Research On The Relationship Among Tropical Cyclone,Subtropical High And Predecessor Rain Events

The interactions between the tropical cyclone (TC), subtropical high (SH) and predecessor rain events (PREs) can be categorized into four patterns according to the distribution characteristics of divergence field, non-rotational wind field and the divergent wind vertical circulation among tropical cyclone (TC), subtropical high (SH) and PREs for the 45 events during June, July and August of 2000-2009. The first pattern is the most important in percentage (59% of the events), and a conceptual model is provided for this pattern. The primary characteristics of the model is as follows:TC, SH and the PREs can interact with each other through the divergent wind secondary circulation at both sides of the ridge line of SH (between SH and TC, and between SH and PREs). At high levels (150 hPa or 200 hPa), the northward non-rotational winds from the TC converge towards the SH ridge line and subside, and at 950 hPa the divergent winds from the ridge line of SH converge to TC, these construct the secondary circulation between TC and SH. The southward non-rotational winds at high levels (150 hPa or 200 hPa) from PREs and the divergent winds at 950 hPa from SH ridge line to PREs construct the secondary circulation between PREs and SH. As the bridge of the interactions between TC and PREs, the SH ridge line usually locates at the zone of the downward flow in the whole layers. The numerical simulation (PRE associated with tropical cyclone numbered "0704") further confirms the interactions among TC, SH and PREs, too. The main difference between the first pattern and the other three patterns depends on the different distributions of the divergent wind secondary circulations between TC and PREs. The secondary circulation associated with PREs mainly occurs at middle and lower-level in the second type (named mid-to low-level interaction,15.3% of all), and the interactions also occurs at middle and lower-level. While in pattern III (named mid-to high-level interaction,8.3% of all), the interaction between TC and PREs mainly occurs from mid-to high-levels. In pattern IV (named TC-on-PREs interaction), there is no significant secondary circulation exit at the south of the PREs and there is only one secondary circulation associated with TC between PREs and TC. In addition, the statistical analyses indicate that the average distance between TC and PREs for Pattern I and Ⅱ is approximately 15° longitude/latitude, the distance between TC and the divergent center near SH ridge line on the 950-hPa surface is approximately 9.4° longitude/latitude, and the distance between the PREs and the divergent center is approximately 5.6° longitude/latitude. The distance between PREs and divergent center is smaller than that between TC and divergent center.Based on the research of the ideal tests (the intensity of the TC in the different initial fields is distinct), it found that the ideal tests simulate the PREs and the interaction relationship among the three entities successfully. The stronger TC in the initial field triggers the stronger quasi-stationary Rossby wave. The positive phase of the wave exactly locates at the east of the mid-latitude cyclone at low levels, the eastward move speed of the cyclone slow down and the intensity also weakens because of the positive phase of the wave. But the subtropical high will be strengthened when the positive phase overlap the subtropical high. Then the divergent wind near the subtropical high ridge line will be strengthened and have impact on TC and PREs. On the whole, the intensity of TC in the initial field have smaller effect on PREs.On the other hand, through the numerical simulation it also found that as the convective heat source on the ocean, TC can trigger the quasi-stationary Rossby wave propagating toward northeast. It is unfavourable for the subtropical high and divergent wind at low levels strengthening when the subtropical high ridge line locates in the negative phase of the wave (negative geopotential height difference); when the ridge line moves into the positive geopotential height difference area, the positive phase of the wave strengthens the ridge line and make the downward flow and the mass accumulation strengthen, then the divergent winds near the ridge line strengthen and flow toward north and south, this is favourable for PREs. In addition, the numerical simulation also indicates that when the divergent wind secondary circulation north of the ridge line strengthens, the PRE will strengthen too. On the contrary, the PRE weakens. If the secondary circulation is weak, but the water vapor channel from TC is strong, the PRE can also strengthen. Therefore, the PREs on the mid-latitude area are influenced by both the secondary circulation and water vapor channel from TC.The prediction significance of the interaction model has been studied in this paper. It found that TC, SH, and PREs construct an "S" along longitude direction at low level streamline field for about 90% times. This indicates that an "S" shape streamline field is favourable to PREs. On the other hand, according to the real data analysis, if SH is too strong, it will block the moisture and energy transported to mid-latitude westerly trough and the water vapor channel will not form and PRE will not start. In addition, the further research results show that the divergence field and the non-rotational wind field at 950-hPa have shown the distribution features like pattern I and II before the PREs start, that is, the non-rotational wind field of TC, SH, and PREs at 950-hPa is corresponding to convergence, divergence, and convergence, respectively, and the divergence fields is corresponding to negative, positive, and negative, respectively. These imply that the rainstorm will start later.