1. 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
Remote sensing of whitecap fraction, W
Method:
Passive radiometric observations of ocean surface [Anguelova and Webster, 2006]
Microwave range, 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)