Production velocity of sea spray droplets

The sea spray generation function dF/dr(0) predicts the rate at which droplets of initial radius r(0) are produced at the sea surface. Because this function is not readily measurable in the marine environment, however, it is often inferred from measurements of the near-surface droplet concentration, C(r(0)), through an assumed velocity scale, the effective spray production velocity. This paper proceeds in reverse, though: It uses a reliable estimate of dF/dr(0) and 13 sets of measurements of C(r(0)) over the ocean to calculate the implied effective production velocity, V-eff, for droplets with initial radii r(0) from 5 to 300 mu m. It then compares these V-eff values with four candidate expressions for this production velocity: the dry-deposition velocity, V-Dh; the mean wind speed at the significant wave amplitude (A(1/3)), U-A1/3; the standard deviation in vertical droplet velocity, sigma(wd); and laboratory measurements of the ejection velocity of jet droplets, V-ej. The velocity scales U-A1/3 and V-ej agree best with the implied V-eff values for 20 <= r(0) <= 300 mu m. The deposition velocity, V-Dh, which is the velocity most commonly used in this application, agrees worst with the Veff values. For droplets with r(0) less than about 20 mu m, the analysis also rejects the main hypothesis: that dF/dr(0) and C(r(0)) can be related through a velocity scale. These smaller droplets simply have residence times that are too long for spray concentrations to be in local equilibrium with the spray production rate.