Similarities and differences of SAR derived wind fields...

Seasar08, Frascati 21-25 january 2008

Similarities and differences of SAR derived wind fields using two different methods: the local gradient and the continuous wavelet transform methods

S. Zecchetto° , F. De Biasio# , F. Nirchio* and S. Di Tomaso*

°Istituto Scienze dell'Atmosfera e del Clima (CNR), Consiglio Nazionale delle Ricerche, Corso Stati Uniti 4, 35127 Padova, Italy, [email protected]#Istituzione Centro Previsioni e Segnalazioni Maree (ICPSM), Comune di Venezia, Palazzo Cavalli, S. Marco 4090, 30124 Venezia, Italy, [email protected]

*Agenzia Spaziale Italiana, Loc. Terlecchia, 75100 Matera, Italy, [email protected]


Summary

outline of the methodologies for wind extraction: the 2 D Continuous Wavelet Transform and the Local Gradient data used and results discussion


Wind field extraction from satellite imagery (SAR): the Local Gradient method (LG)

W. Koch, Directional Analysis of SAR Images Aiming at Wind Direction,IEEE Transactions on Geoscience and Remote Sensing, 42, 4, 2004


The image is divided into sub-images accordingly to the grid on which the wind is requested

Each sub-image is processed separately, as first step the land is masked

The Local Gradient method 1/3


The sub image resolution is reduced down at 200 m

Those features like point targets and slicks, not depending from the wind, are identified and masked



The local gradient is computed on each pixel and the most frequent direction is determined. The wind is orthogonal to the gradient.

After the processing of all the sub-images a local analysis is performed: wind directions differing > 45º from the mean value of the eight nearest neighbours are discharged



Wind field extraction SAR:the 2D-Continuous Wavelet Transform (CWT2)

Zecchetto S. and F. De Biasio, On Shape, Orientation and Structure of Atmospheric cells inside wind rolls in two SAR images, IEEE Transactions on Geoscience and Remote Sensing, 40, 10, 2002Zecchetto S. and F. De Biasio, A Wavelet based Technique for Sea Wind Extraction from SAR images, IEEE Transactions on Geoscience and Remote Sensing, submitted, 2008


CWT2 methodology: image pre-processing 1/6

compression-pixel size 125 or 150 m masking for land mitigation of the incidence angle

effects


CWT2 methodology: the energy map W(sr,s

c) 2/6

Choice of the scales sr,s

c in the spatial

range 300 m 4 km

F s r ,sc j,i = ∑

m x=1

mx=Nx

∑m y=1

my=Ny

ψsr ,j m y ψ s c ,i m x SAR m y ,m x

W sr ,sc =∑j,i

F s r ,sc j,i 2

where ψ is the mother wavelet at scale s

r and applied

to the pixel j


CWT2 methodology: image reconstruction 3/6

where are the wavelet coefficientsW j,i = ∑s r ,sc

F sr ,sc j,i 2

F s r ,sc j,i


CWT2 methodology: choice of aliased direction 4/6

aliased direction: 300 degree with a probability of 71 %


CWT2 methodology: dealiasing 5/6


CWT2 methodology: resulting field 6/6


Data set ERS-2 PRI images over the Mediterranean Sea

Selected according to the availability of scatterometer wind fields in the time window ± 6 hours from the SAR pass time

wind speed ranged from 1 to 10 m/s, with a prevalence of low wind conditions


Results 1/4


Results 2/4


Results 3/4


Results 4/4

orbit-frame CWT2 LG Scatt ECMWF Ws (m s-1) 09051-2835 55˚ 26˚ 32˚ 67˚ 8.0 09051-2853 52˚ 27˚ 61˚ 77˚ 10.4 09051-2871 56˚ 10˚ 46˚ 64˚ 6.4 10912-2871 324˚ 345˚ 321˚ 335˚ 7.8 10912-2889 330˚ 354˚ 341˚ 353˚ 3.5 10912-2907 70˚ 3˚ 358˚ 7˚ 1.4 33229-2907 57˚ 12˚ 320˚ 312˚ 3.6 33229-2925 334˚ 350˚ 325˚ 311˚ 3.4 33229-2943 338˚ 343˚ 318˚ 316˚ 6.6

σCWT2-LG

27˚σ

CWT2-SCATT 34˚

σLG – SCATT

37˚σ

SCATT-ECMWF 14˚


Discussion 1/2


Discussion 2/2


Concluding remarksTwo methods tested over nine ERS-2 images over the Mediterranean Sea

•CWT2 is preferable because it de-aliases the directions•The de-aliased wind directions are similar (σ

CWT2-LG= 27˚)

•Both CWT2 and LG directions differ of about 30˚ from the scatterometer directions: this aspect should be investigated in view of applications (Vachon and Dobson, 1996, Lehner et al., 1998, Fetterer et. al., 1988 found similar discrepancies)•Both CWT2 and LG provide unsatisfactory results over highly variable wind field situations •To assess the quality and shortcomings of the two methods, they should be used in tandem over a large SAR set of images

Similarities and differences of SAR derived wind fields...

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