A laboratory observation of the surface temperature and velocity distributions on a wavy and windy air–water interface

Abstract

Temperature and velocity distributions of the water surface are examined experimentally with infrared imaging techniques. The surface velocity is determined from the movement of the pattern of surface temperature cell. It is found that the mean wind drift is about 30%u * , much less than the widely cited measurements of 55%u * . Although many complicated thermal and dynamical processes control the surface temperature, we found that the probability distribution function ͑PDF͒ of the standardized water surface temperature does not vary significantly with wind speed. The increasing nonlinear effects of wave orbital motions and wave breaking on surface motion are shown by the similarity and trend of change in the PDFs of the water surface velocity normalized by the wave orbital velocity at different wind speeds. The standard deviation of the speed distribution increases with the wind speed while the basic skewed distribution shape remains.Wind blowing over a water surface generates waves and surface currents. It is believed that normal stress ͑pressure͒ is more important in generating long waves, and tangential stress is more efficient in producing surface currents. The momentum near the air-sea interface is then further redistributed among waves, currents and turbulence via such mechanisms as wave breaking 1-3 and Langmuir circulations. 4 In this dynamical process surface water is constantly mixed with the body water below, which is often called surface renewal and is of great importance in heat and gas exchange through air-water interface. As water evaporates, latent heat released to the air produces a thin cool skin thermal layer with temperature of a fraction of a degree less than the bulk water, ⌬TϭT surface ϪT bulk . The cool thin thermal layer and the surface viscous shear layer grow with time until broken up by surface renewal events that occur randomly in space and time. The temperature difference, ⌬T, of a local surface water element is dependent on the time that it is exposed to the air and the rate of heat exchange with the air above. There are very detailed temperature surface structures, and they can be observed with sensitive infrared ͑IR͒ cameras. Recently such cameras have been successfully used in studying surface heat flux, 5 small scale Langmuir circulations 6 and microscale breaking waves. 7 Here we present new results on surface wind drift with this new thermal imaging technology.Early wind drift measurements were made mainly by using tracers such as drifters, drogues, or dye. Wu 8 found that the current immediately below the water surface varies linearly with depth. The value at the water surface was therefore obtained by a linear interpolation. The ratio of the wind surface drift to the wind velocity decreases gradually as the wind fetch increases and approaches a constant value of over 3.5% of wind speed or about a half of surface friction velocity, u * . Several recent studies by particle image velocimetry ͑PIV͒ 9-11 and infrared imaging, 6,12 as compiled in Fig. 1͑a͒, have consistently indi...

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