Measuring Sea Spray Spume Droplets Using a Hydrophone

Abstract Sea spray spume droplets modulate the exchanges of heat, mass, and momentum between the ocean and atmosphere during extreme marine weather conditions. However, large uncertainty remains in the sea spray generation function, the number of droplets generated at the ocean surface, during such conditions as field observations are largely missing. In this study, we present an acoustic method to measure the horizontal sea spray spume droplet number flux. We do so by measuring the impact acoustics of spray droplets colliding with a hydrophone positioned in the air. We show through laboratory experiments that the spray volume flux can be estimated directly from the spectral power level at the resonance frequency of the hydrophone. By identifying the characteristic acoustic signal of a hydrophone to droplet impact, the droplet number flux can be reconstructed by identifying the impact signals within the hydrophone recording. Specifically, we find that droplet number flux for the experiments compares well to the droplet number flux independently estimated using a stroboscopic imaging system for droplets with a diameter larger than 400 μ m. As such, the method proposed here provides significant opportunities in estimating the sea spray spume droplets in situ and close to the ocean surface during extreme marine weather conditions.