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International Journal of Scientific Research and Engineering Development-– Volume 3 Issue 4, July –Aug 2020
Available at www.ijsred.com
ISSN : 2581-7175 ©IJSRED: All Rights are Reserved Page 1
Extinction of Millimeter wave on Two
Dimensional Slices of Foam-Covered Sea-surface
Ayibapreye Kelvin BENJAMIN*, Collins E. OUSERIGHA**
*Department of Electrical/Electronic Engineering, Niger Delta University, P.M.B 071 Yenagoa, Bayelsa State Nigeria
Email:ayibapreyebenjamin@ndu.edu.ng
** Department of Physics, Niger Delta University, P.M.B 071 Yenagoa, Bayelsa State, Nigeria
Email:ouserigha.ec@ndu.edu.ng
----------------------------------------************************----------------------------------
Abstract:
This paper focuses on investigation of millimeter wave (mmW) extinction due to its interaction with 2-D
layers of randomly distributed air-bubbles on the surface of the ocean. The Split-step Fourier method was
adopted for evaluation of the refractive and diffractive effects of scattered mmW due to its interaction with
layers of foam-covered sea-surface. With known varying effective dielectric constant of closely packed
air-bubbles in these layers, estimates of millimetre wave attenuation through layers of sea-foams as
functions of frequency, foam layer thickness, polarization and angle of incidence are reported.
Keywords —Extinction, Sea-foams, Split-step Fourier method.
----------------------------------------************************----------------------------------
I. INTRODUCTION
Due to the incessant demand for precision and
accurate prediction of climate and weather variables
from passive radiometric measurements at WindSat
frequencies, there is need investigate the direct and
indirect effects of air-bubbles (sea-foams)
interactions with millimeter wave at the air-
seawater interface. Evaluation of extinction of
millimeter wave by a flat sea-surface covered by
foam is a significant research problem which
contributes vastly in satellite-based geophysical
retrievals of environmental variables such as sea
surface temperature, sea surface emissivity and
brightness temperature.
Anguelova et.al asserted that bubble plumes in
seawater forms foam layers at the sea surface and in
some instances sea spray suspended closely above
sea surface. The skin depth of sea foam at
microwave frequency narrows the remote sensing
investigations of the sea foam to its surface
expression [1]. Many air-sea interaction processes
are quantified in terms of whitecap fraction
because oceanic whitecaps are the most visible and
direct way of observing breaking of wind waves in
the open ocean. Enhanced by breaking waves,
surface fluxes of momentum, heat, and mass are
critical for ocean atmosphere coupling and thus
affect the accuracy of models used to forecast
weather and predict storm intensification and
climate change. Whitecap fraction is defined as the
fraction of a unit sea surface covered by foam [2]. It
has been traditionally measured by extracting the
high intensity pixels marking white water in still
photographs or video images collected from towers,
ships, and aircrafts. Satellite-based passive remote
sensing of whitecap fraction is a recent
development that allows long term, consistent
observations of white capping on a global scale.
The remote sensing method relies on change of
ocean surface emissivity at microwave frequencies
RESEARCH ARTICLE OPEN ACCESS
International Journal of Scientific Research and Engineering Development-– Volume 3 Issue 4, July –Aug 2020
Available at www.ijsred.com
ISSN : 2581-7175 ©IJSRED: All Rights are Reserved Page 2
(e.g., 6 to 37GHz) due to the presence of sea foam
on a rough sea surface. These changes at the ocean
surface are observed from the satellite as brightness
temperature 	[3]-[4].
The algorithm to obtain from satellite
observations of was developed at the Naval
Research Laboratory within the framework of
WindSat mission. It improved upon the feasibility
study of this remote sensing technique by using
independent sources for the input variables of the
algorithm, physically based models for the
emissivity of rough sea surface and emissivity of
foam, improved rain flag, and improved
atmospheric model necessary for the atmospheric
correction. The database built with this algorithm
compiles for entire year 2006 matched in time
and space with data for the wind vector, wave field
(such as significant wave height and peak wave
period), and environmental parameters (such as sea
surface temperature and atmospheric stability). This
data base has proved useful in analysing and
quantifying the variability of	 . Magdalena et.al
presented an updated algorithm for estimating
from WindSat data using new sources and
products for the input variables. This approach
replaces the originally used QuikSCAT data for
ocean wind vector are replaced with new wind
vector fields [3]-[4].
Xiaobin Tin et.al found large discrepancies when
comparing measurements and model simulations as
wind speed (WS) rise above 12ms-1
. Over the open
ocean and for moderately wind speeds (WSs), the
Soil Moisture and Ocean Salinity (SMOS)
brightness temperature ( ) was initially consistent
with computations made by theoretical
prelaunch models implemented in European Space
Level 2 Ocean Salinity processor [5]. In [5] a new
approach was proposed using new set of parameters
for sea wave spectrum and foam coverage model
that can be used for simulating L-band radiometer
data over a large range of WS based on the
deduction of wind induced components from the
SMOS data.
[6]-[7] used Monte Carlo Simulation and the dense
radiative transfer theory (DMRT) with quasi-
crystalline approximation (QCA) to evaluate direct
and inverse scattering problems. The Monte Carlo
simulation use an exact numerical formulation
based on Foldy-Lax multiple scattering equations of
3-D Maxwell’s equation. Previous calculations
assumed particles with size parameter ( ≪ 1)
using DMRT. An extension to moderate sized
particle is vital for millimetre wave remote sensing
because high frequency range above 10GHz, the
particle size in geographical media are comparable
to the wavelength (Mie Scattering). Contributions
by Chi-Te Chan et.al shows that analytical results
were consistent with Monte Carlo simulations of
exact solutions of Maxwell’s equations of randomly
distributed finite size sphere without adjusting
parameters. Input parameters of the model are all
physical parameters of sizes, concentrations
(volume fractions) and permittivity. The results of
the models were used to obtain brightness
temperatures in passive remote sensing of ocean
foam at 19GHz and 37GHz.
Mie scattering could be used with QCA and
QCA-CP (quasi-crystalline approximation with
coherent potential) to provide solution that is
consistent for particle sizes comparable to or larger
than the wavelength. With results obtained from [7],
the extinction behaviour of sea foam was illustrated,
thermal emission from sea foam was evaluated and
it was shown that the extinction was dominated by
absorption. A physical model of foam emission was
obtained that relates observed brightness
temperature to the microstructure of foam as well as
ocean surface wind vector. The brightness
temperature of sea form can be presented as a
function of observed angle and frequency and
radiative transfer equation could be solved using
derived QCA parameters.
II. ESTIMATE OF THE EFFECTIVE
DIELECTRIC CONSTANT OF SEA-
FORMS
Figures A-E of [8] comprise of 3D (three
dimensions) slices of randomly packed spheres in a
International Journal of Scientific Research and Engineering Development-– Volume 3 Issue 4, July –Aug 2020
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unit cube. These slices were later translated to 2D
slices of randomly packed circles. The conversion
of 3D randomly packed spheres to 2D packed circle
was achieved by calculating the radii of each
individual circle using the concept of intersection of
a sphere and a plane. The 2D slices were discretized
with grid sizes ∆ and ∆ which leads to
intersection of the circles bounded in a unit square
with some grid points. The grid sizes were sampled
such that the edges of the circles circumference
which intersects with grid points farther from the
inner grids bounded by the circles are negligible.
Estimate of the effective dielectric constant of sea
foams were made by modelling the randomly
packed bubbles as concentric circles in 2-D where
the outer circle is a mixture of air and seawater
while the inner circle contains about (80 − 95)	%
air, with these estimates we were able to calculate
the area of the annulus (ring) as the radii of the
outer circles are known.
Table 1 and 2 of [8] illustrate dielectric constant
of seawater at fixed salinity 34	 and sea surface
temperature 20° . The dielectric constant of air is
taken as 1.00005	 + 0.0000 . The area of the
circles in each slice was calculated using the total
number of grid points. The sea-foam effective
dielectric constants at WindSat frequencies 6.8	 !",
18.7	 !", and 23.8	 !" were calculated for five
slices of randomly packed spherical air-bubbles that
are coated with thin layers of seawater.
TABLE I
Results for Dielectric constant of sea foam at frequencies of 6.8	 !",
18.7	 !" and 23.8	 !" for 5 2-D Slices of randomly packed air-bubbles
covered with thin-layer of seawater.
FREQUENCY 6.8	 !" 18.7	 !" 23.8	 !"
Slice 1 1.1294 + 0.1297 1.0657 + 0.0896 1.0008 + 0.1267
Slice 2 1.2032 + 0.1562 1.0937 + 0.1460 1.0117 + 0.1526
Slice 3 1.2122 + 0.1537 1.1252 + 0.1652 1.0263 + 0.1833
Slice 4 1.3060 + 0.2148 1.2094 + 0.1804 1.0427 + 0.2098
Slice 5 1.3457 + 0.2357 1.2547 + 0.1993 1.0567 + 0.2302
III. PARABOLIC WAVE EQUATION
METHOD
Parabolic wave equation method is one of the
numerous computational and numerical
approximations used in modelling the interaction of
EM fields with physical objects and the
environment. Parabolic equation method (PEM)
provides efficient approximation to the Helmholtz
wave equation which is derived by decoupling of
Maxwell’s equations to evaluate radio-wave
propagation in random and inhomogeneous media.
Leontovich and Fock [9,10] introduced the
parabolic wave equation which is very powerful in
its application for analysing the problem of radio-
wave diffraction around the earth or in the
atmosphere. [11] Malyuzhinets, Fock and
Wainstein generalized the PEM based on studies of
ray-coordinates and transverse diffusion.
Malyuzhinets unified geometric optics with the
parabolic approximate method to establish an
efficient theory of diffraction of obstacles. Tappert
et.al [9-11] developed the very powerful split-step
Fourier solution of the parabolic equation for
solving underwater acoustic problems.
PEM applications spans from radio-waves
propagation problems in VHF to millimetre wave
affected by atmospheric refraction, diffraction and
reflection by irregular terrains and rough sea
surface. More reviews and references of the
parabolic equation method can be accessed in [11]
and [9-12].
The principal advantages of the various parabolic
wave equations derived below is that it constitutes
an initial value problem in range and hence can be
solved by a range matching numerical technique,
given a source field distribution over depth at the
initial range.
Over the years, several different solution techniques
have been implemented in computer codes [9-12],
but only the split-step Fourier technique and various
finite-difference / finite-element techniques have
gained widespread use in the underwater acoustic
community.
Before going into details on the numerical solution
schemes, let us briefly point out some advantages
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and disadvantages of these two main solution
techniques.
The split-step algorithm has been extensively used
to solve the SPE ever since it was developed by
Hardin and Tappert [9] in the early 1970s. The
technique is computationally efficient for long-
range, narrow-angle propagation problems with
negligible bottom interactions.
For short-range, deep water problems and shallow
water problems in general, propagation is basically
more wide-angled and bottom interacting paths
become more important.
This requires the use of wide-angle PEs, which can
be solved only by finite difference or finite
elements. Moreover, the strong speed and density
contrasts encountered at the water-bottom interface
adversely affect the computational efficiency of the
split-step technique, which in cases of strong
bottom interaction requires an excessively fine
computational grid(∆ , "). Hence, the advantage of
higher computational efficiency of the split-step
technique is entirely lost in situations with strong
bottom interactions.
Finite-difference and Finite-element solutions are
applicable to PE for arbitrary large angles. The
main drawback of these schemes is that, for long
range the split-step solution is more efficient and
also for narrow angle with minimal or no bottom
interaction. The split-step solution remains the most
adopted technique for performance prediction as it
is more suitable to solving many practical ocean
surface problems.
Conversely, the finite-difference and finite-element
schemes have widespread application for wide-
angle and bottom interacting boundaries. It is
prominent for providing higher accuracy in these
domains.
The most recent development in terms of efficient
PE solution schemes is a split-step Pade’
approximations derived by Collins [13,14]. He uses
higher order Pade’ approximations not the square
root operators. The result is a considerable
efficiency gain through the use of higher range step.
Thus, the scheme is claimed to be more than an
order of magnitude faster than standard FD/FE
solution techniques. This could create a unified PE
solution approach where the accurate high angle
PEs can be solved with the efficiency of the
classical split-step Fourier scheme.
IV. DERIVATION OF THE PARABOLIC
WAVE EQUATION
Using a simple model which describe the
propagation of a reduced function
																	ψ( , ") = ( , ")'()*
(1)
associated with the direction of propagation .
Where ( , ") can be expressed as
( , ") = Ψ( , ")',()*
(2)
The Helmholtz equation of the reduced function is
obtained by decoupling Maxwell’s equations and
can be expressed as
∇.
Ψ + .
/.( , ")Ψ = 0(3)
Where /.( , ") is the refractive index and is the
wavenumber. The refractive index /.( , ") is
assumed to possess smooth variations. The reduced
function implies that the propagation energy varies
slowly at angles close to the paraxial direction.
The Laplacian of Ψ( , ") can be expressed as
													∇.
Ψ = [∇.
+ 2 ∇ − .
]'()*
(4)
Where Ψ is taken as the product of a plane wave
solution, substituting equation (4) into equation (3)
yields
				[	∇.
+ 2 ∇ − .
]'()*
+
.
/.( , ")	 '()*
= 0(5)
	∇.
+ 2 ∇ + .
/.
{( , ") − 1}	 = 0(6)
The Laplacian operator for 2D can be expressed as
(
45
4*5
+
45
465
), in the atmosphere where / − 1	is small,
we neglect
45
4*5 as small (paraxial approximation)
and equation (6) becomes
457
465 + 2
47
4*
+ .
(/.
− 1) = 0 (7)
Where equation (7) is the standard parabolic
equation.
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Equation (7) by method of separation of variables
can be written as
2
47
4*
= (1 − /.) .
−
457
465
(8)
Which can be expressed as
47
4*
= 8(1 − /.) .
−
95
.)
: (9)
Let consider ; =	(1 − /.) .
−
95
.)
and substitute
; into equation (9). This yields
47
4*
= ; (10)
Taking like terms
47
7
= ;< (11)
The solution of the parabolic wave equation
becomes
( = + Δ , ") = ( =)'(?@*
(12)
It is worthy of note that equation (7) can be factored
out to obtain
8
4
4*
+ (1 − A): 8
4
4*
+ (1 + A):(13)
This gives us
47
4*
= − (1 − A)(14)
47
4*
= − (1 + A)(15)
Where A is the pseudo-differential operator and is
defined by
A =		B
C
)5
45
465
+ /.( , ")(16)
Equation (14) is the outgoing parabolic equation
and equation (15) is the incoming parabolic wave
equation.
Considering the propagation medium as
homogeneous with refractive index /, the field
component Ψ satisfies two-dimensional scalar wave
equation
45D
465
+ 2
45D
4*5
+ .
/.
(", ) = 0(16)
The refractive index varies with the range and
height " and equation (16) is a good approximation
provided / varies slowly with wavelength. It is
worth noting that equation (16) is not exact [Levy].
If the propagation medium is vacuum, the standard
parabolic wave equation in (7)
45D
465
+ 2
47
4*
= 0(17)
The solution of equation (1.43) can be expressed as
( = + Δ , ") = ( =)'(
E5
5F
@*
(18)
V. THE SPLIT STEP FOURIER TRANSFORM
SOLUTION
The split-step Fourier method is a very efficient
PEM which separate the refractive effect from the
diffractive part of the propagator. Considering a
two-dimensional scalar wave equation for
horizontally and vertically polarised wave. Hardin
et.al introduced the split-step Fourier method which
transforms the rough surface problem with
propagation through a sequence of phase screens
[14-17].
The standard parabolic equation (SPE) in equation
(7) can be written as
47
4*
=
()
.
8
C
)5
45
465 + (/.
( , ") − 1): (19)
Let
; =
C
)5
45
465(20)
G =	/.
( , ") − 1(21)
Equation (19) becomes
47
4*
=
()
.
{; + G} (22)
The analytic solution of the SPE is
( + ∆ , ") = 	 ( , ")'
HF
5
∆*(?I )
(23)
Using
J =
()∆*
.
(24)
Equation (24) yields
( + ∆ , ") = 	 ( , ")'K(?I )
(25)
Equation (25) is the split-step solution which
represent the field propagating through series of
phase screens. The field is first propagated through
a slice of homogeneous medium characterised by
the exponent of ;.
The SSFM routine was implemented to propagate
the plane wave
L("=, , ) = L(", , )' M * + N + 6"O		(26)
with E(z, x, y) ≈ 1 along the forward +z direction.
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The plane wave was propagated through five (5) 2D
slices of sea foam layers each containing
isotropically distributed bubbles. The slices are
equally dimensioned with area 100mm × 100mm
with layer thickness δt = 10 mm separating adjacent
layers. The foam layer thickness d ≫ λ
to account for attenuation (E-field amplitude
variation) and diffuse scattering (E
variation) as the incident E-field travels through
slices of the sea foam layer. WindSat frequency
channels (6.8 GHz, 18.7 GHz, 23.8 GHz) were used
for propagation of the E-field through slices of sea
foam layers.
Fig. 1 Geometric configuration of foam-covered sea-surface divided into
slices of sea-foam layers.
The incident wave is tilted from the normal so that
there is an initial phase gradient along the surface of
the sea foam model. This is done by assigning
values of 0 ≤ θi≤ 90◦for the zenith angle and fixed
azimuthal angle for example φi= 0◦or 45
For each of the angles θiwe compute the E
that emerges from the foam layer and calculate
FFTs of the fields. The contour plots of the
intensity of the scattered field is displayed. Range
of the depths of foam for realistic scenarios were
observed to investigate the influence of the foam as
a thin phase scattering screen and deep phase
scattering screen.
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The plane wave was propagated through five (5) 2D
of sea foam layers each containing
isotropically distributed bubbles. The slices are
equally dimensioned with area 100mm × 100mm
= 10 mm separating adjacent
λ0 is required
field amplitude
variation) and diffuse scattering (E-field phase
field travels through
slices of the sea foam layer. WindSat frequency
channels (6.8 GHz, 18.7 GHz, 23.8 GHz) were used
field through slices of sea
surface divided into
The incident wave is tilted from the normal so that
ong the surface of
the sea foam model. This is done by assigning
for the zenith angle and fixed
or 45◦.
we compute the E-fields
that emerges from the foam layer and calculate the
FFTs of the fields. The contour plots of the
intensity of the scattered field is displayed. Range
of the depths of foam for realistic scenarios were
observed to investigate the influence of the foam as
a thin phase scattering screen and deep phase
VI. MILLIMETER WAVE
AT LOW AND WINDSAT FREQUENCIES
AS A FUNCTION OF DEPTH
FORM
The attenuation of the field intensity in decibel
(dB) at a low frequency of 6.8
slice thickness JQ = 0.1	RR which describes
phase scattering screen, as phase perturbation
increases as the depth of sea foam layer increases.
Fig. 2 and 3 illustrate the variation of attenuation of
the E-field in dB with depth in mm of sea
layer. This is due to increase in diffuse refl
or scattering as the EM wave interacts with
randomly oriented sea-foams with different
refractive indices. This happens for both TE and
TM polarized E-fields.
Fig. 2 Attenuation of Field Intensity for horizontal polarization (TE) with
zenith S( = 45° and azimuth T = 0° with depth of sea
foam layer thickness JQ = 0.1	RR.
Fig. 3 Attenuation of Field Intensity for vertical polarization (TM) with
zenith S( = 45° and azimuth T = 0° with depth of sea
foam layer thickness JQ = 0.1	RR.
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WAVE ATTENUATION
FREQUENCIES
DEPTH OF SEA-
The attenuation of the field intensity in decibel
8	 !" and sea foam
which describes a thin
phase scattering screen, as phase perturbation
increases as the depth of sea foam layer increases.
Fig. 2 and 3 illustrate the variation of attenuation of
field in dB with depth in mm of sea-foam
layer. This is due to increase in diffuse reflections
or scattering as the EM wave interacts with
foams with different
refractive indices. This happens for both TE and
Attenuation of Field Intensity for horizontal polarization (TE) with
with depth of sea-foam at 6.8	 !" and
Attenuation of Field Intensity for vertical polarization (TM) with
with depth of sea-foam at 6.8	 !" and
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Fig. 4 Attenuation of Field Intensity for horizontal polarization (TE) with
zenith S( = 45° and azimuth T = 0° with depth of sea-foam at
foam layer thickness JQ = 0.1	RR.
Similarly, the attenuation of the field intensity in
decibel (dB) at WindSat frequency of 18
sea-foam slice thickness JQ = 0.1	RR
the depth of sea foam layer increases. Fig. 4 and 5
illustrate the variation of attenuation of the E
in dB with depth in mm of sea-foam layer. This is
also due to increase in diffuse reflections or
scattering as the EM wave interacts with randomly
oriented sea-foams with different refractive indices.
This happens for both TE and TM polarized E
fields.
Fig. 5 Attenuation of Field Intensity for vertical polarization (TM) with
zenith S( = 45° and azimuth T = 0° with depth of sea-foam at
foam layer thickness JQ = 0.1	RR.
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Attenuation of Field Intensity for horizontal polarization (TE) with
foam at 18.7	 !" and
he attenuation of the field intensity in
18.7	 !" and
RR increases as
the depth of sea foam layer increases. Fig. 4 and 5
illustrate the variation of attenuation of the E-field
foam layer. This is
also due to increase in diffuse reflections or
scattering as the EM wave interacts with randomly
foams with different refractive indices.
This happens for both TE and TM polarized E-
Attenuation of Field Intensity for vertical polarization (TM) with
foam at 18.7	 !" and
Fig. 6 Attenuation of Field Intensity for horizontal polarization (TE)
zenith S( = 45° and azimuth T = 0° with depth of sea
foam layer thickness JQ = 0.1	RR.
Fig. 7 Attenuation of Field Intensity for vertical polarization (TE) with
zenith S( = 45° and azimuth T = 0° with depth of sea
foam layer thickness JQ = 0.1	RR.
For thin phase scattering screens
with a given zenith angle S( =
T = 0°, the attenuation U(VG) increases with depth
(mm) of sea-foam and frequency at
attenuation at 18.7	 !" is less than that at
23.8	 !", which agrees with the fact that there is
more interaction of the E-field with the particles at
high frequency. In Fig. 6 and 7, attenuation of the
E-fields for both TE and TM polarizations are due
to phase perturbations due to diffuse scattering of
disordered air-bubbles with varying dielectric
constants
VII. CONCLUSIONS
We have shown from our results that attenuation
of the E-field is due to diffuse scattering when the
incident E-field is multiple reflec
it propagates through the various slices of sea
layer. The E-field travels through different paths
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Attenuation of Field Intensity for horizontal polarization (TE) with
with depth of sea-foam at 23.8	 !" and
Attenuation of Field Intensity for vertical polarization (TE) with
with depth of sea-foam at 23.8	 !" and
For thin phase scattering screens JQ = 0.1	RR
= 45° and azimuth
increases with depth
foam and frequency at 23.8	 !". The
is less than that at
, which agrees with the fact that there is
field with the particles at
high frequency. In Fig. 6 and 7, attenuation of the
fields for both TE and TM polarizations are due
phase perturbations due to diffuse scattering of
bubbles with varying dielectric
We have shown from our results that attenuation
field is due to diffuse scattering when the
field is multiple reflected by sea-foam as
it propagates through the various slices of sea-foam
field travels through different paths
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and is scattered due to re-radiation of the dipole
moments of the disoriented scatterers which
absorbs some of the incident E-field and re-radiates
it at various directions. The varying effective
dielectric constants of the particles enhances diffuse
scattering of the incident and transmitted E-fields
within the sea-foam layer. The split-step Fourier
transform method is a range marching technique
that is suitable for evaluation of the E-field
propagation given an initial range, the expected
depth of the field can be computed as a function of
range and depth. Future works will evaluate
variation of the field intensity due to its interaction
with rough sea-surface covered by foams.
ACKNOWLEDGMENT
Theauthors are gratefulto the reviewers and
Executive Editor-in-chief for their insightful
comments, which have helped improved the paper.
This research work was supported by the Tertiary
Education Trust Fund (TETFUND), Nigeria.
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Extinction of Millimeter wave on Two Dimensional Slices of Foam-Covered Sea-surface

  • 1. International Journal of Scientific Research and Engineering Development-– Volume 3 Issue 4, July –Aug 2020 Available at www.ijsred.com ISSN : 2581-7175 ©IJSRED: All Rights are Reserved Page 1 Extinction of Millimeter wave on Two Dimensional Slices of Foam-Covered Sea-surface Ayibapreye Kelvin BENJAMIN*, Collins E. OUSERIGHA** *Department of Electrical/Electronic Engineering, Niger Delta University, P.M.B 071 Yenagoa, Bayelsa State Nigeria Email:ayibapreyebenjamin@ndu.edu.ng ** Department of Physics, Niger Delta University, P.M.B 071 Yenagoa, Bayelsa State, Nigeria Email:ouserigha.ec@ndu.edu.ng ----------------------------------------************************---------------------------------- Abstract: This paper focuses on investigation of millimeter wave (mmW) extinction due to its interaction with 2-D layers of randomly distributed air-bubbles on the surface of the ocean. The Split-step Fourier method was adopted for evaluation of the refractive and diffractive effects of scattered mmW due to its interaction with layers of foam-covered sea-surface. With known varying effective dielectric constant of closely packed air-bubbles in these layers, estimates of millimetre wave attenuation through layers of sea-foams as functions of frequency, foam layer thickness, polarization and angle of incidence are reported. Keywords —Extinction, Sea-foams, Split-step Fourier method. ----------------------------------------************************---------------------------------- I. INTRODUCTION Due to the incessant demand for precision and accurate prediction of climate and weather variables from passive radiometric measurements at WindSat frequencies, there is need investigate the direct and indirect effects of air-bubbles (sea-foams) interactions with millimeter wave at the air- seawater interface. Evaluation of extinction of millimeter wave by a flat sea-surface covered by foam is a significant research problem which contributes vastly in satellite-based geophysical retrievals of environmental variables such as sea surface temperature, sea surface emissivity and brightness temperature. Anguelova et.al asserted that bubble plumes in seawater forms foam layers at the sea surface and in some instances sea spray suspended closely above sea surface. The skin depth of sea foam at microwave frequency narrows the remote sensing investigations of the sea foam to its surface expression [1]. Many air-sea interaction processes are quantified in terms of whitecap fraction because oceanic whitecaps are the most visible and direct way of observing breaking of wind waves in the open ocean. Enhanced by breaking waves, surface fluxes of momentum, heat, and mass are critical for ocean atmosphere coupling and thus affect the accuracy of models used to forecast weather and predict storm intensification and climate change. Whitecap fraction is defined as the fraction of a unit sea surface covered by foam [2]. It has been traditionally measured by extracting the high intensity pixels marking white water in still photographs or video images collected from towers, ships, and aircrafts. Satellite-based passive remote sensing of whitecap fraction is a recent development that allows long term, consistent observations of white capping on a global scale. The remote sensing method relies on change of ocean surface emissivity at microwave frequencies RESEARCH ARTICLE OPEN ACCESS
  • 2. International Journal of Scientific Research and Engineering Development-– Volume 3 Issue 4, July –Aug 2020 Available at www.ijsred.com ISSN : 2581-7175 ©IJSRED: All Rights are Reserved Page 2 (e.g., 6 to 37GHz) due to the presence of sea foam on a rough sea surface. These changes at the ocean surface are observed from the satellite as brightness temperature [3]-[4]. The algorithm to obtain from satellite observations of was developed at the Naval Research Laboratory within the framework of WindSat mission. It improved upon the feasibility study of this remote sensing technique by using independent sources for the input variables of the algorithm, physically based models for the emissivity of rough sea surface and emissivity of foam, improved rain flag, and improved atmospheric model necessary for the atmospheric correction. The database built with this algorithm compiles for entire year 2006 matched in time and space with data for the wind vector, wave field (such as significant wave height and peak wave period), and environmental parameters (such as sea surface temperature and atmospheric stability). This data base has proved useful in analysing and quantifying the variability of . Magdalena et.al presented an updated algorithm for estimating from WindSat data using new sources and products for the input variables. This approach replaces the originally used QuikSCAT data for ocean wind vector are replaced with new wind vector fields [3]-[4]. Xiaobin Tin et.al found large discrepancies when comparing measurements and model simulations as wind speed (WS) rise above 12ms-1 . Over the open ocean and for moderately wind speeds (WSs), the Soil Moisture and Ocean Salinity (SMOS) brightness temperature ( ) was initially consistent with computations made by theoretical prelaunch models implemented in European Space Level 2 Ocean Salinity processor [5]. In [5] a new approach was proposed using new set of parameters for sea wave spectrum and foam coverage model that can be used for simulating L-band radiometer data over a large range of WS based on the deduction of wind induced components from the SMOS data. [6]-[7] used Monte Carlo Simulation and the dense radiative transfer theory (DMRT) with quasi- crystalline approximation (QCA) to evaluate direct and inverse scattering problems. The Monte Carlo simulation use an exact numerical formulation based on Foldy-Lax multiple scattering equations of 3-D Maxwell’s equation. Previous calculations assumed particles with size parameter ( ≪ 1) using DMRT. An extension to moderate sized particle is vital for millimetre wave remote sensing because high frequency range above 10GHz, the particle size in geographical media are comparable to the wavelength (Mie Scattering). Contributions by Chi-Te Chan et.al shows that analytical results were consistent with Monte Carlo simulations of exact solutions of Maxwell’s equations of randomly distributed finite size sphere without adjusting parameters. Input parameters of the model are all physical parameters of sizes, concentrations (volume fractions) and permittivity. The results of the models were used to obtain brightness temperatures in passive remote sensing of ocean foam at 19GHz and 37GHz. Mie scattering could be used with QCA and QCA-CP (quasi-crystalline approximation with coherent potential) to provide solution that is consistent for particle sizes comparable to or larger than the wavelength. With results obtained from [7], the extinction behaviour of sea foam was illustrated, thermal emission from sea foam was evaluated and it was shown that the extinction was dominated by absorption. A physical model of foam emission was obtained that relates observed brightness temperature to the microstructure of foam as well as ocean surface wind vector. The brightness temperature of sea form can be presented as a function of observed angle and frequency and radiative transfer equation could be solved using derived QCA parameters. II. ESTIMATE OF THE EFFECTIVE DIELECTRIC CONSTANT OF SEA- FORMS Figures A-E of [8] comprise of 3D (three dimensions) slices of randomly packed spheres in a
  • 3. International Journal of Scientific Research and Engineering Development-– Volume 3 Issue 4, July –Aug 2020 Available at www.ijsred.com ISSN : 2581-7175 ©IJSRED: All Rights are Reserved Page 3 unit cube. These slices were later translated to 2D slices of randomly packed circles. The conversion of 3D randomly packed spheres to 2D packed circle was achieved by calculating the radii of each individual circle using the concept of intersection of a sphere and a plane. The 2D slices were discretized with grid sizes ∆ and ∆ which leads to intersection of the circles bounded in a unit square with some grid points. The grid sizes were sampled such that the edges of the circles circumference which intersects with grid points farther from the inner grids bounded by the circles are negligible. Estimate of the effective dielectric constant of sea foams were made by modelling the randomly packed bubbles as concentric circles in 2-D where the outer circle is a mixture of air and seawater while the inner circle contains about (80 − 95) % air, with these estimates we were able to calculate the area of the annulus (ring) as the radii of the outer circles are known. Table 1 and 2 of [8] illustrate dielectric constant of seawater at fixed salinity 34 and sea surface temperature 20° . The dielectric constant of air is taken as 1.00005 + 0.0000 . The area of the circles in each slice was calculated using the total number of grid points. The sea-foam effective dielectric constants at WindSat frequencies 6.8 !", 18.7 !", and 23.8 !" were calculated for five slices of randomly packed spherical air-bubbles that are coated with thin layers of seawater. TABLE I Results for Dielectric constant of sea foam at frequencies of 6.8 !", 18.7 !" and 23.8 !" for 5 2-D Slices of randomly packed air-bubbles covered with thin-layer of seawater. FREQUENCY 6.8 !" 18.7 !" 23.8 !" Slice 1 1.1294 + 0.1297 1.0657 + 0.0896 1.0008 + 0.1267 Slice 2 1.2032 + 0.1562 1.0937 + 0.1460 1.0117 + 0.1526 Slice 3 1.2122 + 0.1537 1.1252 + 0.1652 1.0263 + 0.1833 Slice 4 1.3060 + 0.2148 1.2094 + 0.1804 1.0427 + 0.2098 Slice 5 1.3457 + 0.2357 1.2547 + 0.1993 1.0567 + 0.2302 III. PARABOLIC WAVE EQUATION METHOD Parabolic wave equation method is one of the numerous computational and numerical approximations used in modelling the interaction of EM fields with physical objects and the environment. Parabolic equation method (PEM) provides efficient approximation to the Helmholtz wave equation which is derived by decoupling of Maxwell’s equations to evaluate radio-wave propagation in random and inhomogeneous media. Leontovich and Fock [9,10] introduced the parabolic wave equation which is very powerful in its application for analysing the problem of radio- wave diffraction around the earth or in the atmosphere. [11] Malyuzhinets, Fock and Wainstein generalized the PEM based on studies of ray-coordinates and transverse diffusion. Malyuzhinets unified geometric optics with the parabolic approximate method to establish an efficient theory of diffraction of obstacles. Tappert et.al [9-11] developed the very powerful split-step Fourier solution of the parabolic equation for solving underwater acoustic problems. PEM applications spans from radio-waves propagation problems in VHF to millimetre wave affected by atmospheric refraction, diffraction and reflection by irregular terrains and rough sea surface. More reviews and references of the parabolic equation method can be accessed in [11] and [9-12]. The principal advantages of the various parabolic wave equations derived below is that it constitutes an initial value problem in range and hence can be solved by a range matching numerical technique, given a source field distribution over depth at the initial range. Over the years, several different solution techniques have been implemented in computer codes [9-12], but only the split-step Fourier technique and various finite-difference / finite-element techniques have gained widespread use in the underwater acoustic community. Before going into details on the numerical solution schemes, let us briefly point out some advantages
  • 4. International Journal of Scientific Research and Engineering Development-– Volume 3 Issue 4, July –Aug 2020 Available at www.ijsred.com ISSN : 2581-7175 ©IJSRED: All Rights are Reserved Page 4 and disadvantages of these two main solution techniques. The split-step algorithm has been extensively used to solve the SPE ever since it was developed by Hardin and Tappert [9] in the early 1970s. The technique is computationally efficient for long- range, narrow-angle propagation problems with negligible bottom interactions. For short-range, deep water problems and shallow water problems in general, propagation is basically more wide-angled and bottom interacting paths become more important. This requires the use of wide-angle PEs, which can be solved only by finite difference or finite elements. Moreover, the strong speed and density contrasts encountered at the water-bottom interface adversely affect the computational efficiency of the split-step technique, which in cases of strong bottom interaction requires an excessively fine computational grid(∆ , "). Hence, the advantage of higher computational efficiency of the split-step technique is entirely lost in situations with strong bottom interactions. Finite-difference and Finite-element solutions are applicable to PE for arbitrary large angles. The main drawback of these schemes is that, for long range the split-step solution is more efficient and also for narrow angle with minimal or no bottom interaction. The split-step solution remains the most adopted technique for performance prediction as it is more suitable to solving many practical ocean surface problems. Conversely, the finite-difference and finite-element schemes have widespread application for wide- angle and bottom interacting boundaries. It is prominent for providing higher accuracy in these domains. The most recent development in terms of efficient PE solution schemes is a split-step Pade’ approximations derived by Collins [13,14]. He uses higher order Pade’ approximations not the square root operators. The result is a considerable efficiency gain through the use of higher range step. Thus, the scheme is claimed to be more than an order of magnitude faster than standard FD/FE solution techniques. This could create a unified PE solution approach where the accurate high angle PEs can be solved with the efficiency of the classical split-step Fourier scheme. IV. DERIVATION OF THE PARABOLIC WAVE EQUATION Using a simple model which describe the propagation of a reduced function ψ( , ") = ( , ")'()* (1) associated with the direction of propagation . Where ( , ") can be expressed as ( , ") = Ψ( , ")',()* (2) The Helmholtz equation of the reduced function is obtained by decoupling Maxwell’s equations and can be expressed as ∇. Ψ + . /.( , ")Ψ = 0(3) Where /.( , ") is the refractive index and is the wavenumber. The refractive index /.( , ") is assumed to possess smooth variations. The reduced function implies that the propagation energy varies slowly at angles close to the paraxial direction. The Laplacian of Ψ( , ") can be expressed as ∇. Ψ = [∇. + 2 ∇ − . ]'()* (4) Where Ψ is taken as the product of a plane wave solution, substituting equation (4) into equation (3) yields [ ∇. + 2 ∇ − . ]'()* + . /.( , ") '()* = 0(5) ∇. + 2 ∇ + . /. {( , ") − 1} = 0(6) The Laplacian operator for 2D can be expressed as ( 45 4*5 + 45 465 ), in the atmosphere where / − 1 is small, we neglect 45 4*5 as small (paraxial approximation) and equation (6) becomes 457 465 + 2 47 4* + . (/. − 1) = 0 (7) Where equation (7) is the standard parabolic equation.
  • 5. International Journal of Scientific Research and Engineering Development-– Volume 3 Issue 4, July –Aug 2020 Available at www.ijsred.com ISSN : 2581-7175 ©IJSRED: All Rights are Reserved Page 5 Equation (7) by method of separation of variables can be written as 2 47 4* = (1 − /.) . − 457 465 (8) Which can be expressed as 47 4* = 8(1 − /.) . − 95 .) : (9) Let consider ; = (1 − /.) . − 95 .) and substitute ; into equation (9). This yields 47 4* = ; (10) Taking like terms 47 7 = ;< (11) The solution of the parabolic wave equation becomes ( = + Δ , ") = ( =)'(?@* (12) It is worthy of note that equation (7) can be factored out to obtain 8 4 4* + (1 − A): 8 4 4* + (1 + A):(13) This gives us 47 4* = − (1 − A)(14) 47 4* = − (1 + A)(15) Where A is the pseudo-differential operator and is defined by A = B C )5 45 465 + /.( , ")(16) Equation (14) is the outgoing parabolic equation and equation (15) is the incoming parabolic wave equation. Considering the propagation medium as homogeneous with refractive index /, the field component Ψ satisfies two-dimensional scalar wave equation 45D 465 + 2 45D 4*5 + . /. (", ) = 0(16) The refractive index varies with the range and height " and equation (16) is a good approximation provided / varies slowly with wavelength. It is worth noting that equation (16) is not exact [Levy]. If the propagation medium is vacuum, the standard parabolic wave equation in (7) 45D 465 + 2 47 4* = 0(17) The solution of equation (1.43) can be expressed as ( = + Δ , ") = ( =)'( E5 5F @* (18) V. THE SPLIT STEP FOURIER TRANSFORM SOLUTION The split-step Fourier method is a very efficient PEM which separate the refractive effect from the diffractive part of the propagator. Considering a two-dimensional scalar wave equation for horizontally and vertically polarised wave. Hardin et.al introduced the split-step Fourier method which transforms the rough surface problem with propagation through a sequence of phase screens [14-17]. The standard parabolic equation (SPE) in equation (7) can be written as 47 4* = () . 8 C )5 45 465 + (/. ( , ") − 1): (19) Let ; = C )5 45 465(20) G = /. ( , ") − 1(21) Equation (19) becomes 47 4* = () . {; + G} (22) The analytic solution of the SPE is ( + ∆ , ") = ( , ")' HF 5 ∆*(?I ) (23) Using J = ()∆* . (24) Equation (24) yields ( + ∆ , ") = ( , ")'K(?I ) (25) Equation (25) is the split-step solution which represent the field propagating through series of phase screens. The field is first propagated through a slice of homogeneous medium characterised by the exponent of ;. The SSFM routine was implemented to propagate the plane wave L("=, , ) = L(", , )' M * + N + 6"O (26) with E(z, x, y) ≈ 1 along the forward +z direction.
  • 6. International Journal of Scientific Research and Engineering Development ©IJSRED: All Rights are Reserved The plane wave was propagated through five (5) 2D slices of sea foam layers each containing isotropically distributed bubbles. The slices are equally dimensioned with area 100mm × 100mm with layer thickness δt = 10 mm separating adjacent layers. The foam layer thickness d ≫ λ to account for attenuation (E-field amplitude variation) and diffuse scattering (E variation) as the incident E-field travels through slices of the sea foam layer. WindSat frequency channels (6.8 GHz, 18.7 GHz, 23.8 GHz) were used for propagation of the E-field through slices of sea foam layers. Fig. 1 Geometric configuration of foam-covered sea-surface divided into slices of sea-foam layers. The incident wave is tilted from the normal so that there is an initial phase gradient along the surface of the sea foam model. This is done by assigning values of 0 ≤ θi≤ 90◦for the zenith angle and fixed azimuthal angle for example φi= 0◦or 45 For each of the angles θiwe compute the E that emerges from the foam layer and calculate FFTs of the fields. The contour plots of the intensity of the scattered field is displayed. Range of the depths of foam for realistic scenarios were observed to investigate the influence of the foam as a thin phase scattering screen and deep phase scattering screen. International Journal of Scientific Research and Engineering Development-– Volume 3 Issue Available at www.ijsred.com ©IJSRED: All Rights are Reserved The plane wave was propagated through five (5) 2D of sea foam layers each containing isotropically distributed bubbles. The slices are equally dimensioned with area 100mm × 100mm = 10 mm separating adjacent λ0 is required field amplitude variation) and diffuse scattering (E-field phase field travels through slices of the sea foam layer. WindSat frequency channels (6.8 GHz, 18.7 GHz, 23.8 GHz) were used field through slices of sea surface divided into The incident wave is tilted from the normal so that ong the surface of the sea foam model. This is done by assigning for the zenith angle and fixed or 45◦. we compute the E-fields that emerges from the foam layer and calculate the FFTs of the fields. The contour plots of the intensity of the scattered field is displayed. Range of the depths of foam for realistic scenarios were observed to investigate the influence of the foam as a thin phase scattering screen and deep phase VI. MILLIMETER WAVE AT LOW AND WINDSAT FREQUENCIES AS A FUNCTION OF DEPTH FORM The attenuation of the field intensity in decibel (dB) at a low frequency of 6.8 slice thickness JQ = 0.1 RR which describes phase scattering screen, as phase perturbation increases as the depth of sea foam layer increases. Fig. 2 and 3 illustrate the variation of attenuation of the E-field in dB with depth in mm of sea layer. This is due to increase in diffuse refl or scattering as the EM wave interacts with randomly oriented sea-foams with different refractive indices. This happens for both TE and TM polarized E-fields. Fig. 2 Attenuation of Field Intensity for horizontal polarization (TE) with zenith S( = 45° and azimuth T = 0° with depth of sea foam layer thickness JQ = 0.1 RR. Fig. 3 Attenuation of Field Intensity for vertical polarization (TM) with zenith S( = 45° and azimuth T = 0° with depth of sea foam layer thickness JQ = 0.1 RR. Volume 3 Issue 4, July –Aug 2020 www.ijsred.com Page 6 WAVE ATTENUATION FREQUENCIES DEPTH OF SEA- The attenuation of the field intensity in decibel 8 !" and sea foam which describes a thin phase scattering screen, as phase perturbation increases as the depth of sea foam layer increases. Fig. 2 and 3 illustrate the variation of attenuation of field in dB with depth in mm of sea-foam layer. This is due to increase in diffuse reflections or scattering as the EM wave interacts with foams with different refractive indices. This happens for both TE and Attenuation of Field Intensity for horizontal polarization (TE) with with depth of sea-foam at 6.8 !" and Attenuation of Field Intensity for vertical polarization (TM) with with depth of sea-foam at 6.8 !" and
  • 7. International Journal of Scientific Research and Engineering Development ©IJSRED: All Rights are Reserved Fig. 4 Attenuation of Field Intensity for horizontal polarization (TE) with zenith S( = 45° and azimuth T = 0° with depth of sea-foam at foam layer thickness JQ = 0.1 RR. Similarly, the attenuation of the field intensity in decibel (dB) at WindSat frequency of 18 sea-foam slice thickness JQ = 0.1 RR the depth of sea foam layer increases. Fig. 4 and 5 illustrate the variation of attenuation of the E in dB with depth in mm of sea-foam layer. This is also due to increase in diffuse reflections or scattering as the EM wave interacts with randomly oriented sea-foams with different refractive indices. This happens for both TE and TM polarized E fields. Fig. 5 Attenuation of Field Intensity for vertical polarization (TM) with zenith S( = 45° and azimuth T = 0° with depth of sea-foam at foam layer thickness JQ = 0.1 RR. International Journal of Scientific Research and Engineering Development-– Volume 3 Issue Available at www.ijsred.com ©IJSRED: All Rights are Reserved Attenuation of Field Intensity for horizontal polarization (TE) with foam at 18.7 !" and he attenuation of the field intensity in 18.7 !" and RR increases as the depth of sea foam layer increases. Fig. 4 and 5 illustrate the variation of attenuation of the E-field foam layer. This is also due to increase in diffuse reflections or scattering as the EM wave interacts with randomly foams with different refractive indices. This happens for both TE and TM polarized E- Attenuation of Field Intensity for vertical polarization (TM) with foam at 18.7 !" and Fig. 6 Attenuation of Field Intensity for horizontal polarization (TE) zenith S( = 45° and azimuth T = 0° with depth of sea foam layer thickness JQ = 0.1 RR. Fig. 7 Attenuation of Field Intensity for vertical polarization (TE) with zenith S( = 45° and azimuth T = 0° with depth of sea foam layer thickness JQ = 0.1 RR. For thin phase scattering screens with a given zenith angle S( = T = 0°, the attenuation U(VG) increases with depth (mm) of sea-foam and frequency at attenuation at 18.7 !" is less than that at 23.8 !", which agrees with the fact that there is more interaction of the E-field with the particles at high frequency. In Fig. 6 and 7, attenuation of the E-fields for both TE and TM polarizations are due to phase perturbations due to diffuse scattering of disordered air-bubbles with varying dielectric constants VII. CONCLUSIONS We have shown from our results that attenuation of the E-field is due to diffuse scattering when the incident E-field is multiple reflec it propagates through the various slices of sea layer. The E-field travels through different paths Volume 3 Issue 4, July –Aug 2020 www.ijsred.com Page 7 Attenuation of Field Intensity for horizontal polarization (TE) with with depth of sea-foam at 23.8 !" and Attenuation of Field Intensity for vertical polarization (TE) with with depth of sea-foam at 23.8 !" and For thin phase scattering screens JQ = 0.1 RR = 45° and azimuth increases with depth foam and frequency at 23.8 !". The is less than that at , which agrees with the fact that there is field with the particles at high frequency. In Fig. 6 and 7, attenuation of the fields for both TE and TM polarizations are due phase perturbations due to diffuse scattering of bubbles with varying dielectric We have shown from our results that attenuation field is due to diffuse scattering when the field is multiple reflected by sea-foam as it propagates through the various slices of sea-foam field travels through different paths
  • 8. International Journal of Scientific Research and Engineering Development-– Volume 3 Issue 4, July –Aug 2020 Available at www.ijsred.com ISSN : 2581-7175 ©IJSRED: All Rights are Reserved Page 8 and is scattered due to re-radiation of the dipole moments of the disoriented scatterers which absorbs some of the incident E-field and re-radiates it at various directions. The varying effective dielectric constants of the particles enhances diffuse scattering of the incident and transmitted E-fields within the sea-foam layer. The split-step Fourier transform method is a range marching technique that is suitable for evaluation of the E-field propagation given an initial range, the expected depth of the field can be computed as a function of range and depth. Future works will evaluate variation of the field intensity due to its interaction with rough sea-surface covered by foams. ACKNOWLEDGMENT Theauthors are gratefulto the reviewers and Executive Editor-in-chief for their insightful comments, which have helped improved the paper. This research work was supported by the Tertiary Education Trust Fund (TETFUND), Nigeria. REFERENCES [1] M. D. Anguelova, “Complex Dielectric Constant of Sea Foam at Microwave Frequencies”, Journal of Geophysical Research, Vol. 133, C08001, 2008. [2] M. D. Anguelova, M. Bettenhausen, and P. Gaiser. “Passive remote sensing of sea foam using physically-based models”,2006 IEEE International Symposium on Geoscience and Remote Sensing, IEEE, 2006. [3] M. D. Anguelova, “Passive remote sensing of oceanic whitecaps: Further developments”, AGU Fall Meeting Abstracts. 2013. [4] A. Stogryn, “The emissivity of sea foam at microwave frequencies ”, Journal of Geophysical Research, vol. 77, no. 9, 1658–1666, 1972. [5] Y. Xiaobin, et al. “Optimization of L-band sea surface emissivity models deduced from SMOS data.” IEEE Transactions on Geoscience and Remote Sensing, vol. 50.5, pp. 1414-1426, 2012. [6] D. Chen, L. Tsang, L. Zhou, S. C. Reising, W. E. Asher, L. A. Rose, K.-H. Ding, and C.- T. Chen, “Microwave emission and scattering of foam based on Monte-Carlo simulations of dense media”, Geoscience and Remote sensing, IEEE Transanction on, vol. 41, no. 4, 782–790, 2003. [7] C. Chi-Te, et al. “Analytical and numerical methods for the scattering by dense media. “ IGARSS 2000. IEEE 2000 International Geoscience and Remote Sensing Symposium. Taking the Pulse of the Planet: The Role of Remote Sensing in Managing the Environment. Proceedings(Cat. No. 00CH37120) (vol. 5, pp. 2359-2361). IEEE, 2000. [8] A. Benjamin and D. O. Bebbington, “Millimeter wave propagation and attenuation in closed packed sea foam layer and complex dielectric constant of sea-foam using split-step Fourier transform”, Progress in Electromagnetic Symposium-Fall (PIERS-FALL), IEEE, 2556-2553, 2017. [9] G. Apaydin and L. Sevgi. “The Split-step Fourier and finite element based parabolic equation propagation prediction tools: Canonical tests, systematic comparisons, and calibration,” IEEE Antennas and Propagation Magazine, vol. 52, No. 3, 66-79, 2010. [10] M. Levy. “ Parabolic equation method for electromagnetic propagation,” IEEE Electromagnetic Wave Series, vol. 45, The Institute of Engineering and Technology, Dec. 2000. [11] G.F.,Williams,“Microwave emissivity measurements of bubbles and foam., IEEE Trans. Geosci.., Remote Sensing, Electron”., vol.9, 221- 244, 1971. [12] P.C. Pandey and R. K. Kakar. “An empirical microwave emissivity model for a foam-covered sea”,Remote sensing of the ocean,vol.7, no.3,135-140, IEEE J Oceanic Eng, 1982. [13] X. Z. Huang and Y. Q. Jin, “Scattering and emission from two-scale randomly rough sea surface with foam scatterers”,Proc. Inst. Elect. Eng., Microwave Antenna Propagat., vol.142,109-114, 1995. [14] S. L. Durden and J. F. Versecky, “A physical radar cross-section model for a wind driven sea with swell”, Ocean Engineering, IEEE Journal of, vol.10, no.4,445-451, 1985. [15] K. H. Ding, L. Tsang and Q. Li, “Computational electromagnetic scattering models for microwave remote sensing”, Encyclopedia of RF and Microwave Engineering, 2005. [16] Wilheit Jr.,“A model for the microwave emissivity of the ocean’s surface as a function of wind speed”,Geosci. Electron. IEEE Trans. vol.17, 244-249, 1979. [17] J. Guo, L. Tsang, W. Asher, K.-H. Ding, and C.-T. Chen, “Applications of dense media radiative transfer theory for passive microwave remote sensing of foam-covered ocean,” Geoscience and Remote Sensing,IEEE Transactions on, vol.39, no. 5, 1019–1027, 2001.