Wei YANG, Xianglin CAO, Quan YUAN, Yu YU

Yueyang Meteorological Bureau, Yueyang 414000, China

Different heavy rainstorms often show similar characteristics of weather situation, radar echo and satellite imagery, and the strong precipitation center usually relates to regional geological environment[1].Heavy precipitation is generally in relationship with astronomical weather coupling and astronomical precipita tion cycle[2-4]. This paper analyzes the heavy rainstorm in northeast Dongting Lake on July 4, 2014, discusses the influence of Dongting Lake on the precipitation intensity, and introduces an astronomical precipitation forecast method[5-8], which provides a new idea and method for rainstorm forecast.

Weather Forecast

Precipitation situation

The time from July 3 to 5 in 2014 witnessed rainstorm in Hunan,Guizhou, Guangxi, Hubei, Jiangxi,Anhui and Jiangsu Province, heavy rainstorm in some places and even extreme strong precipitation in certain areas. From 08:00 on July 3 to 20:00 on July 5,there are 203 stations where the accumulated precipitation in automatic station in Yueyang was over 50 mm,95 stations where the accumulated precipitation was over 100 mm,and three stations where the precipitation was over 200 mm.The precipitation in Huaronglongqinlianchang in northeastern part of Dongting Lake was 247.5 mm. The precipitation for three hours on June 4 was 137.5 mm, and the precipitation from 3:00 to 4:00 amounted to 70.4 mm(Fig.1).

Analyses of the causes of such weather

Fig.2 shows the upper-air chart at 200 hPa from 20:00 on June 3 to 20:00 on June 4. Yueyang lay in the east of Qinghai-Xizang high and the strong divergence airstream at the bottom of East Asia trough, which became the most intense at 8:00 on June 4. The strongest precipitation in Yueyang was at the divergence period.At 500 hPa, the low trough of high latitude in southwestern area was steady, and the southwest warm and wet airstream before the trough was prevailing. From 700 to 850 hPa, the southwestern low vortex moved to northeastern slowly. From 20:00 on June 3 to 8:00 on June 4, the south western airstream strengthened. At 8:00 on June 4, the wind speed of southwesterly in Changsha and Huaihua reached 16 to 19 m/s, and it lay the southeastern part of shear and thewind speed convergence was distinct.The wind speed of southerly in Nanyue from 23:00 on June 3 to 8:00 on June 4 was above 10 m/s. The wind speed at 23:00 on June 3 reached 14 m/s,and the wind speed at 8:00 on June 4 amounted to 16 m/s.To summarize, the weather in central and northern Hunan showed following features. The southerly remained in the southern part of the low trough shear of central and lower latitude,and the convergence provides abundant water vapor for the strong precipitation. The divergence at higher latitude and the convergence at the lower latitude add distinctively, which are good for the condensation of water vapor at the lower layers, and the lower pressure system at the middle and lower latitude moved slowly and lasted for longer time.

Analysis of Doppler radar in Yueyang

Analysis of basic reflection rate After 20:00 on June 3, many echo belts from southwest to northeast pointed to Yueyang (Fig.3a), and then combined together to influence most areas of Yueyang. Around 00:00 on June 4, the belt-shaped cloud in Changde in west Dongting Lake began to develop, and it began to affect Yueyang after 1:00 with above 62 dBz of maximum echo intensity. The intensity reached to maximum around 03:12, and the echo intensity was up to 68 dBz (Fig.3b). The precipitation zone moved eastward and leftYueyang until the morning on June 5.

Analysis of velocity field and vertically integrated liquid water As we can see from the basic velocity of radar at 1.5°angle at 03:12 on June 4(Fig.4a), there was a distinct S-form within around 50 km of radar. There was northeaster in the lower part of northwest part, south wind in the southern part of Yueyang, and distinct convergence of lower airstream.There was southwester in the mid-high altitude 50 km outside, and the strongest wind speed located around 150 km southeastern part of Yueyang. It is not hard to see from the velocity color,the southwester was strengthening in the southeastern part of Yueyang and the wind velocity convergence was distinct. According to the vertically integrated liquid water of Yueyang(Fig.4b),there was liquid water from 25 to 50 km in the southwestern part of Yueyang and the maximum liquid water was 45 kg/m2, which fit the strong precipitation zone.

Analysis of satellite image

As is shown in the FY2E infrared image and moisture image at 03:00 on June 4 (Fig.5),there are many pieces of precipitation clouds surrounding Dongting Lake, especially the northeastern Yueyang where there is the most abundant moisture, which is consistent with the radar detection results.

Influence of Dongting Lake on Precipitation Intensity

The thermal content of water in Dongting Lake was larger than that on the island,which made the air temperature above the lake rise slowly, and the temperature rose slowly.The tropi-cal island effect became more distinctly in the night above the lake, and the warm wet degree was higher than that on the island. When the southwestern warm and wet airstream met the warm and wet air above the lake, it became moister and warmer. Judging from the above-mentioned radar echo and satellite image,the radar echo intensity,vertical integrated liquid water, infrared picture and moisture image are more prevailing than the surroundings.Therefore,the warm and wet moisture in Dongting Lake contributes to the precipitation intensity around 03:00 on July 4.

Brief Analysis of the Astro-Nomical Precipitation Weather Forecast

The dynamic and thermal effects of celestial bodies and their operations are the motive power of evolution of weather system. The comprehensive effects of many kinds of astronomical factors determine the precipitation distribution and its intensity on earth. For a certain area,the promotion of astronomical precipitation factors contributes to precipitation, while the restrain of astronomical precipitation factors prevent rainfall. When many astronomical factors work together,there is a huge potential for heavy rainstorm and torrential rainstorm.The effect of each astronomical factor on precipitation is closely related to celestial operation. Therefore, the study of precipitation features should begin with the astronomical cycle.

There are seven astronomical cycles in this method,the period of revolution related to the south warm and wet system, the rotation convergence period, the period related to westerly system, the lunar month, the sidereal month, draconic month, and anomalistic month.According to the daily precipitation data in Yueyang and Huarong, the seven astronomical cycles are treated with permutation and combination. The relation within each cycle fluctuated, as there are times when the relation is close and there are times when the relation is closer.The close relevance among seven astronomic periods selects a certain relation relevance data, and treats those precipitations with daily precipitation evolution sequence. And then, the daily precipitation anomaly is selected by certain methods. As is shown in Fig.6, the precipitation anomaly curve in Yueyang on July 4 and the echo on the first and fifteenth day are on the peak, and its relative position contributed to heavy precipitation.The low vortex echo was distinct,and the solar rotation echo were weak wave crest,which laid the foundation for heavy precipitation. In Huarong the echo on June 4 lay in the front of the wave crest and its position led to heavy precipitation.

Conclusions and Discussions

(1)The rainstorm was the most intensive in northeast Dongting Lake,which showed that the warm and wet effect of water in Dongting Lake gave rise to heavier precipitation under the favorable circulation background.

(2) Yueyang meteorological station had already made precise prediction of the heavy rainfall on July 4,and reported to both the government and public. On the morning of July 4, Qing Jianwei, the general secretary of Yueyang, and other leaders came to Yueyang meteorological station and talked with experts to learn about the real-time precipitation and discuss the weather development.They were satisfied with the weather report and service in Yueyang and encouraged weather reporter to continue to work harder.

(3) The astronomical precipitation report method reported the entire process precisely a month in advance,which suggested that precipitation showed distinct astronomical cycle.Based on this feature, it is possible to deliver long-term weather report as long as there was plentiful historic daily precipitation data.

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