Chapter 2: SEA SURFACE MICROLAYER
According to (2.49) and (2.54) for intermediate and high wind speed
conditions 0
/
u u
/ '
,
Figure 2-12. Non-dimensional wind-induced surface current in the laboratory tank for
different wind friction velocities and in comparison with the renewal model (2.57) at
0.25
a
and 0 7.4
/
, calculated for two surface cooling rates: (a) 0 20
Q
W m
-2 , and (b)
0
200
Q
W m
-2 .
The ratio,
/
u u
'
, is closely related to the wind drift coefficient, 0 1 0
/
u U ,
where 0
u is the averaged current velocity at the sea surface (relative to the
background ocean current), and U 10 is the wind speed at 10 m height. The
current velocity at the sea surface includes the Stokes drift as well, which
provides a relatively small (between 5 and 20%) contribution to the winddrift coefficient however. The difference between the current velocity at the
sea surface 0
u and the Stokes surface drift S
u is the wind-induced surface
drift:
0
wd
S
u
u u
.
(2.62)
The ratio between the wind-induced surface drift wd
u and the water
friction velocity u as measured by Wu (1975) varied between 11 and 20.
101