Numerical Simulations of the Tornadic Supercell in the Heavy Rainfall Event of 21 July 2012 in Beijing

Tornadoes in Beijing are very rare and have never been formally reported in details in literature. According to our damage surveys, an F-1 tornado occurred in Zhangjiawan town in Beijing during the heavy rainfall event of 21 July 2012, which lasted for about 20 minutes and killed 2 people. The tornado was spawned in a supercell that formed at the southern tip of a northwest-southeast oriented convective line at around 1330 local standard time (LST). This work is to examine the environmental features of the tornadic supercell through high-resolution numerical simulation with the CM1 developed and maintained by George Bryan from NCAR.

The experiments use a stretched grid with evenly 100-m horizontal resolution in the inner area and smoothly extended to 700 m in the outer region. The vertical resolution was 100 m at the lowest 3 km and extended to 500 m at the top with a total height of 18 km. The coordinate moves at a constant speed defined by the average movement of the supercell. Idealized experiments were performed with a uniform background generated via real-world rawinsonde observations in Beijing. The supercell is triggered using an initial ellipsoid warm bubble of horizontal and vertical radius of 10 and 1.4 km, with a maximum temperature perturbation of 4 K at the center, decreasing to zero at its edges.

The rawinsonde observations at Beijing from 0800 LST to 1400 LST show that the environment became more favorable for the formation of a supercell. The convective available potential energy increased from 1136 J/kg to 2089 J/kg. The convective inhibition decreased from 47 J/kg to 3.9 J/kg. The 0-3km and 0-6km vertical shear increased from 7 and 13 m/s to 19 and 24 m/s, respectively. The veering curved shape of the hodograph became more typical for the supercell formation.

With the rawinsonde at 1400 LST as the uniform background, the evolution of the supercell was successfully simulated. After a storm splitting at around 50 min, the right mover became stronger and better organized with an apparent hook echo, a strong mesocyclone and bounded weak echo region. It moved to the northeast with an evolution of the morphology similar to the observation. When the rawinsonde of 0800 LST was used, however, though the supercell was still captured, its intensity, morphology, and movement apparently deviated from the observations with a much shorter duration.

A series of sensitivity experiments were then performed to explore the key differences between the two soundings that may control the development of this tornadic supercell through replacing certain variable of 14-LST sounding by that of 08-LST one. Results show that wind profile was the key factor in determining the formation and morphology of the supercell while the temperature and moisture profiles were more important in keeping its intensity and longevity. Winds between 0-3km above the ground were the most important, while the impact of the wind above 6km was trivial. Winds between 1.5-3km levels determined the morphology of the supercell, while 0-1.5km winds determined whether the left or right mover would become dominant in the splitting process.