OBSERVING OCEAN SURFACE WIND AND STRESS BY SCATTEROMETER CONSTELLATION
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OBSERVING OCEAN SURFACE WIND AND STRESS BY SCATTEROMETER CONSTELLATION

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    OBSERVING OCEAN SURFACE WIND AND STRESS BY SCATTEROMETER CONSTELLATION OBSERVING OCEAN SURFACE WIND AND STRESS BY SCATTEROMETER CONSTELLATION Presentation Transcript

      • 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?
    • Scatterometer measures ocean surface wind/stress
    •  
    • Ocean-Atmosphere Interaction with Scatterometers and other Sensors
      • 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.
      W. Timothy Liu
      • 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
      • 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 (U N ). U N , 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, U N 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 U ENW 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 (U N ). U N , 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, U N 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.
    • Strong Temperature Gradient & Current Shear at Ocean Front Kuroshio Agulhas Liu et al. 2007, JC Liu et al. 2008, JO Center of anticyclonic currents Center of cyclonic currents
    • 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 U N -U Jan 2003-2005
    • Center of cyclonic currents SST-Ta U N -U Jul 2003-2005
    • 10/06 6/99 QuickScat USA Operating Post EPS Europe Draft 2dec09 10 11 12 13 14 15 16 17 18 19 20 21 22 Ku-band Combined C- and Ku-band 09 08 C-band Launch Date
    • Center of cyclonic currents
    • See the Whole Elephant See from Space
    • backup
    •  
    • Center of cyclonic currents Surface current & vorticity Filtered AMSR-E Ts & U N from uniform wind (7.5 m/s) AMSR-E Ts & QuikSCAT U N U N Conv. from uniform wind U N vorticity from uniform wind QuikSCAT U N & vorticity
    • Center of cyclonic currents
    • Spaceborne Scatterometers SEASAT ERS-1/2 QuikSCAT NSCAT V-H, V-H V ONLY V, V-H, V V, H Polarization Scan Pattern 50 km 50 km 25 km 25/12.5 km Resolution 500 km 500 km 500 km 600 km 600 km 1800 km Swath Variable 41% 77% 93% Daily Coverage 6/78 – 10/78 Dates 8/91-1/01 8/96 – 6/97 6/99 + 14.6 GHz 5.3 GHz 13.402 GHz 13.995 GHz Frequency 22°-55° Inc. Angle 18°-47°,  24°-57° 18°-57°, 22°-63° 46°, 54° Fixed Doppler Beam Resolution RANGE GATE Variable Doppler Spot