Information about the storm

Cause analysis of rainstorm in Jiangxi caused by tropical storm "Beimian" 12

Jiangxi province environment forecast center Nanchang, Jiangxi

Tropical storm 12 in 2002 was generated in the south China sea at 08: 00 on August 4, and its preparation began. After its formation, it moved to the north at a speed of about 25 km/h and developed into a strong tropical storm at 02: 00 on the 5th. At 06: 0015 on the 5th, it landed on the coast of Jieshiwan, Shanwei City, Guangdong Province, and continued to move to the north after landing. On the 5th 14, it weakened into a tropical storm; It weakened to low pressure at 23: 00 on the 5th. Among them, it entered Anyuan county in the south of our province at 20: 00 on the 5 th, and then continued northward until it weakened in Fuzhou area of our province.

Tropical storm 12 originated in the South China Sea and entered our province northward. Although the intensity is not too strong, it caused three days of heavy rain in our province and heavy rain in some areas.

1 path analysis

Through analysis, the forecast focuses of tropical storm 12' s northward path are as follows: the subtropical high in North China weakened obviously and retreated westward; the southerly wind on the east side of tropical storm increased after the western Pacific subtropical high extended westward; South China changed from a positive high pressure area to a negative high pressure area; and South China changed from a negative vorticity area to a positive vorticity area.

1. 1 The subtropical high in North China weakened obviously and retreated westward.

We know that the path movement of tropical cyclones is mainly influenced by the circulation situation of environmental field, and the most important factor is the position and evolution trend of subtropical high. The influence of the evolution of subtropical high on the tropical storm 12 path is analyzed below.

On August 1 ~ 2, the strong western Pacific subtropical high extended to North China, with the main body stronger than 37 N, and there were 1 592 high pressure centers near Shandong Province. There is a low-value system brewing in the easterly airflow at the southern edge of the South China Sea subtropical high. By 08: 00 on the 3rd, low pressure 584 was formed. At 08: 00 on the 4th, it developed into a tropical storm with a central pressure of 990 hectopascals and a maximum wind speed of 20m/s near the center. On June 5438+0 ~ 2, there was a 1 deep low-pressure area in the northwest of Lake Baikal on the north side of the subtropical high, and the bottom of the low-pressure trough extended to 50 N, with a concentrated westerly belt from the trough bottom to the north edge of the subtropical high (Figure 1). At 20: 00 on the 3rd, the trough separated from the high and low pressure area moved to the southeast, and the westerly belt also moved to the south. The temperature at the northern edge of the subtropical high dropped by 2 ~ 4℃ and the potential dropped by 2 ~ 4 meters, forcing the subtropical high to weaken, the main body to contract and retreat to the south and west. With the deepening of the trough, from 20: 00 on the 4th to 08: 00 on the 5th, the line 588 of the subtropical high in North China weakened obviously, narrowed and retreated westward (Figure 2). At the same time, another 1 subtropical high in the western Pacific extends westward, and its western boundary reaches 128 E at 08588 hours on the 5th. In this way, a saddle-shaped field is formed in East China, which is very conducive to the northward movement of tropical storms. Therefore, the northward movement of tropical storm 12 is mainly due to the obvious weakening of the subtropical high in North China and its westward withdrawal.

1.2 After the western Pacific subtropical high extended westward, the southerly wind on the east side of the tropical storm strengthened.

As mentioned earlier, another subtropical high 1 extended westward in the western Pacific from 4th to 5th, and the western boundary of line 588 extended to 128 E, forming a "square head" (Figure 2). This strengthens the pressure gradient between tropical storm and subtropical high, thus strengthening the southerly wind, which is conducive to guiding tropical storm northward. From the wind fields of 500 HPA, 700 HPA and 850 HPA, the wind speed of southwest wind and southeast wind on the east side of tropical storm from April to May is obviously higher than that of northeast wind and northwest wind on the west side of tropical storm. The wind speed of southwest wind and southeast wind on the east side of 500 hectopascal tropical storm is 8 ~ 12 m/s, and 4 ~ 6 m/s on the west side; East side of 700 hectopascals14 ~ 20m/s and west side of 4 ~ 8m/s; At 850 hPa, the east side is14 ~ 20m/s and the west side is 2 ~ 6m/s, which indicates that the southerly wind on the east side of the tropical storm is greater than the northerly wind on the west side, which is conducive to guiding the tropical storm northward.

1.3 South China changed from a positive altitude area to a negative altitude area.

In South China, the whole province is still getting higher from 20: 00 on the 2nd to 08: 00 on the 3rd. From 08: 00 on the 4 th to 20: 00 on the 4 th, Guangdong turned to the south of our province; From 08: 00 on the 5th to 08: 00 on the 4th, the potential negative altitude zone of Shmi runs from Guangdong to our province in the north-south direction, which is also a sign of tropical storm going northward.

1.4 South China changed from negative vorticity region to positive vorticity region.

From the vorticity field of 500 hPa (all physical quantities in this paper come from the analysis field of T2 13), it can be seen that at 20: 00 on the 3rd, South China was a negative vorticity region, which turned into a positive vorticity region at 20: 00 on the 4th, and the positive vorticity region extended from Guangdong to the north of our province at 20: 00 on the 5th (Figure 3). According to the trend of typhoon moving to the positive vorticity region, this vorticity field is also changing.

Although the intensity of tropical storm 12 was not very strong, it caused heavy rain for three consecutive days in our province and there were heavy rains in some areas.

The rainstorm process caused by tropical storm 12 can be divided into three stages: (1) After the tropical storm entered Anyuan County in the south of our province at 20: 00 on the 5th, it rained heavily in southern Jiangxi from 20: 00 on the 5th to 20: 00 on the 6th, that is, near the center of the tropical storm. (2) The tropical storm moved to Fuzhou on the evening of 6th, but the rainstorm area was mainly in northern Jiangxi, that is, the northern part of tropical depression from 20: 00 on 6th to 20: 00 on 7th. (3) From 20: 00 on the 7th to 20: 00 on the 8th, the rainstorm area once again fell in the central part of Jiangxi, with a northeast and southwest trend. From 20: 00 on the 8th to 20: 00 on the 9th, the precipitation weakened, and there were 1 rainstorm areas in northern Jiangxi and southern Jiangxi respectively.

According to the division of the above three stages and the comprehensive analysis of the weather situation in each stage, we can find that:

(1) At 20 o'clock on the 5th, the tropical depression entered the south of our province. Judging from the wind field in the middle and lower layers at that time, the circulation around the tropical depression was relatively uniform, and there was no shear in all directions (figure omitted). Judging from the divergence field, the convergence center of 850 HPA-2/kloc-0 /×10-5s-1at 20: 00 on the 5th is in the south of our province. According to the vorticity field, the vorticity center of 850 HPA+16×10-5s-1at 20: 00 on the 5th is at the junction of the south of our province and Fujian (Figure 3); From the divergence field of water vapor flux, Gannan also has 1 convergence center-3×10-8g cm-2hpa-1s-1. Therefore, the rainstorm area in this period was in southern Jiangxi.

(2) As the tropical depression moved northward into Fuzhou, 1 weak cold air invaded the northwest periphery of the tropical depression, making the northwest wind direction of the tropical depression change from northeast wind to northeast wind, and the southeast wind of the northeast tropical depression changed to southeast wind, which caused obvious typhoon shear in the northern part of our province. At 08: 00 on the 7th, the southeast wind on the east side of the tropical depression continued to turn to southwest wind, and the wind speed increased. At this time, the weak cold air in the north continued to supplement, which strengthened the shear convergence (Figure 5). From the divergence field, the convergence center of 850 HPA-20×10-5s-1moved to northern Jiangxi at 20: 00 on the 6th. Judging from the divergence field of water vapor flux, the convergence center is also in the north-central Jiangxi. Judging from the divergence field of 200 hectopascals, it is a divergence area in the north-central part of our province from 20: 00 on the 6th to 20: 00 on the 7th, and the divergence center value is strengthened, and the upper layer's suction is beneficial to strengthen the upward movement. In the same period, the rainstorm area was in northern Jiangxi, and its intensity was obviously strengthened. Heavy rain 18 station, and there are also heavy rains in Xing Zi, Lushan and Hukou. Of course, the rainstorm in Lushan Mountain is also related to the uplift of the terrain.

(3) From July to August, the tropical depression itself tends to weaken, but the weak cold air in the north continues to invade, and the southwest side of the tropical depression is still a strong southwest airflow, which makes our province maintain a strong convergent northeast-southwest shear at 850~500 hPa, but its position is pressed to the central part of Jiangxi. The convergence center of 850 hPa divergence field and water vapor flux divergence field also moved southward to central Jiangxi, and the rainstorm area was in central Jiangxi during this period, and the rain belt was very consistent with the trend of northeast-southwest shear.

Meteorologist Li Chongyin pointed out that when the weak cold air affects the periphery of the typhoon, it is beneficial to strengthen the typhoon precipitation, and a rainstorm area will be generated at the junction of the weak cold air and the typhoon. This tropical low-pressure rainstorm process also shows this feature. In other words, the strengthening of the rainstorm on July 8 is closely related to the influence of cold air. According to the θse field of 850 HPA, there are 1 θse dense fronts in 29 ~ 32 N from northwest Jiangxi to southeast Hubei at 20: 00 on the 7th. As this weak cold air continues to supplement the pressure to the south, the dense front of θ se also advances to the south. From 08: 00 on the 8th to 08: 00 on the 20th, the dense front area pressed southward to the north and central Jiangxi, and this two-day rainstorm area followed the θse dense front area.

On the 9th, the weak cold air in the north continued to press southward, and the dense front area of 850 hPa shear and θse also pressed southward. At 20: 00 on the 9th, the shear pressed south to Gannan, and our province was in the northerly airflow on the north side of the shear, so the convergence center of the divergence field of 850 hPa and the divergence field of water vapor flux also moved south and weakened. In addition, on the 9th, the divergence field of 200 hectopascals in our province also turned into convergence area. At this time, the precipitation in our province has obviously weakened, and there are only scattered rainstorms in the province.

3 Conclusion

After the formation of tropical storm (1) 12, the main reason for its northward path is that the subtropical high in North China mainland weakened obviously and retreated westward. In addition, another 1 subtropical high in the western Pacific extends westward, which strengthens the southerly wind between tropical storms and subtropical high and is conducive to guiding tropical storms northward.

(2) When there is no obvious shear around the tropical storm and there is no weak cold air around it, the rainstorm mainly falls near the center of the tropical storm; When there is shear around the tropical storm, the rainstorm falls near the shear; When weak cold air invades tropical storms, there will be heavy rain at the intersection of tropical storms and weak cold air.

(3) When the intensity of tropical depression is not strong, but weak cold air invades, the precipitation will be obviously enhanced. Therefore, weak cold air is an important factor that cannot be ignored when considering typhoon precipitation forecast.

(4) The strengthening of high-altitude divergence is beneficial to the strengthening of tropical low-pressure precipitation.