Management Unit of the North Sea Mathematical Models
MUMM | BMM | UGMM
Vertical variability of suspended matter
and its influence on remote sensing reflectance
Griet Neukermans1,2, Hubert Loisel2, Xavier Mériaux2 and Kevin Ruddick1
1Management
2Université
Unit of the North Sea Mathematical Models (MUMM), Royal Belgian Institute for Natural Sciences (RBINS)
du Littoral Côte d’Opale (ULCO), Laboratoire d’Océanologie et Géoscience (LOG), Maison de la Recherche en Environnement Naturel (MREN)
Retrieval of suspended matter (TSM) concentration from space is done using (semi) empirical algorithms between the remote sensing
reflectance (Rrs) and TSM concentration. Until present, it is unknown to which degree these algorithms are affected by the TSM vertical
variability, although in situ observations indicate that vertical variation is considerable. This study addresses this issue for the first time.
Basic concepts
LIGHT AND SEA WATER
OPTICAL QUANTITIES AND WHAT THEY ARE USED FOR
Rrs ∝
bb
a
Back scattered light
bb :back scattering coefficient
When light interacts with sea
water it can either be:
•Scattered in the forward or
backward directions
(b=bb+bf)
•Absorbed (a) or left
•Unattenuated
bbp:bp
Particulate backscattering ratio
index of refraction (i.e. composition), and particle size distribution
a
Absorption coefficient
concentration of absorbing constituents such as phytoplankton,
dissolved organic matter, and mineral particles
Remote sensing reflectance
concentration and types of various constituents present in the water,
proportional to bb:a
for phytoplankton-dominated waters: about 0.5%, for suspensions of inorganic sediments: a few %
Absorbed light (a)
sea
water molecules
phytoplankton
dissolved organic matter
inorganic particles
bbp
Particulate scattering and backscattering coefficients
1st order dependence: particle concentration (TSM)
2nd order dependence: particle size, index of refraction, shape, and
structure
bp
air
Remote sensing
ing
reflectance
(Rrs)
When a photon of light propagates in water and interacts with
a marine particle it can either be
absorbed, be redirected
(scattered), or continue to propagate in the same direction
(unattenuated). These processes
are schematically shown in
Figure 1.
Forward scattered light
bf: forward scattering coefficient
Rrs
Unattenuated light
Figure 1. Interactions between sunlight and seawater
Objectives
Quantify the vertical variability of bp, bbp and bbp:bp . Are surface
and water column means of these parameters correlated?
Determine the effect of vertical variability of bbp and bbp:bp on the
remote sensing reflectance.
Methodology
IN SITU VERTICAL VARIABILITY OF bp, bbp AND bbp:bp
Rrs FROM RADIATIVE TRANSFER SIMULATIONS
The vertical variability of bp, bbp and bbp:bp over the water column was studied using in situ
measurements performed during 4 sea campaigns in 2004 covering Belgian, UK and
French coastal waters. Over three hundred vertical profiles of bp and bbp were recorded (2
examples are shown in Figure 2a-b). A correlation and regression analysis was performed
between surface (0-4m depth) mean and water column mean values of bp, bbp and bbp:bp.
Rrs (at wavelengths 412-715nm) resulting from a variable vertical profile of bbp
were compared with the Rrs resulting from a uniform vertical distribution with a
bbp value equal to the mean surface bbp value of the variable profile. Rrs spectra
were obtained from radiative transfer simulations with Hydrolight software. We
assumed vertically constant a and b values and discriminated between:
1. strongly scattering (such as the sediment dominated Belgian coastal waters)
and
2. strongly absorbing waters (e.g. in the case of algal blooms).
bbp:bp(650nm)
0.00
0.05
0.10
0.15
0.20
0.00
0
0.10
0.20
0
0.30
0
2
5
(a)
6
8
10
bbp
(b)
10
15
bbp
20
bp/20
b/20
12
bbp:bp
14
Depth (m)
Depth (m)
Depth (m)
4
bbp:bp
25
0.02
0
2
4
6
8
10
12
14
16
18
20
0.04
0.06
(c)
bbp profiles were generated with a bbp maximum at the surface (0m) and the
bottom (20m), with different peak widths and magnitudes, based on in situ observations. Maximum to minimum bbp ratios (and thus also bbp:bp maximum to minimum ratios) ranged from 2 to 8 (see Figure 2c).
max:min=2
max:min=4
max:min=6
max:min=8
Figure 2. Vertical profiles of bp, bbp and bbp:bp recorded on June 19th 2004 (a) and March 16th 2004 (b) at different
stations in French coastal waters, a few kilometers North of Le Havre. (c) Example of simulated bbp:bp profiles for sediment dominated waters, for bbp:bp maximum to minimum ratios of 2, 4, 6 and 8.
Results
IN SITU VERTICAL VARIABILITY OF bp, bbp AND bbp:bp
Rrs FROM RADIATIVE TRANSFER SIMULATIONS
Correlation analysis showed significant (p<0.05) correlations between surface mean and
water column mean bp, bbp and bbp:bp values with correlation coefficients of 0.96, 0.97
and 0.99, respectively. Linear regressions between water column and surface means for
bp, bbp and bbp:bp show that dispersion along the regression line is considerable (see
Figure 3a-b).
We observed relative differences between Rrs resulting from a variable vertical profile of bbp and Rrsresulting from a uniform vertical distribution with bbp equal to the
mean surface (0-4m) bbp value of the variable profile as high as 35% in both
strongly scattering and strongly absorbing waters. Examples of Rrs spectra are given
in Figure 4(a-b).
0.06
0.04
0.02
0
0
-0.02
0.02
0.04
0.06
0.08
0.1
bbp(650nm) mean surface value (m-1)
0.12
variable bbp profile
0.01
0.02
0.03
Depth (m)
0.0015
0
0.04
15
0.03
25
10
15
20
25
0.01
(a)
0.14
0.01
0.02
0.03
0.04
0.05
400
500
600
700
800
Wavelength (nm)
bbp:bp(650nm) mean surface value
Figure 3. Linear regression of the surface mean vs water column mean bbp and bbp:bp obtained from 350 vertical profiles recorded between
March and July 2004 in Belgian, UK and French coastal waters. Standard deviations from the means are shown as vertical and horizontal
error bars.
(b)
0
0.0000
0
-0.01
0.15
0.02
0.0005
0
0.1
5
10
20
0.05
0
uniform bbp profile
0.0010
0.01
bbp(650nm) (m-1)
variable bbp profile
0.04
5
0.03
0.02
0.05
bbp(650nm) (m-1)
0
0
uniform bbp profile
0.0020
Depth (m)
0.08
0.0025
Rrs
0.1
bbp:bp(wc) = 0.981 bbp:bp(surf) + 0.002
RMSE= 0.993
N=350
r=0.99
0.04
Rrs
0.05
bbp(wc) = 0.989 bbp(surf) + 0.002
RMSE= 0.947
r=0.966
N=350
0.12
bbp:bp(650nm) water column mean value
bbp(650nm) mean water column value (m-1)
0.14
400
500
600
700
800
Wavelength (nm)
Figure 4. Rrs obtained from radiative transfer simulations for a variable bbp profile and a uniform bbp profile with the same surface
mean as the variable profile (shown in insets) for (a) strongly absorbing and (b) strongly scattering water.
Vertical variability of bbp can drastically affect Rrs. This directly affects
the TSM concentration retrieval from space, as the relative error on
the estimation of TSM has about the same magnitude than the one on
Rrs for clear to moderately turbid waters (TSM concentrations between
0 and 13mg/l)
Despite the great vertical variability of bp, bbp and
bbp:bp the water column mean values of these parameters can be estimated from their surface values.
Conclusions
This research was supported by the BELCOLOUR-2 project, funded by the STEREO programme of the Belgian Science Policy Office under contract SR/00/104.
This poster was presented at the Young Researcher’s Day of Flanders Marine Institute (VLIZ), Brugge (Belgium), 6th March 2009.