Free Wave Effects in Meteotsunamis

Abstract

Free wave generation due to a ship or storm moving past a shallow, great depth change of the water, is measured on the shore (coast), modeled and compared. The free waves are generated at the depth change where the forced wave and velocity field attached to the moving pressure system adjust to the new depth. The wavenumber is a factor 1/12 smaller in the meteotsunami case compared to the ship case. The subcritical depth Froude numbers are similar in the two cases. The meteotsunami that occurred on the Norwegian Coast on 29–30 June 2019 was driven by a supercell thunderstorm moving at speed 110 km/hr. A localized, strong high pressure feature of width of 60 km and crest of 120 km obtained from weather forecast was used as input for a set of simulations of the water-level response including realistic bathymetry. At the transition between the North Sea and the Norwegian Trench, the storm generated a free depression wave. This arrived at the coast 24 min ahead of the depression attached to the storm. The calculation fits to a period of 23 min, of a series several oscillations of height of 0.3–0.4 m, as measured by the water-level gauge. An impulsive start-up generates an additional forerunning elevation wave. Short waves of period of 1/3 of the main ship-driven waves may originate from the steep gradients of the bow and stern. Similarly, short waves of period 6–7.5 min (0.002–0.003 Hz) are observed in the measured water-level on the coast.