Chapter 4: FINE STRUCTURE AND MICROSTRUCTURE
X Figure 4-32X shows the evolution of the diurnal mixed layer depth during
afternoon and early evening hours estimated from the measurements with a
free-rising profiler on 1 October 1977. Three of seven total profiles taken
during this day are shown in X Figure 4-31X . In cases when a second quasihomogeneous layer was observed, only the upper one is taken into account.
X Figure 4-32X also shows the radiative-convective mixed layer depth
calculated from equation X (4.48)X for the case of no cloud conditions during
POLYMODE (Soloviev, 1979). During daytime the model curve
c
h t is in
good agreement with field data. It is remarkable that c
h practically does not
change during daytime hours although the solar radiation flux does change.
At about 1600 LT, when the insolation drops below some level, c
h , starts a
rapid increase. The model demonstrates an unbounded increase of the
diurnal mixed layer depth around 1700 LT. The experimental mixed layer
depth, however, does not follow the model prediction after 1700 LT. This is
related to the fact that the model ignores entrainment fluxes at the bottom of
the mixed layer, which become important when the diurnal mixed layer
depth increases. The water densification due to evaporation only slightly
increases c
h (by about 5-10% during daytime).
4.5.2 Transition from radiative-convective to wind mixing regime
The wind induced mixing can be accounted for by adding the
momentum balance equations (1.17) and (1.18) and by including the shear
production term according to the equation for turbulent kinetic energy (TKE)
balance (1.24). Under an assumption of horizontal homogeneity and with no
rotation effects, the equations for momentum balance are:
1 zx
u
t
z
W
U
w
w
w
w
,
(4.49)
1 zy
v
t
z
W
U
w
w
w
w
,
(4.50)
Equation (1.24) for the TKE balance in stationary form then reads
1
0
S
T
xz
yz
p
gJ
gQ
u
v
E
z
z
c
z
E
D
W
W
H
U
U
U
w
w
w
w
w
w
,
(4.51)
273
LT showed that by 1700 LT the diurnal mixed layer had substantially
deepened (X Figure 4-31X c). After 1700 LT, the evening deepening of the
mixed layer slowed down significantly.
X Figure 4-32X shows the evolution of the diurnal mixed layer depth during
afternoon and early evening hours estimated from the measurements with a
free-rising profiler on 1 October 1977. Three of seven total profiles taken
during this day are shown in X Figure 4-31X . In cases when a second quasihomogeneous layer was observed, only the upper one is taken into account.
X Figure 4-32X also shows the radiative-convective mixed layer depth
calculated from equation X (4.48)X for the case of no cloud conditions during
POLYMODE (Soloviev, 1979). During daytime the model curve
c
h t is in
good agreement with field data. It is remarkable that c
h practically does not
change during daytime hours although the solar radiation flux does change.
At about 1600 LT, when the insolation drops below some level, c
h , starts a
rapid increase. The model demonstrates an unbounded increase of the
diurnal mixed layer depth around 1700 LT. The experimental mixed layer
depth, however, does not follow the model prediction after 1700 LT. This is
related to the fact that the model ignores entrainment fluxes at the bottom of
the mixed layer, which become important when the diurnal mixed layer
depth increases. The water densification due to evaporation only slightly
increases c
h (by about 5-10% during daytime).
4.5.2 Transition from radiative-convective to wind mixing regime
The wind induced mixing can be accounted for by adding the
momentum balance equations (1.17) and (1.18) and by including the shear
production term according to the equation for turbulent kinetic energy (TKE)
balance (1.24). Under an assumption of horizontal homogeneity and with no
rotation effects, the equations for momentum balance are:
1 zx
u
t
z
W
U
w
w
w
w
,
(4.49)
1 zy
v
t
z
W
U
w
w
w
w
,
(4.50)
Equation (1.24) for the TKE balance in stationary form then reads
1
0
S
T
xz
yz
p
gJ
gQ
u
v
E
z
z
c
z
E
D
W
W
H
U
U
U
w
w
w
w
w
w
,
(4.51)
273
LT showed that by 1700 LT the diurnal mixed layer had substantially
deepened (X Figure 4-31X c). After 1700 LT, the evening deepening of the
mixed layer slowed down significantly.