New Fluorescence Algorithms for the

MERIS SensorYannick Huot and Marcel Babin

Laboratoire d’Océanographie de Villefranche

Antoine Mangin and Odile Fanton d'AndonACRI-ST

28 September 2005

Research funded by: A fellowship from the Natural Sciences and Engineering Research Council (NSERC)

What is fluorescence andwhy would one care?

Phytoplankton fluorescence• Just outside the cells:

F =ϕ ϕ chl aϕ* Qa

* Eo

Fluorescence volume flux

Quantum yieldof fluorescence

Chlorophyllconcentration

Chlorophyll specificabsorption

Reabsorption parameter

Scalar Irradiance

Two products of interest

F =ϕ ϕ chl aϕ* Qa

* Eo

The Biomass

Index of physiological status

Today we are developing algorithms for these two products applicable to case 1 waters

May be useful in regions where the chlorophyll concentration cannot be obtained with standard ocean colour algorithms

Processes studies in case 1 waters

Measurement optics

Luf 0−( ) =

14π

 chl a∗__ϕ ϕ Qa

* Eo

PAR 0−( )1

aϕ + KPURsurϕ

Terms from previous page

Subsurface upwelling radiance due to fluorescence

Geometrical factor

Attenuation of downwelling and upwelling light

Adapting recently published MODIS algorithms to MERIS

The two products

chl =Luϕ 0

−( ) 4π aϕ + KPURsurϕ⎡⎣ ⎤⎦ 

a∗__ϕ ϕ Qa

* Eo

PAR 0−( )

ϕ f =Luf 0

−( ) 4π   a f + KPURsurf⎡⎣ ⎤⎦ 

chl  a∗__ Qa

*  Eo

PAR 0−( )

Fixed quantum yield

Measured chlorophyll concentration

• Satellite algorithms:– : MERIS PAR– Chl: MERIS case 1, blue to green ratio algorithm– Luf(0-): Transform from rw to Lu and baseline method

– Kd(490): “Improved” blue to green ratio algorithm

• Case 1 waters relationships functions of Kd(490)– versus measured Kd(490);

• Bricaud et al. 1998 statistics, vs. chl• Morel et al. 2001, Kd(490) vs. chl

Concept of the algorithm

Luf 0−( ) =

14π

 chl a∗__ϕ ϕ Qa

* Eo

PAR 0−( )1

aϕ + KPURsurϕ

Eo

PAR 0−( )

a∗__

a∗__, Qa

*, KPURsurϕ

WARNING:For today’s presentation

some approximations are made that would not be necessary in standard

algorithms.

Scalar irradiance• MERIS product gives Ed(0-,PAR)

– For fluorescence we want • We assumed: 1) Upwelling irradiance negligible 2) d(0-) = 0.75 for the whole scene

We thus use:

Eo

PAR 0−( )

Eo

PAR 0−( ) =1d

Ed 0−,PAR( )

=1.33 Ed 0−,PAR( )

• Satellite algorithms:– : MERIS PAR– Chl: MERIS case 1, blue to green ratio algorithm– Luf(0-): Transform from rw to Lu and baseline method

– Kd(490): “Improved” blue to green ratio algorithm

• Case 1 waters relationships functions of Kd(490)– versus measured Kd(490);

• Bricaud et al. 1998 statistics, vs. chl• Morel et al. 2001, Kd(490) vs. chl

Concept of the algorithm

Luf 0−( ) =

14π

 chl a∗__ϕ ϕ Qa

* Eo

PAR 0−( )1

aϕ + KPURsurϕ

Eo

PAR 0−( )

a∗__, Qa

*, KPURsurϕ

a∗__

Going from rw to Lu

• Problem: MERIS algorithm returns rw :

rw' =π Lw 0

+( )Ed 0

+( )

Atmospheric transmission (td) has to be approximated when one doesn’t have access to the processing chain intermediate products (probably a small error)

to calculate a quantum yield we need Lu(0-):

Lw 0+

( )=rw'π Ed 0+( )

=rw'π Fo ε cosθsun td ta,t r,θsun( )

First step to Lw(0+)

Lu (0−)=tw−a Lw nw

2

Second step to Lu(0-)

The baseline method• MERIS bands dedicated to the natural

fluorescence measurements are:– 665, 681, and 709 nm (bands 7, 8, 9)

This approximation is good in case 1 waters (Huot et al. 2005) but great care must be taken in case 2 waters (see next talk by Babin and Huot)

Concept of the algorithm• Satellite algorithms:

– : MERIS PAR– Chl: MERIS case 1, blue to green ratio algorithm– Luf(0-): Transform from rw to Lu and baseline method

– Kd(490): “Improved” blue to green ratio algorithm

• Case 1 waters relationships functions of Kd(490)– versus measured Kd(490);

• Bricaud et al. 1998 statistics, vs. chl• Morel et al. 2001, Kd(490) vs. chl

Luf 0−( ) =

14π

 chl a∗__ϕ ϕ Qa

* Eo

PAR 0−( )1

aϕ + KPURsurϕ

Eo

PAR 0−( )

a∗__, Qa

*, KPURsurϕ

a∗__

Kd490: A “MERIS” algorithmNOMAD dataset: see Werdell, P.J. and S.W. Bailey, 2005: An improved bio-optical data set for ocean color algorithm development and satellite data product validation. Remote Sensing of Environment , 98(1), 122-140. Thank you to all contributors…

See also: http://oceancolor.gsfc.nasa.gov/REPROCESSING/Aqua/R1.1/

Checking the Kd(490) algorithm

Morel and Maritorena 2001Best fit polynomial

-The two algorithms are consistent, with some bias at high chl-Waters examined do not depart strongly from case 1 relationships

Concept of the algorithm• Satellite algorithms:

– : MERIS PAR– Chl: MERIS case 1, blue to green ratio algorithm– Luf(0-): Transform from rw to Lu and baseline method

– Kd(490): “Improved” blue to green ratio algorithm

• Case 1 waters relationships functions of Kd(490)– versus measured Kd(490);

• Bricaud et al. 1998 statistics, vs. chl• Morel et al. 2001, Kd(490) vs. chl

Luf 0−

( ) =14π

 chl a∗__φϕ Qa

* Eo

PAR 0−( )1

aϕ +KPURsurϕ

Eo

PAR 0−( )

a∗__, Qa

*, KPURsurϕ

a∗__

Case 1 water relationships

Bricaud, A., H. Claustre, J. Ras, and K. Oubelkheir. 2004. Journal of Geophysical Research 109: C11010,doi:11010.11029/12004JC002419.

Morel, A., and S. Maritorena. 2001. Journal of Geophysical Research 106: 7163-7180.

A little algebra:

A little more algebra:

Some calculus and a numerical model:

a∗=ϕ Kd (490)( )

Qa* =ϕ Kd (490)( )

KPURsurf =ϕ Kd (490)( )

Let’s apply it on an image now…

Noise in the 681nm band

Scene from the Benguela upwelling region measured on July 14, 2003, Second reprocessing.

Measured by MERIS

Chlorophyll algorithm

Best fit 1:1 line

Chlorophyll algorithm map

Quantum yield algorithm

Quantum yield algorithm

Comparison with MODIS: ϕfMERIS MODIS

-Very noisy-Hard to use presently-However, much of the noise is not random and it may be possible to correct for it

- Quantum yield too high?

Are we measuring something real?

No clear reason for this trend

Consistent with non-photochemical quenching

Are we measuring something real?

Answer: Perhaps, but what?

Conclusion• We proposed two algorithms for MERIS fluorescence bands– One for chlorophyll– One for the quantum yield

• MERIS band at 681 nm is more noisy than the 665 and 709 bands

• Algorithms need to be fully validated but preliminary results are encouraging

Future prospects• We hope to implement the algorithms with intermediate products of the processing chain to avoid the limitations of the level two products.

• We are testing iterative fluorescence algorithms using only the fluorescence bands for the retrieval of chlorophyll.

Future prospects: first glimpseFLH Today’s

algorithmFluorescence bands only

It’s potential will depend on our ability to reduce the noise observed

in the 681 nm channel

Thanks to

• David Antoine• Norman Fomferra• André Morel