Structure of Two Adjacent Shear Lines Accompanied by Wind Gusts in the Japan Sea Coastal Region during a Cold-Air Outbreak on 12 December 2010

Abstract

Two adjacent shear lines accompanied by wind gusts exceeding 20 m s −1 passed over the Shonai region on the Japan Sea coast during a moderate cold-air outbreak on 12 December 2010. The structure of the shear lines were examined using simulation results and observational data including high-frequency upper-air soundings. Simulation results revealed that cold-airmass outflow from the Eurasian Continent had a split-flow pattern due to the blocking effect of the Changbai Mountains. One is from the south side of the mountains (SCF), and the other is from the north side (NCF). NCF also has two streams: a shallow southward-meandering flow on the south and a slightly deeper straight flow on the north. A shallow shear line was formed between SCF and the shallow southward-meandering flow of NCF. The deeper straight flow of NCF sharply formed another shear line on the north at around 1-km height with SCF. This result implies that the shear lines accompanied by wind gusts on the Japan Sea coast during a coldair outbreak have various structures and that the horizontal wind shear a little above the surface could cause surface wind gusts.

A Statistical Study of Wind Gusts in Japan Using Surface Observations

“…where W 3s (t ) denotes the maximum 3s mean wind speed in the previous 1 min at time t, and PreW 10m (t ) and PostW 10m (t ) are the 10min mean wind speeds just before and after the wind gust at time t, respec tively. These conditions, where the gust factor and the amounts of increase and decrease in the wind speed before and after the wind gust are much higher than the criteria of previous studies (Wakimoto 1985;Kobayashi et al 2008Kobayashi et al , 2012Kusunoki et al 2010;Tomokiyo and Maeda 2016), are schematically shown in Fig. 2.…”

“…This is the first study in which detailed data from a lot of observatories distributed all over Japan are statistical ly analyzed for such a long period. This study does not focus on particular phenomena, such as tornadoes and downbursts, which cause wind gusts, unlike the previous studies (Wakimoto 1985;Kobayashi et al 2008Kobayashi et al , 2012Kusunoki 2010). The understanding of statistical characteristics of wind gusts is very import ant for science and mitigation of windrelated disas ters.…”

“…Most previous studies about wind gusts focused on not only the maximum instantaneous wind speed, but also the amount of increase and decrease in the wind speed and/or gust factor (Wakimoto 1985;Kobayashi et al 2008Kobayashi et al , 2012Kusunoki et al 2010;Tomokiyo and Maeda 2016). The gust factor is usually defined by the ratio of the maximum instantaneous wind speed to the 10min mean wind speed shortly before the wind gust.…”

“…They showed that most of the wind gusts were accompanied by convective clouds and a tem perature drop. Kusunoki et al (2010) examined the frequency of wind gusts in the Shonai Plain on the Japan Sea side during a winter season and found it more than two orders of magnitude higher than that of tornadoes shown in Niino et al (1997). Taniwaki et al (2012) also detected more than 9,000 gust events in the Shonai Plain over three years using 12 ultrasonic anemometers, discovering that most of the wind gusts occurred in winter under prevailing northwesterly wind during a cold air outbreak.…”

Analysis of the horizontal two‐dimensional near‐surface structure of a winter tornadic vortex using high‐resolution in situ wind and pressure measurements

“…As part of “The Shonai Area Railroad Weather Project” [ Kusunoki et al ., ; Inoue et al ., ; Mashiko , ; Nishihashi , ; Kato et al ., ], winter tornadoes and misovortices have been observed in the Japan Sea coastal region since 2007, mainly using two X band Doppler radars and a network of automated surface weather stations. These vortices can be roughly characterized by winds with V t ≈ c , where V t is the tangential wind speed and c is the translational speed of the vortex, resulting from a modest value of V t and a high value of c associated with the winter monsoon [e.g., Inoue et al ., ].…”