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Petelski , 2003 ), but a systematic and conclusive effort to investigate this is yet to be com-
pleted. For instance, Bortkovskii ( 1997 ) confirms the trend indicated by (3.18) , that the
lifetime of foaming reduces as the temperature increases, but at the same time claims that
whitecap coverage increases at higher temperatures. This creates an additional uncertainty
about coverage W in different meteorological conditions, and in this regard it is also nec-
essary to point out what appears to be a persistent confusion about the dependence of W
on atmospheric stability.
Variations of dependence of W versus U due to atmospheric stability were foreshad-
owed in very early studies ( Monahan , 1971 ), and were even quantified to some extent later
(e.g. Wu , 1979 ) and are still highlighted now (e.g. Stramska & Petelski , 2003 ). It is hard
to justify physically, however, such a connection between the stability of the atmospheric
boundary layer and the whitecap coverage of the ocean surface. Wave breaking, white-
cap coverage and lifetime of the foam are essentially water, not air properties. We see
two possible explanations of the correlations between whitecaps and stability, which are
observable.
First of all, the unstable atmospheric conditions (which signify water being warmer than
the lower atmosphere) affect the wind profile. For the steady low boundary layer such a
profile is described as
ln z
z
L
u
κ
(
) =
z 0 ψ
U
z
(3.19)
κ =
.
ψ
where z is the vertical coordinate,
is a function
of the Monin-Obukhov length scale L (we refer the reader to the literature on atmo-
spheric physics for more details (e.g. Komen et al. , 1994 )). Under stable and neutral
conditions, the profile is logarithmic and U 10 wind in (3.5) and
0
42 is the von Karman constant,
stress in (3.7) , or other
relevant wind characteristics are easily found. In unstable conditions, there can be sig-
nificant deviations from the logarithmic profile because of function
τ
. These are quite
difficult to take into account appropriately, hence bias occurs in estimates of U 10 ,
ψ
and in the corresponding dependences. In this case, some correlations between the white-
cap coverage and atmospheric conditions can be observed which are, however, due to
the bias of surface wind estimates rather than due to dependence of W on stratification
of the air.
The second possible explanation is a misinterpretation of some data. For example, Wu
( 1979 ) analysed whitecap-coverage data measured independently by Monahan ( 1971 ) and
by Toba & Chaen ( 1973 ) and demonstrated a clear separation of the data obtained in stable,
neutral and unstable conditions. In both data sets, for the same wind speeds, whitecap cov-
erage on average was higher when the atmosphere was stable, lower when the stratification
was neutral and even lower in unstable circumstances. Since, however, a significant range
of surface water temperatures was involved (in Monahan ( 1971 ) it was from 17
u
5 Cto
.
55 C), and the surface temperature trend was not removed, it would affect the conclu-
sions. Ordering the stratification as stable, neutral and unstable would correspond, on aver-
age, to increasing the water temperature, and therefore to decreasing whitecap coverage
30
.
 
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