Impacts of Meiyu Front upon the Initiation and Development of Mesoscale Convection Systems Producing Rainstorms

Aiming at the fact that the mesoscale convective systems develop on or in the south vicinity of the Meiyu front, lasting a long time and producing rainstorms along the Yangtze River valley, a series of three\|dimensional idealized numerical experiments are carried out to investigate the effect of moisture difference between the two sides of the Meiyu front, to reveal the impacts of basic flow and wind vertical shear upon the development of the mesoscale convective systems along the Meiyu front, and to show the convection organization by the Meiyu front. The results indicate that the mass imbalance resulting from moisture difference between the two sides of the Meiyu front can generate small-amplitude gravity waves, which may organize upward motions in the lower troposphere, trigger convection when coupled with diabatic heating process and lead to a convective rain band forming along the Meiyu front. The scale of the convective systems is usually 20－60 km. The convection rain band moves southward after its formation. The southward propagation of larger－amplitude gravity waves generated by convective perturbations and the triggering of new convective systems by the low－level backflow of dry downdraft to the north of convective rain band may play an important role in the southward movement of the convection rain band. The Meiyu front plays a certain role for the convection organization in its surrounding areas. The convective perturbations near the Meiyu front may develop into mesocale convective systems with a scale of 100－200 km. When the convective perturbation is affected by low-level northerly winds organized by the moisture front, a convergence band forms in its low level and its flow structure tilts in the vertical direction, which is beneficial to the maintenance and development of convective systems. The advective effect of basic flow plays a role in the movement of the Meiyu front, which can offset the southward movement of Meiyu frontal convective rain band itself. The moisture and energy transportation by it plays an important role in the initiation and development of mesoscale convective systems. The wind vertical shear resulting from wind direction variations is in favor of the convection initiation and mesoscale convective systems organization on the Meiyu front and influences the moving direction of the convective systems, while that from wind velocity variations seems to be not favorable for the convection initiation on the Meiyu front and the long-time maintenance of mesoscale convective systems.