NORMATIVA EN MATERIA DE PROTECCIÓN DE DATOS EN LA

The sensitivity of microwave frequencies to foam layer thicknesses has important impli-cations for the remote sensing of (the surface expression of) whitecaps. Anguelova and Gaiser [2011] show that this sensitivity can be explained physically through consid-eration of radiometric and structural properties of sea foam, and that foam thickness depends on both the environmental conditions and the lifetime stage of the white-caps. The results presented in chapter 5 on the different responses of W₁₀ and W₃₇ to secondary forcings, provide support to the idea that use of different radiometric frequencies in the W (T_B) algorithm can crudely distinguish between different stages of foam lifetime.

The next step is to move from mere detection of the foam at different microwave frequencies, towards inferring further information on the foam layers [Anguelova and Gaiser,2011]. The primary aim would be to qualitatively link the radiometric signature of the foam to its lifetime stage (active or decaying). Though it has been shown that as the radiometric frequency decreases from 37 GHz to 6 GHz its sensitivity to thinner foam decreases, the microwave signature is not a simple function of foam thickness.

More work is needed to investigate the complex relationships between the physical and radiometric properties of oceanic foam and lifetime stage, before more assertive claims regarding this feature can be made.

The focus of this study is the end product of the satellite algorithm, W10 and W37. Though it is clear that ‘resolving’ active and decaying whitecaps using the radiomet-ric approach requires further work, including significant development of the retrieval algorithm, it is worthwhile comparing estimates of W at the two different microwave frequencies. Figure 6.1 shows maps of the ratio W37/W10, illustrating the varying re-lationship between the two W estimates for given forcing conditions (wind, wave, and environmental conditions) on a seasonal timescale. Therefore, variations in W37/W10

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Figure 6.1: Seasonal maps of the ratio W³⁷/W10 for (a) December–February, (b), March–May, (c) June–August, and (d) September–November.

are related to the radiometric signature of the foam layers detected at the two fre-quencies. Following the arguments above, as the ratio increases, the area covered by thinner foam layers is increasing, relative to thicker foam layers. If one assumes that changes to the emissivity signal are driven primarily by foam layer thickness, the ratio of the two estimates is related to the persistence of the thinner foam layers.

W₃₇/W₁₀ is generally larger in mid latitude regions (especially along the Equator) where mean wind speeds are lowest. Localised regions, such as off the western coast of Mexico, where W₃₇/W₁₀> 2.8 throughout the year are apparent. Interestingly, the ratio is appreciably larger over much of the northern high latitude oceans for JJA than for the rest of the year. The ratio reaches its lowest value (approximately 1) in regions where mean wind speeds are highest, such as the Southern Ocean. Visual inspection of these maps reveals distinct patterns that could be related to a variety of factors including the distribution of mean values for U₁₀, SST, and perhaps even Chl a. The December–February mean wind speed (U10) dependence of W37/W10 as a function of seasonal mean SST is shown in Figure 6.2. W₃₇/W₁₀is highest (reaching a value of ∼ 4) when SST is highest and U10lowest. The ratio falls off as U10increases, with a rate that is proportional to the mean SST; the higher mean SST is, the quicker the rate of fall off. In regions where mean wind speeds are highest (U10 > 14 m s⁻¹), the ratio

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Figure 6.2: W37/W10 as a function of U10 for December–February, with data coloured by the mean SST over the period.

approaches 1. The exact cause(s) of this dependence should be investigated in future work. Specifically, more work is required to determine whether the enhancement of W₃₇relative to W₁₀in low wind, high SST conditions has a physical basis, for example, stabilisation of decaying foam by surfactants.

In summary, as a direct result of the radiometric frequency dependence of satellite-based observations of W , comparing the magnitude of W10 and W37 estimates (as the currently available end product of the W (T_B) algorithm) may be related to the per-sistence of the thinnest foam layers. However, to achieve the overall goal of using a radiometric approach to accurately quantify the contribution to total whitecap cover-age from foam layers in different life cycle stcover-ages and with different physical properties, it is likely that more fundamental, low-level, development of the W (T_B) algorithm is required. In a recent experimental study [Savelyev et al., 2014], it was shown that there is a strong sensitivity of the rate of aerosol production from oceanic whitecaps to the brightness temperature polarization difference, a parameter obtained from ra-diometric measurements. This is a significant finding; an air-sea interaction processes (in this case SSA production) has been shown to be well correlated with a parameter related to brightness temperature, but not explicitly related to the areal coverage of

the surface foam layer (i.e., W ). It is therefore possible that SSA source fluxes can be accurately parameterised using radiometric measurements of brightness temperature without converting TB measurements to W estimates. Finally, radiometric measure-ments of oceanic whitecaps are not limited to visible and microwave frequencies; the possibility of using infra-red signature of whitecaps to infer further information on sur-face foam and wave breaking dynamics has previously been illustrated (e.g.,Jessup and Phadnis [2005]; Marmorino and Smith [2005]. Importantly, the observed differences between the infrared signature of whitecaps in active and decaying stages are related to the distinct differences between the structure and evolution of the two foam types, and so can be used to distinguish between the two.