A Comparative Time-Series Analysis of Ocean Color Products from MODIS/Aqua and SeaWiFS

Abstract

Introduction

Data Sources

Subset Definitions

SeaWiFS

MODIS

Log10(Chla), 0.01 - 1.0 mg/m^3

Figure 1: Sample Chlorophyll Images, Deep-Water Subset
Days 137-144 of 2003

In addition to the global subsets, six basin-scale subsets were analyzed. These included regions in the northern Pacific (PacN), north western Pacific (PacNW), south eastern Pacific (PacSE), northern Atlantic (AtlN), southern Atlantic (AtlS), and the southern Indian Ocean (IndS). To these, a smaller region near Hawaii was also added. All of these subset regions were adopted from Fougnie et al. 2002. Their locations are listed in Table 2. Based on the results of the regional analysis, yet another group of subsets was defined to provide a systematic means for investigating latitudinally-dependent differences between the two sensors. A longitudinal segment of the Pacific from 170W to 150W was divided into 10-deg latitude zones. These zonal subsets are summarized in Table 3.

Trending Analysis

For each sensor, for each 8-day product, the filled bins associated with a particular subset were identified and used to compute the mean, standard deviation, and average observation time. Figure 2 shows an example of a typical trend plot derived from this analysis. For the plot on the left, the common MODIS and SeaWiFS bins for the deep-water subset were spatially averaged for each 8-day-binned water-leaving radiance product, and the resulting means were then plotted as a function of time. MODIS is shown as dashed lines. The colors indicate different bands, as summarized in Table 1. The plot on the right shows the same data as a ratio, with MODIS means normalized by SeaWiFS means. Similarly, Figure 3 shows the temporal trends in mean chlorophyll, where the SeaWiFS chlorophylls correspond to the standard chlor_a product (OC4v4 algorithm) and the MODIS chlorophyll corresponds with the standard chlor_a_2 product (OC3M algorithm). The solid vertical lines in the temporal trend plots are provided as a reference to indicate the transitions between MODIS Oceans calibration epochs. These epochs are the independent periods over which MODIS calibration corrections were derived and implemented by the MODIS Oceans group at the University of Miami (RSMAS). In most cases, these periods correspond with the calibration epochs used by the MODIS Calibration Support Team (MCST) for the adjustment of the Level-1B radiances, and they usually correspond with spacecraft safe-hold events or significant instrument state changes.

Figure 2: SeaWiFS and MODIS Water-Leaving Radiance Comparison, Deep-Water Subset

Figure 3: SeaWiFS and MODIS Chlorophyll Comparison, Deep-Water Subset

Links to additional trend plots are provided below, along with tabulated data and global images. The subset images show mapped chlorophyll for each 8-day period, for the common bins associated with each geographic subset. These images allow for a qualitative assessment of the agreement between the two sensors, and indicate the spatial extent of the subsetted, common bins. Full product images of the chlorophyll and radiance data are also provided, allowing comparison between the two sensors prior to subsetting or reduction to common bins. Finally, tabulated results of the mean and standard deviation for each product, for each 8-day subset are provided. The tabulated means are the values plotted in the trend plots.

Time-Series Trend Plots

Subset Images

Full Product Images

Tabulated Results

Discussion of Results

Summary

References

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