IGAARS Honolulu July 24-27.

On Satellite Wind Model Functions and some specific applications of a ScatterometerPMF.

Satellite surface Model Functions are the empirical correlations between observedsatellite backscatter and geophysical parameters. The correlation between backscatter andsurface winds has been the foundation of the scatterometers. The backscatter isanisotropic with respect to wind direction, allowing wind direction information to bederived from multiple looks at the same patch of ocean.

The first problem in establishing an empirical model function is to choose a ‘surfacetruth’ to correlate with the radar backscatter. Unfortunately in situ measurements of themost direct geophysical parameter, wind-stress over the ocean, do not exist. However, thelog-layer solution for the flow in the boundary layer suggests that there exists a closerelation between the surface stress and the wind in the near surface layer. Hencecorrelations between o and near surface wind were sought and found. Currently, buoydata and GCM surface analyses have been used to establish the wind model functions.

In a similar fashion, PBL models suggest a strong relation between the surface stressand the wind at the top of the PBL. Recently the ECMWF and NCEP surface pressureanalyses have been used to establish a surface geostrophic wind model function. Thereare limitations and advantages to both geophysical data sets. They are used here intandem to produce a basic set of smooth vector wind and surface stress fields and surfacepressure fields.

R. A. Brown 2003 U. ConcepciÓn

Nov 9 ‘96 18Z Gulf of Alaska rab

Towards a SurfacePressure Model

Function• A Scatterometer doesn’t measure Winds. It measures

Capillaries & Short Gravity waves, related to roughness parameter zo, or u*.

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

Fortunately, there exists a relation U(zo,…) for the surface layer.

Established over Land, Assumed over ocean .Verified in 23 years since Seasat.

R. A. Brown 2003 U. ConcepciÓn

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

Evolution of a Pressure Model Function

Observation: Surface stress related to surface wind

Surface roughness correlates to Surface (Log-layer) wind

Surface wind correlates toGradient Wind & P

Observations: PBL Model = Surface+Ekman Layer with OLE Works well

Observation: Satellite P correlate with NWP Pressures

Backscatter correlates with P

RAB 2/97, 11/99 R. A. Brown 2003 U. ConcepciÓn

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

Ocean surface

Surface Layer Surface Stress, u*

Geostrophic Flow

U10

Ekman Layer with OLE

Thermal WindNonlinear OLE

Non steady-state

Advection,centrifugal terms

U10(u*) effects

Stratification

Variable Surface Roughness

VG(u*) effects

R. A. Brown 2003 U. ConcepciÓn

Let’s try a direct correlation with pressureSince VG = P / ( f )

Calculate from ECMWF surface pressures; get P and VG ; substitute VG for U10 in the Model Function

VG correlates with o as well as U10

* Better alias selection * High winds appear * Low winds, directions appear * Stratification, Thermal Wind Effects

Results:

Prospects:

R. A. Brown 2003 U. ConcepciÓn

R. A. Brown 2003 U. ConcepciÓn

R. A. Brown 2003 U. ConcepciÓn

Example of o vs look angle for U10 = 20m/s; Incidence = 45

Example of o vs look angle for VG = 27m/s; Incidence = 45

R. A. Brown 2003 U. ConcepciÓn

R. A. Brown 2003 U. ConcepciÓn

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

oV vs look angle, 0 < VG < 50m/s; Incidence angle = 45

R.A. Brown PORSEC 2000

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R. A. Brown 2003 U. ConcepciÓn

QuikSCAT plus UW PBL model

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ECMWF

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Ecmwf analysis

QuikScat analysis

J. Patoux 2002

Surface Pressures

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JPL project winds UW project winds

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Southern Hemisphere Pressures

ECMWF & 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 entirelyR. A. Brown 2000R. A. Brown 2003 U. ConcepciÓn

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)

Swath deviation angles show thermal wind, stratification effects.

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

R. A. Brown 2003 U. ConcepciÓn

• 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 not saturate 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).

CONCLUSIONS

R. A. Brown 2003 U. ConcepciÓn

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

Producing smooth wind fields

R. A. Brown 2003 U. ConcepciÓn

R. A. Brown 2003 U. ConcepciÓn

Raw scatterometer windsPressure field smoothed

Local GCM smoothed - Dirth