Hi,

I ordered SeaWiFS L1A products for an area in Pacific and for the global archive (1997 - 2010). When I downloaded them, the system proposed me to simultanously downloaded ancillary files too.

1) Where can I find documentation on the ancillary files depending on file suffix ?
I have download the following type of files

N2008_32300_MET_NCEPN_6h.hdf.bz2
S201034418_NCEP.MET
N201034400_O3_TOMOSOMI_24h.hdf
N201034400_SEAICE_NSIDC_24h.hdf
N2010344_SST_OIVAVAM_24h.nc.gz
S20031390013923_TOVS.OZONE

2) Do they correspond to data simultaneouly acquired with SeaWiFS data ? or are they derived from a climatology ?

3) Ozone questions
3a) Why it seems to have 2 files for deriving ozone information ? (TOMOSOMI and TOVS)
3b) I note that the number of ozone files is very very low in my archive. Is this normal ? Shall I complete this collection downloading ozone data from an other site ?

4) When I generate a L2 product with l2gen, if an ancillary file is not available for the day of the SeaWiFS product, is the ancillary parameter (e.g. ozone) replaced by a parameter derived from a climatology ?

Thank you for your answer
Véronique

I can give you a general answer if you need it soon, but I need to do some digging to get a full answer. 

I can see with a general response first if it answers to my expectations.
Thanks a lot
Véronique

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:

Parameter               Use
zwind                   Water roughness prediction used for glint estimation
mwind                   same
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
as possible.

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
climatology.

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.

Thank you very much for these clarifications very useful !
Véronique

Olive,

First make sure the file actually exists....I've seen that error when the file isn't present (yes, a odd way to report an error for such a condition).
If it does exist, make sure it is a valid HDF file by running:
ncdump_hdf -h <filename>
This should report the following:
netcdf N200905700_SEAICE_NSIDC_24h {
dimensions:
        north_rows = 448 ;
        north_cols = 304 ;
        south_rows = 332 ;
        south_cols = 316 ;

variables:
        byte north(north_rows, north_cols) ;
                north:long_name = "North Pole Ice Fraction" ;
        byte south(south_rows, south_cols) ;
                south:long_name = "South Pole Ice Fraction" ;

// global attributes:
                :Title = "Sea Ice Concentration Daily" ;
                :Creation Date = "Fri Jul 10 06:22:32 2009" ;
                :Created By = "Don Shea, SAIC, from NSIDC NRT data (http://nsidc.org)" ;
                :Reference1 = "Meier, W., F. Fetterer, K. Knowles, M. Savoie, M. J. Brodzik. 2006, updated quarterly. Sea ice concentrations from Nimbus-7 SMMR and DMSP SSM/I passive microwave data. Boulder, Colorado USA: National Snow and Ice Data Center. Digital media." ;
                :Software Name = "ice2hdf" ;
                :Software Version = "1.0" ;
                :year = 2009 ;
                :day_of_year = 57 ;
}

If that works, and you still have an issue processing with it, please attach the parameter file you are using so we can investigate further.

Regards,
Sean