Ocean Surface Roughness and Remote Sensing Paul Hwang Remote Sensing Division Naval Research Laboratory

Ocean Surface Roughness and Remote Sensing Paul Hwang Remote Sensing Division Naval 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)  T b  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 S in (wind), S ds (breaking), S nl (4-wave) for long scale waves (energetic region up to the equilibrium range) S in (wind, breaking), S ds (vis.,?), S nl (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)

Banner et al Stereo photo ( = 0.2~1.6 m) 5.5–13.3 m/s a 0 =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

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)

Phillips 1984 |S nl |<<|S in |,|S ds | For short waves [several times (~10) shorter than the peak wavelength] 0.02< <6 m k > ~2 k p u * /c: (0.04)(1) [S nl ~10 -4 near spectral peak], Source function balance equilibrium range

Scatterometer (Bragg roughness) Altimeter (LP-Filtered roughness) According to the 2θ deviation, the phase shift causes constructive (left figure) or destructive (right figure) interferences (Wikipedia)

Optical36 GHz 14 GHz 5.5 GHz Altimeter mode: filtered roughness Cox & Munk 1954 Vandemark et al Walsh et al Jackson et al Hauser et al. 2004

Scatterometer mode: spectrometer Old: Hwang: 2008 New: Hwang , 293, 377, 449, 508 rad/m Old

VV HH VH Decreasing sensitivity with wind Saturation/dampening Increasing sensitivity with wind: Linear: low wind Cubic: 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 VHWind field retrieved with VV

Hwang & Sletten 2008

Hwang et al Measure energy dissipation rate from space Hwang & Sletten 2008

 = Johnson and Zhang (1999) SPM/SSA Wave spectrum · Weighting function Flat surface -- Wind/roughness modified Weighting function Emissivity

Hwang et al. 2011

Passive Microwave Brightness Temperature WindSat data (Meissner and Wentz 2009) 50~55 deg polar angle SSS=35psu, SST=290K, average of 50 and 55 deg polar angle HRD U 10 rain rate (m/s)(mm/h) 0~50.1 5~ ~ ~ ~ ~ ~ ~407.8 > 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 models Roughness may fall between cracks (air-sea interaction, wave dynamics, remote sensing, …) but it is NOT someone else’s problem

76, 111, 143, 170, 192 rad/m

Scatterometer mode: spectrometer Hwang & Plant GHz5 – 6 GHz