Effects of Synoptic Scale and Sub-synoptic Scale Systems on Formation of a Heavy Rainfall Process

By means of the mesoscale numerical model MM5V2, the heavy rainfall process during the later June of 1998 in the middle and lower reaches of the Yangtze River is analyzed. By utilizing the scale-separating method and comparing the results of numerical experiments, the characteristics of physical elements of synoptic scale and sub-synoptic scale systems are investigated. The physical mechanisms of heavy rainfall formation are presented as follows: synoptic scale wind and water vapor fields provided long-distance water vapor transportation for the heavy rainfall while the sub-synoptic scale wind field results in strong meridional convergence which provided the steady dynamic condition for the lifting and condensation of water vapor; under the favorable disposition of upper and low level jets, the rainfall region was located to south of the upper level jet and north of the low level jet; the unstable thermal stratification, cooler at lower level and warmer at upper level, of the sub-synoptic scale temperature field facilitated the development of the thermal unstable condition and further strengthened the heavy rainfall; the sub-synoptic scale high-value center of the specific humidity field over the rainfall area was favorable to the emission of latent heat and further enhanced the feedback mechanism of the heavy rainfall. In numerical experiments, message separation was conducted on the systems with different scales in the model initial field to indicate their dynamic effects in the heavy rainfall process. The results show that both intensity and coverage of heavy rainfall will decrease to some extent while there is only a synoptic scale or sub-synoptic scale system during rainfall process. Analyses and results of numerical experiments suggest that heavy rainfall can form and sustain under the comprehensive effects of both synoptic scale and sub-synoptic scale systems.