Effects of Heavy Local Downpours on Wind Waves and Wave-Induced Water Flow

Abstract We present experiments investigating the interaction between wind-generated waves and a strong downpour in a laboratory flume. We used a series of wave gauges to measure the water level at several fetches, and a two-dimensional Particle Image Velocimetry (PIV) technique to measure the fluid velocity beneath the air–water interface. Our aim is to show how the velocity, vorticity, stresses and energy fields are modified by the impact of rain on the water’s surface. We applied a novel technique for separating oscillating components and turbulence. The results demonstrate that a reduction in wave height in the presence of rain is accompanied by the diffusion of fluctuating vorticity in the water column, as well as significant damping of oscillating energy and vorticity. The production and dissipation of turbulent kinetic energy (TKE) are also not locally balanced, requiring a transfer mechanism involving a positive influx and negative outflux of energy if rain is present or absent, respectively. Bursting of turbulent shear stress is enhanced in the presence of rain, with longer-lasting high-energy bursts and a dominant contribution from the first and third quadrants. The principal axes of the oscillating and turbulent Reynolds tensors tend to become collinear in the presence of rain, albeit at an angle significantly different from the 45^ 45 ∘ of the principal axis of the strain rate tensor. Because rainfall is confined to a limited portion of the fetch, the observed wave and flow responses reflect the local impact of a strong downpour on a pre-existing wind–wave field and may represent a non-asymptotic regime rather than a fully developed wind–rain equilibrium.