Identifying meteotsunamis based on lightning observations over the Finnish sea areas

Convective storms passing over sea areas are known to cause meteotsunamis, i.e., rapid changes in the sea level. A fast-moving storm creates sudden changes in the air pressure over large bodies of water. If the speed of the storm is close to the long wave phase speed in shallow water, the resonance increases the wave height. Meteotsunamis can cause flooding and damage in coastal areas, and therefore, predicting their occurrence is crucial.

Scientifically, a meteotsunami cannot be identified solely from sea level height data due to its definition of being caused by a weather disturbance. Hence, a combination of sea level height and weather data is necessary. In our study, we examined the occurrence of meteotsunamis over the Finnish sea areas by combining observations of sea level height, squall lines and lightning. The sea level height data was collected from the Finnish tide gauges, and the lightning observations are based on Nordic lightning location system. Squall lines were identified from weather radar data and wind gust observations from the weather stations.

This presentation will focus on how lightning observations can be used to identify meteotsunamis. We used a convective cell-tracking method to group individual lightning flashes to flash cells based on their spatiotemporal information. Grouping flashes to cells enables the calculation of the speed and direction of the flash cells. For a meteotsunami to form, the speed of the convective storm needs to be similar to the long wave phase speed of the water body. Therefore, the flash cell speed could potentially be used to estimate whether a meteotsunami could occur.

In our investigation, we first collected a list of squall lines observed over the Finnish sea areas during 2007-2024. From these cases, we then examined the sea level height data to identify which cases were associated with rapid sea level changes. After identifying the meteotsunami cases, we applied cell-tracking to lightning flashes during these dates. In this presentation, we will show results of these meteotsunami cases and how the lightning flash cell characteristics vary between cases. In the future, the ultimate aim is to examine whether lightning observation data may be used to predict potentially forming meteotsunamis over the Finnish coasts.

This study is part of MAWECLI project (MArine and WEather events in the changing CLImate as potential external hazards to nuclear safety, 2023-2025) that supports nuclear power plant safety in Finland.