Fine structure of gustfront observed by Doppler radar, Doppler sodar, surface weather station and photogrammetry

A thunderstorm developed around Yokohama near Tokyo at noon on July 11, 2004. A line-shaped radar echo system formed and “bow” echo developed at the south edge of the line echo. After the descending of an echo core, the propagation of a gustfront was detected by Doppler radar and photographic and visual observations. The detailed structure of a gustfront was observed at the passage of Yokosuka observation site using Doppler radar, Doppler sodar, weather station and video cameras. The gustfront propagated to the southeast at the speed of 12 ms-1 and a lifetime of the gustfront was about 50 minutes. Arc cloud generated at the head of gustfront and changed its shape remarkably. The evolution of arc cloud was observed below the gustfront using cameras and a video camera. Two gusts occurred at the passage of the gustfront. First gust was observed about 2 km behind the gustfront leading edge. Second gust occurred about 5 km behind the first gust and maximum wind speed of the second gust was larger than that of the first gust. The vertical circulation of the gustfront “head”, which is an overturning internal circulation, had about 5 km in horizontal scale and 500 m in vertical and made the arc cloud. Both updraft and downdraft exceeding 5 ms-1 observed in the vertical circulation. Two gusts were well corresponded with successive two circulations. Strong wind cores (> 25 m/s) existed between 30 m and 500 m in height at the front of the head circulation. Two gust winds, which were observed at the surface, were well corresponded with the strong wind core near the surface and the front of head structures.