Geoscience Reference
In-Depth Information
Andreas
(
2004
) explained that within this approach the spray's role is to redistribute the
momentum. As a result, the total stress does not change at a given wind speed and keeps
growing towards high winds, but the role of the turbulent stress
(3.7)
, proportional to
u
2
,
∗
diminishes by comparison with
(9.6)
towards winds at around 60m
s as mentioned above.
Still, according to
Andreas
(
2004
), spray can affect the air-sea momentum coupling by
providing a negative feedback on the wind-wave growth in response to increasing wind
forcing. That is, as more spray is generated, which then falls back into the sea and knocks
down the short waves, the relative stress is reduced because those short waves would
support an extra stress should they persist.
Most of the research on the 'dynamic spray impact' lately has concentrated on the role
of the suspended droplets in altering the physics of WBL directly.
Makin
(
2005
)started
such studies by revisiting the old theory of suspended particles of
Barenblatt
(
1955
) and
suggested a model where a thin (of the order of wave height) sublayer, saturated with 'light
water droplets', appears very near the surface. Surface roughness seen by the outer sublayer
changes, and so does the sea drag, with the effect of
C
D
being reduced at
U
10
in excess of
30m
/
/
s as was observed in the field by
Powell
et al.
(
2003
) and
Jarosz
et al.
(
2007
). The
dimensionless 'weight' of the droplets is determined by a combination of the terminal fall
velocity
u
term
and frictional velocity
u
∗
u
term
κ
ω
w
=
,
(9.7)
u
∗
with
1 being light. Later,
Kudryavtsev &Makin
(
2010
) pointed out that the existence
of such a 'light-water-particle' sublayer is lacking experimental evidence at this stage.
Barenblatt
et al.
(
2005
),
Bye & Jenkins
(
2006
),
Kudryavtsev
(
2006
),
Kudryavtsev &
Makin
(
2010
) and
Rastigejev
et al.
(
2011
) presumed that the particles were 'heavy' (
ω
w
<
ω
w
1) and damp the turbulence in the lower WBL. Estimates of
Barenblatt
et al.
(
2005
)led
to almost an order of magnitude increase of the wind speed as a result of this. When
Rastigejev
et al.
(
2011
) attempted to reproduce these results, however, they did not find
such a large growth of the wind. Their acceleration effect significantly depends on a choice
of the maximal-spray-concentration function. From those available, it ranged from 7% for
the slowest production term (
Andreas
,
1998
) to 32% for the fastest (
Wu
,
1993
).
An essential new outcome of the model by
Rastigejev
et al.
(
2011
) is a critical value of the
wind speed, at which the effects start to occur. For the fastest spray-production function of
Wu
(
1993
), it would be
U
10
critical
≈
30-40m
/
s, and for the slowest term of
Andreas
(
1998
)
U
10
critical
≈
s. If so, then the faster model of
Wu
(
1993
) is more realistic since it leads to
reduction of the sea drag at those wind speeds where such a reduction is actually observed.
Kudryavtsev
(
2006
) suggested another way of interpreting and modelling the impact
of sea droplets on turbulence in WBL, by means of the Monin-Obukhov approach to the
temperature-stratified boundary layer (see
Section 3.1
). He argued that both the tempera-
ture stratification and the distribution of suspended spume affect the turbulence through the
buoyancy force, and thus the Monin-Obukhov similarity theory, for the stable stratification,
can be adopted in this situation.
50m
/
Search WWH ::
Custom Search