Ocean Surface Roughness and Remote Ocean Surface Roughness and Remote SensingSensing

Paul HwangPaul HwangRemote Sensing DivisionRemote Sensing Division

Naval Research LaboratoryNaval Research Laboratory

Ocean Surface Roughness and Remote Ocean Surface Roughness and Remote SensingSensing

Paul HwangPaul HwangRemote Sensing DivisionRemote Sensing Division

Naval Research LaboratoryNaval Research Laboratory

Wind, Wave, Current, Bathymetry(SEASAT, 1978)

Modification of ocean surface roughness by

Microwave

• Surface roughness and ocean wind sensing–Active

• Radar backscatter from the ocean surface NRCS (0) roughness wind velocity

• Polarimetric returns (VV, HH; VH=HV), high wind speed and breaking wave measurements from radar backscatter

–Passive• Radiometer brightness temperature of the

ocean surface (difference from flat surface) Tb roughness/foam wind velocity, breaking

• Improving surface roughness spectrum and understanding of physical processes

– Cubic wind speed dependence of short wave spectrum (linear in equilibrium spectrum)

– Source functions• Sin (wind), Sds (breaking), Snl (4-wave) for long

scale waves (energetic region up to the equilibrium range)

• Sin (wind, breaking), Sds (vis.,?), Snl (3-wave, parasitic) for short scale waves (roughness)

– Breaking as a source term: impulses of breaking water jets, surface disturbances and bubble plumes propagating downward from the wave crest and the subsequent rising of the entrained bubble clouds to the water surface Hwang 2011 (JAOT)

0 ( )

* *0; ( )

a ku u

B k A kc c

0.5 2.5 3**

2*

( )

( ) , 5.2 10

e

e

uS k bu g k b k

cu

B k b bc

Banner et al. 1989Stereo photo ( =0.2~1.6 m)5.5–13.3 m/sa0=0.18

Ocean short wave spectrum measured by free-drifting wave gauge array (Hwang and Wang 2004)

Toba 1978; Phillips 1985; Hwang et al. 2000

( )

* *10; ( ) ;

a ku u

B k A k B U kc c

H spectrum

Radar backscatter power-law wind speed exponent (Masuko et al., 1986; Phillips, 1988; Weissman et al., 1994; Colton et al., 1995; summarized in Hwang 1997 and Trokhimovski and Irisov 2000)

Hwang (2005, 2008, 2011)

nl in ds

dNS S S

dt

2

* 4( ) ( )in

u

cS k Bm gk k

2*( / )

/

11 ,d

Bca u

B

4

4

( ) ( )( ) /

B k k kk N k g

Phillips 1984

|Snl|<<|Sin|,|Sds|

For short waves [several

times (~10) shorter than the

peak wavelength]

0.02<<6 mk > ~2 kp

u*/c: 0.3 - 3

(0.04)(1) [Snl ~10-4 near spectral peak]

4 ( )ds gkS f B ( ) dadf B A B,

Source function balance equilibrium range

Scatterometer(Bragg roughness)

24 40

1

16 cos

, Ref. coef. Rough. s

2 sin ,

c

0

pe t.

pq pqk g

NR S

W

f k

k

C

Altimeter (LP-Filtered roughness)

22

40 2

0 tansec exp

2 2i

Guf cf

O i iuf

R

ss s

According to the 2θ deviation, the phase shift causes constructive (left figure) or destructive (right figure) interferences (Wikipedia)

Optical 36 GHz

14 GHz 5.5 GHz

Altimeter mode: filtered roughness

Cox & Munk 1954Vandemark et al. 2004Walsh et al. 1998

Jackson et al. 1992 Hauser et al. 2004

24 40

1

16 cos

, Ref. coef. Rough. s

2 sin ,

c

0

pe t.

pq pqk g

NR S

W

f k

k

C

Scatterometer mode: spectrometer

Old: Hwang: 2008New: Hwang 2011

2 sinB rk k 201, 293, 377, 449, 508 rad/m

Old

VV HHVH

Decreasing sensitivity with wind

Saturation/dampening

Increasing sensitivity with wind:

Linear: low windCubic: high wind

Hwang et al. (2010)

R2 and CB in good agreement R2 >> CB Breaking contribution missing in CB computation

RADARSAT-2 polarimetric data

rms 1.50

rms 1.62rms 1.67

rms 1.65

Hwang et al. 2010

Wind field retrieved with VH Wind field retrieved with VV

Hwang & Sletten 2008

3 3.3 410 # #, 0.20 4 1 10 , 1.2a aU

Hwang et al. 2010Measure energy dissipation rate from space

Hwang & Sletten 2008

3 3.3 410 * *, 0.20 4 1 10 , 1.2a aU

=0 30 60

Johnson and Zhang (1999)

SPM/SSA

0

, , , , ', '', ' ' ' '

, , , , ', 'H swBH

V swBV

g f keW k k d dk

g f ke

Wave spectrum · Weighting function

20

20

1, ,

, , 1

HHBH BHs

BV BVVV

RT f eT

T f eR

Flat surface -- Wind/roughness modified

Weighting function

Emissivity

Hwang et al. 2011

Passive Microwave Brightness TemperatureWindSat data (Meissner and Wentz 2009) 50~55 deg polar angleSSS=35psu, SST=290K, average of 50 and 55 deg polar angle

HRD U10 rain rate(m/s) (mm/h)0~5 0.1

5~10 0.310~15 0.415~20 1.720~25 3.025~30 4.930~35 6.735~40 7.8

>40 11.0

Foam

• Radiometer and scatterometer: global wind retrieval

• Ocean surface roughness (short surface waves): key role

• Empirical parameterizations so far, need better understanding of physics: breaking

• Polarization carries breaking wave information for measurement from space

• Improving global wind product, improving driving force of wave modelsRoughness may fall between cracks (air-sea interaction, wave dynamics, remote sensing, …)

but it is NOT someone else’s problem

2 sinB rk k 76, 111, 143, 170, 192 rad/m

Scatterometer mode: spectrometer

Hwang & Plant 2010

14 GHz 5 – 6 GHz