Observation and Genesis Analysis of a Continuous Downburst Weather

Observation and genesis analysis of a continuous downburst weather

Zhou Houfu^1,2, Diao Xiuguang³, Zhao Qian¹, Li Yaodong⁴

(1. Anhui Institute of Meteorological Sciences, Hefei 230031,China;

2. Key Laboratory of Anhui Provincial Atmospheric Science and Satellite Remote Sensing, Hefei 230031,China;

3. Shandong Meteorological Observatory, Ji’nan 250031,China;

4. Beijing Institute of Aeronautical Meteorology, Beijing 100085,China)

Based on refined Doppler radar, the ground observation data, lightning positioning data and field investigation, the observation resulting in the ground gale and the causes about a continuous downburst in eastern China were analyzed in this paper. The inescapable conclusion was as follows:

⑴ Observation facts. The gale weather occurred at around 15pm (Beijing Local Time) on May 2, 2016, located in Wangjiang County and Dongzhi County, West Anhui Province, China. Wangjiang and Dongzhi both had the phenomenon of the gale and precipitation at the same time the hail was observed in Wangjiang. The maximum instantaneous wind was 9-grade in Wangjiang whereas 11-grade in Dongzhi. The gale of Wangjiang occurred in the range of 4km×3km and within 8min time, and the gale of Dongzhi occurred in the range of 1km×1km and within 3min time. The gale occurred from the northwest to southest through two counties’ one after another. The maximum distance involved in the two gales was about 15km, and both the wind directions were westerly. The two gales were closely related to the rapid decline in VIL (Vertical Integrated Liquid). In addition, the lightning distribution was consistent with the movement and direction of the storm.

⑵ The nature of the storm. Two microbursts were identified according to the rapid decline of storms, the distribution of ground wind direction and influence range. The first downburst was a microburst of divergence wind, and the second was a microburst of linear wind. What’s more, based on the observation of the regional meteorological stations and actual investigation, the time of the continuous downburst occurred during 14:54~15:01, 15:18~ 15:20, respectively. Besides, tracing the storm information with Doppler radar showed that the storm causing two microbursts was the same storm. Among many storms cells, the storm related to the ground gale had a continuous and deep mesocyclone during one hour, therefore this storm characterize as a supercell storm. Meanwhile analysis revealed that only the supercell storm accompanied by lighting distribution and lighting relatively densely distributed in the range of the storm which meant that supercell storm was the strongest in all storms.

⑶ The genesis of downbursts. Generally the maximum temperature daily was around 14 o’clock in the local time. It was noteworthy that the maximum temperature in the gale site was 2~3℃ higher than in surrounding during the day of the downburst which making the atmosphere tend to be unstable and conducing to the convective weather. The supercell storm had the characteristic of decline fastly with storm body, reflectivity factor nuclear and mass center height. The storm top and reflectivity factor nuclear decreased more than 4km, and mass center height decreased more than 3km within a volume-scanning time interval. The radar echoes of the storm vigorous stages presented as a hook-like distribution in the horizontal direction and declining structure in the vertical direction. The relative humidity below 600hPa level was between 80 to 93%, less than 35% from 520 to 470hPa at Wuhan Sounding Station, that was, there was a significant dry layer in the middle troposphere. The rapid decline in VIL corresponded to subsidence of the dry and cold air in the middle of the troposphere when the downbursts occurred. When 6min precipitation was 4mm or more, the downburst occurred. Affleted by the downburst, the temparature dropped more than 4℃, that meant the negative buoyancy of the atmosphere was significantly enhanced. The buoyancy caused by temperature difference of 1℃ could offset gravity drag generated by 4 g·kg^-1 water, so from the perspective of the buyoyancy, the buoyancy effect caused by 4℃ temperature difference was roughly equivalent to 16 g·kg^-1 gravity drag. The wind speed from 500 to 700hPa in the middle troposphere was about 20 m·s^-1, and the direction was the west wind (WSW) at Wuhan Sounding Station so two downburst gale directions of the ground were consistent with the prevailing wind direction of the upstream middle atmosphere. The cause of the hail in Wangjiang was that the VIL density was more than 3.2 g·m^-3. The hail didn’t occur in Xiangyu Town, Dongzhi because the VIL density was less than 3.2 g·m^-3 when the storm moved to Xiangyu. It was generally believed that the VIL density of a hail was 3.2 g·m^-3 and the critical density of big hail was 4 g·m^-3. The VIL density of the hail during the process was 3.6 to 3.7 g·m^-3, that was why the hail occurred rather than the big hail.

Above all it was concluded that the genesis of the downbursts were as followings. On the one hand, the dragging force about the liquid or solid particles at initial stage was the original cause. Subsequently, in the middle-late stages, the thermal instability, momentum downward and compensatory airflow at medium, as well as the increase of the negative buoyancy between the water and the environment played an important role in the development of the downbursts. On the other hand, the momentum downward caused by prevailing wind direction in the middle of the troposphere determined the ground wind directions of two downbursts.

Key words: downburst; refined data; low-level wind shear; thermal instability; momentum downward