Simultaneous particle image velocimetry and infrared imagery of microscale breaking waves

Abstract

We report the results from a laboratory investigation in which microscale breaking waves were detected using an infrared (IR) imager and two-dimensional (2-D) velocity fields were simultaneously measured using particle image velocimetry (PIV). In addition, the local heat transfer velocity was measured using the controlled flux technique. To the best of our knowledge these are the first measurements of the instantaneous 2-D velocity fields generated beneath microscale breaking waves. Careful measurements of the water surface profile enabled us to make accurate estimates of the near-surface velocities using PIV. Previous experiments have shown that behind the leading edge of a microscale breaker the cool skin layer is disrupted creating a thermal signature in the IR image [Jessup et al., J. Geophys. Res. 102, 23145 (1997)]. The simultaneously sampled IR images and PIV data enabled us to show that these disruptions or wakes are typically produced by a series of vortices that form behind the leading edge of the breaker. When the vortices are first formed they are very strong and coherent but as time passes, and they move from the crest region to the back face of the wave, they become weaker and less coherent. The near-surface vorticity was correlated with both the fractional area coverage of microscale breaking waves and the local heat transfer velocity. The strong correlations provide convincing evidence that the wakes produced by microscale breaking waves are regions of high near-surface vorticity that are in turn responsible for enhancing air–water heat transfer rates.

Infrared measurements of surface renewal and subsurface vortices in nearshore breaking waves

“…The rms surface vorticity hjw s ji also gradually increases to 2.0 s À1 in analogy with A B in the renewal term from the phase d. In contrast, in the bore region, the renewed surface fully covers the whole surface after the wavefront temporally recovers the surface at the phase a since significant vorticity, more than twice hjw s ji of the plunging region, disturbs extensively the surface over a long duration. The high temporal correlations between A B and hjw s ji in the both of plunging and bore regions are consistent with the former work by Siddiqui et al [2001].…”

“…[21] On the other hand, a series of studies on sea surface temperatures with infrared imagery by Jessup and his colleagues has revealed that wind-driven microbreaking of ocean waves has a primary role for renewing the surface owing to breaking turbulence, resulting in modulations of the surface temperature [Jessup and Hesany, 1996;Jessup et al, 1997;Wick and Jessup, 1998;Siddiqui et al, 2001;Branch and Jessup, 2007]. The temperature modulation has been observed to appear at different phases in open ocean, which is caused by microbreaking owing to short waves bounded on longer swell waves [Branch and Jessup, 2007].…”

“…Using the same method as Siddiqui et al [2001], the fractional area coverage (A B ) was estimated for finding any relation with the root-mean square of the computed counterrotating vorticity hjw s ji near the surface (see Figure 14). In the plunging region, A B increases exponentially to the maximum value of 0.55 after the phase c when the surface begins to be partially renewed along the vortices (see also Figure 6c) until it rapidly decreases to 0 at the recovery phase a.…”

“…Jessup and his colleagues have also discussed the impact of wind-driven microbreaking at wave crests on the surface renewal process. They examined changes in surface temperature modulation and the size of the renewed surface in a series of laboratory experiments and observations made in the open ocean [Jessup and Hesany, 1996;Siddiqui et al, 2001;Zappa et al, 2004;Branch and Jessup, 2007].…”

Dissipation regimes for short wind waves

“…Microbreaking is then marked at the surface by formation of a bulge on the forward face of the wave crest and a train of capillary ripples riding ahead, as observed experimentally by Ebuchi et al [1987], Duncan et al [1999], and Diorio et al [2009]. After the pioneering works by Banner and Phillips [1974] and Okuda [1982], investigations of the water flow exploiting novel imaging techniques have clearly indicated that these surface disturbances are indeed accompanied by the development of a thin surface turbulent sublayer below the wave crest following a downward intrusion of water at the bulge toe [Jessup et al, 1997;Siddiqui et al, 2001;Peirson and Banner, 2003;Zappa et al, 2004].…”