Closure of particle backscattering coefficient in …...400 450 500 550 600 650 700 0.00 0.03 0.06...

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Closure of particle backscattering coefficient in oligotrophic waters ZhongPing Lee 1 , Yannick Huot 2 1. University of Massachusetts Boston 2. Université de Sherbrooke Acknowledgements: NASA, Jim Sullivan, Robert Brewin, Giorgio Dall’Olmo

Transcript of Closure of particle backscattering coefficient in …...400 450 500 550 600 650 700 0.00 0.03 0.06...

Page 1: Closure of particle backscattering coefficient in …...400 450 500 550 600 650 700 0.00 0.03 0.06 0.09 Col 1 vs Col 3 Col 1 vs Col 4 Col 1 vs Col 5 Col 1 vs Col 6 Col 1 vs Col 7 Col

Closure of particle backscattering coefficient in oligotrophic waters

ZhongPing Lee 1, Yannick Huot 2

1. University of Massachusetts Boston

2. Université de Sherbrooke

Acknowledgements: NASA, Jim Sullivan, Robert Brewin, Giorgio Dall’Olmo

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particle backscattering coefficient: bbp

(Behenfeld et al, Nature, 2006)

bbp Ccc (Balch et al. 2005, 2010)

(Stramski et al, Science, 1999)

Bulk optical property

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NOMAD

0.000 0.001 0.002 0.003 0.004 0.005 0.006

0.000

0.001

0.002

0.003

0.004

0.005

0.006

Rrs_inv

reg_no

R2 = 0.92 Y = 1.07 X + 0.0004

“Excellent” closure …

In-situ bbp(555) [m-1]

Rrs

bb

p(5

55

) [

m-1

]

1:1

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(Huot et al 2008)

However:

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(Brewin et al 2012)

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Rrs

bbp much higher than in-situ bbp

for oligotrophic waters!

No closure for such ‘simple’ waters !!

Chl < 0.1 mg/m3 makes ~50% of the global surface waters

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)555()555(

)555(

)555()555(

)555(

)555()555(

)555()555()555(

10b

b

b

b

b

brs

ba

b

ba

bGG

ba

bGR

Brief review of QAA:

Rrs(555) bbp(555)

Based on:

)555()555()555()555( ww aaaa For oligotrophic waters

For Chl = 0.1 mg/m3, Δa(555) ~ 0.002 m-1, 3% of aw(555).

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Potential sources of error from Rrs inversion:

1. Rrs–IOPs relationship

2. Measured Rrs includes Raman scattering contribution

3. a(555) or aw(555) value

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b

b

b

b

b

brs

ba

b

ba

bGG

ba

bGR

10

is supported by Radiative Transfer Theory (Zaneveld 1995)

bbp555

0.000 0.001 0.002 0.003 0.004 0.005

0.5

1.0

1.5

2.0

2.5

3.0

NOMAD

In-situ bbp(555) [m-1]

Rrs

bb

p(5

55

)/In

-sit

u b

bp(5

55

)

Better for larger bbp(555), but Rrs

bbp(555) is higher

1. Rrs – IOPs relationship

Same bbw(555) used for both determinations.

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Impact of Rrs-model parameters Gordon (0.0949;0.0794) vs QAA (0.09;0.125)

X Data

0.000 0.001 0.002 0.003 0.004 0.005

Y D

ata

1.00

1.05

1.10

1.15

In-situ bbp(555) [m-1]

bb

p(Q

AA

)/b

bp(G

ord

on

)

Not enough to have a factor of 2 impact.

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2. Measured Rrs includes Raman scattering contribution

(Westberry et al 2013)

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RF

RR

Trs

rs

1

Empirical Raman correction (Lee et al 2013):

)(

12

)550()()550(

)440()()(

T

rsTrs

Trs R

R

RRF

RF: Raman Factor

:TrsR Rrs from measurements

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NOMAD

In-situ bbp(555) [m-1]

Rrs

bb

p(5

55

)/In

-sit

u b

bp(5

55

)

bbp555

0.000 0.001 0.002 0.003 0.004 0.005

0.5

1.0

1.5

2.0

2.5

3.0

no Raman corr.

yes Raman corr.

Yes, remove Raman effect reduces bbp from Rrs

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BIOSOPE data

bbp555

0.0000 0.0005 0.0010 0.0015

0.5

1.0

1.5

2.0

2.5

3.0

3.5

no Raman corr.

yes Raman corr.

In-situ bbp(555) [m-1]

Rrs

bb

p(5

55

)/In

-sit

u b

bp(5

55

)

Rrs

bbp(555) is still generally much higher than in-situ bbp(555), especially for waters with very sparse in particles.

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)(

12

)550()()550(

)440()()(

T

rsTrs

Trs R

R

RRF

Imperfect Raman correction?

X Data

0.000 0.001 0.002 0.003 0.004 0.005

Y D

ata

0.95

0.96

0.97

0.98

0.99

1.00

RF increased by 15%

In-situ bbp(555) [m-1]

bb

p(1

5%

mo

re R

F)/b

bp

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3. a(555) or aw(555) value

aw(555) bbp(555)

Reference aw(555)

Pope and Fry (1997) 0.0596

Smith and Baker (1981) ~0.0673

Tom and Patel (1979) ~0.063

Sogandares and Fry (1997) ~0.072

Buiteveld et al (1994) 0.064

The smallest value for aw(555) was used.

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Potential sources of errors from in situ bbp:

1. Calibration

2. Sampling volume?

3. Measurement uncertainty?

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Backscatter (active) sensor

Passive sensor Sampling volume of an active sensor

Sampling volume of a passive sensor

~ 10-6 m3 ~10-1000 m3

2. Sampling volume?

“bulk” property?

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(Stramki and Kiefer 1991) >100 particles will be sampled by the 10-6 m3 sample volume

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(Stramki and Kiefer 1991) Particles could be under-represented (or missed) by 10-6 m3 volume

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(Dall’Olmo and Brewin)

Median vs Mean

Sample volume seems not a big issue, if averaged/handled properly.

Treat 1 min of measurements as “bulk”

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3. Measurement uncertainty?

(Brewin et al 2012)

In-s

itu

bb

p(5

26

) [

m-1

] (Huot et al 2008)

bbp(555): BB3: ~0.0004 m-1

HSCAT: ~0.0007 m-1

For Chl = 0.1 mg/m3

~0.0007 m-1

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bbp555

0.0000 0.0005 0.0010 0.0015

0.5

1.0

1.5

2.0

2.5

insitu bbp

insitu bbp + 0.00025

In-situ bbp(555) [m-1]

Rrs

bb

p(5

55

)/In

-sit

u b

bp(5

55

)

Measured bbp(555) + 0.00025

BIOSOPE data

Much better closure for oligotrophic waters!

If indeed insitu sensor missed (under-measured) bbp

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Updated comparison

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Summary:

1. For oligotrophic ocean, bbp(55x) can be retrieved very well from Rrs. Important to correct Raman effect.

2. We still have a (small) gap between inversion and insitu, though.

• Representation of “bulk” product • Extremely low signal Insitu sensor calibration and data handling

3. If ignoring the ~0.0003 m-1 bias, “excellent” closure is indeed achieved between inverted and insitu bbp(55x).

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Something about spectral resolution

400 500 600 700 800

0.00

0.01

0.02

0.03

Wavelength [nm]

Rrs(λ)

[sr

-1]

(a)

(Lee, Hu, Shang, Zibordi, Applied Optics, in press)

901 spectra

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400 450 500 550 600 650 700

0.90

0.92

0.94

0.96

0.98

1.00

5 nm

10 nm

15 nm

20 nm

25 nm

30 nm

Wavelength [nm]

Correlation coefficient between neighboring bands, for 6 different gaps

r Δλ(λ k

, λl)

Rrs is highly correlated between neighboring bands

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400 450 500 550 600 650 700

0.00

0.03

0.06

0.09

Col 1 vs Col 3

Col 1 vs Col 4

Col 1 vs Col 5

Col 1 vs Col 6

Col 1 vs Col 7

Col 10 vs Col 11

Col 10 vs Col 12

Col 10 vs Col 13

Col 10 vs Col 14

Col 10 vs Col 15

Col 18 vs Col 19

Col 18 vs Col 20

Col 18 vs Col 21

Col 18 vs Col 22

Col 18 vs Col 23

Wavelength [nm]

Rrs

(λ)

[sr

-1]

400 450 500 550 600 650 700

0.000

0.004

0.008

0.012

0.016

Wavelength [nm]

Rrs

(λ)

[sr

-1]

Re-constructed vs measured spectral Rrs

15

1

)()(i

irsjijrcrs RKR

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400 450 500 550 600 650 700

0.9998

0.9999

1.0000

1-nm

10-nm

400 450 500 550 600 650 700

0

1

2

3

1-nm

10-nm

Wavelength [nm]

Co

rrel

atio

n C

oef

fici

ent

Perc

enta

ge d

iffe

ren

ce [

%]

Wavelength [nm]

(a)

(b)

Characteristics between measured and re-constructed spectral Rrs

Hyperspectral (contiguous , 5-nm resolution) Rrs can be reconstructed from 15-band Rrs with negligible error.

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Thank you!