A Study Of The Trigger Mechanism Of A Local Convection Associated With A Record-breaking Rainfall Event Occurring In Guangzhou

The triggering and convection evolution of local and abrupt warm-sector rainfall under weak forcing background is still unclear,which poses a formidable challenge to quantitative precipitation forecast.In this paper,a warm sector heavy rainfall occurred in Guangzhou is selected as a typical example.Through observation analysis,numerical simulation and sensitivity test,mesoscale characteristics and mechanism analysis of the trigger and the evolution of the mesoscale convective systems are illustrated as follow: First of all,the sudden enhancement of the southeast flow caused by the low-level anticyclone in East China moves north in Guangzhou.A convergence and lifting zone formed on its leading edge of the southeast flow promotes initial convection.Besides,the combination of convergence zone and the lifting force caused by Huadu terrain allows severe convection to be triggered.And the heating effect of urban heat island on southeast airflow and the blocking and accumulation of the warm and humid airflow in the southeast of Huadu terrain promotes the development of convective available potential energy(CAPE)and thus creates favorable conditions for severe convection.Furthermore,the uplift caused by the northly outflow boundary and the southeasterly warm moist inflow generates vertical ascending movement and therefore peaks the convection.Moreover,the results of the sensitivity experiment on observational nudging thermodynamic local characteristics of Guangzhou indicate that the near-surface southeast flow below 200 meters along with its water vapor and instability energy can be decisive in the development of the uplifting mechanism,and the occurrence and evolution of convection.The heating of the urban heat island area in Guangzhou on near-surface southeast flow plays a significant role in the rapid CAPE development of the Huadu region.The water vapor convergence triggered by convection is attributed to the sudden intensified southeast flow,whereas the thermodynamic local characteristics of Guangzhou have limited influence on the convergence.