Zhao Qian, Zhou Houfu, Shan Naichao, He Zhiqiang
The low-level wind shear which is generally related to the localized downdraft and downburst of the thunderstorm is one of the prime reasons of the aviation accident. This kind of weather system has the characteristics of short life cycle, small temporal and spatial scale, great intensity and so on. So there are non-negligible difficulties about its monitoring and forecasting.
Hefei Xinqiao Airport is located in the northwest of Hefei, capital of Anhui Province, China. The distance between Xinqiao Airport and Hefei Radar Station is about 29.5 km. A low-level wind shear appeared on 22 p.m., August 6, 2015, Beijing Time (the same below), according to the aircraft report of Hefei Xinqiao Airport. This paper has verified this strong wind shear record of the airport and studied its formation mechanism based on 10 minutes meteorological elements (including temperature, wind direction, wind speed, air pressure) from the regional weather station's observations, Hefei Doppler radar data and 0.25°´0.25°FNL data. Main conclusions are as following:
(1) On the one hand, Xinqiao area was in front of the 500 hPa upper trough and controlled by the deep south-west wind before the low-level wind shear happening, which meaning that Hefei area was plenty of low level warm and moist air flow. On the other hand, several instability indexes were captured by FNL data at 20 o’clock p.m. For example, firstly, the CAPE index’s high value center was above 1600 J·kg-1in the south of Hefei area. Secondly, the lifting index was less than -4.5℃, and thirdly the K index was approximate 34℃. At the same time, strong upward movement was existed from the ground to 100hPa in the south of Hefei area. Therefore, the accumulation of unstable energy and active updraft were advantageous to prompt the development of convective clouds.
(2) The most distinct regional weather stations about wind shear were three towns whose names were Xiaomiao, Guanting and Nangang. Their distances from Xinqiao airport were about 15.7km, 23.3km and 20.8km respectively. The obvious shear began at 21:40, as times goes on, the phenomena about the increasing of wind speed and the apparent shear of wind direction were existed in all three regions. Among them, Xiaomiao turned the wind speed 2.7 m·s-1 and the north wind into southeast wind at the speed 11.6 m·s-1within 20min. Seeing from the space, the divergence of wind direction was apparently between regional meteorological stations at 21:50 and 22:00. This shows that the ground observation data proved the fact of the low-level wind shear that acquired from airport report.
(3) It was noteworthy that two little high pressures were lied in the south and south-west of Hefei whereas a meiobar in the north on the surface layer. Under the influence of the pressure gradient force, two convective zones that consisted of a series thunderstorm cells moved towards Xinqiao and strengthened after merged each other shown on the Doppler radar map. Coincidentally, the cell’s directions of movement were consistent with the pressure gradient force. Actually, the enhanced storm caused the low-level wind shear of Xinqiao Airport.
(4) Although there were multiple convection storms through Xinqiao Airport nearby, but the storm L8 was the decisive factor of the wind shear. Storm L8 generated in the border between the northeast of Shucheng County and northwest of Lujiang County at 20:27. Subsequently, the storm's structure had twice split and merger in the process of moving. Each merger made storm intensive or area increased. Besides, storm L8 had several notable characteristics. First and foremost, storm L8’s heyday was 21:47 when the area of the maximum reflectivity factor greater than 55 dBZ was 128 km2. In the second place, L8 was the thunderstorm system whose convergence and divergence coexisted. Radial velocity diagram at 21:36 revealed the distinct zero velocity line and the positive-negative velocity. Convergence was in the middle-low troposphere in the front-right of the thunderstorm cell while divergence was below 4 km left rear the L8. Deep convergence led to strengthen the updraft which was conducive to the development and maintain of the storm. In addition, zero velocity line and the positive-negative velocity represented that low-level had not only the shear of wind direction but also the strong shear of wind speed. So the results of the radar map verified the observation of low-level wind shear too.
(5) It also had another conclusion about the radar echo. On the whole, the mass center height and the strongest echo height fell sharply corresponding about 2.7km and 5.3km between 21:36 and 21:59. The sharply descend of the mass center height, the strongest echo height and the top of storm indicated the powerful and prosperous of downdraft. In this case, downdraft carrying plenty of cold air reached the ground would arouse wind divergence and a sharp drop in temperature on the ground. Fortunately, the dropping of temperature was observed by the regional station. Start at 21:50 Xiaomiao and Nangang’s temperature dropped above 5℃ whereas 2.3℃ in Guanting within 10min. Last but not the least, the sharply drop of the strongest echo height and the emergence of low-level divergence field was the characteristics of a downburst; moreover the drop of the strongest echo height was sooner than the low-level divergence field. From the above analysis could be concluded that the ground wind shear was caused by the downburst generated by the storm L8.
In conclusion, the downburst generated by the continuous consolidation and development of the storm monomer produced the low-level wind shear. There were not only obvious wind speed and direction shear on the ground and in the lower troposphere, but also severe temperature-decrease by the downdraft.
Keywords: Wind Shear; Doppler Radar; Mass Center Height; The Radial Velocity; Downburst
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