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

Introduction

Description of Analysis

With 8-day composited SeaWiFS and MODIS data products in an equivalent form, the data sets were further divided into several geographic subsets. Three global subsets were defined, corresponding to clear water, deep water, and coastal water. The deep water subset consists of all bins where water depth is greater than 1000 meters. Clear water was defined as deep water where the retrieved chlorophyll is less than 0.15 mg/m^3. For the chlorophyll test, both SeaWiFS and MODIS retrievals were required to be below the clear-water threshold. Coastal water was defined as all bins where water depth is between 30 and 1000 meters, as defined by a shallow water mask and the deep water mask.

When comparing the clear-water subsetted data, it should be noted that anomalously high chlorophyll retrievals from either sensor can significantly alter the geographic distribution of selected bins. In contrast, the deep-water and coastal subsets are purely geographic in selection criteria. The coastal subset, however, is more likely to contain regions of significant variability in water structure and atmospheric conditions, as well as case 2 water types, all of which can lead to greater retrieval uncertainty and larger differences between the two sensors. The deep-water subset is, therefore, the most stable subset for cross-sensor comparison of retrieved oceanic optical properties. The geographic extent of all three global subsets will vary, however, with the seasonal change in earth illumination and thus sensor imaging duty cycle. Figure 1 presents a sample pair of MODIS and SeaWiFS deep water subsetted chlorophyll images for one 8-day period in May of 2003. The images show the geographic extent of the common-binned, deep-water subset, and they provide some insight into the qualitative agreement between the two sensors.

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, a set of zonal 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 2.

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.

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

Tabulated Results

Discussion of Results

Summary

References

Clark, D. K., M. E. Feinholz, M. A. Yarbrough, B. C. Johnson, S. W. Brown, Y. S. Kim, and R. A. Barnes, Overview of the radiometric calibration of MOBY, Proc. Spie, 4483, 64-76 (2001).

Eplee, R.E., Jr., R.A. Barnes, and F.S. Patt, 2003a: "Changes to the on-orbit calibration of SeaWiFS." In: Patt, F.S., R.A. Barnes, R.E. Eplee, Jr., B.A. Franz, W.D. Robinson, G.C. Feldman, S.W. Bailey, P.J. Werdell, R. Frouin, R.P. Stumpf, R.A. Arnone, R.W. Gould, Jr., P.M. Martinolich, and V. Ransibrahmanakul, Algorithm Updates for the Fourth SeaWiFS Data Reprocessing, NASA Tech. Memo. 2003--206892, Vol. 22, S.B. Hooker and E.R. Firestone, Eds., NASA Goddard Space Flight Center, Greenbelt, Maryland, (in press).

Eplee, R.E., Jr., R.A. Barnes, S.W. Bailey, and P.J. Werdell, 2003b: "Changes to the vicarious calibration of SeaWiFS." In: Patt, F.S., R.A. Barnes, R.E. Eplee, Jr., B.A. Franz, W.D. Robinson, G.C. Feldman, S.W. Bailey, P.J. Werdell, R. Frouin, R.P. Stumpf, R.A. Arnone, R.W. Gould, Jr., P.M. Martinolich, and V. Ransibrahmanakul, Algorithm Updates for the Fourth SeaWiFS Data Reprocessing, NASA Tech. Memo. 2003--206892, Vol. 22, S.B. Hooker and E.R. Firestone, Eds., NASA Goddard Space Flight Center, Greenbelt, Maryland, (in press).

O'Reilly, J., S. Maritorena, M. O'Brien, D. Siegel, D. Toole, D. Menzies, R. Smith, J. Mueller, B. Mitchell, M. Kahru, F. CHavez, P. Strutton, G. Cota, S. Hooker, C. McClain, K. Carder, F. Muller-Karger, L. Harding, A. Magnuson, D. Phinney, G. Moore, J. Aiken, K. Arrigo, R. Letelier and M. Culver (2000). SeaWiFS postlaunch technical report series, Volume 11, SeaWiFS postlaunch calibration and validation analyses, Part 3, NASA Technical Memorandum.