Time Series Measurements and Algorithm Development at the FRONT Site on
the New England Continental Shelf.
Funded by NASA SIMBIOS
contract # 99011.
Principal investigators: J. Ru Morrison† Heidi M. Sosik†
Collaborators: Daniel L. Codiga§ and Scott
† Woods Hole Oceanographic Institution Woods Hole, Woods Hole, MA
§ Dept. of Marine Sciences, University of Connecticut, 1084
Shennecosett Rd., Groton, CT 06340
The optical properties of case II coastal ocean are influenced by a complex
mixture of seawater constituents and a wide variety of physical processes.
Especially in regions where formation of physical and optical fronts is
frequent, but temporally and spatially variable, this complexity makes
interpretation of ocean color signals subject to large uncertainty.
The goal of the proposed research is to determine which processes and optically
important constituents must be considered to explain ocean color variations
associated with coastal fronts on the New England continental shelf.
To accomplish this goal we propose to carry out extensive time series sampling
and to perform algorithm development and evaluation in collaboration with
the NOPP-supported FRONT program. FRONT is a three-year multi-disciplinary
effort initiated in late 1999 aimed at understanding and modeling physical
and biological processes associated with frontal formation and persistence
at a study site located at the mouth of Long Island Sound. The observational
network includes an array of acoustically linked physical moorings and
the Autonomous Vertically Profiling Plankton Observatory (AVPPO, a profiling
mooring), which we have outfited with a suite of optical sensors.
Our efforts will focus on collection of a complete set of optical property
observations and constituent analysis necessary for evaluating bio-optical
algorithms for retrieving inherent optical properties and phytoplankton
characteristics. These include band-ratio algorithms, semi-analytical
radiance inversion algorithms and algorithms which incorporate chlorophyll
Front Resolving Network with Telemetry (FRONT).
Located at mouth of Long Island Sound (10 – 65 m).
Analysis of 12 years of AVHRR images suggested area of intense frontal
Strongly affected by estuarine outflows and tidal currents.
FRONT is a network of acoustic modem nodes linking bottom
Autonomous Vertically Profiling Plankton Observatory (AVPPO)
at center of the network
Series of physical (CTD), biological (VPR and fluorometer) and optical
sensors (ac-9 and OC-200’s)
Surface telemetry buoy with mobile phone link allows shore control and
near realtime data acquisition.
Figure 1: FRONT location and structure.
A) SeaWiFS chlorophyll concentration (blue low,
orange high, and clouds gray) from 6 October 1997. The black lines
show the position of fronts using the URI edge detection algorithm.
B) Regions of frontal probability greater than 5
% are shown for January in blue and July in violet. Colored dots
and rings show the location and range of telemetering antenna for the FRONT
C) The FRONT array showing locations of profiling
moorings, ADCPs, and repeater nodes. The distance between nodes is
nominally 5 km.
||Downwelling vector irradiance (413, 443, 490, 511, 555, 666, 683 nm)
||Incident irradiance (413, 443, 490, 511, 555, 666, 683 nm)
||Upwelling radiance (413, 443, 490, 511, 555, 666, 683 nm)
||Absorption (412, 440, 488, 510, 532, 555, 650, 676, 715 nm) from WETLabs
||Beam attenuation (412, 440, 488, 510, 532, 555, 650, 676, 715 nm) from
||Absorption (442, 555) from HOBILabs a-?eta
||Backscattering (442, 555) from HOBILabs a-?eta
||Chlorophyll fluorescence with a Wetstar fluorometer
||Beam attenuation coefficient at 660 nm with SeaTech transmissometer
||Temperature and conductivity with SeaBird probes
Figure 2: Mooring schematic and photos.
Hydrographic data and Inherent Optical Properties
Figure 3: From the initial four day AVPPO deployment, as part
of the NOPP project, hydrographic data from the mooring showed the regular
occurrence of colder, less saline water at semi-diurnal timescales, Panels
A and B. This was also apparent in the bio-optical data, from both
the fluorometer, Panel C, and the ac-9 on the AVPPO, Panels D-F, as well
as a diurnal signal. Decomposition of the spectral absorption measured
by the ac-9, achieved by assigning characteristic shapes to algal and non-algal
fractions and minimizing the SOSD between predicted and observed spectra,
suggested that the majority of the variability in the optical signal was
due to changes in the non-algal component, Panels G and H (absorption values
are at 440 nm). The non-algal slope, S, was also retrieved using
the decomposition and was within the range of previously observations (mean=0.0120
nm-1, std=2.5×10-4, N=1274).
Figure 4: Current vectors, measured using an ADCP mounted on the
AVPPO winch sled, superimposed upon interpolated temperature data indicated
that the colder less saline water was associated with a southeasterly flow.
Both a semi-diurnal and diurnal component can be seen in the current measurements
shown by the greater magnitudes of the southerly flows around the middle
of the days in the data shown. Fourier transformation of the sea
surface elevation showed the presence of M2, S2, O1, and K1 tidal harmonics.
Remote Sensing Reflectance
Figure 5: The remote sensing reflectance from the AVPPO, obtained
from measurements using OC-200 radiometer heads, showed two distinct shapes
most clearly seen by the ratio of 550 to 510 nm, Panel A. On all
days, higher ratios were associated with colder less saline waters and
lower ratios with warmer more saline waters, Panel B. Most of the
variability in the ratio appeared to be associated with variations in the
the non-algal component from decomposition of spectral absorption measurements,
Panel C. There appeared to be no relationship with algal absorption
estimates from the decomposition, Panel D.
Continued verification of AVPPO bio-optical sensor
Mooring deployment in Massachusetts Bay (September
Mooring deployment at FRONT site (October, 2001- June,
Algorithm development using both AVPPO collected and
SeaBASS archived data.
Acknowledgments: This work was supported by NOPP, the SIMBIOS project
(Morrison and Sosik), and NSF, Biological Instrumentation (Gallager).
Anne Canaday assisted with data collection and instrument calibration.