Royal Dutch Meteorological Institute; Ministery Of Infrastructure And The Environment

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Impact of Ocean Spray on the Dynamics of the Marine Atmospheric Boundary Layer
2011
by Kudryavtsev (), V.K. Makin (KNMI),

The impact of ocean spray on the dynamics of the marine near-surface air
boundary layer (MABL) in conditions of very high (hurricane) wind speeds is investigated.
Toward this end, a model of the MABL in the presence of sea-spume droplets is developed.
The model is based on the classical theory of the motion of suspended particles in a turbulent
flow, where the mass concentration of droplets is not mandatory small. Description of the
spume-droplet generation assumes that they, being torn off from breaking waves, are injected
in the form of a jet of spray into the airflow at the altitude of breaking wave crests. The droplets
affect the boundary-layer dynamics in two ways: via the direct impact of droplets on the
airflow momentum forming the so-called spray force, and via the impact of droplets on the
turbulentmixing through stratification. The latter is parametrized applying theMonin–Obukhov
similarity theory. It is found that the dominant impact of droplets on theMABLdynamics
appears through the action of the ‘spray force’ originated from the interaction of the ‘rain of
spray’ with the wind velocity shear, while the efficiency of the stratification mechanism is
weaker. The effect of spray leads to an increase in the wind velocity and suppression of the
turbulent wind stress in the MABL. The key issue of the model is a proper description of
the spume-droplet generation. It is shown that, after the spume-droplet generation is fitted to
the observations, the MABL model is capable of reproducing the fundamental experimental
finding—the suppression of the surface drag at very high wind speeds.We found that, at very
high wind speeds, a thin part of the surface layer adjacent to the surface turns into regime of
limited saturation with the spume droplets, resulting in the levelling off of the friction velocity
and decrease of the drag coefficient as U−2
10 ,U10 being the wind speed at 10-m height.

Bibliographic data
Kudryavtsev and V.K. Makin, Impact of Ocean Spray on the Dynamics of the Marine Atmospheric Boundary Layer
Bound.-Layer Meteorol., 2011, 140, 383-410, doi:10.1007/s10546-011-9624-2.
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