Definition and assessment of a regional Mediterranean Sea ocean colour algorithm for surface chlorophyll

Gianluca Volpe

National Oceanography Centre, Southampton School of Ocean and Earth Science

MPhil/PhD Transfer

• Motivations & Aims

• Ocean Colour Principle

• Development of a NEW MED algorithm

• Regional vs Global datasets

• SeaWiFS data Validation

• Conclusions & Future work

Outline

Marine primary production plays an important role

regulating atmospheric CO2

Biological pumpIn principle …..We can calculate global ocean Primary Production

Earth observation data from satellites(Behrenfeld et al., 2001)

Motivation

Gregg et al. (2003)

blending CZCS & SeaWiFSdata with in situ data

primary production declined by 6% since 1980s

Antoine et al. (2005)

algorithm refinements

chlorophyll concentration increased by 22% since 1980s

Motivation

DISCREPANCIES

Different methodologies

Related assumptions

Highlight NEED

ASSESS and QUANTIFY

large UNCERTAINTIES

satellite retrieved chlorophyll

Motivation

Evaluate SENSITIVITY

Primary Productivity Calculation

with respect to:

• Quality of the input data• Primary production models• Integration length and time scales

Aim of the PhD

The main goal

ASSESS the ACCURACY

Remote-Sensed Chlorophyll

….This is of crucial importance in

determining the MAGNITUDE at which

the OCEANS ABSORB CO2

Aim of this work

Atmospheric Correction

Aerosol space-time variability

Optical properties of seawater

Phytoplankton species composition

Vertical distribution

CDOM

Factors influencing the chlorophyll retrieval

Chlorophyll a&

Fluorescence

In situ OpticalMeasurements

Satellitedata

Fls calibration&

Data quality

Compute OWP(chl seen

from surface)

Algorithms’Evaluation

AssessAtmospheric

Correction errorbudget

Run best Chlalgorithm

over satellite data

Run differentPP models

Are theysignificantlydifferent?

Assess PPchanges and

their significance

yes

no

In situ PP data Select bestalgorithmPRIMARY

PRODUCTION

CHLOROPHYLL

DATA

Done

Still to be done

PhD Conceptual Scheme

Sensitivity analysis on input parameters

Compute ERROR budget

Laboratory basin(Lacombe et al., 1981; Robinson & Golnaraghi, 1995)

Processes controlling the global ocean general

circulation in reduced temporal and spatial scales.

…

Large amount of data

Why in the Mediterranean Sea

•Spectral shape R() defines the so called “Ocean Colour”

•OC is indexed by the Blue-to-Green reflectance ratio In case 1 waters is essentially due to phytoplankton chlorophyll content

•B/G decreases with increasing pigment concentration

Rationale for a Bio-optical Algorithm

d

u

E

LR

Ocean colour algorithms relate surface chlorophyll concentration to B/G

(O'Reilly et al., 1998; O'Reilly et al., 2000; Morel and Maritorena, 2001)

GLOBAL

OCRS of the Mediterranean Sea: Three existing Algorithms

REGIONAL algorithms improve the accuracy of

satellite chlorophyll estimates

(Garcia et al., 2005; Gitelson et al., 1996)

REGIONAL

DORMA (D’Ortenzio et al., 2002 )

R is log10 (490/555) reflectance ratios.

4)( 3

32

21010 cChl RcRcRcc

BRIC (Bricaud et al. 2002)

R is 443/555 Reflectance ratios for Chl < 0.4

OC4v4 is used in the other cases

10

cRcChl

OC4v4 (O’Reilly et al. 2000)

R is log10 of the maximum value between:

443/555 490/555 510/555

)( 44

33

221010 RcRcRcRccChl

Mediterranean Ocean Colour CAL-VAL dataset

Bio-optical measurements137 chl/opt measurements used to define the Mediterranean regional algorithm

10 Mediterranean cruises (1998-2003 ):Organized by GOS in the framework of Italian National Projects

In situ chlorophyll-a data 944 chl profiles used to validate satellite chlorophyll products

Fluorescence Calibration

• Calibration performed cruise by cruise

r2 = 0.98

Clearly Log-normal More Gaussian shape

• Log-transformation

Calibrated Fluorescence OWP

“an accurate representation of the pigment concentration measured by a remote sensor

viewing a stratified ocean”(Clark, 1997)

Optical Weighted Pigment Concentration (OWP)

pd

pd

z

Surf

kz

z

Surf

kz

dze

dzezChlOWP

2

2)(

• Chl = in situ chlorophyll concentration

• Zpd = 1/k

• k = attenuation coefficient of downwelling PAR irradiance

%1001

1

NOWP

OWPAlgRPD

N

i i

ii

%1001

1

NOWP

OWPAlgAPD

N

i i

ii

Statistical Parameters for Algorithms’ Evaluation

r2 = coefficient of determination

Alg = [Chl] estimated from different algorithms

covariance between in situ observations and algorithm derived chlorophyll

Error as function of the chlorophyll values

Algorithms’ Evaluation Analysis

Algorithms’ Evaluation

Algorithms’ Evaluation

Need for a NEW Mediterranean Sea OC Algorithm

The New Mediterranean Algorithm: the MedOC4

MedOC4

R is log10 of the maximum value between:

443/555 490/555 510/555

)( 44

33

221010 RaRaRaRaaChl

The good, the ugly, the bad and the MedOC4

Max Ratio choice similar for MED and Global

Regional VS Global Datasets

Mediterranean

Low Chlorophyll: Med Band Ratio < Global Band Ratio

Is the Mediterranean Greener or less Blue than the Global Ocean?

Global

Global

MED

MED

G

B

G

B

Chlorophyll

Max

imu

m B

and

Rat

io

Chlorophyll

Max

imu

m B

and

Rat

io

Global Ocean

Is the Mediterranean Greener and/or less Bluethan the Global Ocean?

Global 30% BLUER than MED

Global Regional

Global Regional

0.01 < Chl < 0.1

MED 15% GREENER than Global

0.1 < CHL < 0.3

Global 12% BLUER than MED

Global Regional

Global Regional

MED 10% GREENER than Global

Is the Mediterranean Greener and/or less Bluethan the Global Ocean?

CHL > 0.3

Global and MED tend to overlap

Global Regional

Global Regional

Is the Mediterranean Greener and/or less Bluethan the Global Ocean?

•DORMA, BRIC and the New MedOC4 into the SeaDAS Code

•A Match-up dataset between concurrent SeaWiFS passes and in situ measurements was built

SeaWiFS data validation

Matchup criteria

Clouds

944 profiles

290 data points

SeaWiFS data validation

•does not show significant changes from the one calculated using in situ data

•MedOC4 is the most stable algorithm performing better than the other in all the chlorophyll ranges

MedOC4 vs OC4v4: Oligotrophy

OC4v4 MedOC4 OC4v4 – MedOC4

0-0.1 0.20.01 50.01 5

2 July 2004

Same dynamical patterns

Significant difference between the two algorithms

Positive difference for lower values

Negative difference for higher values

MedOC4 vs OC4v4: Meso-eutrophy

0-0.80 0.30.01 5 0.01 5

21 April 2004

OC4v4 MedOC4 OC4v4 – MedOC4

Morel’s Primary Production Model

Impact on Primary Productivity

Plots by courtesy of Simone Colella

Impact on Primary Productivity

Plots by courtesy of Simone Colella

Colella’s Primary Production Model

• Mediterranean Sea DIFFERENT bio-optical properties as compared to the global ocean

• Large Errors associated with Global algorithm

• Need of a NEW Regional algorithm

• MedOC4 IMPROVES chlorophyll retrieval

(17 % error vs 110 % OC4v4)

Conclusions

• Estimate ERROR Budget for MedOC4 evaluation and implementation

– Assess the atmospheric correction impact

• Understand WHY MED bio-optical properties are so DIFFERENT as compared to the GLOBAL ocean ones

• Identify most suitable PP model for Mediterranean basin

Future Work