High-Frequency Wave Measurements in the Baltic Sea

The high-frequency part of the wave field is essential for the understanding of air-sea exchange related processes and the turbulent energy dissipation of breaking waves. The quantification of the dimensionless spectra will aid wave model development and contribute to a better understanding of the fundamental laws governing the evolution of wind driven waves. However, typical wave observation devices, such as wave buoys, are limited to observing frequencies under e.g. 0.6 Hz. Dedicated experiments with devices suitable for high-frequency measurements are, in comparison, rare.

We have made high-frequency wave measurements with capacitive wave staffs from RV Aranda. Air turbulence and wind speed measurements are also available and a full motion correction was applied to all measurements. A frequency rage up to 2-3 Hz is enough to study the tail of the wave spectra even during its early development. The unusually high sampling frequency of 200 Hz guarantees that spurious spectral shapes that could be the joint effect of noise and the anti-aliasing filter can be excluded. Directional measurements were made using four wave staffs located 15 or 50 cm apart in the grid.

The mobility of the research vessel has enabled measurements in a wide variety of conditions from the Baltic Proper to the irregular Finnish coastal archipelagos. The aim is to determine the conditions and frequency ranges when the shape of the dimensionless spectra is wind dependent. Especially, it's still not clear whether the use of the wind speed or the friction velocity as the scaling parameter produces better results, or where the transition to the Phillips spectra takes place. The directional measurements can shed light on theories that use the directional spread of the two-dimensional spectrum to explain the shape of the one-dimensional spectrum.