Emanuel, 2001). In surface-level winds of 10 m s
-1 and higher, droplets with
radii up to 500 Pm reach a speed equal to e
-1 of the local speed within 1 s.
An estimate of droplet residence time for all droplets up to 500 Pm in radius
suggests that this time is sufficient to accelerate to the local wind speed.
Figure 6-15. Time IJ ac required for sea spray droplets to accelerate to wind speed U 10 .
Here, IJ f is the typical atmospheric residence time for droplets of initial radius r 0 at
the indicated wind speed calculated from relation
1/ 3 /
f
t
A w
W
, where 1/ 3
A is the
significant wave amplitude determined as
2
1/ 3
10
0.015
A
U ( 1/ 3
A is in m and 10
U is in
m s
-1
). The air temperature is taken as 20°C, and the barometric pressure as 1000
Thus, spray droplets with relatively short atmospheric residence time
(the re-entrant spray) effectively transfer momentum flux and sensible but
not latent heat flux. Note that the latent heat flux is usually much larger than
the sensible heat flux.
The spray momentum flux is defined from the following formula
(Andreas and Emanuel, 2001):
3
0
0
0
4
3
hi
lo
r
sp
r
dF
U r
dr
dr
S
W
U ³
,
(6.25)
Chapter 6: HIGH WIND SPEED REGIME
427
hPa. Reproduced from Andreas (2004) by permission from American Meteorological
Society.