Post on 21-Jan-2016
transcript
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