Abstract
On 2 April 2007, a strong bow echo struck southern Taiwan, with a peak surface wind speed of 26 m s−1. On observation, the rear inflow jet (RIJ) was located at the northern flank and only one anticyclonic vortex dominated behind the bow structure. This case was different from commonly occurring cyclonic–anticyclonic pairs of bookend vortices, and was investigated through data analysis, model simulation, and vorticity budget diagnostics. The present bow echo formed at the leading edge of a cold front, with favorable ingredients of instability, large west-southwesterly vertical wind shear, and dry air aloft. Farther behind the front, however, stable conditions could not support deep convection and the portion north of the RIJ was therefore missing. Within a frontal flow structure, the developing mechanism of the anticyclonic vortex also differed from typical cases. As the low-level (west-southwesterly) vertical shear pointed from right to left of the line, and the tilting effect of updrafts generated positive (negative) vorticity at the front (rear) side. South of and below the RIJ, the anticyclonic vorticity was enhanced by the stretching and tilting effect of system-generated horizontal vorticity via a sinking motion. These sources of vorticity were then advected downward and southwestward by the postfrontal flow near the surface.
Highlights
Bow echoes are bow-shaped lines of convective cells that are often associated with swaths of damaging straight-line winds and small tornadoes (p. 97, [1]), and are a particular type of severe weather at the mesoscale (p. 677, [1]) [2,3,4,5] (Figure 1, adapted from [6])
Bow echoes are most commonly observed over the Great Plains and Midwest of the United States of America (USA) in spring and summer, and they have a horizontal scale of roughly 20–200 km and can last for 3–6 h or longer [3,4,5,6,7,8,9]
The present bow echo exhibited an interesting and atypical characteristic in its structure on its backside, namely, it did not possess a pair of bookend vortices with the rear inflow jet (RIJ) at the middle, and instead, the RIJ was located near the northern end behind the leading bow and only one predominant anticyclonic vortex appeared
Summary
Bow echoes are bow-shaped lines of convective cells that are often associated with swaths of damaging straight-line winds and small tornadoes (p. 97, [1]), and are a particular type of severe weather at the mesoscale (p. 677, [1]) [2,3,4,5] (Figure 1, adapted from [6]). In a system-relative sense, other structural features include a low-level front-to-rear (FTR) inflow, which feeds the warm and moist air into the updraft [5,12,13,14], an FTR outflow as an Atmosphere 2022, 13, 331 tion can reinforce the pair and, at the same time, maintain a balance with the environm2eonf 2-2 tal vertical wind shear, promoting the longevity and severity of the system [15,16]. On 2 April 2007, another event of over 200 km in size developed and propagated across the Taiwan Strait to hit southern Taiwan (cf Figure 2c), with a recorded maximum w3 oifn2d2 speed of 26 m s−1 Was this latter event bigger and stronger, as will be illustrated later, it had only one bookend vortex that rotated clockwise, i.e., in the anticyclonic directHioenn.cHe,etnhceeb, othweebcohwo iencAhopriinl 2A0p07rilw2a0s0u7nwiqauseuinniiqtus easiynmitms eatsryicmsmtruetcrtiucrset,rwuchtiucrheh, awdhnicoht hbaeednnsoetebnepernevseioeunsplyreivniothuesllyiteinratthuerel.itTerhaetruerfeo.reT,hiteriseftohree,suitbijsectht eofsuoubjreicntvoefsotiugratiinovnesintigthaetpiorenseinntthsteupdrye, saenndt ostuurdmy,aainndobojuecrtmiveaiins otobjuenctdiveresitsantoduhnodwerasntadnwdhhyoswucahndanwahsyymsumchetarinc astsryumctmureetrfiocrsmtreudctounrethfoerbmacekdsoidnetohfetbhaisckbsoiwdeeocfhtoh.is bow echo. For the purpose of verification and comparison with the model results, some of the observational data listed above are used
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