THE NEAR-SURFACE LAYER OF THE OCEAN
result, in a one-dimensional model the concentration of the largest droplets
should drop dramatically with distance from the sea surface. The surface
roughness length scale following from the Charnock (1955) parameterization
is of the order of 1 cm. From (6.19) it follows that the initial concentration at
z = 1 cm should decrease by a factor
10
3
2
10
|
. Though the turbulent
diffusion mechanism apparently is not effective for the largest droplets, after
tearing from the wave crest these droplets are taken up by the wind and may
fly some distance in the horizontal direction before reentering the ocean
surface.
6.2.3 Sea spray generation function
Essential for many applications is the sea spray generation function
defined as the rate at which spray droplets of any given size are produced at
the sea surface. The sea spray generation function, commonly denoted as
0
/
dF dr (e.g., Monahan et al., 1986), where 0
F is total particle flux from the
ocean and 0
r is the radius of a droplet at its formation, has units of number
of droplets produced per square meter of surface per second per micrometer
increment in droplet radius. For the consideration of spray momentum, heat,
and moisture transfer, the generation function as a volume flux,
3
0
0
4
/3
/
r
dF dr
S
, is more relevant.
Andreas (1998) compared the results of different authors only to find
that the existing parameterizations of the primary marine aerosol source
differ 6-10 orders of magnitude (Figure 6-9). He nevertheless found some
consistency in the predicted shape of the spray generation function. The
volume flux is relatively small for droplets with radii less than 2 Pm or
exceeding 500 Pm; there is a 2 to 3 order-of-magnitude peak in the spray
generation function between approximately 10 Pm and 200 Pm. This peak
appears in the spume droplet region, which contributes most to the heat and
momentum fluxes at high wind speeds. Parameterization of spume droplet
production is therefore essential for evaluating the effects of spray on air-sea
heat and moisture transfer.
416
result, in a one-dimensional model the concentration of the largest droplets
should drop dramatically with distance from the sea surface. The surface
roughness length scale following from the Charnock (1955) parameterization
is of the order of 1 cm. From (6.19) it follows that the initial concentration at
z = 1 cm should decrease by a factor
10
3
2
10
|
. Though the turbulent
diffusion mechanism apparently is not effective for the largest droplets, after
tearing from the wave crest these droplets are taken up by the wind and may
fly some distance in the horizontal direction before reentering the ocean
surface.
6.2.3 Sea spray generation function
Essential for many applications is the sea spray generation function
defined as the rate at which spray droplets of any given size are produced at
the sea surface. The sea spray generation function, commonly denoted as
0
/
dF dr (e.g., Monahan et al., 1986), where 0
F is total particle flux from the
ocean and 0
r is the radius of a droplet at its formation, has units of number
of droplets produced per square meter of surface per second per micrometer
increment in droplet radius. For the consideration of spray momentum, heat,
and moisture transfer, the generation function as a volume flux,
3
0
0
4
/3
/
r
dF dr
S
, is more relevant.
Andreas (1998) compared the results of different authors only to find
that the existing parameterizations of the primary marine aerosol source
differ 6-10 orders of magnitude (Figure 6-9). He nevertheless found some
consistency in the predicted shape of the spray generation function. The
volume flux is relatively small for droplets with radii less than 2 Pm or
exceeding 500 Pm; there is a 2 to 3 order-of-magnitude peak in the spray
generation function between approximately 10 Pm and 200 Pm. This peak
appears in the spume droplet region, which contributes most to the heat and
momentum fluxes at high wind speeds. Parameterization of spume droplet
production is therefore essential for evaluating the effects of spray on air-sea
heat and moisture transfer.
416