| Bow echo is one of the important weather systems causing damaging surface winds. Understanding its structure and environmental condition is of great importance for improving its forecast. Jianghuai region(115-122°E,30-36°N) in China is the most frequent area for damaging thunderstorm winds. This study examines the environment conditions of bow echoes and their associated damaging thunderstorm winds characteristics in this region by using the sounding data, surface observations, reanalysis data as well as the severe weather reports from2009-2012. The Doppler weather radar data is used further to investigate the spatial and temporal distribution of bow echoes, as well as their three-dimensional structures and possible mechanisms to cause damaging wind. Statistical results show that bow echoes in Jianghuai region preferentially occur in the northwest of Anhui province, southeast of Jiangsu, the southeast of Shandong province, southwest of Jiangsu, and the plain between two mountains in southern Anhui with the frequency maximum in the late afternoon. The extreme damaging winds (>24.5ms-1) induced by bow echoes account for30%of all the extreme damaging wind events. The typical synoptic systems associated with the bow echo are the Northeast Cold Vortex (NECV) and upper-level trough, which are characterized by the moderate convective available potential energy(CAPE,1780Jkg-1) and the moderate vertical wind shear (11.6ms-1between1000and700hPa), as well as a dry middle level air. Compared with the environmental conditions affected by the upper-level trough systems, NECV is usually accompanied with a larger DCAPE and stronger cold pool on the ground. According to the structure of radar observations, bow echoes in Jianghuai Region are divided into three categories, Classic Bow Echo (BE), Bow Echo Complex (BEC), and Squall Line Bow Echo (SLBE), which account for28.6%,14.3%, and57.1%, respectively.Based on statistical results above, two typical bow echoes, occurred on June3(case1) and19(case2),2009have been selected to represent the systems affected by NECV and the upper-level trough, respectively. And their structures and evolutions have been further studied using Doppler radar observations. The analysis result shows that Case1is a BE type bow echo. It developed from a supercell storm, which was characterized by a typical hook echo at the low level, a strong mesocyclone and a bounded weak-echo region (BWER) above. Accompanied with the dissipation of the supercell, the cold pool formed on the ground due to the high precipitation evaporation and the entrainment of the dry air from the environment. Subsequently, it evolved into the bow echo through merging with other storms, lasting longer for about3h. At the mature stage of the bow echo, the storm-relative rear-inflow jet descended from the back of the convective region at2km, and accelerated to about20ms"1near the leading edge of the system. The cyclonic and anticyclonic vortices existed at midlevel behind the northern and southern ends of the system, respectively, which contributed to about20%of the rear-inflow jet. It is worthy to point out that the anticyclonic vortex extended to about5km, higher than the cyclonic vortex (～4km). At1.5km, there was also a low-level vortex near the apex at the leading edge of the outflow boundary. The ground-relative maximum wind with the magnitude of more than32ms-1located near the apex, and was created by the superposition of the rear-inflow jet and the southwestern periphery of the low-level vortex. During the declining stage, the cold outflow spread to the front of the system and cut off the warm and moist inflow. As a result, the system dissipated quickly. Compared with Case1, Case2was a SLBE type. It was resulted from the cell merge, and accompanied with no extreme damaging wind record on the ground. At its mature stage, the storm-relative rear-inflow jet reached about12ms-1. A strong anticyclonic vortex located near the southern end of the system at mid and low level, but no cyclonic vortex was found near the northern end. Different from the Case1that had a strong cold pool and the damaging wind contributed partly by the cyclonic vortex in both middle and low levels, Case2had a much weaker surface cold pool, and the strongest ground winds enhanced by the the anticyclonic vortex. |
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