Selected Regional Comparisons with SeaWiFS

by Gene Eplee - SAIC


As part of the validation of the SIMBIOS OCTS reprocessing, a comparison of normalized water-leaving radiances (Lwn) and chlorphyll has been conducted for the northern and southern Pacific Ocean using the OCTS data and the current SeaWiFS operational (reprocessing #3) data. The regions involved in this analysis are 15N-30N, 130E-140W (northern Pacific) and 15S-30S, 180-90W (southern Pacific). For the analysis, mean values of Lwn in bands 1-5 and mean values of chorophyll over these regions were computed for each OCTS and SeaWiFS 8-day timebin file. For OCTS, 30 files were analyzed over a timerange of 1 November 1996 to 3 July 1997. For SeaWiFS, 31 files were analyzed each for four similar timeranges starting in November 1997, 1998, 1999, and 2000.

Northern Pacific Radiance Comparison

Figure 1 shows the four SeaWiFS Lwn time series. The plot shows that the interannual variation in the Lwn are small. Seasonal variations are observed, particularly in bands 1 and 2, but these variation repeat from year to year. The stability of the SeaWiFS temporal correction over the course of its mission is apparent in the time series.

Figure 2 shows the OCTS Lwn time series. The gross shape of the time series agree with those from SeaWiFS. Differences in the magnitudes of the curves are due to offsets in the relative calibration of the two instruments and to differences in the bands centers between SeaWiFS and OCTS.

Figure 3 shows the ratio of the OCTS measurements to the mean of the SeaWiFS measurements for the corresponding timebin files. Since the SeaWiFS data should remove any seasonal variations, the trends in these plots should be indicative of any changes in the OCTS calibration over the mission.

Southern Pacific Radiance Comparison

Figure 4, Figure 5 and Figure 6 show a similar set of comparisons between SeaWiFS and OCTS for the southern Pacific. Comparison of figs. 3 and 6 suggests that there is a decrease in the radiometric response of OCTS over the course of its mission.

Chlorophyll Comparison

Figure 7 shows the four SeaWiFS chlorophyll time series for both the northern and southern Pacific Ocean. As for the Lwn, the interannual variation in the chlorophyll concentration is small, and seasonal variations are observed.

Figure 8 shows the OCTS chlorophyll time series. While the general shape of the time series agree with those from SeaWiFS, there is a significant difference in the chlorophyll concentrations between OCTS and SeaWiFs.

Figure 9 shows the ratio of the OCTS chlorophyll to the SeaWiFS chlorophyll. The time series are approximately constant in time, though for both regions the OCTS chlorophyll is less than the SeaWiFS chlorophyll.