Transcript C-2PO CMOD5.N

Cross-Polarized SAR: A New
Potential Technique for Hurricanes
Will Perrie and Biao Zhang
Bedford Institute of Oceanography
Dartmouth, Nova Scotia, Canada
σ 0VV
PR  HH
σ0
SAR-wind models
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 PR ocean wind models
Nonlinear Least Squares fit ….
PR Model I
only incidence angle dependence
PR  A exp(B )  C
[Linearunits]
Coefficient
Fitted values
A
0.5474
B
0.0333
C
-0.0802
PR Model II
with additional wind speed dependence
Coefficient
Fitted values
P1
0.0012
PR  P( )U10Q  [Linearunits]
P2
-0.0162
P3
0.9559
Q1
-0.0006
Q2
-0.0505
P( )  P1*  P2 *  P3
2
Q( )  Q1 *   Q2
Mouche et al #2
Our #1model
Mouche et al #1 downwind
Dataset
Mouche et al #1 crosswind
Elfouhaily
PR
PR from RADARSAT-2
Johnsen et al
Our #2 model
Thompson et al
Vachon&Dobson
SAR winds from CMOD5.N
and our model#I + R2 HH pol image
Dataset
14.1 m/s
12.9 m/s
12.4 m/s
11.6 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.
17
C-band models
 VV
HH
HV
VH
Co-Polarization Model: CMOD5.N
1:wind speed, wind direction, incidence angle.
2: NRCS_VV saturated under high winds.
Cross-Polarization Ocean model: C-2PO
U10
U10
Correlation coefficient for C-2PO = 0.91
1: only wind speed dependence.
2: NRCS_VH not saturated under high winds.
R2 Quad-Polarization Ocean Backscatter Measurements
Dataset
NRCS_VV,
NRCS_HH
depend on
incidence
angle, wind
direction
NRCS_HV,
NRCS_VH
not sensitive
to incidence
angle, wind
direction
NRCS_VV saturates
U10
U10
no NRCS_HV saturation
C-2PO
corr coeff
= 0.91
U10
Hurricane wind-speed retrievals with C-2PO
Hurricane Bertha – 12 July 2008
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
Winds -buoy #41001 is 18.1
(m/s) C-2PO is
16.0
CMOD5.N is
17.4
H*Wind is
16.8
H*Wind (m/s)
CMOD5.N (m/s)
C-2PO model (m/s)
Comparison of C-2PO and CMOD5. SAR wind retrievals
eyewall? rain? gradients?
Along track-SFMR (hr) time series
SFMR-measured 10s rain rates (mm/hr) time series (hr)
Hurricane Earl
C-2PO
CMOD5.N
Comparisons of C-2PO and CMOD5.N SAR-retrieved winds U10
(Sep 02, 2010 at 22:59 UTC) with collocated H*Wind
Hurricane Ike
dual-polarization SAR image
at 23:56 UTC on Sep 10, 2008
VV polarization
VH polarization
CMOD5.N +
wind directions
via H*Wind
C-2PO model U10
Hurricane Ike
C-2PO
CMOD5.N
Comparisons of C-2PO and CMOD5.N SAR-retrieved winds U10 (23:56
UTC, on September 10, 2008) with collocated H*Wind
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
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:
1) CMOD5.N is saturated in high winds
2) co- and cross-polarized NRCS calibration error
3) CMOD5.N and C-2PO do not account for rain, high sea states, eye gradients, e.g.
dampen the NRCS  biases in wind speeds
4) inaccurate wind directions for CMOD5.N.
Some other sources…
• Paul Hwang et al. TU2.T10.4 (11:20) Wind retrieval with crosspolarized 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).