Investigation of the physical scaling of sea spray spume droplet production

Abstract

[1] In this paper we report on a laboratory study, the Spray Production and Dynamics Experiment (SPANDEX), conducted at the University of New South Wales Water Research Laboratory in Australia. The goals of SPANDEX were to illuminate physical aspects of spume droplet production and dispersion; verify theoretical simplifications used to estimate the source function from ambient droplet concentration measurements; and examine the relationship between the implied source strength and forcing parameters such as wind speed, surface turbulent stress, and wave properties. Observations of droplet profiles give reasonable confirmation of the basic power law profile relationship that is commonly used to relate droplet concentrations to the surface source strength. This essentially confirms that, even in a wind tunnel, there is a near balance between droplet production and removal by gravitational settling. The observations also indicate considerable droplet mass may be present for sizes larger than 1.5 mm diameter. Phase Doppler Anemometry observations revealed significant mean horizontal and vertical slip velocities that were larger closer to the surface. The magnitude seems too large to be an acceleration time scale effect. Scaling of the droplet production surface source strength proved to be difficult. The wind speed forcing varied only 23% and the stress increased a factor of 2.2. Yet, the source strength increased by about a factor of 7. We related this to an estimate of surface wave energy flux through calculations of the standard deviation of small-scale water surface disturbance, a wave-stress parameterization, and numerical wave model simulations. This energy index only increased by a factor of 2.3 with the wind forcing. Nonetheless, a graph of spray mass surface flux versus surface disturbance energy is quasi-linear with a substantial threshold.

Impact of Ocean Spray on the Dynamics of the Marine Atmospheric Boundary Layer

“…In the range r > 100 µm, the model overall is consistent with the data that may justify the validity of the spray-generation model. It is noted that the spray-generation function as reported by Fairall et al (2009) is strongly wind speed dependent; the wind-speed exponent of the spray concentration in their experiments varies from 4.5 to 13 (see also the wind-speed dependence in Fig. 3b).…”

“…Results of the extensive measurements of the spray generation in laboratory conditions were recently reported by Fairall et al (2009). The measured spectra of the droplet concentration at a height of z = 0.125 m above the mean water surface at friction velocities u * = 1.35, 1.44 and 1.64 m s −1 are shown in Fig.…”

“…This term simulates the spume-droplet generation by the wind tearing off crests of breaking waves, and was used to investigate the spray-generation problem (Fairall et al 1990(Fairall et al , 2009K06;KM09). Droplets, being torn off from breaking waves, are injected into the airflow at the altitude of breaking waves crests, and since scales of breaking waves are different, the generation of droplets is spread over height.…”

“…Following the experimental results of Koga (1981), Anguelova et al (1999) and Fairall et al (2009) we assume that the maximum radius of spume droplets is O(1000) μm. The model calculations with r 0 taken at 750 µm are shown in Fig.…”

“…3b, the spectral density of wave breaking fronts described by (32) was confined to the wavenumber range from k b to k p , where k p is the wavenumber of the spectral peak given in Table 1 of Fairall et al (2009). The model calculations ofŝ at z = 0.125 m with fixed above values of the tuning constants c r and c s are shown in Fig.…”