Detection of Microscale Breaking in Wind-Driven Waves using a Colour Imaging Slope Gauge (CISG)

Wind-driven gravity–capillary waves play a key role in air–sea interactions and in small-scale energy dissipation across the surface microlayer (SML). Despite decades of studies, the transition from smooth gravity–capillary waves to intermittent microscale breaking under weak wind forcing is still not well understood. 
This study investigates gravity–capillary wave dynamics and micro-breaking in a 24 m long, 1 m wide wind–wave tank with a total height of 1.5 m and a mean water depth of 0.51 m. Measurements were performed using a newly developed Colour Imaging Slope Gauge (CISG), providing high-resolution spatio-temporal observations of surface slopes within a 33.2 cm × 26.8 cm field of view (FOV), at a spatial resolution of 0.024² cm² per pixel and a frame rate of 400 Hz. A total of 18 experiments were conducted over a range of low wind speeds (1.8 m s⁻¹– 4.0 m s⁻¹) with small increments. Wire-wave gauge measurements were used to support three-dimensional surface reconstructions. 
Spectral, wavelet, and band-pass filtering techniques were applied to isolate capillary-scale features associated with micro-breaking. Particular attention was given to surface curvature as a geometric indicator of micro-breaking. The wide FOV enables direct tracking of isolated events and reveals a clear increase in capillary activity and micro-breaking occurrence with increasing wind forcing. 
First results indicate a distinct transitional regime at wind speeds near 2.0 m s⁻¹, where the first clear capillary signatures associated with micro-breaking emerge in the frequency-wavenumber spectra. The CISG successfully captures the spatial onset of 
these micro-breaking induced capillaries with wavelengths between 0.4 cm and 3 cm. 
By applying wavelet and band-pass filtering, these features were isolated, allowing for the identification of the "birth" of micro-breaking induced capillaries within the FOV. 
This work establishes a methodological framework for detecting micro-breaking and provides new insights into the surface conditions governing small-scale dissipation processes in wind-driven wave systems.