Mechanism Analysis and Numerical Simulations of a Series of Back-Building Supercells

After a monsoon rain band had a northward shift, a series of supercells producing heavy rain, strong winds, and large hail occurred in the Yangtze River Delta (YRD) of China on 5 July 2017. Supercells were triggered from east to west, resulting in a phenomenon of convection backward propagation or back-building relative to the westerly steering flow. Convection initiation and propagation mechanisms are analyzed by using observational analyses and numerical simulations.

Under the control of the northern part of the subtropical high after the monsoon rain band moved northward, the intensifying southwesterly flow produced increasing moisture advection, instability, vertical wind shear, and low-level relative helicity.

Convective imitation (CI) analysis shows that the first two cells were triggered by the intersection of two sea-breeze fronts at the apex of the YRD and by the north sea-breeze front passed through the northeastern corner of the urban heat island (UHI) of the Shanghai city, respectively. Then, first supercell developed due to the collision of two cells’ outflows. After that, a combined gust front merged by the sea-breeze and the outflows of former cells was proceeding to the west, and lifted low-level environmental warm-moist air. As a result, new supercells were stimulated after the gust front passed from east to west.

High resolution Cloud Model (CM1) idealized simulations were set up by changing low-level wind, temperature, and humidity before, when, and after sea-breeze front and thunderstorm gust front passing, using automatic weather station and wind profiler data. As a control run without any sea-breeze front or gust front, only ordinary cell was simulated. When sounding data with the same low-level thermal profile but a wind profile after sea-breeze front passing, a short-lived supercell was found in the simulation, revealing that the ambient low-level easterly flows played an important role in supercell development by intensifying relative inflow and storm-relative helicity significantly. When the idealize simulation’s sounding data using the wind profile while strong gust front merged with sea-breeze front, two separate supercells appeared simultaneously in the simulation. More horizontal vorticity in the lower level turned into vertical vorticity with the ascending movement, which is the key factor of long-lived supercell formation. This critical conditions made the convective cells initiating in the cold region live longer than those in the warm region. CM1 simulations were also made by changing low level thermal structure using surface observations. As low-level temperature increased due to an increasing UHI effect in the afternoon, stronger cold pool and enhanced low-level flow were observed in the simulation. As a result, Kelvin-Helmholtz waves atop the stable flow (cold pool) were recognized, which might be one of possible reasons for new cell initiation in the cold region.

A conceptual model is given for the evolution of the boundary layer temperature and humidity profile and the change of convective available potential energy (CAPE) before and after thunderstorm outflow overtakes a sea-breeze front.