The Heavy Rainfall Event Leading to the Large Debris Flow at Zhouqu

Heavy rainfall occurs abruptly at Zhouqu, Gansu Province at night of 7 August 2010, causing disastrous debris flow and bringing about more than a thousand casualties. To find out the possible triggering mechanism of this rainfall, observations by Automatic Meteorological Stations are used to analyze temporal variation of the surface temperature and spatial distribution of rainfall; brightness temperature data from MTSAT satellite are adopted to reveal evolution of convective clouds; NCEP/NCAR 1°×1° reanalysis data are used to investigate the large-scale atmospheric conditions; AIRS satellite observations are examined to analyze the atmospheric instability; and ECMWF 0.125°×0.125° forecast data are employed to study the convection. First, over Zhuoqu and its upstream (northwest) region, the rapid increase of surface air temperature and the cold air advection in the rear area of the upper-level trough significantly enhanced the conditional instability in the morning of 7 August, favoring formation and development of deep convection. Second, several small-scale convergence centers and lines at the ground surface, generated by interactions among the southerly warm and northerly cold air flow near the ground surface and the complex terrain elevation, triggered the formation of the precipitating convective clouds around 14:00 7 August 2010 (Beijing Time). Third, the southerly air flow between the strong Northwest Pacific Subtropical High and the typhoon "Dianmu" changed to easterly at 23°—30°N, transporting water vapor toward the west until reaching the eastern side of Tibetan Plateau, and then changed to northward, supplying abundant moisture for the raining storm over Zhouqu and its upstream region. At last, the convective clouds moved toward southeast following the upper-level air flow, arrived at Zhouqu and produced heavy rainfall at night of 7 August, leading to the large debris flow at Zhouqu.Satellite remote sensing observations play an important role in the diagnosis of this synoptic process. The infrared brightness temperature (TBB) from the MTSAT satellite reveals the occurrence, development, movement and weakening of the convective clouds which directly produced the heavy rainfall at Zhouqu. The air column temperature and moisture data observed by the AIRS satellite around 14:30 7 August 2010 are used to analyze convective available potential energy (CAPE) and level of neutral buoyancy (LNB) height. The results indicate that atmosphere over Zhouqu—Qinghai Lake region is strongly unstable with the area-averaged CAPE of 4393 J·kg^-1 and LNB height of 16.54 km.