Multi-scale Features In The Persistent Heavy Rainfall Events Occurring In The Middle And Lower Reaches Of Yangtze River

The persistent heavy rainfall (PHR) occurring in the middle and lower reaches of Yangtze River (MLYR) is a kind of typical extreme rainfall event which is caused by complex multi-scale processes, with the features of great intensity, wide extent and long duration. The forecast for it is still a difficult point. To further understand the mechanism for its occurrence and development, two PHR events in the MLYR occurring in June 1982 and 1998 are studied and compared.Firstly, the stable circulations favorable for the PHR events and contributions of different scale oscillations are analyzed. The result indicates that, although the climatic background fields are same in the two PHR events, the favorable circulations and their formation processes are both different. During the 1982 PHR event, a strong trough located around the Lake Baikal transports cold and dry air from the high latitude regions to the MLYR, and the intensified and westward shifted Western Pacific Subtropical High (WPSH) conveys more moisture to the MLYR than in normal conditions. Anomalous anticyclones associated with both the 30-60day and 10-30day oscillations reaching the South China Sea (SCS)-Philippine Sea region can strengthen the WPSH significantly. The 10-30day oscillation also contributes greatly to the formation of the Baikal trough. For the 1998 PHR event, there are two troughs occurring in the Baikal region and East China respectively, and the WPSH is also stronger and more westward than normal. It is the 60day low-pass perturbation and 30-60day oscillation that lead to formation of the favorable circulations, which is different from the 1982 case. The 60day low-pass perturbation demonstrates positive impact for both the Baikal trough and the WPSH’s western extension. And the 30-60day oscillation enhances the trough in East China with an anomalous cyclone and WPSH when the anticyclone arrives the SCS-Philippine Sea region. The impacts of different-scale oscillations in the high and low latitudes on the PHR events are further investigated using the WRF model. It’s shown that, the 30-60day oscillation demonstrates positive impact on the both PHR events mainly through the low latitude anomalous circulations.In addition, the conceptual model is given for the evolution process of the 30-60day oscillation, indicating that, the 30-60day oscillation at low latitude in the both PHR events plays an important role for formation of the anomalous anti-cyclone in the SCS-Philippine Sea region, but its evolution process presents different features in the two cases. Before the PHR event in 1982, suppressive convection anomaly associated with the 30-60day oscillation arises in the equatorial West Indian Ocean and propagates eastward. It breaks into two parts when arriving at the East Indian Ocean, one of which moves northward and another continues propagating eastward until it merges with the convection anomaly from the East Pacific. Then the merged convection anomaly moves northwestward to the SCS-Philippine Sea region and the anti-cyclonic anomaly associated with it leads the Subtropical High to extend westward which is in favor of the PHR. Thereafter it gets to the landmass and decays. Before the PHR in 1998, suppressive convection anomaly born in the equatorial West Indian Ocean and the equatorial West Pacific Ocean moves to the northeast and northwest simultaneously and merges in the area of Bay of Bengal to SCS. Then the merged convection anomaly propagates northeastward and the anomalous anti-cyclone accompanied with it helps to transport moisture from the Pacific to the MLYR. Meanwhile, the anomalous convection from the north part of the Pacific moves southwestward and merges with the one from the SCS in the area to the east of the Japan.Since the low latitude systems of the 30-60day Intra-Seasonal Oscillation (ISO) influence the PHT events significantly, the propagation mechanism of it in the Western North Pacific (WNP) is investigated. The propagation of ISO is caused by the low-level convergence and moisture accumulation resulted from the anomalous cyclone-anticyclone wave train. And the propagation mechanism of the anomalous positive vorticity is further investigated, indicating that, development of the anomalous vorticity is mainly owe to the following dynamic factors:anomalous vorticity advection by the large-scale wind field, large-scale relative vorticity and planetary vorticity advection by the anomalous wind and the interaction between the anomalous convergence (or divergence) and the planetary vorticity. For the northwestward propagation of the anomalous vorticity in 1982, both the anomalous vorticity advection by the large-scale wind field and planetary vorticity advection by the anomalous wind generate positive vorticity tendency to the northwest of the cyclonic anomaly. Convergence effect also makes some contribution to its development in the SCS region, after which, divergence makes the positive vorticity located to the east of Japan decay. In 1998, during the early stage of the anomalous vorticity’s northwestward propagation, it is the large-scale wind that transports the anomalous vorticity to the northwest. But during the later stage, convergence of the anomalous wind field enhances the vorticity in the SCS, resulting in propagation of the anomalous vorticity to this region. And when it reaches the SCS, convergence over the region to the east of Japan leads it propagate there. And then the anti-cyclonic circulation spreading from the SCS to Japan advects large-scale positive vorticity and planetary vorticity from the higher latitude to its west, making a westward propagation.