Chapter 4: FINE STRUCTURE AND MICROSTRUCTURE
deployed from a small boat. They found that during low wind speeds the jet
is often localized in the upper few meters of the ocean.
Kudryavtsev and Soloviev (1990) concluded that the stabilizing buoyancy
flux due to absorption of solar radiation reduces the turbulent friction. As a
result, the near-surface warm layer slips over the underlying water mass
practically with no turbulent friction. A similar slippery layer may result
from positive buoyancy flux due to precipitation or due to the lateral
advection (as in the fore mentioned observation by Houghton, 1969).
Kudryavtsev and Soloviev (1990) also noted the somewhat surprising fact
that the speed of the diurnal jet did not drop with decreasing wind speed.
This is nevertheless easy to understand, at least qualitatively. It is due to the
diurnal mixed layer thinning at nearly the same rate as the wind stress
decreases, thus concentrating the smaller momentum flux in a thinner layer.
Figure 4-12X shows the solar radiation and wind speed during the
experiment of Kudryavtsev and Soloviev (1990) in the equatorial Atlantic
alongside with the temperature and velocity differences across the upper 5 m
layer of the ocean. When the wind speed 20
U measured at 20 m height drops
below approximately 6 m sP
-1
P
the temperature difference,
5
T
' , measured
between depths of 0.35 m and 5 m dramatically increases (X Figure 4-12c), as
does the corresponding velocity difference
5
u
'
(X Figure 4-12X d). These
observations suggest that the temperature and velocity differences in the
diurnal jet are interrelated.
Figure 4-12e shows the variability of the wind drift coefficient, 5 2 0
/
u U
'
.
5
a pair of drifters with drogue depths of 0.35 m and 5 m and from the wind
speed 20 m above the sea surface, 20
U (X Figure 4-12e). Solid lines in
diagrams (d) and (e) indicate the current velocity difference,
5
u
' , calculated
between 0.35 m and 5 m depth from the logarithmic layer model as follows:
1/ 2
1/ 2
1
3
5
2 0
2
1
2 0
/
ln
/
0.85 10
a
u C
z z
U
U U N
'
|
u
,
(4.1)
where
3
20
1.3 10
C
| u
is the bulk flux coefficient (for 20 m height),
0.4
N
(the von Karman constant), zB 2 B = 5 m, and zB 1 B = 0.35 m.
The data from 22 February and March 15 (X Figure 4-12) obtained during
wind speeds 20 7
U t m sP
-1
P
show that the measured velocity difference,
5
u
' ,
and the wind drift coefficient,
5
2 0
/
u U
'
, were close to the logarithmic layer
prediction. When the wind speed drops below approximately 6 m sP
-1
P,
the
temperature and velocity differences in the upper 5 m layer of the ocean
caused by the diurnal warming rapidly increase, and the wind drift
coefficient exceeds the logarithmic layer prediction by up to a factor of 5.
237
This coefficient is calculated from the velocity difference 'u measured by
deployed from a small boat. They found that during low wind speeds the jet
is often localized in the upper few meters of the ocean.
Kudryavtsev and Soloviev (1990) concluded that the stabilizing buoyancy
flux due to absorption of solar radiation reduces the turbulent friction. As a
result, the near-surface warm layer slips over the underlying water mass
practically with no turbulent friction. A similar slippery layer may result
from positive buoyancy flux due to precipitation or due to the lateral
advection (as in the fore mentioned observation by Houghton, 1969).
Kudryavtsev and Soloviev (1990) also noted the somewhat surprising fact
that the speed of the diurnal jet did not drop with decreasing wind speed.
This is nevertheless easy to understand, at least qualitatively. It is due to the
diurnal mixed layer thinning at nearly the same rate as the wind stress
decreases, thus concentrating the smaller momentum flux in a thinner layer.
Figure 4-12X shows the solar radiation and wind speed during the
experiment of Kudryavtsev and Soloviev (1990) in the equatorial Atlantic
alongside with the temperature and velocity differences across the upper 5 m
layer of the ocean. When the wind speed 20
U measured at 20 m height drops
below approximately 6 m sP
-1
P
the temperature difference,
5
T
' , measured
between depths of 0.35 m and 5 m dramatically increases (X Figure 4-12c), as
does the corresponding velocity difference
5
u
'
(X Figure 4-12X d). These
observations suggest that the temperature and velocity differences in the
diurnal jet are interrelated.
Figure 4-12e shows the variability of the wind drift coefficient, 5 2 0
/
u U
'
.
5
a pair of drifters with drogue depths of 0.35 m and 5 m and from the wind
speed 20 m above the sea surface, 20
U (X Figure 4-12e). Solid lines in
diagrams (d) and (e) indicate the current velocity difference,
5
u
' , calculated
between 0.35 m and 5 m depth from the logarithmic layer model as follows:
1/ 2
1/ 2
1
3
5
2 0
2
1
2 0
/
ln
/
0.85 10
a
u C
z z
U
U U N
'
|
u
,
(4.1)
where
3
20
1.3 10
C
| u
is the bulk flux coefficient (for 20 m height),
0.4
N
(the von Karman constant), zB 2 B = 5 m, and zB 1 B = 0.35 m.
The data from 22 February and March 15 (X Figure 4-12) obtained during
wind speeds 20 7
U t m sP
-1
P
show that the measured velocity difference,
5
u
' ,
and the wind drift coefficient,
5
2 0
/
u U
'
, were close to the logarithmic layer
prediction. When the wind speed drops below approximately 6 m sP
-1
P,
the
temperature and velocity differences in the upper 5 m layer of the ocean
caused by the diurnal warming rapidly increase, and the wind drift
coefficient exceeds the logarithmic layer prediction by up to a factor of 5.
237
This coefficient is calculated from the velocity difference 'u measured by