Cross-Polarized SAR: A New Cross-Polarized SAR: A New PotentialPotential Technique for Hurricanes Technique for Hurricanes

Will Perrie and Biao ZhangBedford Institute of OceanographyDartmouth, Nova Scotia, Canada

PR vs. incidence angle, from quad-polarization data. Compared to other PR models …

PR dependence on incidence angle

We develop a polarization ratio model to improve wind speed retrievals…

SAR-wind modelsSAR-wind modelsHH0

VV0

σ

σPR =

HH0

VV0

σ

σPR =

SAR PR ocean wind models SAR PR ocean wind models

-0.0802C

0.0333B

0.5474A

Fitted valuesCoefficient

units]Linear [ )exp(PR CBA += θ

Nonlinear Least Squares fit ….

-0.0505Q2

-0.0006Q1

0.9559P3

-0.0162P2

0.0012P1

Fitted valuesCoefficient

( ) units][Linear )(PR 10θθ QUP=

3*2*1)( 2 PPPP ++= θθθ2*1)( QQQ += θθ

PR Model I only incidence angle dependence

PR Model II with additional wind speed dependence

DatasetDatasetMouche et al #2 Our #1model Mouche et al #1 downwind

Mouche et al #1 crosswind Johnsen et al Thompson et al

Elfouhaily Our #2 model Vachon&Dobson

PR

PR from RADARSAT-2

DatasetDataset

17

SAR winds from CMOD5.N and our model#I + R2 HH pol image

14.1 m/s12.9 m/s

12.4 m/s11.6 m/s 15.9 m/s

16.6 m/s

Zhang, B., W. Perrie, and Y. He (2011), Wind speed from RADARSAT-2 quad-polarization images using a new polarization ratio model, J. Geophys. Res., doi:10.1029/2010JC006522.

Co-Polarization Model: CMOD5.N

Cross-Polarization Ocean model: C-2PO

1:wind speed, wind direction, incidence angle.2: NRCS_VV saturated under high winds.

1: only wind speed dependence.2: NRCS_VH not saturated under high winds.

Correlation coefficient for C-2PO = 0.91

σ VVσHH

σHV σVH

U10 U10

C-band models

DatasetDatasetR2 Quad-Polarization Ocean Backscatter Measurements

NRCS_VV saturates

no NRCS_HV saturation

NRCS_HV, NRCS_VHnot sensitive to incidence angle, wind direction

NRCS_VV, NRCS_HHdepend on incidence angle, wind direction

U10U10

U10 U10

C-2PO corr coeff = 0.91

Hurricane Bertha – 12 July 2008

Hurricane wind-speed retrievals with C-2PO

Hurricane Ike – 10 Sept 2008

Hurricane Bill – 22 Aug 2009

Hurricane Danielle – 28 Aug 2010

Hurricane Earl on Sep 02, 2010 at 22:59 UTC

VV polarization VH polarization

RADARSAT-2 dual-polarization SAR image

Hurricane Earl on Sep 02, 2010 at 22:59 UTC

H*Wind (m/s)

Winds -buoy #41001 is 18.1(m/s) C-2PO is 16.0 CMOD5.N is 17.4 H*Wind is 16.8

CMOD5.N (m/s) C-2PO model (m/s)

Comparison of C-2PO and CMOD5. SAR wind retrievals

Along track-SFMR (hr) time series

SFMR-measured 10s rain rates (mm/hr) time series (hr)

eyewall? rain? gradients?

Hurricane Earl

Comparisons of C-2PO and CMOD5.N SAR-retrieved winds U10 (Sep 02, 2010 at 22:59 UTC) with collocated H*Wind

C-2PO CMOD5.N

dual-polarization SAR image at 23:56 UTC on Sep 10, 2008

Hurricane Ike

VV polarization VH polarization

CMOD5.N + wind directions via H*Wind

C-2PO model U10

Comparisons of C-2PO and CMOD5.N SAR-retrieved winds U10 (23:56 UTC, on September 10, 2008) with collocated H*Wind

Hurricane Ike

CMOD5.N bias of -4.89 m/s and RMS error of 6.51 m/s C-2PO bias of -0.88 m/s and RMS error of 4.47 m/s

C-2PO CMOD5.N

Summary Summary

• C-2PO model presented• insensitive to wind direction, radar incidence angle

– easy mapping of observed cross-pol NRCS to wind speed • avoids errors in wind speed retrievals that occur in CMOD5.N • in quad-pol data, C-2PO does not seem to saturate

– potential for hurricane wind retrievals• dual-pol Earl: high wind comparisons R2 SAR – airborne SFMR • Earl: C-2PO bias= -0.89 m/s, RMSE= 3.23 m/s,

CMOD5.N bias= -4.14 m/s, RMSE= 6.24 m/s.

Reasons for under-estimates: • CMOD5.N is saturated in high winds• co- and cross-polarized NRCS calibration error• CMOD5.N and C-2PO do not account for rain, high sea states, eye gradients, e.g.

dampen the NRCS biases in wind speeds• inaccurate wind directions for CMOD5.N.

Some other sources…Some other sources…• Paul Hwang et al. TU2.T10.4 (11:20) Wind retrieval with cross-

polarized SAR returns • Vladimir Zabeline et al. TH2.T02.2 RADARSAT Application in ocean

wind measurements

Hwang, P. A., B. Zhang, and W. Perrie, 2010: Depolarized radar return for breaking wave measurements and hurricane wind retrieval. Geophys. Res. Lett., 37, L01604, doi:10.1029/2009GL041780.

Vachon, P.W. and J. Wolfe, (2010), C-band cross-polarization wind speed retrieval, IEEE Geosci. Remote Sens. Lett., 7, 456-459.

Zhang, B., and W. Perrie (2010), C-band Quad-Polarization ocean backscatter measurements: A new polarization ratio model. SEASAR2010, January 25-29, Frascati, Italy.

Zhang B., W. Perrie, and Y. He, 2011: Wind speed retrieval from RADARSAT-2 quad-polarization images using a new polarization ratio model. J. Geophys. Res., 116, doi:10.1029/2010JC006522.

Zhang, B., W. Perrie (2011), Cross-polarized synthetic aperture radar: a new measurement technique for Hurricanes, under review - Bulletin of the American Meteorological Society (BAMS).