Deep-Sea Research, 1971. Vol. 18, pp. 1041 to 1042. Pergamon Pre~s. Printed in Great Brltnm.

S H O R T E R C O N T R I B U T I O N

A note on sea spray and wind-stress discontinuity

J I N W U *

(Received 6 Apri/1971; accepted 20 May 1971)

Abstract--The sudden rise of spray droplet concentration and the occurrence of the wind-stress discontinuity are found at about the same wind velocity of 15 m/see.

S E A S P R A Y

A RAF~-MOUNTF.D photographic system was used by MON~J~AN (1968) tO sample spray droplets at an altitude o f 13 cm above the sea surface, and aboard the same raft the wind was measured by a cup anemometer at an elevation of 47 cm. The results, reproduced in Fig. la, indicate that the spray droplet concentration increases abruptly as the wind increases from about 8.5 to about 9.5 m/see. This set of results is often used and compared directly with other oceanic measurements although the droplet concentrations were correlated with the wind speed measured at a height much lower than the standard anemometer height, 10 m above the mean sea level. Therefore, it may be worth- while to replot Monahan's data with the corresponding wind speeds measured at the standard anemometer height, based on which most oceanic data are presented.

The logarithmic nature of the wind velocity profile near the sea surface seems to be generally accepted (PHn.ups, 1966). On the basis of such a velocity distribution, the wind velocity at the standard anemometer height U~o can be estimated from the wind velocity Uo.,7 measured at the height of 47 cm, or

I0 o

~ c 8 g -

o

~ ' - 4

o . ~ _

c ~ 2 g

0 .

~a',

i 6

Uo 4;'

c

c

c

C

9 12

Wind velocity,

o

o o e: Cio-0.5 X U I ~ X I0 -~

C,c, t55 XlO -3

0

i

m / s e e

c

c @

o J

. 0

I0 I~

Fig. I. Spray droplet concentration versus wind velocity.

20

*Hydronautics, Incorporated, Laurel, Maryland 20810, 1041

1042 Shorter Contribution

where u. is the shear velocity of the wind and K is the Karman constant. Certain wave-induced air motion is inevitable at an elevation of 47 cm. According to SHEMDtN (1968), the wind velocity Ulo obtained from an extension of the logarithmic velocity distribution should be regarded as a slightly underestimated value.

The wind-stress c~fficient needed to calculate the shear velocity is customarily defined with the wind velocity measured at the standard anemometer height. Earlier data tend to indicate a wind- dependent stress coefficient (Wu, 1969), while recent measurements tend to support a wind-indepen- dent value (SMITH, 1970). The empirical formulae derived from earlier data indicate a wind-stress coefficient C1 o = 0"5 x U1 o ~ x 10- 3 (Ulo expressed in m/see) while the recent measurements provide a value of 1"35 × 10 -3.

The percentage of MONAHA~'S (1968) photo frames bearing droplets with radii greater than 45p at various Uo.4~, are shown in Fig. la. The same percentages are replotted vs. the estimated Ulo in Fig. lb ; two different symbols are used to identify the values estimated with different wind-stress coefficients cited in the previous paragraph. It is seen that adopting different wind-stress coefficients has an insignificant effect on the estimation. Considering the possible slight underestimation discussed earlier, the critical condition, where the spray droplet concentration increases abruptly, is found in Fig. I b approximately at Ut o = 15 m/see.

WIND-S1RESS DISCONTINUITY

In the compilation of wind-stress data (Wu, 1969), a discontinuity of the wind-stress coefficient was found at Ulo = 15 m/see, where the coefficient suddenly increases to a higher value. A probable explanation was offered then by relating this discontinuity to the comparison between the phase velocity of the dominant wave and the wind velocity measured at a height of the dominant wave amplitude from the mean sea surface. This suggestion was based on the conjecture that momentum transfer from air to waves when the dominant waves travel faster than the wind may be different than when the wind travels faster than the dominant wave. More work is definitely required to show that a sudden increase of the wind-stress coefficient results due to this difference. Furthermore, it is restated here (Wu, 1971) that only through careful field experiment can the controversy (existence and cause) of the wind-stress discontinuity be resolved. It may also be worthwhile to mention that the wind-stress discontinuity discussed here and the critical wind velocity proposed by Mtn, a< (1947) are two different matters. The latter is related to the transition process of the oceanic wind boundary layer from laminar to turbulent, while the former is found when the wind boundary layer is not only turbulent but also in the hydrodynamically rough regime.

The importance of sea spray on the wind stress was pointed out by KRAUS (1967). He stated that the spray can affect wind in two ways: the water droplets increase the hydrostatic stability and also produce the so-called 'spray stress' due to the horizontal acceleration of the droplets. The first effect changes to some degree the details of the wind profile, while the second effect undoubtedly intensifies the momentum transfer from air to waves. The latter effect should increase the wind stress applying on the sea surface. Therefore, the sudden rise of spray droplet concentration and the occurrence of the wind-stress discontinuity at about the same wind velocity may be more than just a coincidence. It is hoped more studies, especially simultaneous measurements of sea spray and wind stress, will be conducted to improve our understanding of this phenomenon.

Acknowledgment--This work is partially supported by the Office of Naval Research.

R E F E R E N C E S

K~AUS E. B. (1967) Wind stress along the sea surface. In: Advance in Geophysics, H. E. LANDSBERG. editor. Academic Press, 12, 213-255.

MONAHAN E. C. (1968) Sea spray as a function of low elevation wind speed. J. geophys. Res., 73, 1127-1137.

MtmK W. (1947) A critical wind speed for air-sea boundary processes. J. mar. Res., 6. 203-218. PHILLIPS O. M. (1966) The dynamics o f the upper ocean. Cambridge University Press, 261 pp. SHV~DIN O. H. (1968) Wind velociw profile above progressive water waves. Proc. l l th Cong. Coast.

Engng., 1, 53-70. SMrr8 S. D. (1970) Thrust-anemometer measurea~ents of wind turbulenee, Reynolds stress, and

drag co¢ff~ient. J. geophys. Res., 75, 6758-6770. Wu Jnq (1969) W'md stress and surface roughness at air-sea interface. J. geophys. Res., 74, 444--455. W u J'rN (1971) Reply. J. geophys. Red., 76, 2205.