary within a four-station sounding polygon. Two primary characteristics of the precipitation have been found. First, the major rainfall event were linked to the passage of midlatitude disturbances and typically consisted of both deep convective and stratiform components. Deep convection was primarily prefrontal or frontal, while the stratiform precipitation was postfrontal, presumably in association with overrunning and orographic lifting. Second, there was a pronounced diurnal variability in the rainfall. Vertical motion, heating (Q1), and moistening (Q2) profiles have been used to define the character of the precipitating systems. During periods of deep convection (as indicated by radar and surface rainfall measurements), a separation of the Q1 and Q2 peaks is observed, whereas at times of stratiform precipitation, the Q1 and Q1 peaks are nearly coincident. The findings for Taiwan generally support those of Luo and Yanai, indicating a predominance of stratiform rainfall over the entire southern China and Yangtze regions (including Taiwan) during the Mei-Yu; however, they also suggest that in at least a portion of this region (Taiwan), precipitation may consist of a mixture of deep convective and stratiform components. The occurrence of coincident Q1 and Q1 peaks in the mid- to lower troposphere (600–800 mb) during moderate-to-heavy stratiform rain events indicates the importance of shallow cold-frontal and/or stable orographic lifting. Thus, it appears that in the Taiwan area, heavy rain in stable situations may depend critically on low-level forcing mechanisms. The evolution of the sea breeze, the development of the afternoon mixed layer, and the diurnal cycle of Q1 and Q1 have been examined on a synoptically undisturbed day (24 May) when afternoon thunderstorms occurred over Taiwan. Moistening of the boundary layer by the daytime sea breeze was evident. A high-level heating peak and a midtropospheric drying peak were observed in the afternoon in association with the sea breeze and deep convection. In the evening, heating and drying aloft and cooling and moistening at low levels occurred, suggestive of stratiform precipitation during the decaying stage of the convection.