Author Topic: Meteotsunamis (meteorological tsunamis) also known as Seiche's, milghuba, abiki  (Read 13142 times)


electrobleme

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Meteorological tsunamis (Meteotsunami): Destructive atmosphere-induced waves observed in the World Ocean
Meteorological tsunamis: Destructive atmosphere-induced waves observed in the World Ocean .pdf

"Meteorological tsunamis” are very similar to ordinary tsunamis; they have the same temporal and spatial scales and affect the coast in a similar destructive way, despite that the generation mechanism of these waves is quite different

"Meteorological tsunamis” are produced by atmospheric processes (atmospheric waves, pressure jumps, squalls, frontal passage, etc.) and have strongly resonant nature

"Meteorological tsunamis” are regularly observed at the same sites with pronounced local resonant properties and  commonly have some local names (“rissaga”, “abiki”, “šciga”, “death waves”, “marrubio”, “milghuba”, … )

"Meteorological tsunamis” are produced by the resonant superposition of internal factors (pronounced resonant properties of a specific bay or harbour) and external factors (strong atmospheric disturbance resonantly interacting
with open-ocean waves)

In general, the meteorological tsunamis have a multi-resonant generation mechanism. The needed coincidence of several resonant factors significantly diminishes the possibility of occurring such events, which is the main reason why these phenomena are rare and restricted to specific locations.


electrobleme

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Generation of Meteorological Tsunamis (Large Amplitude Seiches) Near the Balearic and Kuril Islands
ALEXANDER B. RABINOVICH and SEBASTIAN MONSERRAT
Generation of Meteorological Tsunamis (Large Amplitude Seiches) Near the Balearic and Kuril Islands .pdf

paragraphs from various parts of the research paper

Abstract. Extreme atmosphere-induced seiche oscillations occasionally occur in specific inlets and bays of the world ocean causing severe damage to coastal areas, ships and port constructions.

1. Introduction
Extremely strong seiche oscillations are regularly observed in specific sea areas around the world....These oscillations have the same temporal and spatial scales as ordinary tsunami waves and affect coasts in a similarly destructive way, but they are related to meteorological rather than to seismic forcing...Ciutadella Harbour (Menorca Island, Spain) is one of the places where such meteorological tsunamis are quite common. They are known by the local name of ‘rissaga’

Two key questions arose from this analysis:
Why are disastrous seiche oscillations generated only in some specific places
in the world ocean, in particular in Ciutadella inlet?
What kind of external factors (conditions) cause these strong events?


The main purpose of the present study is to use the observation data to find an answer to the second question. Comparative analysis of seiche oscillations and atmospheric pressure fluctuations shows an evident correlation between these two processes. At the same time the nature of this correlation is far from straightforward:
sometimes relatively weak pressure fluctuations generated significant seiches or, vice versa, strong atmospheric events did not cause noticeable seiche oscillations.

It is interesting,however, that for a few cases when relatively strong seiches were observed in Krabovaya (oKS > 7 cm), seiches in Malokurilskaya and Otradnaya bays were much weaker than the regression coefficients in (1) suggest.



The event on 10–11 August 1989 was another and maybe even more interesting example. For this event all instruments were in operation and accurately recorded the corresponding abnormal atmospheric pressure/sea level oscillations
in the investigation area (Figure 4). The PP record clearly shows a strong zigzaglike disturbance of atmospheric pressure with a double amplitude of about 200 Pa (2 mbar) which arrived at the Palma instrument about 19.00 UT on 10 August. This
disturbance caused a noticeable intensification of seiche oscillations in Palma (PS) and Sol de Mallorca (SS). The same disturbance (but about twice the amplitude) reached the region of Ciutadella approximately one hour later, generating largeamplitude seiches in the inlet with a maximum trough-to-crest wave height of 87 cm slowly decaying with time (see CP and CS records in Figure 4). About 4 hours later, another atmospheric disturbance arrived first at Palma and then at Ciutadella, causing a significant increase in seiche oscillations. Just after this second disturbance a maximum wave height of 103.8 cm was recorded in Ciutadella inlet. There were no strong atmospheric pressure disturbances afterwards and subsequently the seiche oscillations in Ciutadella inlet slowly damped over a period of about 20 hours, apparently because of continuous energy pumping from the open sea and the the high Q-factor of this basin. This type was addressed in RM1 as an ‘impulse’ type and we clearly see that the trains of seiche oscillations were caused mainly by two impulses of atmospheric disturbances.

From Figure 5 we see that the first atmospheric disturbance (1P), ranging from 200 Pa (2 mbar) in Palma to 400 Pa (4 mbar) in Ciutadella, generated seiches in the inlet with a maximum height of 87 cm, which was reached during the third
oscillation. The wavelength of the atmospheric disturbance was two orders higher than the length of the inlet (about 1 km), so the direct generation of seiches is highly unlikely (this question was discussed by Gomis et al., 1993). The dominant
atmospheric period (1 hr) was much greater than the resonant period of Ciutadella Harbour (10.6 min). Thus, the observed strong seiches could not be explained by local resonance in the harbour. We may hypothesize that the atmospheric disturbance generated long waves in the open sea and then these waves approached the western coast of Menorca, exciting the large seiches in Ciutadella.


Thus, the increase of seiche intensity normally corresponds to atmospheric activity with the exception of seiches generated by tsunamis (cf. Murty, 1977) or internal sea waves (cf. Chapman and Giese, 1990). However, the opposite is not true: sometimes even very strong atmospheric events do not excite seiches. For example, a train of significant atmospheric waves propagated over Shikotan Island on 15 May 1991 but did not cause any seiche oscillations at Malokurilskaya, Krabovaya, and Otradnaya bays (Figure 2a, b). The same situation was observed in the region of Ciutadella on 31 August, 9 and 13 September 1990 when atmospheric pressure disturbances did not generate noteworthy seiches in the inlet (Figure 6a). We also see that the disturbances of very similar intensity on 24 and 25 September 1990 both generated large seiches, but of different magnitude, in the latter case they were much stronger than in the former (Figure 6a).

The atmospheric pressure fluctuations were weaker in Ciutadella (in 1990) than in the region of Shikotan. However, the sea level oscillations were significantly stronger in Ciutadella. The corresponding regression coefficients for Ciutadella are
approximately 5 times larger than for Shikotan. Thismeans that the extreme seiches observed in Ciutadella are related specifically to certain resonant features of the inlet itself but not to extraordinary atmospheric events occurring in the region. The correlation between atmospheric pressure and sea level is significant but not high. So, we can conclude that there are some other important parameters of atmospheric processes (besides intensity) which determine heights of the generated seiches. In particular, the effectiveness of seiche generation probably depends on the amount of atmospheric energy at resonant frequencies rather than on the total energy of the atmospheric disturbances.

Weak spatial coherence of high-frequency background atmospheric processes demonstrates that they cannot be an effective mechanism of the generation of sea level oscillations. The whole picture changes dramatically during strong atmospheric events.


The general tendency of the correlation between atmospheric pressure and sea level is clear: stronger atmospheric disturbances normally generate larger seiche oscillations. However, the detailed character of this correlation is not so simple: there are several exceptions to this general tendency; now and then even quite powerful disturbances do not excite noticeable seiches.

Also, it was found that the phase speed of atmospheric waves in some spectacular cases of significant rissaga waves was about 30 m/s, i.e. very close to the phase speed of long waves on the shelf of Mallorca. Moreover, for long waves
generated by atmospheric disturbances propagated over the Balearic Islands from the southwest, the southeastern shelf of Mallorca Island acts as a wave guide. The associated wave trains crossing Menorca Channel between Mallorca and Menorca islands would head specifically for the area of Ciutadella, this probably being the reason for the abnormally large seiches generated there. The orientation of Ciutadella inlet itself, in this particular direction, is a key factor amplifying the
corresponding effect.


Another interesting result was the different character of formation of background seiches, continuously existing in the basins, and extraordinary events occurring only occasionally. One of the reasons for this difference is that the background
oscillations are formed by many various mixed sources, including tides, internal waves, wind, wind waves, long waves  coming from remote regions, etc. Generation of particular events (significant seiches) is determined by the specific dominant
forcing, mainly by the atmospheric disturbances moving in the vicinity of the observation regions. Another reason is that, as was found, the spatial scales of coherence of atmospheric processes (and therefore the forcing area) increase significantly
during strong atmospheric events. However, probably the most important reason is that these specific events are the result of the resonance response of the sea surface to the atmospheric forcing, so that relative amplification of such seiches
is much stronger than usual.


electrobleme

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La Rissaga , Menorca - roll cloud linked to tsunami?
« Reply #3 on: September 15, 2009, 07:46:19 »
La Rissaga , Menorca - roll cloud linked to tsunami?

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Just back from holidays in Menorca (one of the Balearic islands) and last Thursday night I observed a very strange cloud formation. I was on the south coast of the island.

Here's the formation approaching us from the south:

and here it is going overhead (at a very fast pace, i must add):



The formation stretched from horizon to horizon, and proceeded northwards in an almost tsunami-wave like manner. It was a jaw dropping sight to see.

That same night , the port of Ciutadella was inundated by an actual 4 metre high tsunami. Luckily , nobody was hurt, but an awful lot of property and boats were damaged.

The phenomenon is known as "La Rissaga" - Here's a page on it with a nice animated GIF mid way through that explains how a wave of athmosphearic pressure can cause a meteotsunami - a tsunami that isnt caused by earthquakes, but rather, a pressure wave.

I suspect that the sky wave i photographed is part of this phenomenon.

Feel free to repost my photos - here are links to the high res versions:

http://static.flickr.com/66/168579159_8dd60c7ea7_b.jpg
http://static.flickr.com/49/168579158_41516d9e5f_b.jpg

Lots of British tourists were snapping away as this thing passed over - many with digital cameras.
I'm not the only one who photographed this, so I expect more pictures of this to surface over the next week or so, as tourists fly back home.

My photos really dont do the formation justice - if only i had one of those panoramic cameras! Believe me, this stretched from horizon to horizon - it was an incredible sight, and it passed over us at , i would estimate, roughly 50 to 70 km/h

The nearest thing i can compare to is a tsunami wave - except , this one was in the sky. it had those kind of qualities - a tight formation, that appeared to be rolling over and over. kicking myself i didnt have enough memory left on my camera to take a video shot.
La Rissaga , Menorca - roll cloud linked to tsunami? - ukweatherworld .co.uk