Geoscience Reference
In-Depth Information
Figure 8.4 Basis for the choice of bottom-pressure threshold for registering breaking events.
b
t
(bottom scale) is the factor used as a multiplier to the run-averaged bottom-pressure-squared
signal mean
(
var
(
p
))
. Data are from record 4 (
U
10
=
6
.
6m
/
s,circles),record10(8
.
1m
/
s, x-symbol)
and record 8 (11
s, pluses) of
Table 8.1
.a)
T
∗
, the observed ratio of breaking duration to total
duration. b) Corresponding dependence of
E
∗
, the relative contribution of breaking waves to the total
energy flux. c) Normalised energy-flux enhancement ratio, defined as the ratio of
E
∗
.
/
9m
=
E
br
/
E
tot
to
T
∗
=
T
br
/
T
tot
. Figure is reproduced from
Babanin
et al.
(
2007b
)
©
American Meteorological
Society. Reprinted with permission
As already mentioned, the threshold
b
t
was sought as a multiplier for the run-averaged
bottom-pressure-squared signal, mean
(
var
(
p
))
. When calculated,
the product
b
t
·
mean
identifies a critical value for the running average of the high-pass-filtered
bottom-pressure signal. If this value is exceeded at any instant, the synchronously recorded
wave is considered breaking, its physical properties are determined by the zero-crossing
analysis as described above, and the total energy flux over such a local wave is obtained
by integration of the instantaneous flux
p
∂η
∂
(
var
(
p
))
t
from the preceding trough to the following
trough. We should mention again that the threshold was chosen such that waves above the
critical-pressure property were definitely breaking, whereas waves below could possibly
have been breaking or non-breaking. In carrying out this analysis, an iterative procedure
was used to suppress the contribution of breaking events to the mean bottom pressure.
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