Turbulent stress observations at Lake IJssel

This report is the result of the analysis of turbulence observations at two locations at Lake IJssel in the Netherlands. The aim of the measurements was to gain a better understanding of the relationship between wind speed, wave height and turbulent stress at the water surface. To this end, measurements were taken during four storm seasons with sonic anemometers at two existing locations on Lake IJssel (FL47 and FL48). At these two locations, essential other parameters are already measured in order to arrive at a good analysis of stress measurements.

The first three storm seasons were used to make improvements to the measurement setup. It concerns two problems: 1) Data loss especially in the most interesting situations with high wind speed; 2) Disruption of the turbulent signal by vibrations in the frame of the sonic anemometers, also especially at the most interesting cases with high wind speeds. While the improvements have mitigated the issues to some extent, they remain problematic in the time series of the fourth and final storm season.

This report describes the observations and discusses the correction methods that have been developed to arrive at useful data. Due to these unforeseen problems with the measurements there was insufficient time within the project to carry out a satisfactory analysis of the data.

The result of this project is therefore: 1) a method to derive stress values and co-spectra from turbulence data with a variable measuring frequency; 2) a methodology to correct turbulent stress observations for vibrational effects on the sonic anemometer; 3) a first but limited substantive analysis of Cd relations on Lake IJssel; 4) a dataset of corrected stress measurements together with relevant additional parameters to perform further wave-wind interaction analysis