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 • Applications of wind and stress • Is there life after QuikSCAT?

2. Scatterometer measures ocean surface wind/stress

3. Trade winds brought Commodore Perry Kamikaze repelled the Mongols Monsoons bring rain Typhoons cause devastation

5. 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.

6. Scatterometer Geophysical Model Function (GMF) σ0 normalized radar cross-section of the sea surface u* friction velicity Χ relative wind direction (the angle, measured in the horizontal plane, between the wind and the radar illumination direction) …… subsidiary factors independent of local stress. θ radar incidence angle p polarization (vertical or horizontal) f radar frequency

7. Schematic representation of the SeaWinds measurement geometry

10. The basic GMF features are: • o(in dB) increases approximately linearly with log(U*) at fixed azimuth; • the azimuthal variation of o can be characterized as harmonics with “upwind-downwind asymmetry” and “upwind-crosswind modulation”. • At least three collocated observations of o differing in “look”, i.e. geometry (, ), in principle allows determination of a unique wind vector. • It has been demonstrated that the azimuthal angle dependence can be separated from the incidence angle and friction velocity functions using the three term Fourier series (Wentz et al., 1984, Freilich, 1996):

12. 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)

13. 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.

14. Center of cyclonic currents

15. Center of cyclonic currents

16. Center of cyclonic currents

17. Center of cyclonic currents

19. 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.

20. Xie et al. 2002, GRL

21. 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.

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

23. 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

24. 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)

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

26. 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

28. SCIENCE SYNERGISM W. Timothy Liu / JPL atmosphere energy and hydrology cycle biogeochemical cycle wind stress ocean Scatterometer data have also important application over land and ice.

29. The Importance to Wind and Stress Winds Marine weather forecast to avoid shipping hazard Monitoring and forecasting hurricane Monitoring and study of monsoon Convergence fuels convection that drives circulation Distribution of wind power for electricity generation Stress Horizontal current driven by wind 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

30. Power density Center of cyclonic currents Using Weibull distribution Scaling factor Shape factor ū is mean wind speed σ is standard deviation of wind speed Г is the gamma function

35. Liu, W.T., W. Tang, and R.S. Dunbar, Scatterometer observes extratropical transition of Pacific typhoons, Eos Trans. of AGU, 78, 237 & 240, 1997. Extratropical Transition of Typhoon Tom  (9/96)

36. Weather

37. Interplay Between Wind and Rain Observed in Hurricane Floyd - W. Timothy Liu, Hua Hu, and Simon Yueh (EOS, 23, 254 & 257, 2000) The surface wind and fresh water flux of the operational numerical weather prediction model with the highest spatial resolution (EDAS) cannot resolve the rain bands of Hurricane Floyd [upper left]. By simply replacing the surface level EDAS wind divergence with QuikSCAT data, the fresh water flux [middle left] became more realistic compared with observations by the TRMM Radar (PR) [lower left]. The vertical profiles of heating and rain rate [right panels] show that QuikSCAT data help to reveal the eye and precipitation walls, in agreement with PR obervations.

38. East Asia Continental rainfall increases sharply at the monsoon onset in May, and lasts until September Time series of precipitation integrated over east Asia agrees well with the temporal variations of moisture advected from the Bay of Bengal However, the sum of moisture influx from the Pacific Ocean occurs in fall, out of phase with the precipitation (black curves) Time series of precipitation over land integrated in six parallel zonal segments in Indochina and China, as indicated by the black boundary lines in inserted map, measured by TRMM from August 1999 to August 2003. (red curves) Moisture transport from the Bay of Bengal (top), southern oceans (middle), and Pacific ocean (bottom), across the boundaries as indicated by yellow arrows.

39. India subcontinent Net moisture advection into India subcontinent is in phase with precipitation over land. During onset of summer monsoon, moisture advected out of India to Bay of Bengal occurs before moisture advected into the subcontinent from Arabian Sea. (a) Time series of moisture transported into India from adjacent oceans (black) and TRMM measured precipitation integrated over India subcontinent (blue), from September 1999 to August 2003. (b) Moisture advection from Arabian Sea (green) and Bay of Bengal (red) over plotted with the net moisture advection (black). (c) Map shows area and the boundaries for rain and moisture transport analysis shown in (a) and (b). (d) Moisture from Arabian Sea and Bay of Bengal during monsoon seasons from 2000 to 2003.

41. We apply the framework previously established to extract tropical cyclone wind data from QuikSCAT to SST measurements from different sources: • AMSR • TRMM/TMI • AVHRR • MODIS

42. Composite of Radial (u) Wind in N. Hemisphere 2000-2007 moving dir. Radial wind component indicates inflow around storm center with maximum from right-rear; also detects area of outflow in front when translation speed picks up

43. Composite of Tangential (v) Wind in N. Hemis. 2000-2007 moving dir. Translation speed (ts) increasing The tangential component is near symmetric for slow-moving storms. The left-right asymmetry induced by and becomes stronger for fast-moving storms.

44. El Niño Teleconnection and Multi-scale Interaction Climate [Liu, W.T., W. Tang, and H. Hu, Eos Trans. AGU, 79, 249 & 252, 1998] [Yu, J.Y., W.T. Liu, and C. R. Mechoso, Geophys. Res. Lett., 27, 1931-1934, 2000] •El Nino (warming of equatorial Pacific) occurs every few years and is preceeded by the collapse of the trade winds and westwind burst that last for a few days. • The westwind burst is connected to super cloud clusters and convection, which has sub-daily time scales • A branch of the equatorial winds, as part of the displaced anticlockwise flow, brings warm and moist air the U.S. west coast • The displacement of atmospheric circulation shifts the position of the decadal ocean temperature dipoles, with strong ecological effect to coastal water •The relation between El Niño index (equatorial temperature anomaly) and dipole index (temperature difference between warm and cold centers) in a long-term (over 50 years) simulation by UCLA coupled ocean-atmosphere model supports the teleconnection revealed by NSCAT.

45. Biology & Carbon QuikSCAT Lin, Liu et al, GRL, 30(13),2003 Typhoons destroy human life and property, but also enhance marine life (ocean productivity) and reduce green house warming (carbon sink). TRMM SeaWiFS

46. Coastal Ecosystem Wind changes coastal ecosystem and excites biochemical response. (Hu & Liu, 2003) Morning minus evening differences in wind speed and direction measured in the first 3 years of the QuikSCAT satellite mission. (S. Gille) High frequency winds describe the full cycle of land-sea breeze and the high variability of coastal ecosystem Scatterometer Constellation - W. Timothy Liu

47. Ocean Physical Response to Wind Forcing-Seasonal GRL, 26 (10), 1999 JGR 108 (C8), 2003

48. Polar Ice Wind-Ice Interaction • QuikSCAT provides continuous monitoring of the ice-extent (gray areas) and the wind vectors (white streamlines and color image of wind speed) in the remote and data-sparse oceans around Antarctica. • It reveals three groups of intense storms (standing atmospheric wave), which are associated with three maxima of sea ice extent (SIE), that persists daily during the Austral winters, indicating positive wind-ice feedback. • It also shows that the SIE maxima provide favorable conditions for cyclogenesis in the ocean. [Liu, W.T., Backscatter, V. 12, No. 1, 2001] Meridional wind Ice edge [Courtesy of Xiaojun Yuan] • Besides seasonal modulations, the relation between winds and SIE are more prominent during anomalous cooling (’96 and ’99 La Niña) and weaker during warming in the equatorial Pacific (’93 El Niño).