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air-sea-interaction dynamics (e.g. Emanuel , 1995 , see also discussion in Section 9.1.2 ),
and lately it was confirmed experimentally ( Powell et al. , 2003 ; Jarosz et al. , 2007 ) where
the critical wind speed was found as
U 10 =
32-33 m
/
s
.
(9.12)
Powell ( 2007 ) reanalysed his earlier data and detailed the previous conclusions. He now
provided distribution of the possible values for sea drag rather than a unique dependence
on the wind speed. The range of wind speeds analysed was U 10 =
/
s. It was now
concluded that the drag coefficient increases linearly with the wind speed until it reaches
the maximum of
20-79 m
10 3
C D =
2
·
,
(9.13)
at U 10 =
41 m
/
s. It then starts decreasing down to the minimum of
10 3
C D =
0
.
6
·
,
(9.14)
at U 10 =
s. Values (9.13) and (9.14) are quite low in absolute rather than relative
terms, and can be exceeded even in benign wind-forcing conditions of U 10
61 m
/
s (e.g.
Babanin & Makin , 2008 ). Therefore, one can wonder whether the high winds indeed
quench the sea drag so much or if there is some systematic measurement bias here.
Qualitatively, however, the account provided by Powell ( 2007 ) for the non-uniform dis-
tribution of the sea drag within the ocean surface covered by a tropical cyclone is most
interesting. Within a 30 km-distance from the hurricane centre, the measurements showed
low values of C D
10 m
/
10 3 , with no dependence on the actual wind speed in the hurricane.
The (9.13) - (9.14) dependence is therefore for the drag in the outer zone of the tropical
cyclone. In this outer zone, the 'increase-then-decrease behaviour was found to be confined
to the front left sector'. As Powell ( 2007 ) points out this is the region where the wave field is
characterised by swell presence (see also Young , 2006 ). Dependence of the sea drag on the
wind, for the wind-sea crossed by swell, is very complicated in general (e.g. Dobson et al. ,
1994 ; Donelan et al. , 1997 ; Drennan et al. , 1999 ; Smedman et al. , 1999 ; Grachev & Fairall ,
2001 ; Grachev et al. , 2003 ; Guo-Larsen et al. , 2003 ; Kudryavtsev & Makin , 2004 , see also
Section 9.1.1 ), and apparently this complexity is greatly enhanced in tropical cyclones (see
Young , 2006 ).
It should be commented here that it is not clear how the peculiar wind-dependent
increase-then-decrease behaviour, confined to one front sector in the outer zone of the hur-
ricane, that is to a relatively small area with respect to the total area of the tropical cyclone,
can define the behaviour of sea drag over the entire hurricane. Whether this is because C D
dependence on the wind in the other sectors is marginal (as in the 30 km inner hurricane
zone) or due to something else, is still to be clarified.
The observation of drag saturation in hurricanes was followed by a number of theoreti-
cal, modelling and experimental efforts which offered explanations, simulations and sum-
mary reviews of this effect ( Andreas , 2004 ; Donelan et al. , 2004 , 2006 ; Barenblatt et al. ,
2005 ; Makin , 2005 ; Bye & Jenkins , 2006 ; Kudryavtsev , 2006 ; Kudryavtsev & Makin ,
1
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