A study of the permanently elongated convective system during the heavy torrential rain event over Jinan on 18 July 2007

Using the conventional observations, the satellite infrared and visible cloud images, the Doppler weather radar data, and the meso-scale automatic weather station data in the Jinan city, we investigate the formation and development processes of a permanently elongated convective system (PECS) that leads to severe rainfall over the southern part of north China on 18-19 July 2007. The results indicate that the formation and development of the PECS was associated with the intrusion of the low-level cold air mass from northeast China and the outward flows from the rear of the weakening meso-scale convective complex. The invaded cold air mass behind a cold front and the southwesterly warm/wet flow in front of the cold front strengthened the convergence of low-level wind over the southern part and triggered the local frontogenesis. The interactions of the frontogenesis with the outward flow from a weakened MCC produced the PECS. The analysis of cloud images shows that the PECS was a mesoαscale convective system with a long life, persisting for 18 h from its formation to extinction. This PECS was successively composed of 10 convection cells, in which a combination of two αscale convective systems resulted in the severe rainfall at the Shanghe county in the north of the Jinan city. The severe rainfall occurred at the developing stage of the PECS and in the rear of PECS, corresponding to a cold cloud lid with a temperature of lower than 201 K. At the same time, a narrow cloud line (also called the squall front) that contained 4 γscale convective cells was seen on visible cloud images. Of them, one cell produced the radar echo up to the height of 13 km above the ground surface. The PECS is accompanied by the formation and development of the meso-scale systems associated with the squall that contain a thunderstorm high, a wake low, and a squall front. A mesoγ scale thunderstorm high occurred behind the cold front with the emergence of the cloud line and then developed into the meso α scale thunderstorm complex, with rainfall appearing between the cold front and the thunderstorm high. The severe rainfall at the Shanghe county began with the emergence of the thunderstorm high and terminated with the emergence of the wake low. Differing from the severe rainfall in the Shanghe county, the severe rainfall in the Jinan city occurred when two overshooting cloud tops in a meso-scale cloud cluster merged into one stronger overshooting cloud top. The rainy echo associated with the development of the PECS showed the features of rear extending and intersecting. The Shanghe heavy rainfall was produced by a main cold intrusion associated with thunderstorm high, while the Jinan heavy rainfall was possibly with the formation and development of two β scale convective cells and one γ scale convective cell in a multi cell thunderstorm that was produced by the combination of a main outflow surge and a sub-outflow surge.