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Structure of a Waterspout Occurred Over Tokyo Bay on May 31, 2007

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Structure of a Waterspout Occurred Over Tokyo Bay on May 31, 2007 SOLA, 2008, Vol. 4, 001‒004, doi:10.2151/sola.2008‒001 1 Structure of a Waterspout Occurred over Tokyo Bay on May 31, 2007 Yuya Sugawara and Fumiaki Kobayashi National Defense Academy, Yokosuka, Japan Abstract On May 31, 2007, a waterspout occurred over Tokyo Bay near the Futtsu Coast, Chiba Prefecture, Japan. Based on Doppler radar observations and the field surveys, the detailed structures of the funnel and the parent cloud were revealed. The funnel diameter was about 25 m near the surface, and expanded as the altitude increased. The misocyclone had a diameter of 0.2 km and vorticity on the order of 10‒1 s‒1 below the cloud base. The lifetime of the funnel was about 7 minutes, while that of misocyclone was about 20 minutes. The waterspout formed over the wind shear zone, which was favorable for the formation of an anticyclonic vortex. 1. Introduction In Japan, tornadoes are commonly accompanied by extratropical cyclones, typhoons and heat thunder- storms in all season (Niino et al. 1997). In particular, many “non-supercell” type tornadoes such as water- spouts have occurred near the coastline of Japan (e.g., Kobayashi et al. 1997). In the United States, some obser- vations of waterspouts have been made with Doppler Fig. 1. Surface weather map at 15:00 JST on May 31, radar (e.g., Wakimoto and Lew 1993), but the detailed 2007 (JMA). Shaded area denotes the cold air less than mechanism and structure of waterspouts remain uncer- ‒18°Cat500 hPa level at 09:00 JST. tain. A tornado (hereafter, waterspout) occurred in Futtsu Coast, Chiba Prefecture at 17:30 JST (Japan Standard Time), on May 31, 2007. X-band Doppler radar observa- tions were carried out at Yokosuka near the Futtsu Coast. Cloud images were also captured by time-lapse video and still photography. This paper presents the structure and generation process of the waterspout. 2. Synoptic situation Figure 1 shows a surface weather map at 15:00 JST with a cold air core of 500 hPa level. Although cyclones were not analyzed over Japan, the ‒18°C cold core had approached to the Kanto District and the state of the atmosphere thus became very unstable at this time. Precipitation echoes were observed near the area from Tohoku to Chubu District since that morning. Hail- storms and heavy precipitation were occurred in many areas that day (e.g., 90 mm/h precipitation in Kawasaki City, Kanagawa Prefecture; a hailstorm in Ohta-ku, Tokyo Prefecture). After 15:00 JST, cumulonimbus with a strong radar echo (exceeding 32 dBZ) were generated over the Boso Peninsula along the coast of Tokyo Bay. Figure 2 shows a CAPPI (Constant Altitude PPI) image (intensity) at 1 km AGL, calculated from 17:28 to Fig. 2. CAPPI radar echo image (intensity) at 1 km AGL, calculated from the 17:28 to 17:34 JST volume scan data on May 31, 2007. Arrows indicate AMeDAS wind at Corresponding author: Fumiaki Kobayashi, Department of Geo- science, National Defense Academy, Hashirimizu, Yokosuka 17:30 JST. A small square denotes the region expanded 239-8686, Japan. E-mail: [email protected]. ©2008, the Meteo- in Fig. 4. The origin is the location of the NDA radar site rological Society of Japan. at Yokosuka. 2 Sugawara and Kobayashi, Structure of a Waterspout Occurred over Tokyo Bay Fig. 3. Photograph of the funnel taken from the Futtsu Coast at 17:30 JST on May 31, 2007. 17:34 JST when the waterspout was generated at almost stationary, the funnel inclined to the southward. Futtsu. The waterspout was formed at the northern This feature may suggest that the shape of the water- edge of a developing cumulonimbus over the Boso spout was under the influence of the gust front that the Peninsula (a rectangular area in Fig. 2). According to cold air intruded to the location of the waterspout near AMeDAS (Automated Meteorological Data Acquisition the surface. System) wind observations, there was a distinct wind shear line between easterly and westerly winds over 4. Doppler radar observations of the parent Tokyo Bay at that time. cloud 3. Structure of the funnel cloud The range of the NDA X-band Doppler radar at Yokosuka is 64 km in radius. Plan position indicator Numerous witness testimonies and images of the wa- (PPI) volume scans of 20 steps, from an elevation (EL) terspout were obtained in field investigations around of 0.5° to 20.5°, were carried out at 8-minute intervals Futtsu City. The location of waterspout generation was (Kobayashi et al. 2007a). The distance between the radar determined to be 1.5 km offshore from Kazusa-minato, site and the generation point of the waterspout was Futtsu City (35°13´N and 139°51´E), based on the pho- approximately 10 km, while the spatial resolution of the tographs that had been obtained from three different radar beam was about 100 m. These measurements points. The location of the waterspout corresponded successfully detected the mesocyclone (on the order of well with the center of the vortex observed by the several km) in the parent cloud. Doppler radar. Figure 4 shows the Doppler velocity patterns at Figure 3 shows a photo of the waterspout taken 17:29 JST (EL = 2.5°, about 500 m ASL on the Futtsu from Kazusa-minato at 17:30 JST. A funnel cloud began Coast) immediately after the generation of the water- to form at the cloud base from 17:25 JST, as water spray spout. The positive (7 m s‒1 ) and negative (‒9ms‒1 ) began rising into the air near the sea surface. Although opposite peak winds in the Doppler velocity, which were the funnel cloud formed only at levels higher than 400 located in the same position as the funnel generation, m ASL, the rising sea water made it possible to see the indicate the presence of the vortex signature (wide funnel vortex (see Supplements 1 and 2). The lifetime of arrows in Fig. 4). These observations suggest the exis- the funnel was approximately 7 minutes. The funnel tence of a vertically-oriented vortex tube, which implies diameter near the surface was estimated to be approxi- that a misocyclone (less than 4 km in diameter; Fujita mately 25 m based on the image as shown in Fig. 3. The 1981) formed in the north edge of the developing echo. height of the cloud base (800 m ASL) was observed by The misocyclone, which was approximately 200 m in a ceilometer at Yokosuka. A dark shear-band, similar to diameter below the cloud base, corresponded well with that reported by Golden (1974), was observed at the sea the location of the parent cloud, shown in Fig. 3. surface (Fig. 3). According to video images, the funnel Figure 5 summarizes the time-height sections of the circulation was anticyclonic, which corresponded with vorticity and diameter calculated from the tornado the vortex observed by the Doppler radar. The tangen- vortex signatures. The vortex signature (misocyclone) tial velocity and vorticity were estimated from the lasted from 17:29 to 17:51 JST. The misocyclone frames of video data. The averaged tangential velocity reached to a height of 4 km (from EL = 0.5° to EL = of the funnel near the surface was estimated to be 20 m 17.5°). The diameter of the misocyclone was about 200 s‒1 and the vorticity was estimated to be ‒3 × 100 s‒1 . m near the surface (below 1.5 km ASL). It expanded Although the parent cloud and the echo system were with an increase in altitude and reached 1 km at higher SOLA, 2008, Vol. 4, 001‒004, doi:10.2151/sola.2008‒001 3 Fig. 4. Doppler velocity pattern of PPI (EL = 2.5°)at 17:29 JST on May 31, 2007. The parallel wide arrows indicate the position of the opposite peak Doppler velocity. Solid lines denote the radar reflectivity of 16 and 32 dBZ. Broken line denotes the wind shear line of the Doppler velocity pattern. levels at 17:30 JST. The maximum tangential velocity Fig. 5. Time-height cross section of the vorticity (top) corresponding to the misocyclone in the cloud was 15 and diameter of the misocyclone (bottom). × denotes no ms‒1. The vorticity, calculated by peak to peak Doppler vortex signature. velocity assuming an axially symmetric vortex, varied from ‒1 × 10‒1 to ‒3 × 10‒1 s‒1 below 1.5 km ASL and was approximately ‒5 × 10‒2 s‒1 at higher levels. The tornado vortex disappeared below the cloud base around 17:40 similar to the Denver shear line described by Wakimoto JST, while the misocyclone persisted in the cloud. After and Wilson (1989), which accompanied the formation of the funnel cloud decayed, the misocyclone existed both non-supercell tornadoes. Additionally, a local wind below the cloud base and in the cloud around 17:50 JST. shear formed at the northern edge of the echo cell The diameter of the misocyclone expanded as the (broken line in Fig. 4). This wind shear line, which corre- altitude decreased in the decaying stage (around 17:50 sponded well with the dark shear-band shown in Fig. 3, JST), possibly indicating the breakdown of the vortex. indicates a convergence of downdrafts between the The vorticity was on the order of 10‒2 s‒1 in the all layers. northern and the southern parts of the developing cells. No mesocyclone with the size of 10 km in diameter was This wind shear pattern was favorable for the formation observed in the cloud. of an anticyclonic vortex.
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