The Configuration Change and Train Effect Mechanism of Multi-Cell Storms

Several multi-cell storm systems occurring in the Beijing area have been investigated by using the low-level thermal and dynamical retrieval system of four-dimensional variational assimilation (4Dvar) based on the Variational Doppler Radar Analysis System (VDRAS). This paper examines the physical mechanism of configuration processes, intensity changes, and train-effect phenomena that occur during the transmission of linear multi-cell storms. The following results are reported: (1) In a multi-cell storm system in which the transmission direction of the cells is constant with a convective band such as a squall line, the configuration changes such that linear reflectivity becomes arched, and intensity changes of the cells are related to interaction of cells’ transmitting process and environmental low-level air flow. If the frontal inflow is a warm and moist air stream, and stronger vapor convergence is present, the cell flourishes more easily and transmits faster; otherwise, the cell weakens and transmits slowly. Therefore, in squall-line system nowcasting, the intersection of the multi-cell transmitting direction and the environmental vapor convergent line should be closely followed because the cell will travel faster, and the most severe effects of convection will occur. (2) In a multi-cell system that shows characteristics of a train effect, the cell’s transmitting feature and developing environment differ almost completely from those of the multi-cell previously mentioned. Train-effect phenomena generally occur in low-level warm and moist air streams or neighboring low-level jets in unstable ambient atmospheric conditions. The transmitting mechanism of the storm cell can be connected with the propagation and stimulation of an inertial gravity wave such that the warm and moist air stream or low-level jet is a transmitting band for warm vapor advection. When inertial gravity wave propagates from high to lowθ se , the wave receives energy from the ambient atmosphere and develops constantly. Therefore, during storm cell transmission, cells gradually enhance and become arranged in a row to produce the torrential rain center in the leading end of the wave train.