The first two convective cells initiated in the early afternoon of that day were associated with pre-existing sea-breeze fronts. One cell was triggered at the intersection of two sea-breeze fronts at the apex of the Yangtze River Delta. The other cell initiated when the north sea-breeze front passed through the northeastern corner of the urban heat island. Both cells were located in the cold zone behind the sea-breeze front and were moving east-northeast under a south-southwest prevailing flow. The first supercell was triggered just at the intersection of the outflows generated by the downdrafts of the first two cells, which was the point where the curvature of gust front was the largest. This supercell produced heavy rainfall, strong winds, and large hail. After that, the outflow of the supercell merged with the sea-breeze front to become a larger bow-shaped gust front with a moving direction opposite to the ambient wind direction. In the area where the gust front passed, two supercells were triggered successively after the gust front passed through. Compared with the mean west-to-east flow, the series of supercells developed from east to west, similar to "back-propagation" or "back-building". In the end, this gust front collided with a thunderstorm outflow moving northeast out of Zhejiang Province into western Shanghai, triggering the last supercell with a 47.1m/s downburst. Radar velocity data showed there were strong low-level divergence features, as well as reports of large hailstones and suspected tornado vortices (a citizen video report). During the whole case, the convective cells that developed in the cold zone behind the gust front showed many characteristics of supercells and continued for a long time, while the convective cells that were initiated in the warm zone quickly weaken after entering the cold zone.
This case occurred just after the end of the Meiyu (monsoon) season. Under the control of the northern part of the subtropical high, the intensifying southwesterly flow produced increasing moisture advection, instability, vertical wind shear, and low-level relative helicity in Shanghai, which were the key environmental factors for this series of supercells. Under the sub-tropical high, the triangle-shaped coastline of the Yangtze River Delta usually causes the two sea-breeze fronts, one in northwest-southeast direction along the north coast, and the other in southwest-northwest direction along the south coast due to an enhanced land-sea thermal contrast in the late morning. On July 5, the two sea breeze fronts met at the apex of the Yangtze River Delta with large curvature, resulting in one of the earliest convective cells. Along the northern coast of the city, a sea-breeze front was approaching the central urban heat island in the early afternoon. After arriving at the northeast corner of the city, the sea-breeze front developed a sharp increase in the curvature due the blockage of the city. The hot air atop the heat island was lifted and one of the two earliest convective cells was initiated.
Radar data show that the series of supercells in this case exhibited hook echo, overhang, bounded weak echo region (BWER), and mesocyclone features. The three-body scattering spike (TBSS) and the large hail region had strong reflectivity (R) and weak differential reflectivity (ZDR). The strong ZDR column near the BWER reveals the existence of large raindrops in the mixed layer, providing abundant, super-cooled droplets for the formation of large hailstones.
Surface wind and wind profiler data also showed that the combined gust front formed by the merging of the sea-breeze front and supercells' outflow (east to northeast wind) was proceeding to the west. The low-level easterly wind lifted low-level warm air which in turn stimulated convection. One possible reason for convection initiation was that the cold pool enhanced the low-level flow dramatically, and Kelvin-Helmholtz instability might be available atop the stable flow (cold pool). The triggering mechanism continued to move westward and triggered convection constantly, resulting in the phenomenon of backward cell re-development (backward propagation) relative to the westerly steering flow. When those cells that developed in the warm area close to the gust front moved into the cold pool of the earlier thunderstorms behind the sea breeze front, they tended to decay very fast due to losing the water vapor and heat supply. The combined gust front moving westward also intensified the relative inflow of storms with the low-level environmental southwesterly air flow, which made the relative helicity of the storm significantly increase. More horizontal vorticity in the lower level turned into vertical vorticity with the ascending movement, which is the key factor of supercell formation. The critical conditions also made the convective cell initiated in the cold region live longer than those initiated in the warm region.
Based on this case, 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.
Keywords: supercell, convection initiation, sea-breeze front, urban heat island, dual-polarization radar