Scatterometer’s Unique Capability in Measuring Ocean

Wind & Stress

W. Timothy Liu, Wenqing Tang, and Xiaosu Xie

Basics of scatterometry and air-sea turbulent exchanges Reduced wind sensitivity at strong winds Spatial coherence with surface temperature and currentIs there life after QuikSCAT?

Scatterometer measures ocean surface wind/stress

• A Scatterometer sends microwave pulses to the Earth's surface, and measures the power scattered back from surface roughness.

• Over oceans, roughness is caused by small waves in equilibrium with the wind stress.

• Measuring both wind and stress vector is a unique capability of a scatterometer.

Ocean-Atmosphere Interaction with Scatterometers and other Sensors

W. Timothy Liu

The Importance to Wind and StressWindsMarine weather forecast to avoid shipping hazardMonitoring and forecasting hurricaneMonitoring and studying monsoonConvergence fuels convection that drives circulationDistribution of wind power for electricity generation

StressHorizontal current driven by stress distributes heat and carbon stored in oceanConvergence and vorticity control ocean mixingMixing brings short-term momentum and heat trapped in the surface into deep to be stored over timeIt brings nutrient and carbon stored in the deep to surface for photosynthesisAffects air-sea turbulent transfer of heat, moisture, and gases

Definition and Basics• Wind is air in motion. Stress is the turbulent transport of

momentum. • Turbulence is generated by instability caused by vertical wind

shear and buoyancy• We do not have any large-scale stress measurements; our concept

of stress distribution is largely influenced by our wind knowledge.• Wind influence stress but does not uniquely define stress; stress

depends on ocean temperature and current that drives instability.They have smaller-scale variability than winds.

• Scatterometer measures roughness which is in equilibrium with stress.

• The geophysical product is the equivalent neutral wind (UN). UN, by definition, has an unambiguous relation with surface stress, provided that ocean surface current is negligible, while the relation between actual wind and surface stress depends also on atmospheric density stratification

• It is generally assumed that, over most ocean, atmosphere is neutral and current is negligible, UN has been used as the actual wind, particularly in operational applications.

Center of cyclonic currents

Center of cyclonic currents

Center of cyclonic currents

QuikSCAT demonstrates flow separation in hurricanes( Similar for ASCAT)

QSCAT UENW

QSCAT o-V

QSCAT o-H

Hwind Speed (m/s)

Hurricanes in 2005 were used. Those with more than 50% chance of coincident rain occurrence were removed.

ASCAT shows similar behavior at C-band (Bentamy presented at OVWST meeting)

Definition and Basics• The geophysical product is the equivalent neutral wind (UN). UN,

by definition, has an unambiguous relation with surface stress, provided that ocean surface current is negligible, while the relation between actual wind and surface stress depends also on atmospheric density stratification

• It is generally assumed that, over most ocean, atmosphere is neutral and current is negligible, UN has been used as the actual wind, particularly in operational applications.

ENW is higher than actual wind under unstable condition

Xie et al. 2002, GRL

Wind and SST Coupling

Lin, I.-I.; W.T. Liu, C.-C. Wu, J.C. Chiang, and C.-H. Sui, 2003: Satellite observations of modulation of surface winds by typhoon-induced upper ocean cooling. Geophys. Res. Lett., Vol. 30(3), 1131, doi:10.1029/2002GL015674.

QuikSCAT ENW (color) & AMSR-E SST (contour)

Liu et al. 2008, GRL

Spatial coherence between scatterometer measurements and SST is ubiquitous, under all kinds of atmospheric circulation and boundary layer conditions.

• Tropical Instability Waves [Xie et al. 1998;Cronin et al, 2003; Hashizume et al. 2002; Chelton et al. 2004] Kuroshio [Nonaka and Xie 2003] Circumpolar Current [White and Annis, 2003; O’Neill 2003] Indian Ocean [Vecchi et al., 2004] Winter outbreak in East China Sea [Xie et al.2002] Gulf Stream Ring [Park and Cornillon 2002] Typhoon wake [Lin et al., 2003] Numerical model simulation[Yu and Liu, 2003;Song et al. 2004]

Failure to find a generally applicable wind (boundary layer) theory. The reason is that at small turbulent scales, factors such as Coriolis force, pressure gradient force, baroclincity, cloud entrainment, boundary height, are not important, and ocean factors that generate turbulence (stress) are neglected.

Center of anticyclonic currentsCenter of cyclonic currents

Strong Temperature Gradient & Current Shear at Ocean Front

Kuroshio

Agulhas

Liu et al. 2007, JC

Liu et al. 2008, JO

Collocation of ENW magnitude with SST is inherent in the definition of ENW and turbulent mixing theory. (Liu et al. 2007, JC)

Observation from satellite

Computed from uniform wind field at 10m

Agulhas

Observation from satellite

Computed from uniform wind field at 10m

Liu et al. 2008, JO

Kuroshio

Center of cyclonic currents

Stability Parameter = buoyancy / shear

Center of cyclonic currents

SST-Ta

UN-UJan 2003-2005

Center of cyclonic currents

SST-Ta

UN-UJul 2003-2005

10 11 12 13 14 15 16 17 18 19 20 21 22

Ku-band

Combined C- and Ku-band

0908

C-band

LaunchDate

10/06

6/99 QuickScat USA QuickScat USA

Operating

Post EPS Europe

Draft 2dec09

Center of cyclonic currents

See the Whole Elephant

See from Space

backup

Center of cyclonic currents

Surface current & vorticity

Filtered AMSR-E Ts & UN from uniform wind (7.5 m/s)

AMSR-E Ts & QuikSCAT UN

UN Conv. from uniform wind

UN vorticity from uniform wind

QuikSCAT UN & vorticity

Center of cyclonic currents

Spaceborne ScatterometersSEASAT ERS-1/2 QuikSCATNSCAT

V-H, V-H V ONLY V, V-H, V V, HPolarization

Scan Pattern

22°-55°Inc. Angle 18°-47°,24°-57° 18°-57°, 22°-63° 46°, 54°

50 km 50 km 25 km 25/12.5 kmResolution

500 km 500 km 500 km 600 km 600 km 1800 km

Swath

Variable 41% 77% 93%Daily Coverage

6/78 – 10/78Dates 8/91-1/01 8/96 – 6/97 6/99 +

14.6 GHz 5.3 GHz 13.402 GHz13.995 GHzFrequency

Fixed DopplerBeam Resolution RANGE GATE Variable Doppler Spot