why do satellite-based estimates of whitecap fraction depend on
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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. [email protected]. - PowerPoint PPT PresentationTRANSCRIPT
Magdalena D. Anguelova and Peter W. GaiserRemote Sensing Division, Naval Research Laboratory, Washington, DC, USA
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