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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 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
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).
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).
Figure 4. Ramp model of Chl distribution across SSF.
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 of Chl distribution across SSF.
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
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.
REFERENCESBelkin, 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.
ACKNOWLEDGEMENTSWe are grateful to NOAA for funding this project under the Research to Operations program and through a contract to the University of Rhode Island.
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.
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.
Before MF
After standard MF 3x3 After contextual MF 3x3 in 5x5
before MFafter MF
A B
C
Contextual Median Filtering of Spikes and Blobs
Contextual median filtering of peaks and spikes. Contextual MF3in5 removes 1-point spikes but leaves intact sharp 3- and 5-point peaks.
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).
SpreadspikesbeforeMF3
SpreadspikesafterMF3
SwapspikesbeforeMF3
SwapspikesafterMF3
Median Filtering of the Model Gulf Stream and its Rings
