Bio-optical Measurements, Algorithms &
Analyses
Summarized by W.M. Balch
Inherent Optical Properties (Zaneveld)
For the measurement of absorption, this group
recommends that filter-pad measurements be
made in parallel with ac-9 measurements in
order to verify spectral shape, and also
to understand the relative importance of
particulate and dissolved absorption. Calibration
of ac-9's should be according to Wetlabs
protocols. Absorption values of pure water
should be according to Pope and Frye (1998).
AC-9 users should be careful to check the
stability of interference filters annually.
Finally, when reporting absorption values,
one should be sure to designate whether values
are for total, particulate, or dissolved
fractions. Many of the above comments apply
to attenuation. In situ backscattering measurements
can now be made with the commercially-available
Hydroscat instrument. Regular calibration
of the instrument in the field and factory
are critical to the proper interpretation
of the results. Moreover, "sanity checks"
of the data can be done if reflectance and
absorption data are available, using the
reflectance model of Gordon et al (1983;
Barnard and Zaneveld, pers comm.). Other
issues that Hydroscat users should be aware
of are: 1) data can be affected by instrument
orientation (horizontal vs. vertical mounting),
especially when the instrument is close to
the sea floor, and 2) bubbles can affect
any comparison between in situ Hydroscat
measurements and shipboard scattering measurements.
Remote Sensing Reflectance (Mueller)
A meeting was convened at Old Dominion University
earlier this year for compiling data on RRS+
and RRS-, with the goal of comparing such
data and defining protocols. Data are still
being collected but is clear that reliable
reflectance measurements in the red are rare.
As with the above discussion on backscattering,
microbubbles near the surface can severely
impact surface reflectance measurements.
In order to minimize the variance of F/Q,
summer match-ups with the sun at peak elevation
are ideal. Closure in reflectance measurements
can be checked using the reflectance relationship
of Gordon et al. (1988) 2 R/Q= S li (bb/(a+bb))i
i=1 where Q is the variable distribution
function (often set equal to p), and l1 =
0.0949 and l2 = 0.0794. The impact of wind
speed on surface reflectance will be addressed
by Jim Mueller using tank experiments. A
meeting will be held at the Santa Fe ASLO
meeting in February, 1999 to discuss the
status of the reflectance data set.
Particulate Absorption (Mitchell)
A "round robin" experiment was
conducted at Scripps Institute of Oceanography.
The goal of this workshop was compare the
estimates of b, the path length elongation
factor. It was observed that estimates were
good to 15-20% for cells>1 µm diameter.
Interestingly, Prochlorococcus had b values
significantly different from other algal
species used in this workshop. This is likely
due to its small size. Vicarious checks are
recommended for resulting cholorophyll-specific
absorption by comparing the 683nm specific
absorption with maximum a*683 for chlorophyll
a & c containing cells (max a*683 ~ 0.025
m2 mg chl-1) or for chlorophyll b containing
cells ( max a*683 ~0.03 m2 mg chl -1) Another
"sanity check" is to use a and
k values to estimate the mean cosine (mu
bar = a/k). It was pointed out that the protocols
need to address b . Comparisons of ac-9 vs.
filter pad absorption should be done in eutrophic
waters where the ac-9 is not limited by path
length. Absorption of colored dissolved material
requires a preliminary filtration process
to remove particulate material. Glass fiber
filters can contain significant quantities
of CDOM, and must be either pre-combusted
at 5000 C (as suggested by Bricaud) or treated
with 10% HCl. In either case, pure water
should be run through a filter prior to sample
filtration. Just as glass fiber filters can
leach CDOM, they can also absorb it. This
issue makes use of GFF filters for dissolved
absorption measurements questionable, and
it is preferable to use pre-rinsed polycarbonate
filters.
Pigments (Clark)
There was considerable discussion on HPLC
vs. fluorometric chlorophyll a determinations.
Some of the important issues are that ratios
of HPLC chl a: fluorometric chlorophyll can
range from to 0.6. Another important issue
is that HPLC pigments are difficult to do
consistently due to the considerable expertise
required. Some other questions raised were
1) what combination of HPLC derived pigments
are needed to compare to fluorometrically-derived
chlorophyll? 2) what protocols must be used
to process HPLC measurements? Several recommendations
were made concerning the measurement of pigments.
1) If the SIMBIOS team deems that HPLC pigments
are a priority, then funds need to be made
available for a laboratory to run such samples
for all SIMBIOS investigators. The sample
burden for such a laboratory could be simply
surface samples from clear; SeaWiFS overpasses
rather than entire water column profiles
from all stations. 2) A second recommendation
is to write out protocols for processing
HPLC pigments. International JGOFS protocols
exist for flourometric chlorophyll measurements
and are sufficient for SIMBIOS. 3) Water
soluble pigments should be included in the
measurement of accessory pigments. 4)Fluorometric
chlorophylls are mandatory for all SIMBIOS
measurements while HPLC measurements should
be done when possible. 5) Investigators need
to be wary of limitations of both techniques.
In situ match-ups (Schieber)
Discussion centered around how to improve
match-ups. It was suggested that viewing
geometry be also taken in consideration in
comparing satellite and ship derived Lw's.
Moreover, recording all pertinent information
for each of 9 pixels centered over the ship
station is essential, in order to adequately
describe the statistics. It was also pointed
out that there are strong non-linearities
involved in taking regional averages, which
bias the statistics to underestimate chlorophyll
a. Lastly, in order to adequately describe
match-up statistics, it is essential that
all data are available to all SIMBIOS investigators.
Minimizing error due to residual atmospheric
noise (Frouin)
It was pointed out that significant errors
in chlorophyll prediction can result from
using algorithms based on R443/R550 or R490/R550.
The NDPI algorithm ([R443-R555]/R490) has
less error from residual atmospheric noise,
especially at low chlorophyll concentrations.
Atmospheric Measurements, Algorithms &
Analyses
Summarized by John N. Porter
Many suggestions and lively discussions took
place during this session. In general the
discussion/suggestions can be grouped into
how the atmospheric validation measurements
could be improved and secondly on how the
atmospheric correction could be improved
in the SeaWifs algorithm. Although many suggestions
were made, it was realized that the data
sets collected over the past year are just
now being processed. This means we do not
yet have an overall assessment of the SeaWifs
atmospheric correction. In particular we
need to better understand under what conditions
the SeaWifs approach might works well and
under what conditions it works poorly.
Suggestions for better validation measurements
Several comparisons between satellite and
ground based optical depth (865nm) measurements
were shown during the SIMBIOS meeting. It
appeared that for certain sites the agreement
was better than at other sites. During the
meeting, it was suggested that the best validation
measurements appeared to come from ship observations
and that this might be due to land adjacency
effects in the satellite image. At this point
this hypothesis has not been tested. It would
be good to look into this further to see
if land adjacency is creating a problem in
the validation effort. . The Cimel at the
Dry Tortugas for example showed to poorest
comparisons with the SeaWifs optical depths.
It was suggested that the Dry Tortugas ground
system be compared by sending a MicroTops
handheld sun photometer to the site for simultaneous
measurements. This would eliminate questions
about the quality of the measurement at this
important site. Another Cimel coastal site
was Bahrain, which had fairly good agreement
between SeaWifs and ground based optical
depths (875nm). This good agreement is rather
surprising as dust is likely to be poorly
modeled by the SeaWifs aerosol models. The
Hawaii HOT site had generally good agreement
but certain days were significantly off for
no apparent reason. Overall, it was decided
that this error in these comparisons could
be due several possibilities including incorrect
aerosol models, poor quality surface validation
measurements, improper correction of the
foam in the satellite algorithm, land adjacency
effects, and satellite calibration problems.
The aerosol model problem is an obvious one.
In order to see under which aerosol types
the SeaWifs algorithm might fail, it was
suggested that surface measurements should
provide additional information which could
detect the presence of dust (non-spherical
particles) or highly absorbing aerosol (anthropogenic
soot). M. Wang suggested polarization measurement
capability be added to some of the Cimel
instruments. The retrieval approach could
follow a paper from Howard Gordon et al..
Unfortunately some questions are still present
about the durability of polarization filters
(B. Holben). It was also suggested that sky
radiances from the Cimel network could be
used to look for variations in sky radiances.
Currently the sky radiances from the Cimel
instruments are not being used. In the future,
some effort should be made to see if the
presence of non-spherical particles can be
detected in sky radiances. The data sets
for this approach are already available through
the Cimel network. An agreed upon standard
algorithm would be needed.
Aerosol absorption is another area which
is poorly derived from surface or satellite
measurements. It was suggested that aerosol
properties derived from the TOMS satellite
could be combined with SeaWifs satellite
images to derive information on the aerosol
absorption. Irregardless of how well the
aerosol single scatter albedo can be derived,
the TOMS data would allow for the detection
of regions with significant absorbing aerosol
which is more than we now know. It was also
suggested that making ground based optical
depth measurements at high air mass would
allow for a TOMS type analysis to derive
the aerosol absorption optical depth. Both
the TOMS and surface approach are new and
will require further effort.
Several members also suggested that air mass
trajectories could be used to identify which
type of aerosol were present. This might
help in choosing appropriate aerosol models.
While this approach offers promise, it was
felt that this approach was more suitable
for case studies at this point.
It was also realized that the best possibility
for validating atmospheric corrections was
through aircraft measurements but that these
required a major investment in time and money.
Therefore SIMBIOS should take advantage of
several upcoming major experiments. These
include the INDOEX (spring 1999) experiment
in the Indian Ocean (Piotr Flatau is a SIMBIOS
funded participant). This experiment will
include a suite of ship and aircraft measurements
which will be ideally suited for ocean color
validation. The ACE-Asia experiment off Asia
(~2000-2001) also offers another opportunity
for ocean color validation. Also several
smaller ongoing experiments in Alaska (K.
Stamnes) and Hawaii (J. Porter) offer the
opportunuty for ocean color validation from
aircraft platforms.
The handheld MicroTops sunphotometers are
currently the main instrument being used
for optical depth measurements on cruises.
Currently no standard protocol exist for
the use and processing of these measurements.
J. Porter and M. Miller have agreed to write
up a report on their experience with these
instruments. Contributions from others who
have had experience with these instruments
are welcome. There was also discussion about
other ship based sun photometer instruments
by M. Miller (ship based shadowband radiometer),
J. Porter (gimballed shadowband radiometer).
These instruments offer autonomous operation
but have not yet matured enough for broad
use. R. Fruin has developed and comercialized
a unique instrument (SIMBAD) which can measure
aerosol optical depths and surface reflectance.
The SIMBIOS instrument pool also has several
other instruments (Prede, Cimel). At some
point a comparison between the various types
of instruments should be carried out and
written up.
Suggestions for SeaWifs algorithm
The suggestions for improvements on the atmospheric
correction focused on the SeaWifs satellite.
R. Fruin suggested the spectral dependence
of white caps should be included in the model.
He argued that his data as well as a body
of more recent data show similar effects.
During M. Wang's talk he mentioned that a
new aerosol model has been included in the
SeaWifs algorithm so that water leaving radiances
were not negative after subtracting aerosol
radiances. J. Porter suggested that the new
aerosol model should be tried which he argues
are more realistic. Several newer aerosol
models are available with larger coarse mode
aerosol than the Shettle and Fenn aerosol
model currently used.
Other Suggestions
- It was suggested that SeaDAS training classes
be continued. Many of the investigators have
not yet taken advantage of this opportunity
- It was suggested that a set of "problem
images" be saved with matchup data sets.
This would allow for different test to see
where the problems might exist. It was suggested
that the TOMS data set be included in these
problem cases when possible.
- It was suggested that funded SIMBIOS investigators
be allowed to run the QC algorithm. This
will allow them to test the model in detail.
There was also some discussion of which products
the users might want. From our meeting, the
ability to determine which aerosol model
was being chosen and the water leaving radiance
at 670nm were desired items.
- K. Stamnes asked for suggestions on the inherent
optical properties of the ocean and atmosphere
which he could use in his modeling studies
(phase function, single scatter albedo, etc.).
He did not receive a clear answer as there
was not agreement on this matter.