Multi-scale Structure Characteritics Of Return-flow Events Over North Of China

Using historical weather charts, conventional and non-conventional observational data, re-analysis data of NCEP, Meso-scale model output data ,etc, the return-flow events occurred over North China are documented. And many methods are uesd, such as, synoptic analysis, diagnosis, numerical simulation and dynamic theoretical research, to investigate the characteristics of the weather situation, the nature of return flow events occurring over North China and their Doppler radar characteristics. In the process of return flow events with heavy rain, the structure of Meso-scale weather system is also studied. Main conclusions are:In order to estimate the nature of return flow in North China , NCEP re-analysis data, the precipitable water vapor (PWV) data from GPS/MET, wind and moisture data observed by 255m meteorological tower located in Tianjin are analyzed. The results show that cold air coming from North-East Plain, though the Bohai Sea, is also relatively dry. The sensitive tests of numerical simulation for water vapor prove that return flow is dry and cold. The moisture comes from South part with south-west air flow. Contrast with easterly wind the speed of south-west air flow play an important role on the precipitation in return-flow events. And the lower level Northeast airflow acts as a cold wedge. Taihang mountain is in favor of rainfall.Based on 4 cases Doppler weather radar data of return-flow events, the radar echo characteristics are studied by base reflectivity, base velocity and VAD wind profile etc. After the cold advection appeared in low level at VAD wind profile, the heavy precipitation occurs. The Adverse Wind Region is found in rain storm. And the AWR nearly overlap on the center of heavy rain . Cold advection and AWR are valuable in nowcasting of rain storm.An analysis on the mesoscale feathers of a return flow events with rain storm over North China on October 10th—12th ,2003 is performed by using Automatic Weather Station data, NCEP re-analysis data and Doppler radar images. It shows that the new high time-space resolution observation data can well exhibit the signature of the event.the results show that the mesoscale rain clusters move with mesoccale low and shear.And these mesoscale systems are found on the convergence belt forming between warm wet southwesterly and cold dry return flow.Summing up a concept model for return flow events occurring over North China. The trough located in the north of 40°N on 500 hPa map moves eastward fastly, causes a large numbers of low-level cold air southward, the cold air enter to North China Plain through the Bohai Sea, and in the surface acts as a cold front. The warm and wet air before an eastward upper trough from Hexi creep up on the Wedge of cold air, and produce the heavy rainfall. And in the return flow events with rain-storm , the upper and low jet exits. Mesoscale systems are form in the convergence belt which warm and wet southwesterly meet the cold and dry return flow.