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| Titel |
Effect of sea sprays on air-sea momentum exchange at severe wind conditions |
| VerfasserIn |
Yu. Troitskaya, E. Ezhova, A. Semenova, I. Soustova |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250069674
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| Zusammenfassung |
| Wind-wave interaction at extreme wind speed is of special interest now in connection with
the problem of explanation of the sea surface drag saturation at the wind speed exceeding 30
m/s. The idea on saturation (and even reduction) of the coefficient of aerodynamic resistance
of the sea surface at hurricane wind speed was first suggested in [1] on the basis of theoretical
analysis of sensitivity of maximum wind speed in a hurricane to the ratio of the enthalpy and
momentum exchange coefficients. Both field [2-4] and laboratory [5] experiments
confirmed that at hurricane wind speed the sea surface drag coefficient is significantly
reduced in comparison with the parameterization obtained at moderate to strong wind
conditions.
Two groups of possible theoretical mechanisms for explanation of the effect of the sea
surface drag reduction can be specified. In the first group of models developed in [6,7], the
sea surface drag reduction is explained by peculiarities of the air flow over breaking waves.
Another approach more appropriate for the conditions of developed sea exploits the effect of
sea drops and sprays on the wind-wave momentum exchange. Papers[8,9] focused on the
effect of the sea drops on stratification of the air-sea boundary layer similar to the model of
turbulent boundary layer with the suspended particles [10], while papers [11-13] estimated
the momentum exchange of sea drops and air-flow. A mandatory element of the spray
induced momentum flux is a parameterization of the momentum exchange between droplets
and air flow, which determines the “source function” in the momentum balance
equation.
In this paper a model describing the motion of a spume droplet, the wind tear
away from the crest of a steep surface wave, and then falling into the water. We
consider two models for the injection of droplets into the air flow. The first one
assumes that the drop starts from the surface at the orbital velocity of the wave. In the
second model we consider droplets from the water jet formed at the base of the
collapsing cavity of the air bubbles in whitecaps of breaking waves. In both models, we
calculate the momentum that acquires the droplet in the interaction with the air
flow. Depending on the particular airflow velocity field, on the wave parameters
and on the radius of the droplet, it can as receive and deliver momentum to the
airflow during its life cycle from the separation from the surface of the water to
fall into the water. Large droplets with a radius greater than 100 microns can as
deliver momentum to the air flow and acquire it, but small droplets with a radius of
less than 100 microns after detachment from the surface of water levitate in the air
flow. However, they are accelerated (or decelerated) from the speed they had on
the water surface (the orbital velocity of waves in the first model and the ejection
velocity of the top jet droplet in the second model) to the air flow velocity, on average
taking away its momentum. The experimental data, compilation of which is given in
[14] show that the droplet generation function dF
da exponentially decreases with
increasing a dF
da = be-a-a0, where a0 = 38.5 mm. As a result, the main contribution
to the momentum flux is due to small droplets, taking away the momentum from
the air flow, and then spray formation slows down the airflow and, consequently,
increase aerodynamic drag of the sea surface. It should be noted, however, that
the effect of momentum transfer by spume droplets torn away from the crests of
breaking waves, is small compared with the turbulent momentum flux in most realistic
conditions, including hurricane winds, but it grows rapidly with increasing wind
speed. The wave-induced momentum flux and can be compared in magnitude with
the turbulent momentum flux, when the wind friction velocity u* is about 2.5 m /
c.
Thus, the direct calculation of the momentum exchange between sea sprays and wind
shows that this mechanism leads to an increase of aerodynamic resistance of the sea surface
with increasing wind speed. To explain the effect of the sea surface drag reduction at high
wind speed conditions, apparently, changing of the form drag, possibly associated with the
influence of foam should be considered.
This paper was supported by RFBR (project codes 10-05-00339-a, 11-05-00455-a). |
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