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Radar I maging of Curre nt Features: A Syne rgy of Bac kscatte r Powe r and Dopple r Shift :

V. Kudryavtsev (1,2,3), B. Chapron (4), J.A.Johannessen (3,5), F. Collard (6), D. Akim ov (1), and K.-F. Dagestad (3)
(1) Nansen International Environmental and Remote Sensing Center, St. Petersburg, Russia (2) Marine Hydrophysical Institute, Sepastopol, Ukraine (3) Nansen Environmental and Remote Sensing Center, Bergen, Norway (4) Institute Francais de Recherche pour lґExploitation de la Mer, Brest, France (5) Geophysical Institute, University of Bergen, Norway (6) BOOST Technologies, Brest, France


MOTIVATION Doppler centroid anomaly will be soon a product routinely produci ng fro m space-born SAR. Syner getic analysis of SAR NRCS and Doppler shift could signi ficantly i mprove quality of the ocean current mo nitori ng fro m t he space OBJECTIVE Develop SAR imagi ng model whic h should consistently reproduce observed modulations of NRSC and Dopper shifts related to ocean current features, fronts, and ot her phe no mena (e.g. oil spills) APPROACH Recently t wo ne w methods for qua ntitative studies of SAR i magi ng of curre nt features were published in J.Geophys.Res. (Ocean). They are the Doppler shift approach (Chapro n et al. 2005), and the radar ima gi ng model (RIM), (Kudryavtsev et al. 2005; Johannessen et al. 2005). This study combines these two approaches in a unified DopRim model.
f
k
D R

Doppl er S hift

=

(u sin - w cos ) 0 ( + ) 0 ( + )

+
Radar Im a ging M od el

DopRIModel


Dopple r ce nt roid anoma ly by S AR: Some Res ults
Chapron B., F. Collard, and F. Ardhum, 2005, Di rect measurements of ocean surface vel ocity from space: Interpretati on and validati on, J.Geoph.Res., 110, C07008, 2005 ASAR ENVISAT Global Observations

For C- ba nd 1m /s s ur fac e c urre nt Corres p on ds t o D op ple r s hi ft F=15- 20 Hz at 2 0-3 0 d eg i nc id enc e


ASAR ENV ISAT Observation of the Gulf Stream ENVISAT


Doppler Shift : Governing Equat ions :
f
k
D

=

R

(u sin - w cos ) 0 ( + ) 0 ( + )

~~ ~~ 1 w 0 u 0 VD = f D / k R sin = c + u S - + 0 tan 0 VD = uS + c - 1 1 0 tan 0



k
cos( R - )cB(k )d ln kd +
d



k < kd

cos( R - )cB (k )Md ln kd

If one suggests that long waves are narrow, then

2 C 1 0 2 C P VD = c + u S - + |MH | 2 tan 0 2
as proposed in Chapron et al, JGR, 2005


SAR Signature of Sea Su rface Cur rent s

Model approach developed by
Kudryavtsev V., D.Akimov, A.Johannessen, and B. Chapron, On radar imaging of current features. Part 1: Model and comparison w ith observations, J.Geoph.Res., VOL. 110, C07016, doi:10.1029/2004JC002505, 2005





Obs erv ed fro nt al fea tur e

Inp ut: c urre nt v eloc it y, S ST, ge os tro ph ic wi nd a nd free atm os ph ere tem per atu re

Atm os ph eric Bou nd ar y L ay er m ode l o ut put

W ave s pec tr um tr ans fo rm ati on m od el o utp ut: Brag g s p ec trum , MSS, W CC

Me as ur ed NR CS c ontr as t
com pa rison

NRC S m o de l out pu t

Radar Im aging M odel


Comparison with Observ ations: Mode le d frontal e ddy
:


Merger of RIM and Doppler models. RIM
RIM NRCS

0p = 0pR (1 - q ) + 0bq
Ve locity of Face ts
P c P = PbP cbr + Psp c sl + PwP cb r b

c sl =

c

wb

=



k
cos( R - ) cB(k )d ln k d
d

k
cos( R - ) B(k) d ln k d
d

k < kw k
cos(R - ) c (k ) B(k ) d ln k d
b

cos( R - ) (k) B(k )d ln k d
wb

VD = u S + c -

1 1 0 tan 0



k
cos( R - ) cB(k)d ln k d +
d



k
cos( R - )cB (k)Md ln k d
d

Hydrodynam ic M TF si2 sp si2 bPr P M S + PwP M M = P M B + P M Si + Psp b 2 2 i sp si br si
P br q


Some ba ckground fea tures:

4 3 2 1 ] s / m [
D

0 -1 -2 -3 -4 -2 0

U

Fo r C-b an d 1 m/s su rface cu rren t co rres po nds to Do p p ler s h ift F=1 5- 20 H z at 20- 3 0 d eg in cid en ce

-1 0

0 U 10 [m /s ]

10

20


ENVISAT AS AR Brest Coast


Model S urface Current Field use d for t he M odel Ca lculat ions
,


RIM simulation of SAR signature of tidal current near Atlantic coast of France


NRCS 35 [d B ]

-2 2 -2 4 -2 6

30 25

-2 8 ] m k [ 20 -3 0 15 10 -3 2 -3 4 -3 6 5 -3 8 0 0 5 10 15 20 [km ] 25 30 35

ENV ISAT ASAR im age w ith strong manifestations of tidal current

RIM simulation of the SAR image


Project ion of Current Velocit y on Ra dar Look Direct ion
U
r

[m /s ] 0. 5

35 0 30 -0 . 5 25 -1 ] m k [ 20 -1 . 5 15 -2 10 -2 . 5 5 -3 0 0 5 10 15 20 [k m ] 25 30 35


Cont ribut ion of S urface Roughne ss Modul at ion t o t he Doppler Velocit y
C
VV

-U

r

[m /s ] 1.2 1.1

35

30

1 0.9 0.8

25 20

] m k [

0.7 15 0.6 0.5 0.4 5 0 0 5 10 15 20 [km ] 25 30 35 0.3 0.2

10


The DopR im model Doppler Velo cit y
C
VV

[m /s ]

35

4 3.5 3 2.5

30 25

] m k [

20 2 15 1.5 10 1 0.5 0 0 5 10 15 20 [km ] 25 30 35

5

0





EN VISAT A SAR English Ch ann el (Cherbourg)


Model S urface Current Field use d for t he M odel Ca lculat ions
,


RIM simulation of SAR signature of tidal current: English Channel (Cherbourg)
NRCS 45 -1 1 40 -1 2 35 -1 3 30 ] m k [ 25 20 15 10 5 0 0 10 20 [km ] 30 40 -1 4 -1 5 -1 6 -1 7 -1 8 -1 9 -2 0 [d B] -1 0


Project ion of Current Velocit y on Ra dar Look Direct ion

U 45 40 35 30 ] m k [ 25 20 15 10 5 0 0 1 2
r

[m /s ]

1.5

0.5

-0 .5 0 10 20 [km ] 30 40


Cont ribut ion of S urface Roughne ss Modul at ion t o t he Doppler Velocit y

C 45 40 35 30 -0 .6 ] m k [ 25 20 15 10 5 -1 .2 0 0 10 20 [km ] 30 40 -0 .7 -0 .8 -0 .9 -1 -1 .1
VV

-U

r

[m /s ] -0 .2 -0 .3 -0 .4 -0 .5


The DopR im model Doppler Velo cit y
C
VV

[m /s ] 1.5

45 40 35 30 ] m k [ 25 20

1

0.5

0

-0 .5 15 10 5 0 -1

-1 .5 0 10 20 [km ] 30 40





Does Doppler velocity inevitably corresponds to "real" surface velocity? T he answer is · YES when modulations of surface roughness by current is weak, and · NO when modulations are strong.

In order to illustrate this case we consider the ocean surface wave field in the Gulf Stream zone at a special conditions whe n wind blows opposite to the current. At such conditio ns effect of the trapping of surface waves by jet-current with strong enha nceme nt of wave energy may take place. This phe nome no n had been observed and investigated in field experime nts by Kudryavtsev et al. JGR, 100, C10, 1995. ? -, - , , , . ( Kudryavtse v et al. JGR, 100, C10, 1995)



Jet-Like Current (wind waves and swell)
5 m/s Wind and Swell are Opposite to Jet-Current (Wdir=270; CUdir=90) 6 5 4 3 1 2 1 0 -200 0 200 0 -1 -2 -200 0 200 0 -0.5 -1 Doppler R=225 0 200 E 5 4 Doppler R=270 3 2 2 1.5 1 0.5

WIND

-1.5 -200

1.6 1.5 1.4 1.3 1.2 1.1 1 0.9 -200 0 200 MSS

4 3.5 3 2.5 2 1.5 1 0.5 -200 0 200
0

1.6 1.5 1.4 1.3 1.2 1.1 1 0.9 -200 0 200 0

CURRE NT

Condit ions : Wind 5 m/s oppos ite to jet ; Jet w idth is 80 km, V eloc ity is 1.5 m/s ; Sw ell: KA =0.05, L=200 m Radar obs er v ations : C- band, V V ; Inc idenc e angle 30 deg.; Radar loo k: a) dow n-w ind and b) 45 deg c loc kw is e off w ind dir ec tion


Let-Like Current (wind waves only)
5 m/s Wind (NO Swell) is Opposite to Jet-Current (Wdir=270; CUdir=90) 1.8 E 1.6 1.4 1.2 -0.5 1 0.8 -200 -1 0 200 -1.5 -200 0 200 -1 Doppler R=225 -1.5 -200 0 200 1.5 1 Doppler 0.5 R=270 0 -0.5 1 0.5 0

WIND

CURRE NT
1.2 MSS 1.15 1.1 1.05 1 0.95 -200 1.15 1.1 1.05 1 1 0.95 -200 0.95 -200 1.2 0 1.05 1.1 0
Condit ions : Wind 5 m/s oppos ite to jet ; Jet w idth is 80 km, V eloc ity is 1.5 m/s ; No sw ell Radar obs er v ations : C- band, V V ; Inc idenc e angle 30 deg.; Radar loo k: a) dow n-w ind and b) 45 deg c loc kw is e off w ind dir ec tion

0

200

0

200

0

200


Radar Im aging of Oil Spills - DopRim Approach

Modulatio n of radar return and Doppler shift by slicks: Ka-band, VV. Wind speed 7m/s

Experiment by Ermakov et al. 2002 (Dokl. RAS, 388)

Model Simlation


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