Measuring rain-induced momentum exchange at the ocean surface under low wind speed conditions

Rainfall is known to affect many aspects of the air-sea interaction problem including damping waves, generating turbulence, and enhancing air-sea gas exchanges. Despite these facts, the dynamic effects of rainfall entering the ocean have traditionally been ignored. We present results from laboratory experiments on the generation of turbulence and the damping of the wind-wave field by rain. These experiments were conducted in a wind-wave flume at the University of Delaware's Air-Sea Interaction Laboratory. The Plexiglas flume is 7.3 m long, 0.48 m wide, and 0.61 m tall, with the mean water level maintained at 0.4 m. A rain module, 0.86 m by 0.38 m, was suspended 4.98 m above the flume to allow droplets to reach approximately 91% of terminal velocity at impact. Turbulence was measured using both optical particle image velocimetry and planar laser induced fluorescence. Rain effects on the wave field were investigated with an optical wave gauge directly beneath the rain, as well as with a single point laser slope gauge downwind of the rain impact area. We have conducted experiments for several low to moderate wind speed conditions, and for different rain rates of the order O(100)mm h^-1, with both mono- and poly-disperse drop size distributions, corresponding to conditions that would be typical of the tropics. Experiments were completed with both fresh and salt water, 35 ppm, within the flume. We find that rainfall generates intense turbulence and mixing. We show that the mixing occurs in bursts associated with vortex rings generated by single drops, which are then sheared by the background current. In addition, this shearing and loss of coherence in the vortices generated by the drop impacts prevents significant vortex pairing thereby limiting the depth of the mixing region to a few 10s of cm. The depth of this mixed region is further reduced when raining on salt water at low wind. Incidentally, this leads to kinetic energy dissipation levels in this shallow layer that are comparable to those typically found in active surface breaking conditions. Using measurements made with several rain kinetic energy fluxes, we scale our laboratory results to field, using measured drop size distributions. We also explore the possible links between the turbulence and the wave damping and will also discuss the results in the context of surface turbulence and mixing with its implications in air-sea heat and gas transfers.