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Surface Pressures

Surface Pressures. R. A. Brown 2003 U. Concepci Ó n. Nov 9 ‘96 18Z Gulf of Alaska rab. Towards a Surface Pressure Model Function. A Scatterometer doesn’t measure Winds. It measures Capillaries & Short Gravity waves, related to roughness parameter z o , or u*.

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Surface Pressures

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  1. Surface Pressures R. A. Brown 2003 U. ConcepciÓn

  2. Nov 9 ‘96 18Z Gulf of Alaska rab

  3. Towards a SurfacePressure Model Function • A Scatterometer doesn’t measure Winds. It measures Capillaries & Short Gravity waves, related to roughness parameter zo, or u*. • Fortunately, there exists a relation U(zo,…) for the surface layer. • Established over Land, Assumed over ocean . • Verified in 23 years since Seasat. This is Easily Extrapolated to: • * Fortunately, there exists a relation P( zo, .…) for the PBL. • - Established in UW PBL Models • - Verified in 10 years of scatterometer & SAR data/GCM comparisons R. A. Brown 2003 U. ConcepciÓn

  4. Toward a Surface Pressure Model Function • A Scatterometer doesn’t measure Winds. It measures Capillaries & Short Gravity waves, related to zo., u* • Fortunately, there exists a relation U10/u* = F(z, zo, stratification…). • Established over Land, Assumed over Ocean . • Verified in 19 years since Seasat. • There’s an Easy Extrapolation to: • Fortunately, there exists a relation UG/u* = F(z, zo, stratification, , ……). • Established in UW PBL_LIB • Verified in 20 years scatterometer data R. A. Brown 2003 U. ConcepciÓn

  5. Evolution of a Pressure Model Function Observation: Surface stress related to surface wind Surface roughness correlates to Surface (Log-layer) wind Observations: PBL Model = Surface+Ekman Layer with OLE Works well Surface wind correlates to Gradient Wind & P Observation: Satellite P correlate with NWP Pressures Backscatter correlates with P R. A. Brown 2003 U. ConcepciÓn RAB 2/97, 11/99

  6. Why must we consider a PBL (planetary boundary layer) model? • The satellite measures the mean density of the capillaries and short gravity waves on the ocean surface. There is no good theory relating this to anything geophysically worthwhile. • There exists a raw empirical parameterization between surface roughness and near surface winds (for over flat, smooth land surface). • There is a nonlinear analytic solution of the PBL in a rotating frame of reference (but it contains OLE). R.A. Brown PORSEC 2000 R. A. Brown 2003 U. ConcepciÓn

  7. Geostrophic Flow VG(u*) effects Thermal Wind Ekman Layer with OLE Nonlinear OLE Non steady-state Advection,centrifugal terms U10(u*) effects Stratification U10 Surface Layer Surface Stress, u*  Variable Surface Roughness Ocean surface R. A. Brown 2003 U. ConcepciÓn

  8. Let’s try a direct correlation with pressure Since VG=P / ( f ) Calculate from ECMWF surface pressures; getP and VG ; substitute VG for U10 in the Model Function Results: VGcorrelates with o as well as U10 Prospects: * Better alias selection * High winds appear * Low winds, directions appear * Stratification, Thermal Wind Effects R. A. Brown 2003 U. ConcepciÓn

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  11. Example of o vs look angle for VG = 27m/s; Incidence = 45 Example of o vs look angle for U10 = 20m/s; Incidence = 45 R. A. Brown 2003 U. ConcepciÓn

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  13. Results from Satellite Scatterometer surface pressure analyses: Agreement with ECMWF pressure fields indicate that both Scat winds and the PBL model are accurate within 2 m/s. 3-month, zonally averaged offset angle (VG, U10) of 19° suggests the mean PBL state is near neutral. Swath deviation angles show thermal wind, stratification effects. Higher winds (than GCM or buoys) from pressure gradients agree with OLE effect predictions.VG rather than U10 could be used to initialize GCMs R. A. Brown 2003 U. ConcepciÓn

  14. oV vs look angle, 0 < VG < 50m/s; Incidence angle = 45 R.A. Brown PORSEC 2000 R. A. Brown 2003 U. ConcepciÓn

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  16. QuikSCAT plus UW PBL model R. A. Brown 2003 U. ConcepciÓn

  17. ECMWF R. A. Brown 2003 U. ConcepciÓn

  18. Surface Pressures Ecmwf analysis QuikScat analysis J. Patoux 2002 R. A. Brown 2003 U. ConcepciÓn

  19. JPL project winds UW project winds R. A. Brown 2003 U. ConcepciÓn

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  21. Southern Hemisphere PressuresECMWF & NSCAT Comparison • Surface Pressure Fields of 102 Storms surveyed for 1996: • 25% good matches (-3 mb ave. diff.) • 70% misplaced average 280 km • 5% missed entirely R. A. Brown 2003 U. ConcepciÓn R. A. Brown 2000

  22. Early Results from Satellite Scatterometer surface pressure analyses:  Agreement with ‘Surface Truth’ pressure fields indicate that both Scat winds and the PBL model are accurate within 2 m/s.  3-month, zonally averaged offset angle (VG, U10) of 19° suggests mean PBL state is near neutral (1996) Higher winds (than GCM or buoys) from pressure gradients agree with nonlinear equilibrium with OLE model predictions. VG rather than U10 should be used to initialize GCMs  Swath deviation angles show thermal wind, stratification effects. R. A. Brown 2003 U. ConcepciÓn

  23. CONCLUSIONS • Buoy winds are not good surface truth • GCM PBL models still have wrong physics, too-low winds • The oV saturates (due to white water) @ U10 ~ 35 m/s, but the oH does notsaturate at U10 ~ 65 m/s • The winds are higher, the lows lower & more frequent, heat fluxes greater and stress greater than climatology states. • Scatterometer derived pressure fields can be used to de-alias winds, and correct (smooth) o single or small area anomalies (rain or nadir/edge ambiguities). R. A. Brown 2003 U. ConcepciÓn

  24. Results from Satellite Scatterometer surface pressure analyses: Agreement with ECMWF pressure fields indicate that both Scat winds and the PBL model are accurate within 2 m/s. 3-month, zonally averaged offset angle (VG, U10) of 19° suggests the mean PBL state is near neutral. Swath deviation angles show thermal wind, stratification effects. Higher winds (than GCM or buoys) from pressure gradients agree with OLE effect predictions.VG rather than U10 could be used to initialize GCMs R. A. Brown 2003 U. ConcepciÓn

  25. Producing smooth wind fields R. A. Brown 2003 U. ConcepciÓn

  26. Raw scatterometer winds Local GCM smoothed - Dirth Pressure field smoothed R. A. Brown 2003 U. ConcepciÓn

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