Preliminary results of assimilation of ASAR wave spectra level 2b products in WAM

L. Aouf (1), J.M. Lefevre (1) and D. Hauser (2)

(1) Meteo France; (2) CETP/CNRS1. Introduction

Assimilation of directional spectra 　 in wave model in Meteo France

Better estimate of the wave parameters(Synthetic directional spectra)

2. Description of the assimilation scheme

Partitioning of the wave spectrum (first guess and observed)

Cross-assignement between equivalent wave systems (fg and obs)

Optimal interpolation for the selected partitions

Reconstitution of the analysed wave spectrum

Data set 1 data set 2 data set 3 data set 4

May 3 at 6:00 may 3 at 12:00 may 4 at 0:00 may 8 at 12:00

3. Case studies

ASAR-ENVISAT wave spectra

Wave model WAM (global scale 1x1 degrees)Discretization 24 frequency (0.044-0.22 Hz) and 36 directions

4. Limitation for the observed ASAR wave spectrum

Two versions of the assimilation scheme :1- all wave spectrum2- limited wave spectrum (cut-off fc=0.1 Hz)

0 100 200 3000.05

0.1

0.15

0.2

wave direction (degrees)

frequ

ency

(H

z)WAM-FG

0

10

20

30

0 100 200 3000.05

0.1

0.15

0.2

wave direction (degrees)

frequ

ency

(H

z)

ASSIMILATED

0

5

10

15

20

0 100 200 3000.05

0.1

0.15

0.2

wave direction (degrees)

frequ

ency

(H

z)

ASAR-ENVISAT

0

5

10

15

0 0.1 0.2 0.3 0.40

5

10

15

20

frequency (Hz)

ener

gy d

ensi

ty (

m2 /H

z)

long=282 deg; lat=-26 deg

WAMASSIASAR

Comparison between wave spectra:Without limitation in the scheme (may 3 at 6:00)Long=282 deg; lat=-26 deg

Comparison between wave spectra:Without limitation in the scheme (f=0.1 Hz)Long=282 deg;lat=-26 deg

0 100 200 3000.05

0.1

0.15

0.2

wave direction (degrees)

frequ

ency

(H

z)

WAM-FG

0

10

20

30

0 100 200 3000.05

0.1

0.15

0.2

wave direction (degrees)

frequ

ency

(H

z)

ASSIMILATED

0

5

10

15

20

0 100 200 3000.05

0.1

0.15

0.2

wave direction (degrees)

frequ

ency

(H

z)

ASAR-ENVISAT

0

5

10

15

0 0.1 0.2 0.3 0.40

5

10

15

20

frequency (Hz)

ener

gy d

ensi

ty (

m2 /H

z)

long=282 deg; lat=-26 deg

WAMASSIASAR

0 100 200 3000.05

0.1

0.15

0.2

wave direction (degrees)

frequ

ency

(H

z)

WAM-FG

0

2

4

6

8

10

0 100 200 3000.05

0.1

0.15

0.2

wave direction (degrees)

frequ

ency

(H

z)

ASSIMILATED

0

5

10

15

0 100 200 3000.05

0.1

0.15

0.2

wave direction (degrees)

frequ

ency

(H

z)

ASAR-ENVISAT

0

5

10

0 0.1 0.2 0.3 0.40

2

4

6

8

10

frequency (Hz)

ener

gy d

ensi

ty (

m2 /H

z)

long=278 deg; lat=-11 deg

WAMASSIASAR

Comparison between wave spectra:Without limitation in the scheme Long=278 deg; lat=-11 deg

0 100 200 3000.05

0.1

0.15

0.2

wave direction (degrees)

frequ

ency

(H

z)WAM-FG

0

2

4

6

8

10

0 100 200 3000.05

0.1

0.15

0.2

wave direction (degrees)

frequ

ency

(H

z)

ASSIMILATED

0

5

10

0 100 200 3000.05

0.1

0.15

0.2

wave direction (degrees)

frequ

ency

(H

z)

ASAR-ENVISAT

0

5

10

0 0.1 0.2 0.3 0.40

2

4

6

8

10

frequency (Hz)

ener

gy d

ensi

ty (

m2 /H

z)

long=278 deg; lat=-11 deg

WAMASSIASAR

Comparison between wave spectra:Without limitation in the scheme (f=0.1 Hz)Long=278 deg; lat=-11 deg

0 10 20 30 40 500

0.5

1

1.5

2

2.5

3

3.5

4

observation index

low

fre

qu

en

cy w

ave

he

igh

t (m

)

may 3, 2002 at 6:00

assi-totalasar-envisatnoassiassi-limit10

Bias(m) RMS(m) STD(m)Assi total spectrum

Assi spectrum cut-off0.1 Hz

Without assim

-0.102 0.292 0.277

0.037 0.243 0.245

-0.369 0.648 0.539 Statistic analysis for low frequency wave height Hs>10s(set 1)Date : may 3 at 6:00

240 250 260 270 280 290-80

-60

-40

-20

0

20

40

longitude (degrees)

lati

tud

e (

de

gre

es)

locations of observations; may 3 at 6:00

0 20 40 60 800

1

2

3

4

5

observation index

low

fre

qu

en

cy w

ave

he

igh

t (m

)

may 3, 2002 at 12:00

assi-totalasar-envisatnoassiassi-limit10

Statistic analysis for low frequency wave height Hs>10s(set 2)Date : may 3 at 12:00

Assi total spectrum

Assi spectrum cut-off0.1 Hz

Without assim

-0.057 0.475 0.475

-0.092 0.341 0.329

-0.461 0.688 0.513

100 150 200 250 300 350-80

-60

-40

-20

0

20

40

60

80

longitude (degrees)

lati

tud

e (

de

gre

es)

total spectrum; may 3 at 12:00

Bias(m) RMS(m) STD(m)

1 2 3 4 50

0.5

1

1.5

2

2.5

3

3.5

4

observation index

low

fre

qu

en

cy w

ave

he

igh

t (m

)

may 4, 2002 at 0:00

assi-totalasar-envisatnoassiassi-limit10

Statistic analysis for low frequency wave height Hs>10s (set 3)Date : may 4 at 0:00

Assi total spectrum

Assi spectrum cut-off0.1 Hz

Without assim

0.010 0.592 0.596

-0.077 0.349 0.342

-0.438 0.7 0.548

0 100 200 300 4000

10

20

30

40

50

60

70

80

longitude (degrees)

lati

tud

e (

de

gre

es)

obs locations; may 4 at 0:00

Bias(m) RMS(m) STD(m)

0 10 20 30 40 50 60 700

1

2

3

4

5

6

observation index

may 3, 2002 at 6:00, limited

SW

H in

me

ters

NOASSIERS-ALTIASAR-ENVISATASSI=SPEC

Bias (m) RMS (m)ERS2alti-asar-level2 1.18 1.35ERS2alti-wam 0.28 0.89ERS2alti-assi-cutoff 0.47 0.87

Results on significant Wave height may 3 at 6:00

Comparison to ERS2 Altimeter significant wave height

0 20 40 60 80 1000

1

2

3

4

5

6

7

8

observation index

may 3, 2002 at 12:00, limited

SW

H in

me

ters

NOASSIERS-ALTIASAR-ENVISATASSI=SPEC

Bias (m) RMS (m)Ers2alti-asar-level2 0.58 1.24Ers2alti-wam -0.75 1.37Ers2alti-assi-cutoff -0.42 1.25

Results on significant Wave height may 3 at 12:00

Comparison to ERS2 Altimeter significant wave height

0 1 2 3 4 5 6 7 80

1

2

3

4

5

observation index

may 4, 2002 at 0:00, limited

SW

H in

me

ters

NOASSIERS-ALTIASAR-ENVISATASSI=SPEC

Bias (m) RMS (m)Ers2alti-asar-level2 0.62 0.73Ers2alti-wam -0.66 0.68Ers2alti-assi-cutoff -0.52 0.55

Results on significant Wave height may 4 at 0:00

Comparison to ERS2 Altimeter significant wave height

Assimilation impact on SWH may 3 at 12:00

Assimilation impact on mean Frequency may 3 at 12:00

Assimilation index : (%) SWH 53

mean period 41.6 mean direction 51.3

RMS with assi without assiSWH (m) 0.385 0.817Period (s) 1.56 2.6Direction(deg) 32.7 49.4

Assimilation impact on SWH may 8 at 12:00

Assimilation mpact on mean frequency may 8 at 12:00

Concluding remarks and future works

Assimilation scheme in wave model is efficient and works correctlyperformance will be improved

The limitation of the spectral information (frequency cut-off) is very important for the wave forecast otherwise the analysed wave parameters will be deteriorated

Long period assimilation for validation and analyse of the Impact on the wave forecast (one week complete data, if Nick can provide us)

Combined assimilation ASAR and RA2

Financial support to continue this study and to have more conclusiveInterpretations (ESA, CNES)