A Comparison and Relationship between Two Consecutive Heavy-Rainfall Events during SCMREX-2014

Two heavy-rainfall events that occurred consecutively on 22 and 23 May 2014 over Guangdong Province in South China are studied using high-resolution observations collected during the Southern China Monsoon Rainfall Experiment (SCMREX) field campaign and a series of convection-permitting model simulations. Observational analyses reveal significant differences in the characteristics of heavy-rain-producing mesoscale convective systems (MCSs) between the two days. Heavy rainfall on 22 May was overspread, and has a distinct center of about 300 mm over the northern mountainous regions that was produced by a quasi-stationary MCS and the passage of a southeastward-moving quasi-linear MCS in the early morning hours. The two MCSs merged to form a larger MCS at noon, which exhibited a classic mesoscale structure of the leading convective line and trailing stratiform precipitation. This merged MCS produced well-organized cold outflow boundaries, where new convective elements were continuously initiated, accounting for its continued southeastward movement. Of significance is that this MCS left behind a mesoscale northwest-southeast oriented cold pool on the windward side of the northern mountains during the early evening hours. Convective elements, albeit much less intense than those occurring during the daytime, began to be continuously initiated over the “old” cold pool around midnight. At the same time, weak convective elements were initiated over the sloping mountains on the northwest of the cold pool and then moved over the cold pool, leading to the formation of a northwest-southeast oriented MCS in a few hours. The MCS at the mature stage consisted of multiple convective rainbands that were embedded in stratiform precipitation regions. Little evidence of cold outflows could be observed at the surface, suggesting the presence of elevated convection associated with the rainbands. Despite its weaker convective intensity, as suggested by both lightning flashes and vertical profiles of reflectivity in convective cores, this MCS persisted locally for about 20 hours, producing more than 300 mm precipitation on the windward side of the mountains in central Guangdong on May 23.

Both heavy rainfall events occurred under favorable large-scale settings including high precipitable water (about 60 mm), a high θ_e tongue from the South China Sea, southerly low-level jets (LLJs), and warm advection in the planetary boundary layer (PBL). However, the environmental dynamics and thermodynamics of the two events differ in a few ways. On 22 May there was a northeast-southwest oriented shear line in the lower troposphere extending to north Guangxi, where the moving MCS was initiated. The MCS’s subsequent growth and movement were driven by convectively generated cold outflow boundaries associated with middle-level dry intrusions. In contrast, the heavy-rain-producing MCS on 23 May was significantly influenced by the “old” cold pool (depth of 1~1.5km) that facilitated the continuous lifting of high-θ_e air of tropical origin from the southerly LLJ, and the generation of elevated convection with little cold downdraft air.

The above observation-based understanding, especially the connection between the two events, is supported by a series of numerical simulations. Results confirm that the formation, long-duration, and stagnation of the heavy-rain-producing MCS on 23 May are attributable partly to the cold dome left behind by the 22 May MCSs and partly to its own convective feedbacks (i.e., rain evaporative cooling and cloud shielding). The northeastern mountains in Guangdong tended to block the MCS’s movement, accounting for the stagnation of the 23 May MCS. Moreover, the weakening of the LLJ in the evening hours of 23 May seems to play an important role in the dissipation of the MCS.