Hello,

We are using 3-level satellite products (chlorophyll concentration, Cromophoric Dissolved Organic Matter index and two SST products) from the Aqua MODIS sensor available at http://oceancolor.gsfc.nasa.gov/cgi/l3. We use these data to improve our understanding of the processes causing the natural emission of halogenated molecules (mainly organic molecules, i.e. halocarbons) from marine sources. These species undergo oxidation in the atmosphere and are relevant to different aspects of the atmospheric chemistry. For the moment we have worked with all the daily and 3-day composites of these datasets at 9 km from Sept 2010 to May 2011. 

I am just a user and therefore do not know anything about the technical aspects of the production of these data. I have two questions which might be very trivial to you:

(1) We found a downward trend in the Cromophoric Dissolved Organic Matter (CDOM) index in Aqua MODIS (this is clear e.g. in the thumbnails of the annual composites). Because of that we tried to work with the CDOM index from Terra MODIS. However the values in the Terra product look much higher than in the AQUA one. As an example see:
http://oceancolor.gsfc.nasa.gov/cgi/l3/A20100012010365.L3m_YR_CDOM_cdom_index_9km.png?sub=img
http://oceancolor.gsfc.nasa.gov/cgi/l3/T20100012010365.L3m_YR_CDOM_cdom_index_9km.png?sub=img

In addition, I see there is lot of noise (or at least lot of spatial variability) in the Terra product.

Could somebody tell me why are these products so different? Should I trust on of them more than the other one?

(2) I read that MODIS Aqua products will be reprocessed to correct for temporal drift in the calibration during the latter period of the mission. I had a look at the info at http://oceancolor.gsfc.nasa.gov/WIKI/OCReproc20100MA.html and in a pdf attached there, but I find it difficult to understand all the technical details... So I go to the point: The calculations for which I use chlorophyll / CDOM / SST data from Aqua MODIS take quite long so I would only like to repeat them if the differences between the old and the new dataset were relevant. Could somebody tell me how much (in percentage if possible) these versions of such products might differ? Would you recommend me to download the new data files when they are available and repeat all my analyses?

In case this info is relevant: I have worked with these data for different geographical regions and periods, but at this moment I am mainly interested in an area around the tropical east Pacific (around 10 N - 20 S, 100-70 W) for the period Sept 2010 - June 2011.

Any help will be very appreciated. Thanks!

Carlos Ordóñez
Laboratory for Atmospheric and Climate Science (CIAC)
CSIC - JCCM
Toledo, Spain

Carlos,

the largest change of the MODIS Aqua R2010.0 reprocessing is in band 8 (412nm) in 2011. CDOM is calculated using the water-leaving radiances from that wavelength. Since you are using CDOM from 2011,  it is very likely you should redo your calculations.

SST data did not change. The chlorophyll changes are documented on the webpage you mentioned (e.g. 50% at the beginning of 2011 in eutrophic regions, about 30% in oligotrophic regions).

The reprocessed MODIS Aqua CDOM data should agree much better with the MODIS Terra CDOM data. MODIS Terra radiometric sensitivity has degraded severely in the blue bands, which should explain the increased noise in the MODIS Terra CDOM product.

Gerhard

Gerhard,

Thanks a lot for your helpful reply. Following your recommendations we will redo our analyses for CDOM and chlorophyll from MODIS Aqua.

Cheers,
Carlos