Strong winds are usually caused by downbursts and/or tornadoes. Which of these two systems caused the Zhangjiawan wind damage was a really difficult question to answer. Tornadoes have never been recorded in the meteorological history in Beijing, while vision evidences, however, showed conflict results: Some local people said they saw a funnel cloud while some said they did not. There are no weather stations in that area. Though we are in a world of tons of camera phones, no one took a single picture of the cloud system. In this situation, the only way to prove whether it was a tornado or not is damage survey.
We performed twice on-site damage surveys on Aug.3 and Aug. 24, talked to 20 local residences, farmers, factory workers, and village committee members, checked almost all pictures and videos available on the internet to try to locate the damage swath and the falling directions of the trees and electricity poles in order to rebuild the surface wind when the strong wind occurred. Through careful examination, we found two tale-telling signatures of a tornado: a narrow wind damage swath (10-km long 100-500-m wide) and surface convergent winds at several locations in the developing, mature and dissipating stages of the wind system. These two signatures demonstrate that the wind damage was caused by a tornado with a scale of F-1 according to Fujita scale classification. This is the first detailed tornado damage survey in China.
The environment of the tornado was then examined based on the NCEP analyses, rawinsonde, and radar observations. Result shows that this tornado was produced near the mesocyclone of a supercell that formed on the southern tip of a northwest-southeast oriented linear convective line in front of a mid-latitude trough. An S-band radar 30-km away from the supercell clearly showed a typical morphology of supercell including hook-echo, a strong mesocyclone, and a bounded weak echo region. Radiosondes show that the environment was very favorable for the development of a supercell at 14 LST with the convective available potential energy of 2089 J/kg, the convective inhibition of 3.9 J/kg, the 0-3 km and 0-6 km vertical wind shear of 19 and 24 m/s. The key factors in the development of the supercell will be presented in another presentation of this conference by Dan Yao based on numerical simulations with CM1.