THE NEAR-SURFACE LAYER OF THE OCEAN
the convectively unstable atmospheric boundary layer confirms that
temperature has a characteristic ramp profile—a gradual rise followed by a
relatively sharp decrease. Temperature ramps in the atmospheric boundary
layer were suggested to be a signature of organized large-scale motion
(Phong-Anant et al., 1980). In the unstably stratified atmospheric turbulent
boundary layer, the coherent structures of this type have vertical scale
proportional to the Oboukhov length scale (L O ), while their horizontal size is
an order of magnitude bigger than the vertical.
An important question concerns the size of the contribution of the
coherent structures to the vertical Reynolds shear stress and heat flux. From
measurements in the atmospheric boundary layer, Phong-Anant et al. (1980)
estimated the contribution of the organized motion to the vertical heat flux
for unstable stratification conditions as being over 40% of the average
vertical heat flux and about 20% of the average Reynolds stress. The relative
contribution to the average vertical heat flux and Reynolds stress is less than
10% for nearly neutral and moderately stable conditions.
There are good reasons to assume that ramp-like coherent structures are
important in the dynamics of the upper ocean turbulent boundary layer as
well (Thorpe, 1985). At the same time, free-surface effects can modify the
properties of the organized motion. Observation and theory of the coherent
structures in the near-surface layer of the ocean are more complicated than in
the atmosphere. One reason is that due to significant difference in density
and specific heat capacity in air and water, the boundary layer temperature
and velocity scales in the ocean are much smaller than in the atmosphere.
Thorpe (1985) observed ramp-like structures in the stably stratified nearsurface ocean. Soloviev (1990) and Wijesekera et al. (1999b) reported the
presence of ramp-like structures in the near-surface layer of the ocean under
unstably stratified conditions as well. The next three subsections are devoted
to the analysis of data on ramp-like structures taken in the upper ocean.
5.6.2 Observation of ramp-like coherent structures with bowmounted sensors
A schematic diagram of Soloviev’s (1990) experiment is shown in
Figure 5-42. The underwater probe was mounted on the bow of the research
vessel Akademik Kurchtov at 2 m depth and it measured temperature and
conductivity fluctuations ahead of the moving vessel. Due to a “knife-edge”
hull, the research vessel produced practically no bow wave ahead of itself, as
confirmed by photographs and following from hydrodynamic estimates. In
front of a moving sphere, streamlines are not appreciably disturbed at a
distance greater than ~3 radii (see discussion of these techniques in Section
3.2.5b). At the probe mount location, the radius of curvature of the vessel’s
356
the convectively unstable atmospheric boundary layer confirms that
temperature has a characteristic ramp profile—a gradual rise followed by a
relatively sharp decrease. Temperature ramps in the atmospheric boundary
layer were suggested to be a signature of organized large-scale motion
(Phong-Anant et al., 1980). In the unstably stratified atmospheric turbulent
boundary layer, the coherent structures of this type have vertical scale
proportional to the Oboukhov length scale (L O ), while their horizontal size is
an order of magnitude bigger than the vertical.
An important question concerns the size of the contribution of the
coherent structures to the vertical Reynolds shear stress and heat flux. From
measurements in the atmospheric boundary layer, Phong-Anant et al. (1980)
estimated the contribution of the organized motion to the vertical heat flux
for unstable stratification conditions as being over 40% of the average
vertical heat flux and about 20% of the average Reynolds stress. The relative
contribution to the average vertical heat flux and Reynolds stress is less than
10% for nearly neutral and moderately stable conditions.
There are good reasons to assume that ramp-like coherent structures are
important in the dynamics of the upper ocean turbulent boundary layer as
well (Thorpe, 1985). At the same time, free-surface effects can modify the
properties of the organized motion. Observation and theory of the coherent
structures in the near-surface layer of the ocean are more complicated than in
the atmosphere. One reason is that due to significant difference in density
and specific heat capacity in air and water, the boundary layer temperature
and velocity scales in the ocean are much smaller than in the atmosphere.
Thorpe (1985) observed ramp-like structures in the stably stratified nearsurface ocean. Soloviev (1990) and Wijesekera et al. (1999b) reported the
presence of ramp-like structures in the near-surface layer of the ocean under
unstably stratified conditions as well. The next three subsections are devoted
to the analysis of data on ramp-like structures taken in the upper ocean.
5.6.2 Observation of ramp-like coherent structures with bowmounted sensors
A schematic diagram of Soloviev’s (1990) experiment is shown in
Figure 5-42. The underwater probe was mounted on the bow of the research
vessel Akademik Kurchtov at 2 m depth and it measured temperature and
conductivity fluctuations ahead of the moving vessel. Due to a “knife-edge”
hull, the research vessel produced practically no bow wave ahead of itself, as
confirmed by photographs and following from hydrodynamic estimates. In
front of a moving sphere, streamlines are not appreciably disturbed at a
distance greater than ~3 radii (see discussion of these techniques in Section
3.2.5b). At the probe mount location, the radius of curvature of the vessel’s
356