Convection initiation and organization Mechanism Research in a Extreme Rainfall Event over Southern China on August 2018

An extreme daily rainfall (maximum of 1056.7mm) produced by a long-lived mesoscale convective system (MCS) occurred over Gaotan town of Guangdong province during 30-31 August 2018, which broke the historical record in Guangdong province, caused severe flash flood and aroused social concern. Analysis of the extreme rainfall based on various observation data and NCEP/NCER_FNL reanalysis include property of the precipitation, environmental conditions, initiation and maintenance of the β-MCS. It is shown that the record-breaking rainfall is characteristic of high intensity and ultra-long duration with high-thermal, high-humidity and high-CAPE on background of monsoon depression. New convective cells (γ-MCSs) are continuously initiated from the meso-small scale valley, propagating and developing along the background wind field at low troposphere constantly, then merging and enhancing. These γ-MCSs line up to form a linear-shaped β-MCS, with characteristics that low-echo top, low-echo-centroid and train effect. The organization of β-MCS is closely related to near-surface wind field, which is affected by multi-scale systems that we qualitatively analyze using rotation rate equation of the direction of sea and land breezes. The southerly flow is able to sustain for a long period that is determined by the forcing of monsoon depression and local topography, and the southerly flow on the side of river valley over slope topography enhancement helps the warm-ridge development of temperature field, thus outflow from cold pool on the side of mountain over slope topography cannot pushes the boundary moving southeastward, leading to sharp temperature gradient over that region. Quantitative diagnosis using Boussiniesq equation shows the dynamic mechanism to sustain convection maintenance and β-MCS organization stems from local vertical wind shear at 0-3km, causing by the sharp temperature gradient.