MIXING RATE AND TEMPERATURE IN STRATIFIED WATERS
349
microstructure since the microstructure Richardson number always will be
less than the overall Richardson number.
For the Lake Ontario observations the density structure can be
determined with the same resolution as the dye distribution. The following
discussion is therefore focused on these observations.
It seems likely that the shear instability mechanism can account for most
of the layer formation. To investigate this, the Lake Ontario experiments
have been examined and the shear required for the Richardson number
Ri = b has been calculated for a number of layers falling more or less clearly
into the categories 1-4 defined above. It is found that (Table 11):
(1 ) Pulse-formed layers with sharp boundaries are generally observed in
connection with sharp density gradients where the required shear is large
relative to the observed overall shear. If breaking occurs, the shear is so large
that the dye outside the pulse is swept away rapidly and not detected. In this
way the pulse form is maintained. The strong gradients prevent any step
formation. The pulse-formed layers can be either trapped on a density sheet
or between two density sheets. The overall shear in the layer between two
sheets is generally large enough to create a homogeneous distribution in the
layer, so that entrained clean water will be rather rapidly absorbed in the
layer.
(2) The layers with ragged boundaries are situated in regions where
breaking is more likely to occur. The required shear is not so large implying
that its “cleaning” effect is not so pronounced, and indications of a step
formation can be detected.
(3) Leaf structure, or separated sheets. are predominantly found in regions where shear instability is likely to occur. The persisting dye sheets are
trapped in local disturbances in the density distribution. The dye between is
more rapidly mixed due to the relatively more efficient shear in the locally
weak density gradients.
(4) The well-defined, step-formed distributions are invariably connected
to very weak density gradients. The shear required for breaking is correspondingly weak. The step-formed structure can be observed only because
the pulsation and the shear of the horizontal current are not intense enough
to destroy it. The one-sided step distributions are very significant. No steps
are developed at the boundary connected to a sharp density gradient while
well-marked steps are developed at the other boundary.
The above classification is somewhat simplified in the sense that layers of
more complicated structure occur. Nothing definite can be said about the
dominating processes in the sea. Considering the similarity of the distributions found in the sea and the lake it is likely that the same interpretation
holds for the sea. It is noted that a step-formed structure like that in Fig. 9
has not been observed in the sea.
349
microstructure since the microstructure Richardson number always will be
less than the overall Richardson number.
For the Lake Ontario observations the density structure can be
determined with the same resolution as the dye distribution. The following
discussion is therefore focused on these observations.
It seems likely that the shear instability mechanism can account for most
of the layer formation. To investigate this, the Lake Ontario experiments
have been examined and the shear required for the Richardson number
Ri = b has been calculated for a number of layers falling more or less clearly
into the categories 1-4 defined above. It is found that (Table 11):
(1 ) Pulse-formed layers with sharp boundaries are generally observed in
connection with sharp density gradients where the required shear is large
relative to the observed overall shear. If breaking occurs, the shear is so large
that the dye outside the pulse is swept away rapidly and not detected. In this
way the pulse form is maintained. The strong gradients prevent any step
formation. The pulse-formed layers can be either trapped on a density sheet
or between two density sheets. The overall shear in the layer between two
sheets is generally large enough to create a homogeneous distribution in the
layer, so that entrained clean water will be rather rapidly absorbed in the
layer.
(2) The layers with ragged boundaries are situated in regions where
breaking is more likely to occur. The required shear is not so large implying
that its “cleaning” effect is not so pronounced, and indications of a step
formation can be detected.
(3) Leaf structure, or separated sheets. are predominantly found in regions where shear instability is likely to occur. The persisting dye sheets are
trapped in local disturbances in the density distribution. The dye between is
more rapidly mixed due to the relatively more efficient shear in the locally
weak density gradients.
(4) The well-defined, step-formed distributions are invariably connected
to very weak density gradients. The shear required for breaking is correspondingly weak. The step-formed structure can be observed only because
the pulsation and the shear of the horizontal current are not intense enough
to destroy it. The one-sided step distributions are very significant. No steps
are developed at the boundary connected to a sharp density gradient while
well-marked steps are developed at the other boundary.
The above classification is somewhat simplified in the sense that layers of
more complicated structure occur. Nothing definite can be said about the
dominating processes in the sea. Considering the similarity of the distributions found in the sea and the lake it is likely that the same interpretation
holds for the sea. It is noted that a step-formed structure like that in Fig. 9
has not been observed in the sea.