On the upward flux of sea‐spray spume droplets in high‐wind conditions

Abstract

It has been speculated that the spume droplets torn off from ocean waves in high wind speeds (larger than about 9 m s−1) enhance air‐sea fluxes. However, the vertical transport of spume droplets in the high‐wind atmospheric boundary layer (ABL) is little researched and lacks the support of numerical simulations. Based on observational data and the characteristics of high‐wind ABL above the ocean, the vertical transport of spume droplets is studied and simulated. It is revealed that the existence of gusty wind with coherent structure during high winds is favorable for the vertical transport of the large‐size droplets. The trajectories of 1000 spume droplets with different sizes under different wind speeds in the range 10–35 m s−1 are calculated, and the uplifting fraction of spume droplets that can reach the 100 m height in the ABL is parameterized by using a nondimensional factor U102/(r80g), where U10 is the wind speed at 10 m height, r80 the equilibrium radius of droplet at an equivalent 80% relative humidity, and g the gravitational acceleration. Then, the upward flux of large‐size spume droplets and the mass concentration at 100 m height are evaluated. These numerical results are comparable to some observational data and show that the large‐size spume droplets can be entrained into the ABL during high winds and then contribute to the air‐sea mass fluxes.

Aerosol Optical Depth and Burden From Large Sea Salt Particles

“…Previous assumption to exclude coarse mode sea salt with r d > 10 μm is that it settles too quickly after emitted from ocean surface. However, new observation data shows that strong wind is generally accompanied by positive vertical velocity and coherent gusts, which enable some of large sea salt particles to rise into atmospheric boundary layer (Wu et al, ). The new scheme is based on Andreas () by including an uplifting factor k up based on the work of Wu et al ().…”

“…Moreover, Cheng et al () found that strong winds are always accompanied with positive upward vertical velocity and large amplitude gusts in the marine‐atmospheric boundary layer. They can pick up some of the emitted large droplets into marine boundary layer (Wu et al, ).…”

“…The sea salt emissions in CAM5 are calculated using Mårtensson et al () scheme for dry diameter D p < 2.8 μm and Gong () scheme D p > 2.8 μm. Our main purpose is focused on extending the range SSA emission distribution to up to 250 μm based on the researches of Andreas () and Wu et al (). In order to distinguish this larger coarse mode of SSA from original three modes in CAM5, we define the additional coarse mode SSA as SSLT4 (Sea salt mode 4) in this paper.…”

“…And the ratio of elevated large droplets to total emitted large droplets must be taken into consideration. By the use of parameterizations of vertical velocity w and the necessary characteristics of gusts and turbulences in strongly windy marine atmospheric boundary layer (Cheng et al, ; Zeng et al, ), Wu et al () used a Lagrangian stochastic model to simulate trajectories of droplets and calculated the ratio of droplets transported into the 100 m height level, k up , as a function of a nondimensional parameter λ as follows:

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“…By using some observations of marine atmospheric boundary layer during 10‐m wind speed as strong as 10–40 m/s in the South China Sea regions, Cheng et al () and Zeng et al () have obtained some vertical structures of mean wind and characteristics of high frequent turbulences and gusts with periods of 1–5 min. On the basis of this data, Wu et al () have constructed parameterization equations of marine atmospheric boundary layer characteristics and applied them to calculate the trajectories of large spume droplets and obtained the emission parameterization. Based on these results, we introduce a new emission parameterization into Community Earth System Model (CESM) in order to assess the contribution of the large SSA to the AOD and aerosol burden.…”