THE NEAR-SURFACE LAYER OF THE OCEAN
can interfere with air-sea interaction (for instance, via capillary-gravity
waves) and affect the properties of molecular sublayers.
Around breaking waves, the sea surface is not well defined–it includes
the surface of bubbles and the outside of spray droplets, and the overhang of
plunging waves. The height values of the sea surface can be multiply
defined. In this case, the sea surface can be defined only in the topological
sense and the concept of microlayer needs to be extended.
As a first approximation, the thickness of the viscous, thermal, and
diffusion molecular sublayer at the ocean surface can be associated with the
internal (Kolmogorov’s) length scale of turbulence,
1/ 4
3 /
Q
K
Q H ,
(2.1)
where Q is the molecular kinematic viscosity, and H is the dissipation rate
of the turbulent kinetic energy. Similar length scales also exist for thermal
and diffusive turbulent processes,
1/ 4
1/ 2
3
Pr
/
T
v
K
H
(2.2)
and
1/ 4
1/ 2
3 /
D
Sc
v
K
H
(2.3)
where Pr
/ T
Q N is the Prandtl number ( Pr 7.1 for water at 20
o C),
/
Sc Q P is the Schmidt number (Sc~10
3 ), T
N is the molecular coefficient
of kinematic thermal diffusivity, and P is the molecular coefficient of
kinematic salinity or gas diffusivity.
An instructive schematic diagram is shown in Figure 2-1. The
logarithmic scale ranges from the diameter of a molecule to the maximum
depth of the world ocean emphasizing the top millimeter of the ocean.
Molecular sublayers extend from the surface to typical depths of about 1500
µm (viscous sublayer), 500 µm (thermal sublayer), and 50 µm (diffusion
sublayer). There are also organic films on the sea surface.
These are of course only nominal values. The thickness of molecular
sublayers depends substantially on the air-sea interaction regime. In fact, the
structure of the molecular sublayers are quite complex. It depends on wind
stress acting on the sea surface, on shortwave radiation absorbed in the upper
millimeters of the ocean, on heat, salt, freshwater, and gas fluxes crossing
68
can interfere with air-sea interaction (for instance, via capillary-gravity
waves) and affect the properties of molecular sublayers.
Around breaking waves, the sea surface is not well defined–it includes
the surface of bubbles and the outside of spray droplets, and the overhang of
plunging waves. The height values of the sea surface can be multiply
defined. In this case, the sea surface can be defined only in the topological
sense and the concept of microlayer needs to be extended.
As a first approximation, the thickness of the viscous, thermal, and
diffusion molecular sublayer at the ocean surface can be associated with the
internal (Kolmogorov’s) length scale of turbulence,
1/ 4
3 /
Q
K
Q H ,
(2.1)
where Q is the molecular kinematic viscosity, and H is the dissipation rate
of the turbulent kinetic energy. Similar length scales also exist for thermal
and diffusive turbulent processes,
1/ 4
1/ 2
3
Pr
/
T
v
K
H
(2.2)
and
1/ 4
1/ 2
3 /
D
Sc
v
K
H
(2.3)
where Pr
/ T
Q N is the Prandtl number ( Pr 7.1 for water at 20
o C),
/
Sc Q P is the Schmidt number (Sc~10
3 ), T
N is the molecular coefficient
of kinematic thermal diffusivity, and P is the molecular coefficient of
kinematic salinity or gas diffusivity.
An instructive schematic diagram is shown in Figure 2-1. The
logarithmic scale ranges from the diameter of a molecule to the maximum
depth of the world ocean emphasizing the top millimeter of the ocean.
Molecular sublayers extend from the surface to typical depths of about 1500
µm (viscous sublayer), 500 µm (thermal sublayer), and 50 µm (diffusion
sublayer). There are also organic films on the sea surface.
These are of course only nominal values. The thickness of molecular
sublayers depends substantially on the air-sea interaction regime. In fact, the
structure of the molecular sublayers are quite complex. It depends on wind
stress acting on the sea surface, on shortwave radiation absorbed in the upper
millimeters of the ocean, on heat, salt, freshwater, and gas fluxes crossing
68