THE NEAR-SURFACE LAYER OF THE OCEAN
7:49 LST (immediately after sunrise), the relic thermocline had disappeared,
though the new diurnal mixed layer and diurnal thermocline had not yet
formed.
The temperature profile at 7:49 LST was practically homogeneous
within the upper 10 m. This measurement is indicated in X Figure 4-33X a by a
vertical dashed line. The temperature profiles taken later this day show the
formation of a shallow diurnal mixed layer and diurnal thermocline and their
subsequent evening deepening. By 19:06 LST, the diurnal thermocline
disappeared from the depth range of the free-rising profiler. This
measurement is indicated in X Figure 4-33X a by a second vertical dashed line.
The depth of the diurnal mixed layer calculated from equation X (4.55)X is
in remarkable agreement with the experimental data even in the presence of
substantial variability of the solar radiation due to clouds (X Figure 4-33X b).
The situation, however, radically changes during the evening hours when the
model depth tends to infinity and is no longer able to predict the actual depth
of the diurnal mixed layer. This is because entrainment fluxes at the bottom
of the diurnal mixed layer are not taken into account in X (4.53)X .
Soloviev (1982) demonstrated that the effect of volume absorption of
solar radiation leads to a reduced sensitivity of the model diurnal mixed
layer depth to changes of external parameters. (This is true only at relatively
small depths of the mixed layer, D
h < 2 m.) For larger mixed layer depths,
the effect works in the opposite direction, increasing the sensitivity of the
mixed layer depth to changes in the air-sea heat fluxes and the wind speed.
This, in particular, explains the characteristic 3 shape of the diurnal mixed
In order to predict the diurnal mixed layer dynamics during the evening
hours as well as the temperature difference across the diurnal thermocline, a
numerical model is required that accounts for entrainment fluxes (see
4.5.3 Parameterizations for the diurnal SST range
Early empirical parameterizations for diurnal SST amplitudes (for
instance, Hasse, 1971; Deschamps and Frouin, 1984) operated with
instantaneous wind velocities and instantaneous or peak solar radiation
fluxes. Since the diurnal warming is a cumulative process, which depends on
the “history” of the heat and momentum fluxes during the hours after
sunrise, Lukas (1991) proposed connecting the SST range ( max
T
'
) with daily
average cloud fraction ( C ) and daily average wind speed (U ) by a
regression formula:
276
layer dependence on the time of the day clearly seen in Figure 4-33a.
Section 4.5.4). Rotation effects can also then be included.
was made at 5:57 LST (before sunrise). There is a diurnal thermocline left
from the previous diurnal cycle. The depth of this “relic” diurnal mixed layer
is ~ 6 m (see X Figure 4-33X a). At the time of the temperature profile taken at
7:49 LST (immediately after sunrise), the relic thermocline had disappeared,
though the new diurnal mixed layer and diurnal thermocline had not yet
formed.
The temperature profile at 7:49 LST was practically homogeneous
within the upper 10 m. This measurement is indicated in X Figure 4-33X a by a
vertical dashed line. The temperature profiles taken later this day show the
formation of a shallow diurnal mixed layer and diurnal thermocline and their
subsequent evening deepening. By 19:06 LST, the diurnal thermocline
disappeared from the depth range of the free-rising profiler. This
measurement is indicated in X Figure 4-33X a by a second vertical dashed line.
The depth of the diurnal mixed layer calculated from equation X (4.55)X is
in remarkable agreement with the experimental data even in the presence of
substantial variability of the solar radiation due to clouds (X Figure 4-33X b).
The situation, however, radically changes during the evening hours when the
model depth tends to infinity and is no longer able to predict the actual depth
of the diurnal mixed layer. This is because entrainment fluxes at the bottom
of the diurnal mixed layer are not taken into account in X (4.53)X .
Soloviev (1982) demonstrated that the effect of volume absorption of
solar radiation leads to a reduced sensitivity of the model diurnal mixed
layer depth to changes of external parameters. (This is true only at relatively
small depths of the mixed layer, D
h < 2 m.) For larger mixed layer depths,
the effect works in the opposite direction, increasing the sensitivity of the
mixed layer depth to changes in the air-sea heat fluxes and the wind speed.
This, in particular, explains the characteristic 3 shape of the diurnal mixed
In order to predict the diurnal mixed layer dynamics during the evening
hours as well as the temperature difference across the diurnal thermocline, a
numerical model is required that accounts for entrainment fluxes (see
4.5.3 Parameterizations for the diurnal SST range
Early empirical parameterizations for diurnal SST amplitudes (for
instance, Hasse, 1971; Deschamps and Frouin, 1984) operated with
instantaneous wind velocities and instantaneous or peak solar radiation
fluxes. Since the diurnal warming is a cumulative process, which depends on
the “history” of the heat and momentum fluxes during the hours after
sunrise, Lukas (1991) proposed connecting the SST range ( max
T
'
) with daily
average cloud fraction ( C ) and daily average wind speed (U ) by a
regression formula:
276
layer dependence on the time of the day clearly seen in Figure 4-33a.
Section 4.5.4). Rotation effects can also then be included.
was made at 5:57 LST (before sunrise). There is a diurnal thermocline left
from the previous diurnal cycle. The depth of this “relic” diurnal mixed layer
is ~ 6 m (see X Figure 4-33X a). At the time of the temperature profile taken at