how are 8d/monthly data averaged?

[bwwolfe]

I am interested in the Chlorophyll-a Aqua MODIS NPP L3SMI Global 4km data. I was planning on calculating a monthly moving average each day, however comparing that averaging grid cells from the daily composite data does not yield the same result as the monthly composite — See attached comparison plots of my average of the daily data from June 2021 vs the 06-16-2021 monthly composite over the same spatial extent. Could anyone advise how the monthly (and 8day) composites are created?

[OB General Science - guoqingw]

Hi,

It sounds like you may be planning to average the daily mapped files to get 8-day and monthly mapped files. You should use l3 BIN data instead of SMI for that purpose. After binning, you can use the SeaDAS l3mapgen to produce the mapped images. All the standard mean/composite products are calculated from the ISIN grid bin level then mapped using a specific projection (plate carree - also called lat-lon - for our standard mapped image files).

If you are interested in the binning procedure, here are some useful materials and previous questions/answers from the forum that can help.

https://ioccg.org/wp-content/uploads/2015/10/ioccg-report-04.pdf

https://oceancolor.gsfc.nasa.gov/forum/oceancolor/topic_show.pl?pid=14958;hl=binning

viewtopic.php?t=1037

Guoqing