**5. Pressure jump of Nagasaki 1979 'Abiki' event**

of Philadelphia (see the upper left panel in **Figure 6**) and moved eastward over the continental shelf. The wave was reflected along with the edge of the continental shelf and back to the coast. The body wave propagated across the edge after 17:00Z, crossing with the reflected wave. The body wave seemed to have disappeared at 20:00Z, which travelled longer than 300 km along the coastal shelf. The area of the maximum wave height higher than 10 cm corresponded with

**Figure 5.** A schematic of the vertical structure of derecho (upper panel) and observed barometric pressure anomaly during the passage of derecho (lower panel) after case study on June 13, 2013 event, by Wertman et al. (2014) [37].

**Figure 6.** A sequence of the meteorological tsunami propagation computed by NOAA Pacific Tsunami Warning Cen-

tre. (Available from https://www.youtube.com/watch?v=ykABRe5Yt3c [38]).

the area passing the body wave with resonance effect.

22 Tsunami

The system with the strong intrusion of the cold dry air can cause the abrupt pressure jumps with weak precipitation or without precipitation. **Figure 7** shows the satellite image and the observation records of meteorological tsunami by abrupt pressure jumps at Nagasaki Bay in west Kyushu, Japan [25]. The maximum wave height showed 278 cm at the third wave in tidal station of Nagasaki port. Some references cited the maximum wave of 4.8 m at this event [14], but that value was estimated value using Green's formula in Eq. (3). Abrupt pressure jump of 5.9 hPa per 30 min was recorded at Meshima about 140 km southwest from Nagasaki. The pressure jump decayed as the system moved to east and the intensity was 3.0 hPa per 20 min. Hibiya and Kajiura (1982) [5] proposed that the pressure jump was generated at the region near Shanghai, China, while Akamatsu (1982) [25] suggested that pressure jump was generated more towards the east side near Meshima. The satellite visible image showed that there was clear boundary between the stratiform cloud and the dry air in the middle of the East China Sea.

**Figure 7.** Meteorological tsunami of Nagasaki Bay caused by abrupt pressure jump on March 31, 1979. Satellite visible image by Geostationary Meteorological Satellite (left), barometric pressure recorded at Meshima (middle) and tidal amplitude observed at Nagasaki Bay (right) (Akamatsu 1982) [25].

**Figure 8.** Distribution of the wet and dry air and horizontal wind in 500 hPa (left), 700 hPa (middle) and 925 hPa (right) isobar surface on 00Z, March 31, 1979. The reanalysis data of JRA-55 (Kobayashi et al. 2015) [39] was used.

The long-term reanalysis data from Japan Meteorological Agency (JRA-55) [39] in **Figure 8** indicated that the lower troposphere over the East China Sea was very dry (smaller than 3 g/kg in mixing ratio) and moist air came from south China region in 700 hPa isobar surfaces. The moist air lifted upward orographically in the mountain range of south China and vapour front formed in the mid-troposphere. Around the vapour front, the dry air cooled by re-

**Figure 9.** Air particle distribution into East China Sea. The left panel indicates the analysis area with initial particles located in region X. The middle panel shows the distribution of the individual particles 72 h before the meteorological tsunami over the East China Sea. The colour markers indicate the altitude of each air particles. The right panel indicates the particle proportion in each sector shown in the left panel [26].

evaporation of the cloud liquid water and settled down. The pressure jump was generated in the dry sector of the vapour front in the mid-troposphere. The back trajectory analysis of the air particle [39] depicted in **Figure 9** showed that the air particle in the East China Sea during the meteotsunami event came from mainly two or three regions. The warm moist air mainly came from South China Sea, Indochina Peninsula and Bengal Bay. Those air particles moved northeast along with the high-pressure system located in the Philippines, and lifted orographically over the inland area of the South China (region 'SW' and 'S'). Other particles came from upper dry air of northwest Eurasian continent or along with the subtropical jets via Tibetan Plateau (region 'NE' and 'E'). Further analysis in **Figure 10** showed that the anomalous mass of the moist air in the lower troposphere was transported from the tropical regions within 3 days before the meteotsunami event on March 31, 1979. The peak value of the moisture transport in the end of March (~200 kgm/s) was much greater than the peak value in the summer monsoon season (~150 kgm/s). The massive transport of the lower moist air into inland China can be seen especially in the late winter and early spring (February–April) for nearly every year. In the same period, the secondary oscillations larger than 1.0 m in amplitude were

**Figure 10.** The northward component of water vapour flux across the line of South China coasts (22.5N, 105–120E) after the vertical integration between 850- and 1000-hPa isobar surface. Red line indicates the 3-day average from 6-h data in the year 1979. Blue line indicates the 10-day average of climatic value for 55 years (1958–2012) [26].

measured in the west Kyushu [27] nearly for every year. Hence, fluctuation of the vapour transport from tropical region to inland China can be key information to provide the first guess of the atmospheric disturbance causing meteorological tsunamis.
