A very high-impact local extreme weather event – The devastating heavy-rainfall and tornado of 21July 2012 in Beijing,China

On 21 July 2012, an extreme weather event occurred in Beijing, China, which killed 81 people. The extreme of this event is two folds: 1) a very heavy rainfall last more than 10 hours in urban area with a record-breaking 24-h accumulated rainfall of 460mm, which is close to the annual mean rainfall of 420-660mm of Beijing; 2) a tornado, which has never been recorded in the meteorological history of Beijing, was triggered during the heavy rainfall event and caused 2 fatalities. The operational forecast and all operational numerical forecasting models substantially underestimated the maximum rainfall with huge timing error. This work examines these two extreme features of this high-impact local weather event.

The heavy rainfall was extremely extensive and intensive. Most area in Beijing was covered by a 24-h accumulated rainfall of > 100 mm and an hourly rainfall of > 50mm. The mean 24-h rainfall of all the stations in Beijing was 190 mm, which broke the historical record. Severe urban flooding and suburb landslide and mud flow affected about 16 million people and more than 20000 square hm crops. 525 flights were cancelled. The direct economic loss was about 11.6 billion RMB. The heavy rainfall consisted of two stages: warm-sector convective precipitation and squall lines immediately ahead of a front. Its extreme is examined from both climatology and weather aspects. This event happened in an environment with the co-existence of three major weather systems that usually cause extreme rainfall in China: tropical cyclone, front and mesoscale vortex. The complicated interaction among these three weather systems in the favorable juxtaposition of the quasi-stationary cold vortex and subtropical high with favorable topographical effect and extremely rich moisture supply (beyond 6-sigma of climatology) resulted in the quasi-stationary pre-frontal rainband featured of heavy-rainfall-producing backbuilding and train echo.

Besides the warm sector heavy rainfall, the favorable convective environment ahead of the front triggered another extreme weather phenomenon that ever happened in Beijing at 1330 LST: a tornado. Whether was the wind damages caused by a tornado or not was a hard question to answer at that time since: 1) some local people said they saw a funnel cloud while some said they did not; 2) there was no picture evidence of the cloud system; 3) even the meteorological bureau was not sure about what was happening. In that situation, the only way to prove whether it was a tornado or not is damage survey. The authors performed twice on-site detailed damage surveys, checked almost all pictures and videos available on the internet, and found two tale-telling signatures of a tornado: a narrow wind damage swath and surface convergent winds at several locations along the path.

Observational analyses show that this tornado was produced near the mesocyclone of a supercell that formed on the southern tip of a linear convective line. The super cell had an apparent hook-echo, a strong mesocyclone, and a bounded weak echo region. It developed in an environment that was very favorable for tornadic supercells in comparison to the climatological statistics performed by previous studies based on the cases in the U.S. Significant changes in the environment of Beijing occurred from 0800 LST to 1400 LST. 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. Cloud-resolving numerical experiments are then performed to explore the key differences between the two rawinsondes that may control the development of this tornadic supercell. Results show that wind profile was more important in determining the formation and morphology of the supercell while the temperature and moisture profiles were more important in keeping its intensity and longevity. The winds between 0-3km above the ground may have played a key role in the formation and development of the supercell that triggered the tornado.