On the Possibility of Vertical Flux of Sea Spray Spume

The generation of sea-salt aerosols by the ocean is the first of many processes included in climate models, and have been studied for many years and many models have been developed. The starting point to model the emission of sea-salt aerosols from the ocean surface is to model the sea-spray generation function (SSGF). A SSGF predicts how many film, jet, and spume droplets are produced for each size of interest depending on the air-sea states such as the wind speed, usually in unit (m^-2 s^-1 μm^-1). The equilibrium radius at an equivalent 80% RH, r₈₀, is the common convention of specifying the size of sea salt aerosol particles. For droplets with r₈₀≤10–20 μm, a great deal of progress in determining the SSGF has been made in the past decade. The uncertainties in previous SSGFs have been narrowed down partly because of the improved measurement techniques, as well as improved estimates of the whitecap coverage over a large range of wind speed. However, the present SSGFs concentrates little on spume droplets, which torn from the waves are many and large, with minimum radius generally about 20 μm, and no definite maximum radius.

The present parameterizations of air-sea turbulent fluxes are reasonably valid up to wind speeds of about 25 m/s. This wind speed range covers the vast majority of oceanic wind climatology; however, the hurricane wind speed can reach up to 70 m/s. At high wind speeds, the ocean is a major source of droplets produced by bursting bubbles and spume (i.e., from sheared-off wave tops) to the lower troposphere. Spume production is a special case because droplets produced at the interface near the peak of a breaking wave are easily blown off the top of the wave by turbulent wind gusts. This was regarded as probably the only way the larger droplets can influence the atmosphere. Because of their much larger sizes (and larger mass flux) spume droplets are expected to dominate the hurricane droplet flux problem. Spume droplets may play a large role in latent heat transfer between the ocean and atmosphere and under extremely high winds such as found in hurricanes, may also have a large effect on the air-sea exchange of momentum. To access the air-sea exchange induced by spume droplet, it is essential to understand its generation and movement mechanisms in the atmospheric boundary layer (ABL). However, in contrast to the situation in bubble-mediated droplet fluxes, spume droplet production physics is still much less explored. Laboratory experiments provided the evidence that the numerous large drops are not accounted for in theoretical estimates of the spray generation function and their relevance to the total air-sea fluxes remains undetermined. The transport of sea spray in the ABL is not yet a fully resolved problem. It is difficult to reveal this problem thoroughly, and we only try to study the movement of spume droplets in the ABL during strong winds, and discuss their possible contribution to the sea-salt flux.

The ABL near sea surface during strong winds had been observed during the Marine Meteorological Experiment Complex (MMEC) at Bohe, Guangdong Province, in cooperative efforts by the Institute of Tropical and Marine Meteorology/China Meteorological Administration, the Maoming Meteorological Bureau/Guangdong Province, the Institute of Atmospheric Physics/Chinese Academy of Sciences, and China Ocean University. The MMEC at Bohe, Guangdong Province, comprises three sites: (1) Beishan Station, which is located on Lotus Head (a small and very narrow peninsula, like an arrow intruding into the sea) at a height of about 10 m; (2) Near Beishan Station, there is an observation platform (21°26′21″N, 111°23′44″E) above the sea surface and 6.5 km from the shore, where the depth of the water is 16 m; (3) A 100 m meteorological tower (21°27′3″N, 111°22′28″E) installed on a very small rocky island named Zhizi, which is located at a height of 10 m and 4.4 km from the shore, where the depth of the surrounding sea is 6–10 m. There is a 25 m high tower on the observation platform with three sets of Gill R3-50 ultrasonic anemometers, which are installed at 24 m, 16 m and 9 m above the tower base, and the tower base is 11 m above the sea surface. Besides, there is also a small annex tower with one set of Gill R3-50 ultrasonic anemometers at 8 m above sea level. On Zhizi Island there is a meteorological tower with six levels of NRG-Symphonic anemometers at 10, 20, 40, 60, 80 and 100 m above the island and three levels of NRG anemoscopes at 10, 60 and 100 m above the island. Through the data obtained from the MMEC, we provided the ABL characteristics during strong winds which contain both Typhoon Hagupit and cold surge cases.

Based on the structure analysis of the observed typhoon data, the vertical transport of spume droplets were calculated and the vertical flux was provided. Different from the classical turbulence, there exist gusty wind disturbances in the ABL near the sea surface during strong winds. They are anisotropic with rather strong coherency and play an important role in the transport of droplets. Then the Lagrangian stochastic model was modified by considering the gusty wind disturbances. Using the actual observational data, the trajectories of spume droplets (the size r₀≥20 μm) were calculated and the results showed that a certain percentage of spume droplets could fly into the 100 m height in ABL. And the coherent gusty disturbances provide one mechanical way to lift up the spume insteading of returning to the sea surface immediately. Therefore an uplifting ratio that how many spume droplets can fly into the 100 m height in ABL was obtained and parameterized through a dimensionless factor (U₁₀)² /(r₀g). Combining the current SSGF of spume droplets and this uplifting ratio, the vertical flux of spume droplets were calculated for the Hagupit month September 2008. The heat flux of the uplifted spume droplets during typhoon period will be the next work.