METEOTSUNAMI IN MALI LOŠINJ

Nataša Strelec Mahović, Meteorological and Hydrological Service of Croatia

Danijel Belušić, Department of Geophysics, Faculty of Science, University of Zagreb


INTRODUCTION

On 15 August 2008, at about 17 UTC, a sea wave flooded the harbor of Mali Lošinj on the island Lošinj in the northern Adriatic. The flood was about 50 to 80 cm high above the ground what, together with the height of the ground above mean sea level, gives the wave height (trough-to-crest) of more than 2 m.

This study is aimed to show that the flooding was caused by a METEOTSUNAMI.

What is a METEOTSUNAMI?

Meteotsunamis are large amplitude sea waves generated by a propagating atmospheric disturbance. They are known in all parts of the world ocean, usually under their local names such as Rissaga in Spain, Milghuba on Malta, Marrubio in Italy and Abiki in Japan. However, their impact to the coastal regions is more pronounced in the low-tidal basin, as the infrastructure is not adopted to the large and rapid sea level variations and associated ocean currents.

Conditions for the generation of such a wave are:

  • Strong small-scale atmospheric disturbance
  • Propagation of the atmospheric disturbance toward the entrance of a harbor
  • External resonance – between the atmospheric disturbance and ocean waves
  • Large amplification factor of the harbour of bay
  • Internal resonance – between the dominant frequency of the arriving open-sea waves and the harbor mode frequency

It appears from previous Adriatic studies that the speed of the propagating atmospheric disturbance was ~22 m/s to cause the resonance and amplification of the long ocean waves. This speed is dependent on the depth of the basin - sqrt(gravity x depth), which is in the northern Adriatic approximately 50 m. However, the speed may be higher (e.g. around 30 m/s, as estimated for the Balearic meteotsunamis which are being generated over 70-120 m shelf) or lower (e.g. off the coast of Florida) - the important condition is that the speed of the atmospheric disturbance and the speed of the long ocean waves are quite similar!

In Mali Lošinj case the atmospheric disturbance was present in form of a pressure jump, as seen in the microbarogramme:

The pressure was falling slowly throughout the day, but then, around 13 UTC it started to fall more rapidly and until 17 UTC it fell for about 5 hPa. Then at 17 UTC it suddenly rose for about 4 hPa.

The question is: What was the cause of the abrupt pressure change?

To answer this, meteorological set-up of the case is presented.

SYNOPTIC SITUATION

On 15 August 2008 the upper level flow over the Adriatic was south-westerly, on the leading side of a deep upper-level trough. Frontal parameter shows that convective development occurred within the frontal zone, during the passage of the front. A maximum of warm air advection is found in the region of the Northern Adriatic and Showalter index value is negative and less than -3 in the whole region!

Satellite images reveal the presence of a large MCS over the Northern Adriatic and Alpine region. In the southern part of the system overshooting tops (seen as red dots) can be noticed traveling towards the island Lošinj an reaching it at about 16:45 UTC. Cloud-top temperature in this spots is about -70°C. Overshooting tops develop due to very strong updrafts in the convective cloud.

The mean-sea level pressure chart from the ALADIN model (ALARO/HR) show that the model forecast the development of a small low near the island Lošinj at 16 UTC.

At nearly the same time convective development is expected by the model, as seen in CAPE image for 17 UTC.

Looking at the wind field, a line of convergence forming 16 UTC in the northern Adriatic and traveling towards the island Lošinj appears as the probable cause for convective development.

INVESTIGATION

From the analysis and model data presented it can be assumed that the passage of a convective cloud had something to do with meteotsunami development. Since the meteotsunami, as mentioned in the introduction is caused by the traveling small-scale atmospheric disturbance it is clear that the general synoptic situation could not be the cause of the wave development. There must be a mechanism producing a small scale pressure or wind disturbance that can then, assuming that it propagates with the right speed and towards a bay or an inlet, cause meteotsunami development. The mechanism of development may have something to do with the convective cloud aloft, since the strong vertical motion necessary have pressure change in the lower levels as consequence. If we assume that the pressure (or wind) disturbance has formed due to the processes within the convective system, we still need an information about the speed of the system and it's direction. For the case of Mali Lošinj meteotsunami this has been derived using the nowcasting tools based on satellite data.

In the image below IR satellite image overlayed with atmospheric motion vectors is shown.

The southern part of the system was moving towards the island Lošinj. As the speed is not clearly readable from that image, here is the image with velocities in which the speed is more clearly seen:

In the time preceding the inundation of the harbor, the velocity of the system moving towards the island Lošinj was between 20 and 25 m/s.

DISCUSSION

Besides the case of Mali Lošinj there are three more cases known in the Adriatic in the past. They have all been analyzed in details and the analyzes have been published. The image below shows the locations of all known meteotsunami cases in the Adriatic.

Synoptic situations were similar in three of four cases, whereas in the case of 27 June 2003 large scale streaming was a bit different, as seen in the image below:

But in all these cases a convective system was present in the area! For example, one can easily detect it during the 2007 Ist meteotsunami:

Therefore, a new idea that came up from the analysis of all known cases is that convective system traveling above the sea causes the pressure or wind disturbance which, if other circumstances are favorable, can cause meteotsunami formation. The reason for that idea was the fact that in all Adriatic cases a convective cloud or system was present and moving towards the affected area.

The question is: What is the atmospheric origin of meteotsunami development in other areas of the world? and another: If we assume that there is connection with convective system aloft, are there cases in other parts of the world ocean that developed in similar situations?

To answer these questions an international conference on meteorological tsunamis has been organized in Vela Luka on the island of Korčula, Croatia, on the 30th anniversary of the strongest Adriatic meteotsunami that hit Vela Luka on 21 June 1978. As the result of this conference, a special issue of the journal Physics and Chemistry of the Earth was published, with 12 papers documenting and explaining the meteotsunami occurence in different seas all over the world. The papers can be found on http://www.sciencedirect.com/science/journal/14747065.

SOME REFERENCES

Šepić, J., Vilibić, I., Belušić, D., 2009. The source of the 2007 Ist meteotsunami (Adriatic Sea). Journal of Geophysical Research – Oceans, 114, C03016, doi:10.1029/2008JC005092.

Drago, A., 2008. Numerical modeling of coastal seiches in Malta, Physics and Chemistry of the Earth, 33, 260-275.

Vilibić, I., Monserrat, S., Rabinovich, A.B., Mihanović, H., 2008. Numerical modelling of the destructive meteotsunami of 15 June 2006 on the coast of the Balearic Islands. Pure and Applied Geophysics, 165, 2169-2195.

Vilibić, I., 2008. Numerical simulations of the Proudman resonance. Continental Shelf Research, 28, 574-581.

Belušić D., Grisogono, B., Klaić, Z.B. 2007. Atmospheric origin of the devastating coupled air-sea event in the east Adriatic, Journal of Geophysical Research - Atmospheres, 112, D17111, doi:10.1029/2006JD008204.

Monserrat, S., Vilibić, I., Rabinovich, A.B., 2006. Meteotsunamis: atmospherically induced destructive ocean waves in the tsunami frequency band. Natural Hazards and Earth System Sciences, 6, 1035-1051.

Vilibić, I. , Domijan, N., Orlić, M., Leder, N., Pasarić, M., 2004. Resonant coupling of a traveling air-pressure disturbance with the east Adriatic coastal waters. Journal of Geophysical Research – Oceans, 109, C10001, doi:10.1029/2004JC002279.

Monserrat, S., Thorpe, A.J. 1992. Gravity-wave observations using an array of microbarographs in the Balearic Islands, Quarterly Journal of the Royal Meteorological Soceaty, 118, 259-282.

Hibiya, T., Kajiura, K. 1982. Origin of “Abiki” phenomenon (a kind of seiche) in Nagasaki Bay, Journal of the Oceanographic Society of Japan, 38, 172-182.

Lindzen, R. S., Tung, K.-K. 1976. Banded convective activity and ducted gravity waves, Monthly Weather Review, 104, 1602-1617.

 
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