Geoscience Reference
In-Depth Information
Vertical strati
cation of a lake has a major role in the cooling process. The quality of
the strati
le: in stable, neutral or unstable
situation, the temperature gradient toward increasing depth is positive, zero or negative,
respectively.
1
In very deep lakes, such as Lake Baikal, the compressibility of water needs
to be also considered. Unstable situations are very short living and lead to vertical con-
vection to the depth across where a neutral or stable strati
cation is given by the vertical temperature pro
cation is recovered. The density
of lake water depends in general on the temperature, salinity, and pressure, given by the
equation of state (see Sect.
2.12
).
The strength of the stable strati
cation can be expressed by the vertical density gra-
dient, which scales to the V
ä
is
ä
l
ä
frequency N by
s
q
0
@
q
N ¼
ð
7
:
1
Þ
@
z
where
ρ
0
is reference density and z is the vertical co-ordinate positive downward. The
quantity N gives the highest frequency of free oscillation of a water particle in a strati
ed
10
−
2
s
−
1
. In turbulent boundary layers, the stability of
fl
fluid. In pre-winter conditions, N
*
the strati
cation is the ratio of buoyant production to shear production of turbulence, given
by the Richardson number
@
q
@
z
@
u
@
z
N
2
Ri ¼
q
0
2
¼
ð
7
:
2
Þ
2
@
u
@
z
In stable (unstable) situation, Ri >0(Ri < 0). According to observations, for Ri > 0.2
turbulence would be damped by the buoyancy.
C is 0.13 kg m
−
3
lower than at 4
Example 7.1
. For fresh water, the density at 0
°
°
C. For
temperature change from 0 to 4
frequency is
N = 0.011 s
−
1
and to damp the turbulence, the vertical shear needs to be less than
0.025 s
−
1
. This is valid for very weak winds over open lakes or beneath a full ice cover. In
brackish water with salinity 10
°
C across a 10-m water layer, V
ä
is
ä
l
ä
T
f
= 0.045 kg m
−
3
, and the
‰
, the density difference Tf
m
−
limiting shear is half of that in the fresh water case.
Thus, in pre-winter, strong winds can easily destroy the inverse strati
cation, which
then returns in weak wind periods. Several mixing
cation phases may occur. This
process determines the distribution of temperature in the water
restrati
-
sediment system of a lake
at the time of freezing and also controls the oxygen storage for the winter, since the
oxygen saturation level depends on the temperature (Fig.
7.1
) (Terzhevik et al. 2009).
Mechanically forced mixing can extend over a large temperature range, from T
m
down to
-
1
Strictly speaking, stable stratification means that a water parcel returns to its original position after
a vertical disturbance, while in unstable stratification a disturbed parcel goes further away.
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