Study on the Formation Mechanism and Microphysical Characteristics of WarmSector Convective System with Multiple-Rain-Bands Organizational Mode

Based on ERA5 reanalysis data and multi-source observations,including polarimetric radar and automatic weather stations,this study analyzes the formation mechanism and microphysical characteristics of a warm-sector heavy rainfall event caused by a convective system with multiple-rain-bands organizational mode over the western coast of south China.In the early stage,under the influence of coastal convergence and topography,convection was triggered in the coastal mountainous areas and moved north-eastwards.Nocturnal cooling induced the north winds in the inland mountainous area.A mesoscale convergence line was formed in the middle of Yangjiang city between the inland north and coastal south winds,which facilitated the developing and merging of convective storms into a linear convective band along the convergence line.This relatively long convective band presented a quasi-stationary state in the south of Mt.Ehuangzhang and Mt.Tianlu,which results in the first precipitation peak.At this stage,the convection developed to a higher level,with relatively larger raindrops,producing larger amounts of rainfall,which was probably related to the active merging of convection.In the later phase,as the environmental winds shifted,convective bands tended to move southeastwards,accompanied with the cold pools.At the same time,the multiple short convective bands were formed,which were almost parallel to the shear line,and a multiple-rain-bands organizational mode occurred.The mesoscale convergence line maintained due to the outflows of cold pools caused by precipitation in the preceding period,and then gradually moved southwards.Under the influence of the mesoscale convergence and topography,convection was continuously triggered at the southern end of the short convective bands.This back-building characteristic favored the development of the convective system.The multiple rain bands passed through the same place in a“rainband-training”form,resulting in the second peak of precipitation.The collision process was active in the low levels during this event.

Mesoscale Analysis of a Heavy Rainfall Event over Hong Kong During a Pre-rainy Season in South China

During the Heavy Rainfall Experiment in South China （HUAMEX） of 1998, a record heavy rainfall event occurred in the delta of the Pearl River during the 24 hours from 1200 UTC 8 June to 1200 UTC 9 June, 1998, and a 24-hour precipitation maximum of 574 mm was reported in Hong Kong. In this paper, some mesoscale characteristics of this heavy rainfall event are studied using data from satellites, Doppler radar, wind profilers, and automatic meteorological stations collected during HUAMEX. The following conclusions are drawn： （1） During this heavy rainfall event, there existed a favorable large-scale environment, that included a front with weak baroclinity in the heavy rain area and with an upward motion branch ahead of the front. （2） Unlike most extratropical or subtropical systems, the closed low in the geopotential height field does not exited. The obvious feature was that a southerly branch trough in the westerlies existed and Hong Kong was located ahead of the trough. （3） The rainfall areas were located in the warm sector ahead of the front, rather than in the frontal zone, which is one of the characteristics of heavy rainfalls during the pre-rainy season of South China. A southerly warm and moist current contributed to the heavy rainfall formation, including the transportation of rich water vapor and the creation of strong horizontal wind convergence. （4） The observations show that the heavy rainfall in Hong Kong was directly caused by a series of meso β systems rather than a mesoscale convective complex （MCC）. These meso β systems moved with the steering current in the lower-mid troposphere, their life cycles were 3-6 hours, and their horizontal sizes were 10-100 km. （5） The disturbances in the lower and mid troposphere, especially that in the planetary boundary layer （PBL） were very shallow. However, they are a possible trigger mechanism for the occurrence and development of the mesoscale convective systems and related heavy rainfalls. Finally, a conceptual model of the heavy rainfall in the warm sector ahead of the front in South China is proposed.