**4. Mesoscale convective systems**

The west coast of United States has complicated shoreline shapes, similar to the Adriatic

East Asia region is one of the major spots of meteorological tsunami. There are several characteristics that can enhance meteotsunami much easier than other coastal regions. The unstable layer in the middle troposphere can easily form because a couple of jets in the upper air get closed in the leeward of the Tibetan Plateau. The lifting of the wet moist air from lower latitude and the descending of the upper dry air from continental region makes the so-called wave-ducting layer often. A wide coastal continental shelf is extended from China to the Islands of Japan. The equivalent speed of the ocean long wave ranged 20–40 m/s except the Okinawa trough near the Kyushu Island, Japan. What is more, there are many small inlets having eigen periods ranging between several minutes and several hours. In March 1979, a giant meteorological tsunami (*abiki* in local name) hit on the Nagasaki Bay in Kyushu, Japan, by passing abrupt pressure jump of 5.9 hPa per 30 min. The tide gauge observed the maximum wave height of 2.78 m [25]. A large meteorological tsunami also caused by train of pressure wave with the amplitude of 0.5–2.0 hPa, with the wave length of 20–100 km [26, 27]. In the west Kyushu, most of meteorological tsunami are likely observed in the season between February and early April [28, 29]; however, the severe squall system such as Baiu front sometimes generates the strong downdraft to meteorological tsunamis [30]. On the west of the Korean Peninsula, maximum amplitude of 1.4–1.6 m recorded in March 2007 and May 2008 with the pressure jump of 2–5 hPa travelled over the Yellow Sea [31]. In the Bohai Sea of China, sea level oscillations with the amplitude higher than 1.0 m are often observed; however, the oscillation period was much longer (56–160 min) than that reported in other coasts (nearly 5–20 min). Such large amplified oscillation remained unclear as to what kind of the atmospheric sources induces such a long period wave [32]. The meteorological tsunami hit on the Kuril Island and the Kamchatka when the developing low approached the area from Hokkaido, Japan. In recent

Coasts. However, the wave amplitude for each event was not so large [24].

20 Tsunami

years, the wave height recorded was higher than 1.2 m in February 2010 [33].

across the shoreline due to run-up of the wave [36].

meteotsunamis in the tropical region such as Sri Lanka and India.

Some destructive events have been introduced in recent papers, which occurred on the southern hemisphere. The west coast of Australia caused the meteorological tsunami resulting from squall line or frontal passage in the low pressure system. The wave height of 1.0 m was recorded in the frontal passage on June 12, 2012 [34]. Similar magnitude of meteorological tsunami was measured on the coast of New Zealand in April 2002 (*rissaga* in local name) [35]. A large flooding damage by meteorological tsunami 2.9-m wave height occurred on the west coast of South Africa on August 7, 1969. In that event, the flooding damage of housings and parked automobiles was brought within the area of 2 km along the shoreline and 100–200 m

It is possible that other coastal areas hit the meteorological tsunami if the multiple resonant conditions were satisfied. However, the recorded wave height were smaller than 1-m in those coastal areas. For example, Pattriaratki and Wijeratne (2015) [14] introduced smaller The mesoscale convective system (MCS) is one of the most important phenomena to generate pressure jumps at sea level. The horizontal scale of the individual convective cell is an order of ~10 km, with the lifetime of several tens of minutes. The vertical wind speed sometimes becomes as high as 20 m/s under the strong unstable atmosphere with the convective available potential energy (CAPE) higher than 1000 J/kg. A high pressure anomaly, of the order of 1–2 hPa, generates at the surface level inside the convective cell due to the downdraft. To travel a long distance satisfying the Proudman resonance, it is significant to form a series of the convective system including multiple cells such as squall line.

Derecho is one of the systems with a long lifetime, which can move fast and long enough to have Proudman resonance on the ocean surface. The lifetime of the derecho is longer than 6 h travelling hundreds to thousands of kilometres as seen in **Figure 4** [20]. Some of the severe systems bring very strong gusts higher than 40 m/s (~90 miles/h) with the destructive damage of housings in the land area. A schematic of the derecho structure and sea level pressure fluctuation is shown in **Figure 5** [37]. The organized convective systems move towards the warm sector parallel to low-level thickness lines with the mean tropospheric flow. The system moves as high as 15–25 m/s (50–90 km/h), when the velocity of the mean tropospheric flow is very fast against the horizontal wind velocity of the low-level moist air into the convective system.

**Figure 4.** A mosaic composite of radar reflectivity (dBZ) image indicating the development and evolution of the derecho during June 29 and 30, 2012. Unitless numerical values indicate observed wind gusts (m/s). (Image by G. Carbin, NOAA/NWS Storm Prediction Center [22]).

In the US East Coast, the derecho had moved from land to ocean and the tsunami propagated towards offshore with Proudman resonance. **Figure 6** shows a sequence of the wave propagation simulated by the Pacific Tsunami Warning Centre, NOAA, as an example of June 13, 2013 [38]. The body wave, surrounded by a rectangle marked as 'a', generated along the coast 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 the area passing the body wave with resonance effect.

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