Scatterometer ’ s Unique Capability in Measuring Ocean Wind & Stress

1. 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 current • Is there life after QuikSCAT?

2. Scatterometer measures ocean surface wind/stress

4. Ocean-Atmosphere Interaction with Scatterometers and other Sensors W. Timothy Liu • 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.

5. The Importance to Wind and Stress Winds Marine weather forecast to avoid shipping hazard Monitoring and forecasting hurricane Monitoring and studying monsoon Convergence fuels convection that drives circulation Distribution of wind power for electricity generation Stress Horizontal current driven by stress distributes heat and carbon stored in ocean Convergence and vorticity control ocean mixing Mixing brings short-term momentum and heat trapped in the surface into deep to be stored over time It brings nutrient and carbon stored in the deep to surface for photosynthesis Affects air-sea turbulent transfer of heat, moisture, and gases

6. 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.

7. Center of cyclonic currents

8. Center of cyclonic currents

9. Center of cyclonic currents

10. QuikSCAT demonstrates flow separation in hurricanes ( Similar for ASCAT) QSCAT UENW ASCAT shows similar behavior at C-band (Bentamy presented at OVWST meeting) QSCAT o-V Hurricanes in 2005 were used. Those with more than 50% chance of coincident rain occurrence were removed. QSCAT o-H Hwind Speed (m/s)

11. 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.

12. ENW is higher than actual wind under unstable condition

14. Xie et al. 2002, GRL

15. 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.

16. QuikSCAT ENW (color) & AMSR-E SST (contour) Liu et al. 2008, GRL

17. 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.

18. Center of anticyclonic currents Center of cyclonic currents Strong Temperature Gradient & Current Shear at Ocean Front Liu et al. 2008, JO Kuroshio Agulhas Liu et al. 2007, JC

19. Observation from satellite Agulhas Computed from uniform wind field at 10m Collocation of ENW magnitude with SST is inherent in the definition of ENW and turbulent mixing theory. (Liu et al. 2007, JC)

20. Observation from satellite Kuroshio Computed from uniform wind field at 10m Liu et al. 2008, JO

21. Center of cyclonic currents

22. Stability Parameter = buoyancy / shear

25. Jan 2003-2005 UN-U Center of cyclonic currents SST-Ta

26. Jul 2003-2005 UN-U Center of cyclonic currents SST-Ta

27. 10 11 12 13 14 15 16 17 18 19 20 22 HY-2AChina CFOSatChina/France HY-2B China Meteor-M3 Russia GCOM-W2 with DFS Japan/u.s. Oceansat-2India METOP-AEurope GCOM-W3 Japan/u.s. Oceansat-3 India Launch Date 08 09 21 C-band 10/06 Post EPS Europe METOP-B Europe METOP-C Europe Ku-band 6/99 QuickScatUSA Combined C- and Ku-band Design Life Extended Life Proposed Needed Approved Operating Draft 2dec09

28. Center of cyclonic currents

29. See the Whole Elephant See from Space

30. backup

32. Filtered AMSR-E Ts & UN from uniform wind (7.5 m/s) UNvorticity from uniform wind Center of cyclonic currents QuikSCAT UN & vorticity AMSR-E Ts & QuikSCAT UN UNConv.from uniform wind Surface current & vorticity

33. Center of cyclonic currents

34. Beam Resolution Fixed Doppler Variable Doppler Spot RANGE GATE Inc. Angle 22°-55° 18°-47°,24°-57° 18°-57°, 22°-63° 46°, 54° Spaceborne Scatterometers SEASAT ERS-1/2 NSCAT QuikSCAT Frequency 14.6 GHz 5.3 GHz 13.995 GHz 13.402 GHz Scan Pattern Polarization V-H, V-H V, V-H, V V, H V ONLY Resolution 50 km 25 km 25/12.5 km 50 km 500 km 500 km 600 km 600 km 1800 km 500 km Swath Daily Coverage Variable 41% 77% 93% Dates 6/78 – 10/78 8/91-1/01 8/96 – 6/97 6/99 +