Chapter 4: FINE STRUCTURE AND MICROSTRUCTURE
Wind-wave mixing and/or nighttime convection produce a well-mixed
layer (X Figure 4-5X a). With no rain, salinity in the molecular diffusion sublayer
just below the sea surface is slightly increased due to evaporation.
The rain forcing consists of a volume flux and a surface flux of
freshwater (Section 1.5). When rain starts, the kinetic energy of raindrops, as
well as the associated buoyancy flux, contributes to the turbulent kinetic
energy balance in the near-surface layer. The rain-formed mixed layer may
not be clearly seen in the vertical salinity profile (as in the sketch shown in
X Figure 4-5X b) during strong rain. However, when the rain ceases, the rainformed mixed layer is usually well defined from the vertical salinity profile
(as schematically shown in X Figure 4-5X c).
Rains are often intermittent in time and space. As a result, the rainformed halocline usually varies spatially. Examples of the vertical and
horizontal structure of the rain-formed halocline are given in the next
section.
4.1.5 Low salinity patches due to convective rains
Convective rains produce low salinity patches in the upper ocean.
Measurements with bow sensors across such a patch (associated with strong
rain event) are shown in X Figure 4-6X . Averaged vertical profiles of
temperature (T), salinity (S) and density (VB t B ) are shown in X Figure 4-7X . They
are calculated by sorting the data in pressure (P) and averaging over the 10
minutes intervals, indicated on the time axis of X Figure 4-6X by heavy line
segments. Segments marked by rectangles in X Figure 4-6X are shown in X Figure
4-8X in more detail.
This low salinity patch is also traced in the temperature record because
the temperature of the raindrops is lower than the SST (Gosnell et al., 1996).
High frequency fluctuations of T, S, and VB t B primarily result from depth
variations of the probes.
Figure 4-8X gives a more detailed picture of the core of the rain patch (the
T, S, and VB t B records for the corresponding segment in X Figure 4-6X are marked
by rectangles). The contour plot of VB t B versus depth (X Figure 4-8X d) reveals
disturbances that look like internal waves with apparent wavelengths of ~200
m or more. The true wavelengths cannot be estimated from these
measurements because the ship‘s motion relative to the internal wave
propagation is unknown.
Averaged vertical profiles of T, S, and VB t B at 6:49LT (X Figure 4-7X ) just
before entering the rain zone show a well-mixed layer within the depth range
of the bow probes. The wind speed was 7.8 m sP
-1
P
and there were no
signatures of diurnal heating or previous rain events in the upper 4 m of the
ocean.
227
Wind-wave mixing and/or nighttime convection produce a well-mixed
layer (X Figure 4-5X a). With no rain, salinity in the molecular diffusion sublayer
just below the sea surface is slightly increased due to evaporation.
The rain forcing consists of a volume flux and a surface flux of
freshwater (Section 1.5). When rain starts, the kinetic energy of raindrops, as
well as the associated buoyancy flux, contributes to the turbulent kinetic
energy balance in the near-surface layer. The rain-formed mixed layer may
not be clearly seen in the vertical salinity profile (as in the sketch shown in
X Figure 4-5X b) during strong rain. However, when the rain ceases, the rainformed mixed layer is usually well defined from the vertical salinity profile
(as schematically shown in X Figure 4-5X c).
Rains are often intermittent in time and space. As a result, the rainformed halocline usually varies spatially. Examples of the vertical and
horizontal structure of the rain-formed halocline are given in the next
section.
4.1.5 Low salinity patches due to convective rains
Convective rains produce low salinity patches in the upper ocean.
Measurements with bow sensors across such a patch (associated with strong
rain event) are shown in X Figure 4-6X . Averaged vertical profiles of
temperature (T), salinity (S) and density (VB t B ) are shown in X Figure 4-7X . They
are calculated by sorting the data in pressure (P) and averaging over the 10
minutes intervals, indicated on the time axis of X Figure 4-6X by heavy line
segments. Segments marked by rectangles in X Figure 4-6X are shown in X Figure
4-8X in more detail.
This low salinity patch is also traced in the temperature record because
the temperature of the raindrops is lower than the SST (Gosnell et al., 1996).
High frequency fluctuations of T, S, and VB t B primarily result from depth
variations of the probes.
Figure 4-8X gives a more detailed picture of the core of the rain patch (the
T, S, and VB t B records for the corresponding segment in X Figure 4-6X are marked
by rectangles). The contour plot of VB t B versus depth (X Figure 4-8X d) reveals
disturbances that look like internal waves with apparent wavelengths of ~200
m or more. The true wavelengths cannot be estimated from these
measurements because the ship‘s motion relative to the internal wave
propagation is unknown.
Averaged vertical profiles of T, S, and VB t B at 6:49LT (X Figure 4-7X ) just
before entering the rain zone show a well-mixed layer within the depth range
of the bow probes. The wind speed was 7.8 m sP
-1
P
and there were no
signatures of diurnal heating or previous rain events in the upper 4 m of the
ocean.
227