Why do satellite-based estimates of whitecap fraction depend on the probing frequency and how to use this for air-sea interaction studies? Magdalena D. Anguelova and Peter W. Gaiser Remote Sensing Division, Naval Research Laboratory, Washington, DC, USA maggie.anguelova@nrl.navy.mil Remote sensing of whitecap fraction, W Method: Passive radiometric observations of ocean surface [Anguelova and Webster, 2006] Microwave range, 1 - 37 GHz [Anguelova et al., 2006] Motivation: Measure W over a wide range of meteorological and oceanographic conditions Extensive database for better parameterizations of W Results: W values at various frequencies differ: Analogous to W from photographs at various intensity thresholds. W from photographs Whitecap fraction from WindSat observations 10 GHz, H pol., gridded data (0.50.5) Whitecap fraction from WindSat observations 18 GHz, H pol., gridded data (0.50.5) Whitecap fraction from WindSat observations 10 GHz, H pol., gridded data (0.50.5) Anguelova, M.D., M.H. Bettenhausen, and P.W. Gaiser (2006), IGARSS’06 Proceed. , 7, pp. 3676–3679 Anguelova, M. D, and F. Webster (2006), J. Geophys. Res., 111, C03017 What is the reason for the frequency dependence of satellite-based W? Foam skin depth , d, and its dependences Fa (z) profiles Foam skin depth Effect of fa(z) profile choice d is the medium thickness over which the propagating electromagnetic radiation decreases by 86% from its initial value, i.e., 1-e-2 Foam layer α(z)–attenuation coefficient f (z)–foam permittivity fa (z)–foam void fraction F–frequency (Hz) c–speed of light z–vertical coordinate t–foam layer thickness Effect of SST (a) and salinity (b) via f Effect of fa(z) lower (a) and upper (b) limits Active breaking foam 1 cm < t < 12 cm Residual decaying foam 0.1 cm < t ≤ 1 cm Frequency sensitivity to foam thickness and its implications Frequency limits for thin foam Foam of t = 1 cm detected by various frequencies (see blue lines in the figure) 37 GHz 6.8 GHz 1.4 GHz t = 1 cm t > d t = 1 cm t ≈ d t = 1 cm t < d Red lines show that: 37.0 GHz senses all expected t 10.7 GHz senses t > 0.5 cm 6.8 GHz senses t > 0.9 cm 1.4 GHz senses t > 2.5 cm Distinguish active and residual foam using various frequencies Foam of 1 cm is: “Radiometrically thick” for 37.0 GHz Signal comes from part of the layer If t varied, emissivity (ef) is the same Use to detect active and residual whitecaps Foam of 1 cm is: “Radiometrically nominal” for 6.8 GHz Signal comes from the layer only If t varied, ef is specific for each t Use to detect foam thickness if database of ef (t) is available Foam of 1 cm is: “Radiometrically thin” for 1.4 GHz Signal comes from the layer + seawater If t varied, ef changes Use to detect aeration below the foam if knowing ef (t)