Transcript Document 7606679

An Algorithm for Oceanic Front Detection in Chlorophyll and SST Satellite Imagery
Igor M. Belkin, University of Rhode Island, and John E. O’Reilly, NMFS/NOAA
[email protected] and [email protected]
1. Abstract. An algorithm is described for oceanic front detection in chlorophyll (Chl) and
3. INTELLIGENT MEDIAN FILTER:
REMOVES SPIKES (IMPULSE NOISE)
PRESERVES FEATURES:
STEP-LIKE FRONTS;
CHL ENCHANCEMENT at FRONTS;
CHL PEAKS AND LOCALIZED BLOOMS;
ITERATES UNTIL CONVERGENCE;
DETECTS OSCILLATIONS
sea surface temperature (SST) satellite imagery (Belkin and O’Reilly, 2008). The algorithm is
based on gradient approach. The main novelty is a shape-preserving, scale-sensitive,
contextual median filter applied selectively and iteratively until convergence. This filter has
been developed specifically for Chl since these fields have spatial patterns such as
chlorophyll enhancement at thermohaline fronts and small- and meso-scale chlorophyll
blooms that are not present in SST fields. Linear Chl enhancements and point-wise blooms
are modeled as ridges and peaks respectively, whereas conventional fronts in Chl and SST
fields are modeled as steps or ramps. Examples are presented of the algorithm performance
over a broad range of spatial and temporal scales, using modeled (synthetic) images as well
as Chl and SST imagery. Satellite data from several thermal and color sensors (AVHRR,
SeaWiFS and MODIS/Terra and Aqua) were processed with the new algorithm to generate
climatology of SST and Chl fronts off the U.S. Northeast, encompassing the Mid-Atlantic
Bight, Georges Bank and Gulf of Maine (Belkin et al., 2008).
2. Transversal structure of Chlorophyll and SST fronts
Contextual median filtering of ridges. Thin (1-pixel wide) and wide (3-pixel
wide) spiral ridges are shown before MF (left), after standard MF (center),
and after contextual MF(right). Insets are enlarged in the next figure below.
Median Filtering of the Model Gulf Stream and its Rings
Spread
spikes
before
MF3
Before MF
Spread
spikes
after
MF3
Figure 1. Schematic of the Shelf-Slope Front
(SSF).
Figure 2. Spatially-averaged Chl
concentration as a function of
frontal swath width for the ramp and
peak models (see below).
Swap
spikes
before
MF3
Swap
spikes
after
MF3
After standard MF 3x3
After contextual MF 3x3 in 5x5
Contextual median filtering of ridges. Thin (1-pixel wide) and wide (3-pixel wide)
spiral ridges (top panel) are processed with standard MF (left) and contextual MF
(right). Standard MF removes thin ridge and blunts the crest of wide ridge (bottom
left panel), whereas contextual MF leaves both ridges intact (bottom right panel).
Median filtering of the model Gulf Stream and its rings. The model Gulf Stream’s
edges are frayed with horizontal spread and swap spikes that are smoothed by
standard MF3 (right column). Contextual MF3in5 leaves the Gulf Stream front’s edge
intact. Isolated vertical spikes within rings are removed by both standard MF3 and
contextual MF3in5.
Figure 3. Chl distribution in the NW
Atlantic,
September
2002.
Chl
concentration over the shelf is relatively
uniform and significantly higher than
offshore. This distribution is described
by the ramp model (Figure 4).
Contextual Median Filtering of Spikes and Blobs
Figure 4. Ramp model of Chl
distribution across SSF.
A
B
before MF
after MF
C
Examples of the algorithm performance on synoptic satellite images of SST (top
row; 3 May 2001) and chlorophyll (bottom row; 14 October 2000). Left column,
original images. Right column, gradient magnitude.
Figure 5. Chl distribution in the NW
Atlantic, April 2001. Chl concentration
peaks at SSF. This type of Chl
distribution is described by the peak
model (Figure 6).
Figure 6. Peak model
distribution across SSF.
of
Chl
Contextual median filtering of peaks and spikes. Contextual MF3in5
removes 1-point spikes but leaves intact sharp 3- and 5-point peaks.
REFERENCES
Belkin, I.M. and J.E. O’Reilly (2008). An algorithm for front
detection in chlorophyll and sea surface temperature
satellite imagery. Journal of Marine Systems.
Belkin, I.M., J.E. O’Reilly, K.J.W. Hyde, and T. Ducas (2008).
Satellite climatology of chlorophyll and sea surface
temperature fronts in the Northeast U.S. Large Marine
Ecosystem. In preparation.
ACKNOWLEDGEMENTS
We are grateful to NOAA for funding this project
under the Research to Operations program and
through a contract to the University of Rhode
Island.