2007 Volume 85 Issue 3 Pages 233-253
In the morning of 26 Nov 1997, a squall line in association with a tapering, or carrot-shaped cloud, formed along an ENE-WSW oriented cold front north of Taiwan, and moved toward the south-southeast. A segment embedded within the squall line subsequently evolved into a bow echo and made landfall over northern Taiwan. Being a very rare event and the first case in subtropical East Asia reported, this wintertime bow echo was examined in its environment, structure, and evolution mainly using the observations from the single-Doppler radar located at the Taiwan Taoyuan International Airport (TTIA). The bow echo developed in an environment of unusually large low-level vertical wind shear of 28 m s−1 over 0-3 km, with a moderate Convective Available Potential Energy (CAPE) of 1288 m2 s−2. The approaching baroclinic cold front was essential in providing not only a means to organize the convection, but also the vertical shear and thermodynamic conditions suitable for bow echo development. Two aspects of the present bow echo were quite different from those in the United States over mid-latitude continents: It occurred in cold season, and was accompanied by heavy rainfall, which was linked to its weaker rear inflow and slower propagation speed of about 16 m s−1.
The bow echo had a length of 60-90 km, and a lifespan of about 4 h. After formation, it propagated rapidly eastward along the squall line and passed TTIA near 0600 LST, producing northwesterly surface gusts of 18.5 m s−1. The bow echo also exhibited many typical structural features; including a bulged apex, a reflectivity notch, the mid-level rear-to-front (RTF) inflow, reaching 25 m s−1, and a pair of cyclonic-anticyclonic bookend vortices. During the mature stage, the updraft tilted up-shear, and the RTF inflow jet was elevated (near 3-4 km) until very close to the gust front. The bookend vortices lasted for about 2-3 h, and were most evident at 2 km.