Radar Analysis and Numerical Simulation of Strong Convective Weather for "Oriental Star" Depression

At about 2132 BT 1 June 2015, "Oriental Star" cruise from Nanjing for Chongqing suffers severe stormy weather when sailing in the Yangtze River near Jianli County, resulting in 442 people killed. The investigation team finds that the occurrence of the catastrophic accident is caused by a sudden downburst of squall line weather. Using ARPS (the Advanced Regional Prediction System) model and assimilation of conventional data and four-radar data in the surrounding area, the severe weather process is simulated. Combined with the high-resolution radar observation, the structure and strength of the squall line are analyzed synthetically.The atmosphere of dry ambient at low level and humid ambient at middle level is favorable for the occurrence of convective weather phenomena. Radar observations show that the squall line convective system is northeast-southwest direction, and moves fast to southeast, the life duration is about 6 hours, and the squall line transit lasts about an hour. During the evolution of the squall line, convections developed to their strongest at the intensifying stage, when the strong convection region reached its maximum. At its mature stage, gust front appears in front of ground thunderstorms, and the horizontal scale of the squall line system reaches the upper limit. The instability of the stratification and the flat terrain of the Jianghan Plain are important causes for the strong convective activity and the downburst phenomenon.The numerical simulation shows that the high-speed wind area above the ground, the position of instantaneous maximum speed of horizontal and vertical wind, cumulative rainfall maximum center, the composite reflectivity high value area of 200 m resolution show a zonal distribution to uniform, which coordinates with the time and spatial distribution of the accident. Influenced by the direct impact of the down burst, intensity of thunderstorm near the accident point increases rapidly, and a narrow gust wind appears near the ground where wind shear increased significantly. Ship wreck is affected by above 10 m·s^-1 downdraft and above 18 m·s^-1 strong westerly wind from 2132 BT to 2134 BT. The precipitation intensity begins to increase at 2131 BT, and the center of heavy precipitation is located just above the accident spot from 2131 BT to 2135 BT, with a maximum precipitation of more than 10 mm per minute. A gust wind makes great contribution to the transportation of momentum from the middle and low levels to the land surface, accompanied by raindrops of drag and the sinking air flow, enhancing the speed of the wind further.