Downbursts in the Northwestern Part of Saitama Prefecture on 8

Home , Toyama Airport

Journal of the MeteorologicalSociety of Japan, Vol. 75, No. 4, pp. 885-905, 1997 885

Downbursts in the Northwestern Part of Saitama Prefecture

on 8 September 1994

By Hajime Takayama

MeteorologicalResearch Institute, Tsukuba, Ibaraki 305, Japan

Hiroshi Niino

Ocean Research Institute, University of Tokyo, Nakano, Tokyo 164, Japan

Shinji Watanabe

TokyoDistrict MeteorologicalObservatory, Chiyoda, Tokyo 100, Japan

Juhei Sugaya

New TokyoAirport Local MeteorologicalObservatory, Narita, Chiba 282, Japan

and

Members of Tsukuba Area Precipitation Studies

(Manuscript received 10 May 1996, in revised form 9 June 1997

Abstract on the afternoon of 8 September 1994, a severe thunderstorm passed over the Gunma and Saitama prefectures, north of Tokyo. It lasted for more than 3 hours and produced a strong gusty wind associated with hail. The wind produced damage to window glass of the Misato Junior High School (MJHS), injuring 2 teachers and 71 students. The damage-producing wind and its parent thunderstorm were analyzed by using data from a damage survey, surface meteorological stations, upper air soundings, satellite images and a conventional radar. It is identified that at least three downbursts occurred in association with the storm. The storm moved toward the east-southeast direction at a speed of about 8 m s-1. It was accompanied with a temperature drop of about 10 K and a divergent wind., though not initially of damage-producing intensity, below its cloud base from its developing stage. Its cloud-top reached about 15 km AGL in its mature stage. The radar reflectivity data showed that it had a marked overhang in the direction of the movement. The width of the region with damage-producing hail below the storm markedly increased twice. The times and locations of the increases coincided with those when and where descents of a reflectivity core were indicated from the radar. Furthermore, time-space-converted wind data at several surface stations showed that divergent wind fields, corresponding to Downbursts A and C, occurred near the times and locations of these events. Downburst A occurred right in the middle of Kodama environmental monitoring station (KD) and Kodama District Fire Station which are only 3 km apart, and was clearly delineated from the wind record at these two stations. The temperature drop due to the passage of the storm was largest near the two stations and was more than 11 K. The cool air originating from Downburst A later spread over a region of 40 km in diameter. After the occurrence of Downburst A, the storm quickly dissipated. Downburst B occurred near KD, where another indication of a divergent wind field and a corresponding further temperature drop were recorded. The portion of the storm where Downburst B was generated passed over MJHS at about the time of the damage-producing wind. Though no observational data exist near MJHS, it is conjectured from the damage survey and these facts that the fourth downburst may have occurred near MJHS. All of the downbursts occurred 6-10 km behind the gust front.

1997,Meteorological Society of Japan 886 Journal of the Meteorological Society of Japan Vol. 75, No. 4

CAPE of the storm environment decreased from more than 1800 m-2 s-2 to less than 700 m-2 s-2 during 3 hours of the passage of the storm. The difference of equivalent potential temperature between the low- and mid-levels similarly decreased from more than 26 K to less than 16 K.

1. Introduction on the afternoon of 8 September 1994, a strong and snowflakesfall from the cloud base, a stronger thunderstorm passed over the Gunma and Saitama downdraft is expected because of the slow fallspeed prefectures in the northwestern part of the Kanto of snow flakesand the additional cooling during their plain (see, Fig. 1). It produced a strong gusty melting; 2) In a wet microburst environment, where wind associated with hail when it approached the the cloud base is low, the mid-level is relatively dry boundary between the two prefectures. The wind and CAPE is large, the formation of large precipita- broke window glass of the Misato Junior High School tion particles (especially in the form of hailstones) (MJHS) (Fig. lb) and injured two teachers and 71 is important to generate the microburst (Srivastava, students. Furthermore, several factories and houses 1987; Wakimoto and Bringi, 1988; Proctor, 1989). were damaged in Kodama and Misato towns by the These large particles fall against a strong updraft, wind, and crops in the farms by both wind and hail. entrain the dry air at mid-level, and evaporate or The estimated damage in Saitama prefecture alone sublimate to cool the air. reached a total of about 687 million yen. This paper Scientific documentation of downbursts in Japan reports the analysis of the damage-producing wind started only 10 years ago. The first evidence that and its parent storm. a downburst does occur in Japan was presented by According to the Japan Meteorological Agency Nakayama and Izeki (1985) who analyzed meteo- (1972), which is based on the data between 1954 rological data associated with a microburst on 27 and 1963, the northwestern part of the Kanto plain, July 1983 at Toyama airport. Later, Nakayama and the largest plain in the Japanese islands, experiences Aoyama (1990) analyzed a microburst, which oc- more than 26 days with thunder or lightning dur- curred during the take-off of an A-300 airplane on ing summer months (June, July and August). This 10 July 1988 at Kagoshima airport, using data of number is nearly the same as that in the northeast- the DFDR (Digital Flight Data Recorder) and of ern part of the Gifu prefecture, north of Nagoyacity, meteorological instruments by the runway (includ- and is the largest in Japan. Accordingly, there have ing the wind data at two anemometers within 1.5 km been a number of records on wind hazard associated distance). with thunderstorms in the Gunma, Saitama and The climate of Japan is relatively humid and may Tochigiprefectures, in that part of the Kanto plain, be rather similar to that of the southeastern part since the beginning of this century (e.g., Saitama of the United States. In fact, a recent statistical Prefecture and Kumagaya Local MeteorologicalOb- study on downbursts in Japan shows that a total servatory, 1970). of 70 downbursts reported between 1981 and 1994 The generation mechanism of such a strong wind were all wet (Ohno et al., 1996). Several Doppler that causes the hazard, however, was not clarified radar studies on downbursts have been reported re- until 1976 when Fujita (1976) analyzed the East- cently (Shirookaand Uyeda,1991; Ohno et al., 1993; ern 66 accident at the John F. Kennedy, New York Kusunoki et al., 1994;Akaeda and Sakai, 1994;Ishi- airport and found that it was induced by a damage- hara and Tabata, 1996). These studies confirm that causing downdraft, "downburst". Fujita and Waki- most of the Doppler-radar observed characteristics moto (1983) defined a downburst as a strong down- of downbursts in the United States are also observed draft which induced an outburst of damaging winds in Japan (e.g., a divergent wind pattern (Wilson et near the ground. Downbursts are classified into al., 1984; Fujita, 1985; Ohno et al., 1993), descent macro- and micro-bursts (Fujita, 1980; Fujita and of a reflectivity core prior to a burst of wind at the Wakimoto, 1983;Wilson et al., 1984). The latter are surface (Wakimoto and Bringi, 1988; Ishihara and further divided into two categories: dry microbursts Tabata, 1996), and a mid-level convergence prior and wet microbursts (Wolfson, 1983; Caracena et to a low-leveldivergence maximum (Kropfli, 1986; al., 1983;Wakimoto, 1985). Roberts and Wilson, 1989; Ohno et al., 1993)). Various numerical studies (e.g., Srivastava, 1987; As was mentioned above, damage to structures Proctor, 1988, 1989;Knupp, 1989; Straka and An- associated with downbursts have been examined by derson, 1993) revealed that, 1) In a dry microburst several authors. In most of these previous case environment, where a thick dry adiabatic layer exists studies of downbursts, however, either meteorologi- below the cloud base, evaporation of precipitation cal traces, which delineate the downburst itself, are particles is the most important forcing to drive the lacking (Kobayashi and Kikuchi, 1989) or a nearby microburst. If the situation is such that the temper- sounding, which characterizes the rapidly chang- ature at the cloud base is below the freezing point August 1997 H. Takayama, H. Niino, S. Wanatabe and et al. 887

Fig. 1. (a) Map around Japan. (b) Map around the Saitama prefecture. The open circles show the upper-air sounding locations and the solid triangle denotes the location of Misato Junior High School (MJHS). The smallest rectangle denotes the region shown in Fig. 10. (c) Map of the MJHS region. The solid circles indicate several major surface stations (SK:Saku AMeDAS station, FO:Fujioka environmental monitoring station (EMS), KD:Kodama EMS, FKD: Kodama District Fire Station, HJ: Honjoh EMS and YI:Yorii EMS). 888 Journal of the Meteorological Society of Japan Vol. 75, No. 4

Fig. 2. Weather charts at 500 hPa height at (a) 0900 JST, (b) 2100 JST on 8 September 1994. The full lines show geopotential height for every 60 gpm, the thick line denotes 5700 gpm. The dashed lines show temperature (C). ing storm environment was not available (Omoto, data from prefectures, satellite images and a con- 1987; Kajikawa, 1988; Ohno and Suzuki, 1991a,b; ventional radar data at Mt. Fuji were analyzed to Iwashita, 1992; Ohno et al., 1994; Iwashita, 1995). reveal the characteristics of the parent storm and the The latter is mainly caused by the fact that, al- damaging wind. As will be shown in the following, though. downbursts in Japan have the highest fre- it was found that the damaging wind was caused by quency around 1500 JST (Japan Standard Time; a downburst and that at least three wet downbursts hereafter all the reference to time will be given in occurred behind a gust front of the storm. JST; 0900 JST is 0000 UTC) (Ohno et al., 1996), In the following section, the damage character- the routine upper air soundings of thermodynamic istics as revealed from the damage survey are de- quantities by the observatories of the Japan Meteo- scribed. Section 3 describes the synoptic situation rological Agency (JMA) are made at 0900 JST and and the storm environment. In Section 4, the char- 2100 JST. acteristics of the damaging wind and the parent The day of 8 September 1994 was during an in- storm are analyzed. -In the fast section, the results tensive observation period of TAPS (Tsukuba Area are summarized. Precipitation Study'). In addition to the routine upper air soundings at Tateno Aerological Obser- 2. Damage characteristics vatory of JMA, extra soundings by scientists were Damage survey was made twice by the scientists taken near the time and location of the occurrence of MRI over the area of about 3 km x 5 km around of the strong gusty wind. This provided a unique MJHS : firstly, on 9 September 1994 to examine opportunity that good data on the storm environ- well-preserved damage characteristics and secondly ment were obtained. A detailed damage survey was on 27 September 1994 to obtain some supplemen- also made by the scientists of Meteorological Re- tary data. According to eyewitness accounts, the search Institute (MRI) on the next morning. In this damage-producing wind occurred under a thunder- paper, these data together with those at surface sta- storm which produced hail. A number of holes in tions of AMeDAS (Automated MeteorologicalData leaves of crops and on plastic corrugated sheets con- Acquisition System of JMA), surface environmen- firm the accounts. In MJHS, the strong gusty wind tal monitoring stations of prefectures, hail damage with hail broke 51 sheets of window glass on the northern side wall of a three-story building at about 1 TAPS was initiated in 1993 as a part of three-year project on mesoscale precipitation systems over the Kanto plain, 1440 JST on 8 September. Two teachers and 71 where research institutions and routine observational net- students in a utility hall on the second floor were works are distributed with the highest density in Japan. injured by the broken glass. In a classroom, the It is a unique project with respect to the following point northern side windows of which were broken, sev- : It is not a project sponsored by a certain organization to study a particular phenomenon, but is maintained by eral chairs were blown out of the windows on the a loose cooperation among various scientists of different southern side wall. organizations who are interested in different mesoscale Around MJHS, several factories were demolished phenomena and are sponsored by different funds. and several houses lost roof tiles. Some sheds August 1997 H. Takayama, H. Niino, S. Wanatabe and et al. 889 were blown away southeastward. Several trees of 10-20 cm diameter were blown down and a consid- erable number of trees lost their branches. Several telephone poles made of concrete were also blown down. According to the survey by the Misato town office, one house was completely destroyed and 49 houses were partially destroyed. From these facts, the wind speed is estimated to be Fl according to the Fujita scale (Fujita, 1971). In fact, an engineering-type calculation based on a distortion of a steel support of a rectangular signboard near Kodama environmental monitoring station (EMS) (see Fig. 14) gives an estimate that the maximum wind speed was more than 47 m s-1. This estimate of the maximum wind does not contradict the estimate of F1 based on the general damage characteristics. The broken windows on the northern wall of the utility hall of MJHS were made of ordinary glass of 3 mm thick, 154.5 cm in height and 88 cm in width. According to Asahi Glass Company (1993), more than 50 % of new glass plates of this type are broken at a load of 4412 N. Dividing this load by the area of the window and equating the result to the pressure force due to the gusty wind, one obtains the wind speed of 50 m s-1. If the wind speed were 60 m s-1, about 89 % of new glass plates would be broken. The wind direction estimated from the direction of fallen trees and rice-plants turned out to be almost uniformly northwesterly in the area and any indication of a convergent wind was not observed. This shows that the strong gusty wind was not caused by a tornado. 3. Synoptic situation and storm environment on the afternoon of 7 September 1994, a cold front extending from a low over the Sea of Okhotsk passed over the Japan Islands and moved toward the Pacific Ocean. On 8 September, no synoptic distur- bances such as a low or a front existed around the Kanto district at least until 0300 JST 9 September, when a tail of a cold front extending from another low over the northern part of the Japan Sea, passed the northern part of the Kanto district (Fig. 3). An upper level trough moved over the Japan Sea and Fig. 3. Surface weather charts at (a) accompanying cold-air intruded into the north and 0900 JST on 8, (b) 2100 JST on 8, (c) east parts of Japan at levels above 500 hPa in the 0900 JST on 9 September 1994. The full afternoon of 8 September (Fig. 2). The wind di- lines show isobars at sea level for every rection was nearly southwest around Honshu Island 4 hPa, and the dashed lines for every between 300 hPa and 850 hPa. 2 hPa. A line-shaped cloud area appeared by 1200JST on 8 September on an infrared image of Geostationary MeteorologicalSatellite (GMS)-4. It became gradu- bashi, this temperature corresponds to about 15 km ally clearer and moved eastward with time. Figure 4 AGL. showsthe infrared image of GMS-4 at 1400 JST. The A line-shaped echo corresponding to the cloud lowest temperature of the cloud-top near Gunma- area was also observed on Mt. Fuji radar after Saitama prefectural border reached -65.0C. Ac- 1200 JST on 8 September. Figure 5 shows the line- cording to the sounding data on 1500 JST at Mae- shaped echo which extends from the Fukushima pre- 890 Journal of the Meteorological Society of Japan Vol. 75, No. 4

Fig. 4. Infrared image of the meteorological satellite (GMS-4) at 1400 JST on 8 September. fecture to the Aichi-Shizuokaprefectural border at 950 hPa) and its minimum at the middle level (typi- 1437 JST near the time of the damage-producing cally 600 hPa), at Tateno and Maebashi. At Tateno, wind at MJHS (see Fig. 4 for the locations of the pre- CAPE was larger than 1000 m-2 s-2 after 2100 JST fectures and Fig. 1 for the location of MJHS). The on 7 September. After reaching the maximum value strong echo near the boundary between the Gunma of 1808 m-2 s-2 at 1600 JST on 8 September, the and the Saitama prefectures was associated with the nearest sounding time to the occurrenceof the gusty lowest cloud-top temperature in Fig. 4 and caused wind in Misato town, CAPE rapidly decreased to the damage-producing wind. 686 m-2 s-2 by 1800 JST on 8 September. Me The day of 8 September was during one of the was larger than 20 K after 1400 JST on 7 Septem- intensive observation periods of TAPS. In addition ber. Atkins and Wakimoto (1991) suggested that to the routine soundings at Tateno by the Aerologi- a wet downburst in the United States often occurs cal Observatory, JMA, at 0900 and 2100 JST, extra when the criterion AGe > 20 K is satisfied. After soundings were taken by scientists at Maebashi, at recording a maximum value of 27.6 K at 1600 JST 1500 and 1800 JST, and at Tateno, at 1600 and on 8 September, AGe rapidly decreased to 15.5 K 1800 JST. at 1800JST on 8 September. At Maebashi, CAPE Figure 6 shows the time series of the convective was 3280 m-2 s-2 and AGe was 26.9 K at 1500 JST available potential energy (CAPE) and AGe,the dif- on 8 September, and they decreased to 117 m-2 s-2 ference of the equivalent potential temperature be- and 13.9 K, respectively, at 1800 JST. At around tween its maximum value at the low level (typically 1430 JST, when the balloon was released, Maebashi August 1997 H. Takayama, H. Niino, S. Wanatabe and et al. 891

Fig. 5. Precipitation intensity distribution observed by Mount Fuji, Tokyo, Nagoya and Niigata radars at 1437 JST on 8 September. The precipitation intensity is categorized into six discrete levels according to the notation shown in the bottom. was located in the line-shaped echo in which the record wind, and a limited number of stations record storm that caused the gusty wind was embedded temperature and humidity additionally. The second (see, Figs. 5, 7 and 11). Furthermore, the moist cold is a fire station which records wind, temperature, outflowfrom the storm reached Maebashi at around humidity, pressure and precipitation. The third is 1430 JST. Therefore, the sounding at Maebashi may AMeDASstations which observe wind, temperature, not properly represent the storm environment. precipitation and insolation every 10 minutes. At Tateno, the bulk Richardson number (Weis- The reflectivitydata by the Mt. Fuji radar of JMA man and Klemp, 1986) changed from 121 at were used to examine the evolution of the storm. 0900 JST to 30 at 2100 JST on 8 September (not The radar is located at the top of Mt. Fuji (3786 m shown) . This shows that the wind shear was mod- above the sea level) about 100 km south-southwest erately strong during this period. The reduction in of MJHS. The radar provides PPI data at three ele- the bulk Richardson number during the 12 hours was vation angles for each 2.5 x 2.5 km horizontal square mainly caused by the decrease in the instability. mesh. Elevation I corresponds to about 2000 m, II to 3000 m and III to 4000 m AGL in the area of 4. Storm characteristics interest in the present paper. The radar starts to 4.1 Data scan from elevation I. It takes about 1 min to finish Figure 7 shows the distribution of the major sur- scanning at each elevation and 30 sec to change ele- face observation points, the data of which were vation from I to II or from II to III. Therefore, there used for the present analysis. These observation is a time lag of about 3 min between the scans at points consist of three categories. One is monitoring elevations I and III. The reflectivity data from the stations of atmospheric environment (EMS) in the Tokyo radar were not used for the present analysis, Saitama and Gunma prefectures. All of the stations because a strong echo cell located between the area 892 Journal of the Meteorological Society of Japan Vol. 75, No. 4

of our interest and the radar attenuated the radar beam. Information about hail damage to rice and several crops has been collected from the prefectural governments of the Gunma and Saitama. 4.2 Evolution of the storm The radar reflectivity data and the AMeDASdata show that the line-shaped echo emerged quickly between 1100 and 1200 JST on 8 September. In the line-shaped echo, there were three areas of strong reflectivity: at 1437, for example, one is located around Fukushima prefecture, one near the boundary of the Gunma and Saitama prefectures and the other in the Shizuoka prefecture (Fig. 5). The line- shaped echo system moved toward east while developing. The storm which caused the damaging wind around MJHS was generated over the eastern part of the Nagano Prefecture before 1200 JST. It corresponded to the middle of the three areas of strong reflectivity. At the Saku AMeDASstation (see Fig. 1b or 7) in this area, precipitation of 45 mm and a temperature drop of 11C were recorded within one hour from 1200 to 1300 JST. The temperature drop was only temporary: it recovered from 18C to 21C at 1400 JST and then to 25C at 1500 JST. The movement of the storm as observed at 2000 m AGL by the Mt. Fuji radar is shown in Fig. 7. The storm crossed the boundary between the Nagano and Gunma prefectures by 1254 JST and moved eastward over the southwestern part of the Gunma prefecture while increasing both its size and intensity. After 1354 JST, when the storm approached the boundary between the Gunma and Saitama prefectures, it changed the direction of the motion to the east-southeast and its size and intensity further increased. The storm caused the damaging wind around MJHS at about 1440 JST, and after 1454 JST it weakened rapidly. Figure 8 shows the region where hail damage to farm crops were reported to the Agriculture Divi- sion of the Gunma and Saitama prefectures. The time when the damage-producing hail was observed Fig. 6. Time series of (a) the convective is also shown in the Gunma prefecture. Since available potential energy (CAPE) and the damage-producing hail was only reported from (b) the differencebetween the maximum farming areas, the possibility that some hail oc- value of the equivalent potential temper- curred outside the regions indicated in the figure ature at the low level and its minimum may not be excluded. However, the region of hail at the middle level (AGe), at Tateno damage corresponds fairly well to that of a strong and Maebashi from 0900 JST on 7 to reflectivitylarger than 40 dBZ (ef., Fig. 7). Damage- 0900 JST on 9 September. The solid circles connected by the full lines de- producing hail started in the southwestern part of note the data at Tateno, and the open the Gunma prefecture after 1340 JST on 8 Septem- circles connected by the dashed lines ber. At this time the diameter of the hail was show those at Maebashi. The arrow between 0.51.5 cm. The region of hail dam- points to the occurrence time of the age marched eastward with the movement of the damage-producing wind. storm. The width of the damage-producing hail region increased markedly as the storm approached the boundary between the Gunma and Saitama pre- August 1997 H. Takayama, H. Niino, S. Wanatabe and et al. 893

Fig. 7. The time sequence of radar echo with reflectivity exceeding 30 dBZ observed by the Mt. Fuji radar about every twenty minutes from 1215 JST to 1519 JST on 8 September. The area enclosed by the full line has reflectivity larger than 30 dBZ. The area of reflectivity larger than 40 dBZ is shaded. The triangle denotes the location of MJHS, and the circles indicate several major observational stations on the surface (MB:Maebashi EMS, TO:Tomioka EMS and KG:Kumagaya EMS). The dashed lines denote boundaries of prefectures.

Fig. 8. The region where hail damage to farm crops was reported. In the Gunma Prefecture, the time when the damage-producing hail was observed is also shown : the area for which hail was reported between 1340 and 1400 JST is shaded and that between 1410 and 1430 JST is crosshatched. The triangle and the circles denote the same as in Fig. 7. 894 Journal of the Meteorological Society of Japan Vol. 75, No. 4

▲---▲: elevation III

■----■: elevation II

●-●: elevation I

Fig. 9. The time evolution of the radar echo-intensity at three elevation angles by the Mt. Fuji radar from 1310 to 1510 JST. The circles with the full lines denote the value at elevation I, the squares with the dotted lines elevation II and the triangles with the dashed lines elevation III. Elevation I corresponds to about 2000 m AGL, II to 3000 m and III to 4000 m in the area of interest. fectures. The diameter of the hail reached between descent of the reflectivity core was caused by falling 2.5 and 3.0 cm after 1410 JST. The width of the hail. A similar change of the maximum reflectiv- damage-producing hail region attained a maximum ity in the storm occurred once again between 1429 of more than 13 km at the northwestern edge of the and 1446 JST: Before 1425 JST, the echo-intensity Saitama prefecture. The width once showed a slight at elevation III slightly increased and that at ele- decrease, but it increased again near MJHS. Shortly vation I rapidly decreased. This was followed by after passing over MJHS, the storm stopped pro- rapid decrease of the echo-intensity at elevation III ducing hail. The eastern boundary of the damage- and an increase of the echo-intensity at elevation I producing hail region nearly coincided with that of after 1428 JST. As a result, the echo-intensity at the strong echo region at 1438 JST. elevation I became larger than that at elevation III Figure 9 shows time evolution of the radar echo at 1437 JST. Such a reversal in the echo-intensity intensity at three elevation angles, where the echo lasted until 1446 JST. The echo-intensities rapidly intensity was defined by the maximum reflectivity decreased after 1500 JST in accord with the dissi- for a 2.5 x 2.5 km mesh in the storm. The echo inten- pation of the storm. sity increased rapidly after 1345 JST and attained As was mentioned above, the width of the maxima of more than 55 dBZ for elevation angles II damage-producing hail region twice showed marked and III at 1407 and 1408 JST, respectively. After increases near the boundary between the Gunma recording the maxima in the echo intensity at eleva- and Saitama prefectures: firstly, at the prefectural tions II and III (the beam centers of which are about boundary and secondly near MJHS. The width for 3000 and 4000 m AGL, respectively), the echo inten- the first case is nearly comparable to that of the sity started to decrease rapidly. On the other hand, eastern edge of the strong echo region (>40 dBZ) at elevation I, the beam center of which is about at 1415 JST (see Fig. 7) when the first descent of 2000 m AGL, the reflectivity continued to increase the reflectivity core is indicated in Fig. 9. Similarly, and became nearly equal to the echo-intensity at el- the width for the second case is comparable to that evation III at 1415 JST. Since we do not have a po- of the strong echo region at 1434 JST (see Fig. 7), larized beam radar, we cannot infer the type of the which is nearly the time of the second descent of the precipitation particles. However, it is likely that the reflectivity core (see Fig. 9). August 1997 H. Takayama, H. Niino, S. Wanatabe and et al. 895

■40～45dBZ ■45～50dBZ ■≧50dBZ

Fig. 10. The time evolution of the horizontal distribution of the strong radar reflectivity area (larger than 40 dBZ) at elevation I and III by the Mt. Fuji radar from 1400 to 1500 JST. The reflective intensity is categorized into three discrete levels according to the notation at the bottom. The triangle denotes the location of MJHS and the dashed lines denote the boundary of prefectures. The region shown in this figure is indicated by the rectangle in the lower-right corner of Fig. 11.

Figure 10 shows the time evolution of the hori- vations I and III. This distance is less than the mesh zontal distribution of the strong radar reflectivity size of 2.5 km. Therefore, it is evident that the storm (>40 dBZ) at elevations I and III from 1406 to had a marked overhang in the direction of its move- 1459JST. A comparisonbetween the reflectivitydis- ment. Such a structure may have been caused by tribution at elevations I and III shows that the re- a strong updraft which could prevent precipitation gion of strong reflectivity at 4000 m AGL was about particles from falling (Browning and Ludlam, 1962). 2.5-5 km ahead of that at 2000 m AGL. Since the Between 1428 and 1437 JST, when the second de- storm moved 30 km between 1400 and 1500 JST scent of the reflectivity core is indicated from Fig. (Fig. 7), its average speed is estimated to be about 9, the region of the strong reflectivity larger than 8.3 m s-1. This means that the storm propagated 50 dBZ at 4000 m AGL disappeared whereas it ap- about 1.5 km during 3 min between the scans at ele- peared at 2000 m AGL. Note that the mesh of strong 896 Journal of the Meteorological Society of Japan Vol. 75, No. 4 reflectivity larger than 50dBZ at 2000 m AGL at the intensity of the downdraft. A recent statistical 1434 JST coincides with the location of MJHS, and study on downbursts in Japan from 1981 to 1994 the time of the occurrence of the damage-producing (Ohno et al., 1996) also shows that downbursts ac- wind at MJHS was around 1440 JST. The first de- companied by hail tend to be stronger than those scent of the reflectivity core as noticed between 1406 without hail. and 1415 JST from Fig. 9 is not evident in Fig. 10 because the gray scale is same for reflectivity larger 4.3 Downbursts Figure 11 shows the time evolution of the surface than 50 dBZ. However,one may notice that the area wind field around the storm for every 10 min from of reflectivity larger than 40 dBZ at 2000 m AGL in- 1400 to 1500 JST (note that the wind at time t is an creased markedly between 1406 and 1415 JST. average between t-10 min and t). The region with These facts support the above speculation that the strong radar reflectivity (> 40 dBZ) at 2000 m the descents of the reflectivity cores were caused by AGL at the nearest time is also shown. At 1400 JST falling hail, and showsthat, though hail was continu- the storm was located south of Tomioka EMS of ously produced by the storm, significantfractions of the Gunma prefecture (see Fig. 7). The wind to hail fell down at around 1415 and around 1434 JST. the southeast side of the storm was nearly uniform The speed of the descent of the reflecting core be- and was east-southeasterly. Meteorological traces tween 1409 and 1415 JST is roughly estimated to be about 2000m/ 6min=5.6mifweas- (not shown) at Tomioka EMS show that a strong northwesterly wind (larger than 9 m s-1 in 10 min- sumed that the portion of the core observed at ele- average) started at about 1340 JST and at the same vation I was the same as that observed at elevation time the temperature started to drop rapidly. The III. This value is much less than that of Wakimoto wind direction changed counterclockwisewith time, and Bringi (1988) (see Fig. lOd and 10e of their pa- and a strong southeasterly wind of about 7 m s-1 per), in which the speed was about 1800 m/162 s = was observed between 1350 and 1400 JST (see also 11.1 m s-1. However,a simple comparison of these Fig. 11). This shows that a cool divergent air flow two cases may not be appropriate : In Wakimoto existed below the storm. and Bringi (1988),the storm lasted 30 min at most By 1410 JST, the storm approached the bound- and the horizontal scale of the area of reflectivity larger than 40 dBZ was only about 5 km in its ma- ary between the Gunma and Saitama prefectures. On the southeast side of the storm, the general ture stage. On the other hand, the present storm lasted more than 2 h and the area of reflectivity wind continued to be east-southeasterly. However,a larger than 40 dBZ was about 10 km in its mature clear indication of a divergent wind field beneath the stage (see Fig. 7). Furthermore, the time resolution storm is seen: a westerly wind at Fujioka EMS at of the reflectivity data was 2 to 3 min in Wakimoto the northeast edge of the storm and a southeasterly and Bringi (1988), but it was 9 to 10 min in our one at Tomioka EMS at its northwest edge. There case. In Fig. lOc and lOd of Wakimoto and Bringi was a fairly strong convergenceon the southeastern edge of the storm, which is indicative of the presence (1988), the reflectivity core (>60 dBZ), which is considered to be mainly composed of ice particles, of a strong updraft on the forward flank of the storm had a vertical scale of about 4000 m. If we assume as was speculated in Section 4.2 from the radar re- that the reflectivity core in the present storm had flectivity pattern in Fig. 7. a similar vertical scale and that the top of the core The divergent wind started to strengthen remark- was observed at elevation I 6 min after the bottom ably at 1420 JST. This is immediately after the first of the core was observed at elevation III, the core descent of the reflectivity cores (Fig. 9) occurred. is considered to have descended about 6000 m in At Fujioka EMS, which was located about 5 km 6 min. This gives an estimate for the speed of the (2 meshes) north of the region with strong reflec- descent 6000 m/6 min = 16.7 m s-1, which is larger tivity (> 50 dBZ), a strong southwesterly wind of than that of Wakimoto and Bringi (1988). These about 7 m s-1 is seen. At Kodama EMS and Ko- calculation suggest the descent speed of the reflec- dama District Fire Station (hereafter referred to tivity core does not contradict the speculation that as FKD) of the Saitama prefecture, a wind with it is composed of falling hail. northerly component intensified. The area with According to Byers and Braham (1949), the im- divergent wind field started to spread rapidly after 1430 JST, when the center of the strong echo portant factors in the development of the downdraft are entrainment of dry air into the updraft and load- reached around Kodama. It eventually spread over ing by precipitation particles. Using a simple nu- an area of about 40 km diameter by 1500 JST, when merical model, Srivastava (1985) has shown that a the center of the divergent wind did not collocate downburst can be driven below the cloud base by with the region with strong reflectivity. This clearly evaporative cooling of raindrops and loading due to indicates that the storm stopped producing a strong downdraft before 1500 JST. Tabata et al. (1991)an- precipitation particles. Srivastava (1987)has further shown that precipitation in the form of ice increases alyzed a downburst on 31 July 1987 in Tokyo and August 1997 H. Takayama, H. Niino, S. wanatabe and et al. 897

Fig. 11. The time evolution of the surface wind field for every 10 minutes from 1400 to 1500 JST. The region with the strong radar reflectivity (larger than 40 dBZ) at 2000 m AGL at the nearest time is also shown. The reflectivity intensity is categorized into three discrete levels according to the notation in the lower-right corner. The triangle denotes the location of MJHS and the dashed lines denote the boundary of prefectures. The rectangle shown in the lower-right corner denotes the region shown in Fig. 10. found that the cold air associated with the down- attained a maximum of 11.9 m s-1 in the west- burst eventually spread over a diameter of 30 km. northwest direction at 1432 JST. The maximum in- Figure 12 shows the distribution of a tempera- stantaneous wind speed printed out numerically to- ture drop observed during the passage of the storm, gether with hourly values of other meteorological el- where the temperature drop was defined by the max- ements was 25.3 m s-1 at 1428 JST from a northerly imum temperature minus the minimum temperature direction. The temperature was about 29C before between 1200 and 1600 JST on 8 September. The the storm approached. It started to decrease sud- region with damage-producing hail (Fig. 8) is also denly by 7C after 1420 JST. The relative humid- indicated in the figure. The contour lines of the ity suddenly started to rise at the same time. The temperature drop (drawn somewhat subjectively by rain started at 1422 JST, slightly after the sudden eye) are nearly elliptic with their long axis in the changes of temperature and humidity. The pres- southeast direction. This figure clearly shows that sure then started to rise about 1hPa at 1423 JST. a temperature drop associated with the storm was These records show that a gust front passed FKD largest near Kodama EMS, where the temperature at 1420 JST. Note that the strongest gusty wind drop of 11.9C was observed. The region with the was observed 8 min after the passage of the gust largest temperature drop was not only close to the front. The precipitation at FKD stopped at about region with damaging wind around MJHS, but also 1500 JST. In accord with the stoppage of the pre- close to the hail damage region with the maximum cipitation, the temperature rose by 2.2C. At the width. same time, the humidity showed a marked decrease Figure 13 shows the meteorological traces of about 13 %. recorded by FKD and Kodama EMS. The anemome- At Kodama EMS (Fig. 13b), the wind direc- ter at FKD was mounted on a tower of 55 m tion rotated suddenly clockwise from southeast to height. At FKD (Fig. 13a), the wind direction west at 1410 JST. Then it shifted slowly to west- changed counterclockwisefrom east after 1400 JST northwest until 1425 JST. In the following 10 min, : it became northerly at 1425 JST and then rotated the wind direction showed a considerably complex to the southeast by 1500 JST. The 10-min mean variation : At 1427 JST it started to change from wind speed started to increase after 1420 JST and west-northwest to north-northwest. At 1429 JST it 898 Journal of the Meteorological Society of Japan Vol. 75, No. 4

Fig. 12. The distribution of the temperature drop due to the storm. The temperature drop is defined by the difference of the maximum- and the minimum-temperatures (C) between 1200 and 1600 JST. The crosshatched area shows the region of damage-producing hail. The triangle denotes the location of MJHS and the dashed lines denote the boundary of prefectures.

started to recover to west-northwest. At 1431 JST 1407 JST, it started to rise suddenly. It attained it started to rotate again clockwise and it almost 96 % at 1417 JST, and then it almost remained con- settled down to east at 1437 JST. The mean wind stant. speed continued to increase from 1410 JST and ex- These records show that the gust front passed Ko- ceeded the recording range of 10 m s-1 from 1422 dama EMS at 1406 JST. The gusty wind started to 1430 JST. The complex variation of the wind di- 4 min after the gust front passed. The strongest rection started while the wind speed was more than wind occurred at least 16 min after the passage of 10 m s-1 in a 10 min-average. A simple extrapola- the gust front; if a uniform transitional speed of tion would give a mean wind speed of 20 m s-1 at 8 m s-1 is assumed for the storm, this means that 1425JST. the strongest wind was located about 7.7 km behind The temperature at Kodama EMS started to de- the gust front. crease suddenly from 29.5C after 1406JST. The Although Kodama EMS and FKD are only 3km time rate of temperature drop was gradually re- apart, Fig. 13 shows that variations of the meteoro- duced as the temperature decreased. At 1421 JST, logical elements at these stations were considerably however, the temperature dropped almost instanta- different. At FKD the temperature drop after the neously by 1.2C. This discontinuous change in the passage of the gust front was 8C, but at Kodama it time rate of temperature drop must be associated was 10 C. This difference was mainly caused by the with some distinct phenomenon. The mean wind sudden temperature drop at 1421 JST at Kodama. speed started to exceed the recording range within Such a sudden drop was not observed at FKD. The 2 min after the discontinuous change occurred. The temperature decrease after the passage of the gust temperature remained less than 20C from 1422 to front continued for about 38 min at FKD, but it 1430 JST and attained a minimum of 19.3C at lasted only 24 min at Kodama EMS. The period of 1425 JST. At 1430 JST the temperature instanta- the temperature decrease at FKD nearly coincides neously rose by 0.7C and at the same time the wind with that of precipitation. If this was the case at Ko- weakened to reenter the recording range. The hu- dama EMS, the period of precipitation would have midity increased slowly after about 1330 JST. After been shorter there. a slight fall of about 1 percent between 1405 and The wind speed at FKD increased immediately af- August 1997 H. Takayama, H. Niino, S. Wanatabe and et al. 899

Fig. 13. The meteorological traces recorded by FKD ((a) : wind direction, wind speed, humidity, temperature, pressure and precipitation) and Kodama EMS ((b) : wind direction, wind speed, temperature and humidity from 1300 to 1600 JST. Time, which proceeds from right to left, is shown below each of the traces. 900 Journal of the Meteorological Society of Japan Vol. 75, No. 4

Fig. 14. Hovmoller diagrams for wind data at several observational stations at 1436 JST when the damage-producing wind occurred at MJHS. The location of MJHS is indicated by the triangle and that of the observational stations by the large solid circles. The characters of A, B and C surrounded by dotted lines denote the area of the divergent wind originated in downburst A, B and C, respectively. The direction and the speed of the storm motion is assumed to be 125 degrees and 8 m s-1 respectively from the movement of the echo observed by the Mt. Fuji radar. The square denotes the location of the damaged signboard. ter the passage of the gust front and reached a max- storm motion within the Saitama prefecture was as- imum within 12 min. On the other hand, the wind sumed to be 125 degrees and 8 m s-l, respectively. speed at Kodama EMS started to increase 4 min af- Figure 14 shows a hypothetical wind field thus ob- ter the passage of the gust front and reached a max- tained around Kodama at 1436 JST. A strong diver- imum at least 12 min later. The complex variation gent wind whose center is located between Kodama of the wind direction at Kodama EMS at around and FKD (hereafter referred to as downburst A) is 1425 JST did not exist at FKD. Furthermore, the seen. This figure clearly shows that a strong down- humidity decrease at the time when precipitation burst occurred between these two points around this stopped at FKD was not observed at Kodama EMS. time. To examine the detailed structure of the wind In addition to downburst A, a divergent wind pat- field that caused the damage in Kodama and Misato tern whosehorizontal scale is less than 5 km is found towns at around 1440 JST (see, Section 2), a time- on the data at Kodama EMS between 1420 and space conversion(Fujita, 1963) of wind data at sev- 1430 JST. This shows that another downburst oc- eral observational points around Kodama was made curred right near Kodama EMS (hereafter referred (Fig. 14). To this end, the direction and speed of the to as downburst B). Such a divergent wind pattern August 1997 H. Takayama, H. Niino, S. Wanatabe and et al. 901 except for the major downburst at around 1436 JST cated about 6-10 km behind the gust front (as was is not found on the wind data at FKD. It was noted estimated above from meteorological traces at Ko- in the temperature record at Kodama EMS that dama EMS). the temperature decreased suddenly at 1421 JST by A downburst is inherently an unsteady phenom- 1.2C. Figure 7 shows that the reflectivity contour ena. Therefore, strictly speaking, a time-space con- line of 40 dBZ was about to reach Kodama EMS at version is not applicable. We can only hope that about this time. These facts suggest that the tem- the method may delineate qualitative features of the perature drop was caused by a downburst initiated phenomena if the time interval for which the time- by falling hail. The heat loss due to melting of hail space conversionis applied is short enough to assure is likely to have caused an additional temperature that the flow field has changed little. In Fig. 14, drop. However,it is not possible to clarify why FKD the time-space-converted wind data for a period of did not experience a downburst similar to the one more than an hour is presented. It is obvious that at Kodama EMS from the. data we have. The region the method is not valid for such a long time inter- with reflectivity larger than 40 dBZ also reached val. Therefore, one should interpret the wind field FKD nearly at the same time. Note that FKD was shown in Fig. 14 with some reservation. If one puts located in the northeastern part of the storm. This a circle, the radius of which varies with the lifetime part may not have been suitable for mid-leveldry air of a particular phenomenon, and looks only within to be entrained and to cause stronger downdraft. the circle, however, this illustration is much more The region with reflectivitylarger than 50 dBZ at advantageous to a time-space conversion at a sin- 2000 m AGL at 1415 JST (Fig. 10) seems to have gle observational station. One can obtain some idea moved over Kodama EMS and have reached over of the two-dimensional distribution of the phenom- MJHS at 1434 JST. It is interesting to note that, ena, and yet it retains the time-space conversionat if the wind record showing the divergent wind pat- a single station in its original form. For example, it tern of downburst B at Kodama at around 1420 JST is clear that the wind fields associated with down- were to be shifted to the east-southeast direction by burst A and B are correctly captured from the data assuming the transitional velocity of the storm, it during several minutes around 1436 JST and those would have been located near MJHS at 1440 JST around 1425 JST, respectively. (see Fig. 14). This means that Kodama EMS at Now let us look at the time-space conversion of 1420 JST and MJHS at 1440 JST were located the wind data for somewhat longer time interval. A nearly at the same portion of the storm where a strong southerly divergent wind at Honjoh EMS be- strong downdraft was produced over the former. It tween 1450 and 1510 JST and a strong northerly is difficultto believe that downburst B which started divergent wind at Yorii EMS between 1440 and at Kodama EMS lasted more than 20 min (An av- 1500 JST are considered to originate from down- erage lifetime for downbursts in Colorado is about burst A which occurred between Kodama and FKD. 15 min (Wilson et al., 1984)). Rather, it is likely The spreading speed of the gusty wind originating that another downburst (downburst D) started from from downburst A was about 11 m s-1 to Honjoh the same portion of the storm near MJHS at around EMS and about 21 m s-1 to Yorii EMS respectively. 1440 JST and produced the damage at MJHS. Since The storm's movement at a speed of 8 m s-1 toward there was no observationalstation right near MJHS, east-southeast direction is considered to be respon- it is not possibleto prove the existence of downburst sible for this asymmetry in the propagation speed. D based on any meteorological data, except that At Fujioka EMS, a divergent wind pattern is seen Fl damage to houses was concentrated near MJHS between 1404 and 1420 JST. Since this is much ear- and a wind speed of about 50 m s-1 was estimated lier than the occurrence of downburst A at about from the broken windows of MJHS. If downburst A 1440 JST, it is considered to show another diver- or B, which were located 7 or 8 km distance from gent wind. In fact, the time when the divergent MJHS, had produced the damage near MJHS, there wind occurred corresponds nearly to the time of the would have been much more serious damage between first descent of the reflectivity core. If one exam- Kodama EMS and MJHS. ines the location of strong radar reflectivity in Figs. Figure 14 shows that, at FKD, the convergence 7,10 and 11, it seems that the descent occurred im- of the weak northerly wind is detected at around mediately south of Fujioka. Therefore, it is likely 1420 JST. At Kodama EMS, the convergence of that the divergent southerly wind between 1404 and the weak southerly wind can be seen at around 1420 JST was caused by another downburst (down- 1410 JST. These convergent winds at FKD and burst C) associated with this descent of the reflectiv- Kodama EMS nearly correspond to the time when ity core. The distribution of the temperature drop the temperature started to decrease and humidity (Fig. 12) and the extent of the divergent wind (Fig. started to rise: The wind convergence line corre- 11) shows that downburst C was not as strong as sponds to the gust front. It is evident from this downburst A, which occurred around Kodama EMS figure that two (possibly three) downbursts were lo- and FKD. The cold air supplied by downburst C also 902 Journal of the Meteorological Society of Japan Vol. 75, No. 4 is located behind the gust front of the storm (indi- age scale of F1 seems to be indicative of another cated by the convergenceline at 1350 JST in Fig. 14) downburst near MJHS. and is considered to have contributed to the main- The third downburst occurred between Kodama tenance of the gust front. These facts show that the EMS and Kodama District Fire Station at around storm produced at least three (possibly four) down- 1434 JST. A remarkable divergent wind was iden- bursts from 1400 to 1440 JST in the southeast part tified both from the instantaneous wind data and of the Gunma prefecture and in the north part of the time-space converted data at these two sta- the Saitama prefecture. tions. Again an increasein the width of the damage- producing hail region and a descent of a reflectiv- 5. Summary and Concluding remarks ity core was observed near the time and location Evolution of the strong damage-producing wind of the downburst. The downburst center was lo- which occurred in the northwestern part of the cated about 6-8 km behind the gust front. The di- Saitama prefecture at about 1440 JST on 8 Septem- vergent wind originated from this downburst spread ber 1994 and the storm which produced the strong over the region of 40km diameter. After the occur- wind were analyzed by using the data from a dam- rence of this large downburst, the storm dissipated age survey, surface meteorological stations, Mt. very quickly. Fuji radar, satellite picture and upper air sound- The upper air soundings showed that environ- ings. It was concluded that the storm produced mental condition before the passage of the storm at least three downbursts in the south part of the was suitable for the occurrence of wet downbursts: Gunma prefecture and in the northwestern part of CAPE remained larger than 1000 m-2 s-2 after the Saitama prefecture between 1400 and 1440 JST. 2100 JST on 7 September and reached more than Within 8 km distance from Misato Junior High 1800m-2 s-2 right before the storm. It decreasedto School (MJHS), where 71 students and 2 teachers less than 700 m-2 s-2 during the 3 hours of the pas- were injured by the glass broken by the gusty wind, sage of the storm. The difference of equivalent po- at least two downbursts occurred; one between Ko- tential temperature between low- and middle-levels dama environmental monitoring station (EMS) and continued to be larger than 20.0 K after 1400 JST Kodama District Fire Station (FKD), which are less on 7 September (cf. Atkins and Wakimoto, 1991). It than 3 km apart, and the other right near Kodama decreased to less than 16 K after the storm passed. EMS. Unfortunately, the Doppler radar at Meteorolog- The storm which caused the downbursts was ical Research Institute was not in operation at the generated as a part of a line-shaped echo around time of the downburst so that information on air 1200 ST on 8 September. It moved eastward and flowsin the storm or detailed vertical and horizontal lasted until 1500 JST. The storm had a marked over- structures of the storm were not obtained. Never- hang in the direction of its movement. It started to theless, the dense network of the surface meteorolog- produce hail after 1340 JST. The cloud-top reached ical stations in this area were effectivein identifying 15 km AGL at 1400 JST. The region of the damage- surface airflowsand temperature change associated producing hail corresponded well to that of strong with the downbursts. As was mentioned in the in- reflectivity larger than 40 dBZ observed by the Mt. troduction, the northwestern part of the Kanto plain Fuji radar. The first downburst occurred to the has the largest frequency of thunderstorms in Japan. south of Fujioka EMS when and where the width At the same time, the Kanto plain has the densest of the radar reflectivity larger than 40 dBZ and the network of surface weather stations and other me- width of the damage-producinghail region increased teorological instruments including radars. There- markedly. The descent of a reflectivity core was ob- fore, a further project with Doppler radars and up- served near the time of the downburst. The down- per air soundings such as TAPS should be continued burst was located about 9 km behind the gust front. in this area to accumulate detailed case studies and The second downburst occurred near Kodama to reveal statistical characteristics of downbursts in EMS at around 1425 JST. Before the second down- Japan2. A preliminary study in this direction has burst started, the temperature was already 9 K been put forward by Ohno et al. (1996). colder than it was before the gust front passed (i.e., the downburst was located about 6 km behind the Acknowledgment gust front). When the downburst started, how- The authors are grateful to the prefectural offices ever, a further temperature decrease of 1 K was of Saitama and Gunma; the town offices of Misato observed. As the storm moved east-southeast, the and Kodama; the Kodama District Fire Station; the portion of the storm which produced the second Misato Junior High School; and the Tsukuba Area downburst in Kodama passed right over MJHS at around 1440 JST when the damage-producingwind 2 Afterthis paperwas submitted, a remarkabledownburst occurredin ShimodateCity, Ibarakiprefecture, on 15 occurred there. Though there is no meteorological July1996, causing 20 injuries (one person died afterward) record near MJHS, this fact and the estimated dam- and damaging425 houses. August 1997 H. Takayama, H. Niino, S. Wanatabe and et al. 903

Precipitation Studies group. The authors would like Iwashita, H., 1995: The downbursts at the Osaka Inter- to express their thanks to Mr. N. Yamaguchi of the national Airport (Itami) on 24 August 1988. Tenki, Building Research Institute for cooperation in the 42, 833-842 (in Japanese). estimation of the maximum wind speed of the down- Japan Meteorological Agency, 1972: Climatic Atlas of burst, to Messrs. O. Suzuki and H. Seko for provid- Japan, 90pp. ing the program for calculating CAPE, and to Mrs. Kajikawa, 1988: Disasters caused by hailstorms and downbursts in the southern Akita prefecture on Au- K. Yoshiye for drawing figures. Thanks are extended gust 6, 1985. Natural Disaster Sci., 7-1, 37-48. to Dr. H. Ohno and Mr. K. Kusunoki, whose com- Kessinger, C.J., RD. Roberts and K.L. Elmore, 1986: ments on the early version of the manuscript were A summary of microburst characteristics from low- invaluable. reflectivity storms. Preprints, 23rd Conf. on Radar Meteorology, Snowmass, Amer. 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Izeki, 1985: An example of mi- Fujita, T.T., 1971: Proposed characterization of torna- croburst at Toyama Airport on July 27 1983. Tenki, does and hurricanes by area and intensity. SMRP 32, 329-332 (in Japanese). Res. Paper 91, Univ. of Chicago, 42 pp. Ohno, H. and O. Suzuki, 1991a: Downburst north of Fujita, T.T., 1976: Spearhead echo and downburst near Lake Kasumigaura in the Kanto plain. Preprints, the approach end of a John F. Kennedy Airport run- the Spring Meeting of the Meteor. Soc. Japan, A113 way. SMRP Res. Paper 137, Univ. of Chicago, 51 (in Japanese). pp. Ohno, H. and O. Suzuki, 1991b: Microburst? Haz- Fujita, T.T., 1978: Manual of downburst identification ardous divergent wind in Kanto plain summer af- for Project NIMROD. SMRP Res. Paper, 156, Univ. ternoon 1990. Preprints, 4th International Conf. on of Chicago, 104pp. Aviation Wea. System, Paris, 167-167. Fujita, T.T., 1980: Downbursts and microbursts - An Ohno, H., O. Suzuki, K. Kusunoki, H. Nirasawa and aviation hazard -. Preprints 19th Conf. on Radar K. Nakai, 1993: A severe downburst in Mito city Meteorology,Miami, Amer. Meteor. Soc., 94-101. on 4 September 1992. Preprints, 26th Conf. on Fujita, T.T., 1985: The Downburst: Microburst and Radar Meteorology,Norman, Oklahoma, Amer. Me- Macroburst. SMRP Res. Paper, 210, Univ. of teor. Soc., 221-222. Chicago, 112 pp. Ohno, H., O. Suzuki, H. Nirasawa, M. Yoshizaki, N. Fujita, T.T., 1992: The Mystery of Severe Storms. Univ. Hasegawa, Y. Tanaka, Y. Muramatsu and Y. Ogura, of Chicago, 298 pp. 1994: Okayama downbursts on 27 June 1991: Down- Fujita, T.T. and R. Wakimoto, 1983: Microbursts in burst identifications and environmental conditions. JAWS depicted by Doppler radars, PAM and aerial J. Meteor. Soc. Japan, 72, 197-222. photographs. Preprints, 21st Conf. on Radar Me- Ohno, H., O. Suzuki and K. Kusunoki, 1996: Climatol- teorology, Edmonton, Alberta, Amer. Meteor. Soc., ogy of downburst occurrence in Japan. 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1994年9月8日埼玉県北西部で発生したダウンバースト

高山大 (気象研究所) 新野宏 (東京大学海洋研究所) 渡辺真二 (東京管区気象台) 菅谷重平 (新東京航空地方気象台) つくば域降雨観測実験グループ

1994年9月8日の午後、突風と降電を伴う強い雷雲が群馬および埼玉県を通過した。埼玉県北西部の美里町立美里中学校では、校舎の窓ガラスが突風で割れ、教師2人と71人の生徒が負傷した。被害調査、地上観測、高層観測、静止衛星、現業レーダーなどのデータを用いて、雷雲とこれに伴う突風の解析を行った結果、この雷雲に伴って少なくとも3つのダウンバーストが発生したことがわかった。主な解析結果は以下の通りである。 3時間以上長続きしたこの雷雲は、約8m/sのスピードで東南東進し、発達期以降はその直下で10度近くの気温低下と発散風を伴っていた。成熟期には雲頂は約15kmに達し、レーダーによる反射強度の分布は進行方向に対してオーバーハング構造を呈していた。被害調査による雷雲下の降電域の幅は2カ所で顕著な拡がりを示した。この拡がりの見られた場所と時刻は、レーダーで観測された反射強度の核の降下の場所・時刻と一致していた。更に、数地点の地上気象観測データの時空間変換から求めた水平風の分布には、降電域の拡がりにほぼ対応した場所・時間に明瞭 August 1997 H. Takayama, H. Niino, S. Wanatabe and et al. 905

な発散風(ダウンバーストAおよびC)が見られた。ダウンバーストAは、児玉環境大気測定局(KD)とそこから約3kmに位置する児玉郡市広域消防本部の中間で生じたことが、両地点の風向風速記録から明瞭に読みとれる。雷雲通過による降温はこの2地点付近で最も大きく、11度以上に達した。ダウンバーストA は最終的には差し渡し40kmの範囲にまで広がった。雷雲はダウンバーストAを生じた後、急速に衰弱した。 KDの自記紙にはダウンバーストAとは別の更なる気温降下と風の発散が記録されており、近くでダウンバーストBが発生したことを示している。KDでダウンバーストBを発生させた雷雲の部分は、被害を引き起こした突風が吹いた時刻には約8km離れた美里中上空をちょうど通過していた。美里中付近の気象観測資料はないが、被害調査やこれらの事実から、美里中近くで第4のダウンバーストが発生した可能性が示唆される。これらのダウンバーストはすべて、ガストフロントの6～10km後方で発生した。

雷雲周辺のCAPEは、雷雲通過前後の3時間で1800 m2/s2から700 m2/s2以下に減少した。相当温位の下層の極大値と中層の極小値との差も同様に、26Kから16 Kに低下した。