why do satellite-based estimates of whitecap fraction depend on

Magdalena D. Anguelova and Peter W. GaiserRemote Sensing Division, Naval Research Laboratory, Washington, DC, USA

maggie.anguelova@nrl.navy.mil

Remote sensing of whitecap fraction, Remote sensing of whitecap fraction, WW

Foam skin depth , Foam skin depth , dd, and its dependences, and its dependences

Frequency sensitivity to foam thickness and its implicationsFrequency sensitivity to foam thickness and its implications

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.Whitecap fraction from WindSat observations

10 GHz, H pol., gridded data (0.50.5)Whitecap fraction from WindSat observations

18 GHz, H pol., gridded data (0.50.5)Whitecap fraction from WindSat observations

10 GHz, H pol., gridded data (0.50.5)

What is the reason for the frequency dependence of satellite-based What is the reason for the frequency dependence of satellite-based WW??

t@z.

@z.(z)f

emazf

,εzfFzε

zε.cFπ

zα

dzzα

a

bza

af

f

d

010

0990

.)(

Im2

10

α(z)–attenuation coefficientf (z)–foam permittivityfa (z)–foam void fractionF–frequency (Hz)c–speed of light z–vertical coordinatet–foam layer thickness

Fa (z) profiles Foam skin depth

Foam layer

Frequency, F (GHz)

0 10 20 30 40

Foam

ski

n de

pth,

d (

cm)

0.01

0.10

1.00

10.00

Exponential (m=1)QuadraticLinear

t = 0.2 cmt = 10 cm

Ts = 20 oC

S = 34 psu

Effect of fa(z) profile choice

Effect of fa(z) lower (a) and upper (b) limitsEffect of SST (a) and salinity (b) via f

Frequency limits for thin foam Foam of t = 1 cm detected by various frequencies (see blue lines in the figure)

t = 1

cm

t > d

t = 1

cm

t ≈ d

t = 1

cm

t < d

Active breaking foam1 cm < t < 12 cm

Residual decaying foam0.1 cm < t ≤ 1 cm

37 GHz 6.8 GHz 1.4 GHz

Red lines show that: 37.0 GHz senses all expected t10.7 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)

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

W from photographs

d is the medium thickness over which the propagating electromagnetic radiation decreases by 86% from its initial value, i.e., 1-e-2