OK, I hope this will answer your questions enough:
Here are some answers for you. Our format descriptions for the files are not
online, but they may be TMI anyway.
1. The basics of the ancillary files
A. Meteorological data. These are the files with meteorological global
analyzed data in them selected from the National Weather Service's
National Center for Environmental Prediction, hence the NCEP in the
name. The same data is in the NCEP and NCEPN files but our naming
convention changed. Each file contains data taken at one of the 4
times of day: 00, 06, 12 and 18 GMT. Grids of the data are at 1
degree resolution. Parameters and their use are:
zwind Water roughness prediction used for glint estimation
precipitable water currently unused
pressure compute the atmospheric depth for Rayleigh radiance
removal from total radiances
pressure aids in computing the contribution of Rayleigh
radiance to the top of atmosphere radiances
humidity used to select the proper aerosol correction table
B. Ozone data. The ozone concentration is used to remove the effects
of ozone absorption from the total radiances. Our source for the ozone
is from ozone measuring satellites. Our main sources have been TOMS
aboard the Earth probe (the EPTOMS files), OMI (the OMI files), and the
TOAST (a combination of TOVS and SBUV instrument data), hence the TOAST,
EPTOMS and TOMSOMI in those file names. The data from these is a global
grid at 1 degree resolution generally, once per day.
C. Sea Ice data. The sea ice is information about the ice concentration
in the oceans. The file from the National Snow and Ice Data Center
(hence the SEAICE_NSIDC in the file name) contains ice concentration.
If the concentration exceeds an input threshold, the SEAICE level-2 flag,
SEAICE is set for that pixel.
D. Sea Surface Temperature data. The sea surface temperature file
contains global estimates of the sea surface temperature. It is used
as a first-guess in determining SST for the MODIS SST products.
These files are chosen, then interpolated to match the time of the
SeaWiFS or MODIS data it is applied to. If they are not provided, a
climatology file is used as the default. Correction of the radiances
for NO2 can also be done, but only using a climatology file at this time.
2. The ancillary files that come when ordering a level-1 file should be the
best files to use when processing that file. They are as close in time
3. 2 ozone files. We prefer to use the TOMS-derived ozone data if possible
as the TOAST data was derived with more assumptions.
4. If there is not ancillary data for the day of your data, that ancillary
file name input to l2gen should be left blank. Processing defaults to the
For some online information on the ancillary data see:http://oceancolor.gsfc.nasa.gov/DOCS/ocformats.html#8
for some discussion about ancillary and climatology data and:http://oceancolor.gsfc.nasa.gov/DOCS/MSL12/MSl12_prod.html
for a description of some of the ancillary parameters.