MODIS/Aqua Validation Analysis

The effect of the proposed MODIS/Aqua reprocessing changes on the comparison with in situ data are presented here. A description of the methods employed for these analyses, with minor modifications, can be found in Volume 10, Chapter 7 of the SeaWiFS Post Launch Technical Memorandum Series.

There are approximately 55 radiance and 200 chlorophyll matchups which pass all exclusion criteria.

The matchup analysis was performed three ways, using:

  • the current operational parameters
  • the proposed reprocessing parameters with out the f/Q correction applied
  • the proposed reprocessing parameters with the f/Q correction applied

Statistics Summary:

Current Operational Processing Proposed Reprocessing (without f/Q) Proposed Reprocessing (with f/Q)
Band N Mean Ratio* % Difference** Bias rms+
nLw_412 54 0.742 26.005 -0.271 0.453
nLw_443 65 0.807 20.418 -0.225 0.388
nLw_488 46 0.986 15.757 -0.058 0.268
nLw_531 15 0.963 13.933 -0.074 0.264
nLw_551 54 0.993 11.128 0.044 0.194
nLw_667 46 0.815 27.702 -0.012 0.039
chl 200 1.268 48.235 0.299 0.763
Current Operational Processing Proposed Reprocessing (without f/Q) Proposed Reprocessing (with f/Q)
N Mean Ratio* % Difference** Bias rms+
55 0.637 37.088 -0.393 0.565
66 0.828 19.427 -0.191 0.375
47 0.934 18.852 -0.111 0.285
14 0.955 13.404 -0.105 0.272
55 0.989 10.599 -0.008 0.185
46 0.689 37.449 -0.018 0.042
197 1.331 48.614 0.345 0.819
Current Operational Processing Proposed Reprocessing (without f/Q) Proposed Reprocessing (with f/Q)
N Mean Ratio* % Difference** Bias rms+
53 0.633 39.311 -0.398 0.568
64 0.804 20.428 -0.230 0.395
45 0.823 21.873 -0.189 0.315
14 0.822 19.017 -0.218 0.321
53 0.874 17.135 -0.123 0.217
45 0.621 41.341 -0.025 0.045
197 1.139 45.339 0.219 0.775

* arithmetic mean of all satellite / in situ ratios
** median absolute percent difference
+ sqrt ( &sum(satellite - in situ)^2 / n )

chlorophyll data were log transformed prior to statistics calculation

The Plots

Discussion

Unfortunately, the quantity of validation data available for MODIS-Aqua is sparse and dominated by regions with challenging aerosol conditions. In fact, the radiance matchups are dominated (30 of the 55) by cruises in the Adriatic Sea. Given these limitations, it is difficult to draw solid conclusions from the available dataset. However, there are some general trends which are worth noting:

  • There is a noticeable negative bias in the validation results with the f/Q correction applied vs. results without the correction
  • The overall median percent difference and rms values are coming close to those achieved by SeaWiFS
  • As was the case with SeaWiFS, the 'green' bands are closer to in situ values than the 'blue' bands. This points to an atmospheric correction problem and not necessarily a sensor calibration issue, although sensor calibration cannot be ruled out.