- Ih Measurement of G a s Transfer , Whitecap Coverage , and Brightness Temperature in a Surf Pool : An Overview of WABEX - 93

Ocean regions that global carbon cycle Aodels show are large sinks of carbon dioxide (Cod are known to have high wind speeds. TherefoE, estimates of the global ocean uptake of CO, could be improved through better prediction of the air-sea gas transfer velocity, kb at high wind speeds. Because high winds are also associated with increased whitecapping, development of an accurate method for estimating kL for CO, from remote measurements of whitecap coverage, Wc, could provide a method for predicting kL at high wind speed. It could be possible to measure Wc using the remote-sensing technique of passive microwave radiometry. G a s transfer data from laboratory experiments conducted in a "tipping-bucket" whitecap simulation tank (WST) have been used to develop a parameterization of kL in terms of Wc; Schmidt number, SC; and aqueousphase solubility, a. Due to the difficulty in measuring kL under oceanic conditions, it would be advantageous to apply the parameterization from the WST to air-sea gas exchange rather than to acquire enough field data to independently derive a similar relation for the ocean. However, it must first be shown that the results from the WST can be applied to more realistic breaking waves. Extension of the WST results to oceanic conditions should be made using direct oceanic measurements of kL and W,. However, this would require a dual-tracer gas exchange experiment with its associated cost and difficulty. In addition, with such a field experiment there is a risk that the wind conditions during the measurement period would not be high enough to cause significant whitecapping. As an alternative to an oceanic experiment, gas transfer measurements were made in a large outdoor surf pool capable of generating breaking waves that were very similar to one in the ocean. The use of a wave basin provided the additional benefit of a controlled, reproducible field of breaking waves. Furthermore, gas transfer velocities, whitecap coverage, brightness temperature, aqueous-phase turbulence, bubble populations, and other-data necessary to apply the WST data to large-scale waves could be measured concurrently. AIthough the surf pool experiment was not a direct replacement for the oceanic measurements necessary to verify the whitecap-passive microwave gas exchange model, it allowed the empirical relation developed from the WST data to be tested using breaking waves that were more characteristic of oceanic whitecaps. More importantly, the reproducible waves in the surf pool allowed systematic study of the effect of breaking waves on air-water transfer. Because the surf pool was outdoors, a passive microwave radiometer could be used to measure microwave brightness temperature, u. With this data, the remote-sensing applications of this research could be studied. A passive radiometer could not be used effectively in a windbave tunnel because blackbody mim*wave emissions from interior walls and surfaces of the tunnel would mask the radiometric signal from the breaking waves. Although the experiment was principally designed to verify and refine a model for estimating kL from Wc developed using gas exchange data acquired in the WST, measurement of a also will permit correlation of u with Wc for developing a remote-sensing-based method for estimating kLThe surf pool called "Humcane Harbor" is located at Wild Rivers Waterpark in Imine, California. It is an unheated, freshwater pool approximately 20-m wide by 70-m long with a 1.2-m average depth. It can generate a 0.3-m to 13-m plunging or spilling breaking wave every 3.5 s. Wc could be vaned from 0% to 7% and was measured by grey-scale computer analysis of video records of the water surface. Microwave brightness temperature was measured by a 19-GHz dual-polarization radiometer at an incidence angle of 0.925 rad and an i -ID elevation of 18 m. Basin-averaged values of u ranged from 120 K to 180 K at 19 GHz, horizontal polarization @-pol) and 150 K to 210 K at 19 GHz, vertical polarization (v-pol). Gas fluxes were measured for CO,, nitrous oxide (NzO), oxygen (09, helium (He), and sulfur hexafluoride (SF,). Other data collected included bubble size spectra, aqueous-phase turbulence, wave height spectra, void fraction, and wind speed and direction. linearly with Wc. This is an important result, because the surf pool wave conditions provide an order of magnitude increase in the range of available Wc compared to the WST, and the linear correlation with the surf pool data was the same for both plunging and spilling breaking waves. Furthermore, the physical characteristics of the waves in Humcane Harbor are very similar to oceanic breaking waves, suggesting that kL could be linearly correlated with Wc under oceanic conditions. In the absence of bubbles, kL is proportional to Sc-', and increases in Sc will decrease k, Therefore, for transfer across an unbroken water surface kL measured for SF,, kL(SF6), will be less than kL measured under the same conditions for O,, kL(02), because &(SF,) is greater than Sc(0Z). When wc is less than 1%, the data agree with this analysis and kL(SF6) is less than kL(O,J. In contrast to the flat-surface case, bubble-mediated transfer processes cause k, to be a function of both Sc and a, with kL increasing as a decreases at a constant Sc. At constant a however, k, will decrease with increasing Sc. Although Sc(SF6) is 1.8 times larger than Sc(OZ), a(SF6) is 5.5 times smaller than a(O& When btbble-mediated transfer processes are important, it might be expected that the effect of decreasing a will dominate the effect of the increase in Sc and kL(SF6) will be p a t e r than kL(OZ). In support of this hypothesis, the surf pool data show that kL(SF6) is greater than kL(OZ) when Wc is greater than 1%. This demonstrates that bubbles were an important gas transfer pathway in the surf pool. Wc and u values were averaged over the surface of the pool for both breaking wave patterns at wave heights of 0 3 m, 0.6 m, 0.9 m, and 1.2 m. For both wave patterns and at both polarizations, Wc was found to be linearly correlated with u. The radiometer data also show that kL is linearly correlated with u measured at either h-pol or v-pol for both plunging and spilling breaking waves. The observed linear correlation between k, and u is very significant because it shows that measurements of aqueous-phase transfer velocities can be linearly correlated with a directly measured remote-sensing parameter. The correlation of Wc with u and kL with u is discussed in detail elsewhere (see Wang et al., this volume). One of the goals of the experiment was to determine whether the empirical parameterization of kL for bubble-plume driven gas transfer developed using data collected in the WST could be applied to breaking waves. The WST data have shown that k, can be expressed in terms of Wc as (Asher et al., 1994) The surf pool data agrees with the results of Asher et ai. (1992) from the WST in that kL increases . kL = (k+Wc(kT-kM)) + WCh (1) where kM is defined by transfer from mechanically generated turbulence, kT defines transfer from turbulence generated by the breaking wave, and kB defines transfer from the bubble-mediated gas flux. For freshwater, WST measurements have shown kB is given by (Karle et al., 1994)