Full-resolution
satellite time series of the California Current
area
Mati Kahru, mkahru@ucsd.edu
Abstract
For several satellite
products reduced resolution mapped datasets are available for downloading at
the DAACs. For example, mapped SeaWiFS Level-3 products are available at 9 km
resolution and MODIS-Aqua products at 4 and 9 km resolutions. Full-resolution
data (e.g. approximately 1-km for SeaWiFS and
The processed files are
available from http://spg.ucsd.edu/Satellite_data/California_Current/.
Please note that the files are not available through FTP but through HTTP. For
downloading multiple files at once you can use the widely available utility wget (e.g. http://www.gnu.org/software/wget/).
Methods
·
CZCS, OCTS, SeaWiFS
MLAC (Merged Local Area Coverage and GAC) Level-2 data and MODIS-Aqua Level-2
data were downloaded from the Goddard DAAC. The sub-setting limits for the area
were: 16-45 N; 140-100 W.
·
Albers Conic
Equal Area projection generated with the Terascan (SeaSpace Corp) utility master2 was chosen as the standard map. The
advantage of using an equal area projection is that calculating areas,
integrated masses and fluxes is very convenient. The selected map projection
has 3840 columns and 3405 rows of pixels with an area of 1.0 km2.
The projection is defined in a HDF file cal_aco_3840.hdf.
·
A mapped composite
was generated for each calendar day using all available passes with a WAM
utility wam_l2_map. Level-2 flags ATMFAIL, LAND, HIGLINT, HILT, HISATZEN,
STRAYLIGHT, CLDICE, HISOLZEN, HITAU, LOWLW, CHLFAIL, NAVWARN,
CHLWARN, DARKPIXEL, SEAICE, NAVFAIL were
used to eliminate low-quality pixels. Other flags (BADANC,
COASTZ, NEGLW, COCCOLITH, TURBIDW, ABSAER, TRICHO, MAXAERITER, MODGLINT,
ATMWARN, FILTER, SSTWARN, SSTFAIL) were
ignored. It was found that the pixels marked with the latter flags were
statistically not different from neighboring pixels while their elimination
would have decreased the amount of usable data. In addition to eliminating the flagged
pixels, the cloud image determined with the flag CLDICE was dilated (expanded) to eliminate contaminated
pixels near cloud edges. Cloud edges are often associated with erroneously high
Chl-a values. This is obvious when looking at transects in low-chlorophyll
areas in the open ocean where cloud edges produce elevated chlorophyll values. While
this procedure eliminates a large fraction of the contaminated pixels near
cloud edges, some elevated pixels probably still slip through. The NASA Ocean
Color Processing Group is aware of the problem but the current standard SeaWiFS/MODIS-Aqua
compositing procedures ignore this problem. A typical day has 3 SeaWiFS passes
over the area of interest. As pixels from the swath edges are eliminated (using
flags as described above), the multiple daily passes never overlap. Therefore a
daily composite of high-quality pixels has separate patches of pixels from
individual passes. The resulting mapped daily composite is saved as HDF with
the standard chlorophyll log-scaling. In addition, 8-times reduced and
annotated HDF and PNG quick-looks are saved. The file names (e.g. S2003001_chl_a_mapped.hdf,
S2003001_chl_a_mapped_annotated.hdf , S2003001_chl_a_mapped.png for SeaWiFS)
show the year (2003), the Julian day of the year (001) and the variable (chl_a).
For Aqua the first letter of the filename is “A”. For CZCS the first letter is
“C”, for OCTS it is “O”. “C” is also used for th emerged SeaWiFS and Aqua Chl-a.
Additional information is stored as HDF attributes. A typical application of
the command is the following:
wam_l2_map S*.hdf
cal_aco_3840.hdf cal_aco_reduced.hdf 8
186 15
·
The following
example is a daily composite for January 1, 2003 showing contributions from three
separate passes (west, center and the south-east corner). The Chl-a
concentration scale is logarithmic from 0.05 to 10 mg m-3.
·
For years
2002-2004 with both SeaWiFS and MODIS-Aqua data available the daily composited
images from the individual sensors were combined into a merged into a daily
mapped Chl-a datasets with the WAM
utility wam_composite_2sensors. The
same naming convention was used, except the first letter of the filenames was
replaced with “C”. Orbits and the respective swaths of individual sensors are
different; therefore the merged data have better coverage. Also, because of the
moving clouds between the satellite passes at different times (the SeaWiFS pass
approximately at noon and the Aqua pass at 13:30 local time) the merged dataset
has less area obscured by clouds. The following is a merged SeaWiFS-Aqua data
from January 1, 2003. Note the increased coverage compared to SeaWiFS alone.
·
Daily mapped composite
images were composited into 5-day composites with a WAM utility wam_composite_5day. This operation
creates 3 files: HDF files of the composited average concentration and of the count
of valid pixels used for averaging each pixel. The filename pattern (e.g.
C2003001_C2003005_chl_a_comp.hdf and C003001_C2003005_chl_a_count.hdf) shows
the start year and day (2003-001) and the end year and day (2003-005) of
compositing. For each average image an 8-times reduced annotated HDF and PNG
quick-looks were created as well. A typical application of the command is the
following:
wam_composite_5day
C*.hdf cal_aco_reduced.hdf 8 186 15
·
The following
example shows a 5-day average Chl-a image from the merged SeaWiFS-Aqua data.
Note the increased coverage compared to the previous daily image.
·
As even the
5-day composites are partly blank due to clouds, a 15-day overlapping
compositing was performed with a WAM utility wam_composite_2x. This averages three adjacent 5-day composites,
e.g. days 1-5, 6-10 and 11-15. The next 15-day composite overlaps partly with
the previous 15-day composite, e.g. it uses the following days 6-10, 11-15 and
16-20. The partial overlap is good for producing image loops by reducing the
“flicker” and making smoother transitions. The filename pattern is similar to
the 5-day composites (e.g. C2003001_C2003015_chl_a_comp.hdf) and shows the
year, start and end days of compositing. The reduced annotated HDF and PNG
images are also saved. A typical application of the command is the following:
wam_composite_2x C*.hdf
15 cal_aco_reduced.hdf 8 186 15
·
The following
example shows a 15-day average Chl-a. Note the almost full coverage compared to
the 5-day and daily composite. For the years with only SeaWiFS data the
northwest and the southwest corners are often missing data as a result of the
sub-setting when ordering the MLAC passes.
·
An animated GIF
showing the annual chl-a dynamics using the 15-day overlapping composites is shown
at http://www.spg.ucsd.edu/Satellite_Projects/chla_2003_overlapping_15day.gif.
·
The 1-km daily,
5-day, 15-day and monthly images are all available as HDF or PNG. The HDF files
have the numerical values for each pixel and can be used to generate statistics
for any selected area. The HDF files can be read with any HDF-aware software
(e.g. Matlab, IDL, and WIM). When read with WIM the geo-location and scaling
are automatically retrieved. For geo-location with other software the latitude
and longitude arrays corresponding to each pixel can be easily created.
·
This dataset
can have many applications. For example, you may be interested in a smaller
sub-area of the domain, may-be in a different projection. This can be easily accomplished by remapping
the full-resolution 1-km data to another projection, and or compositing
further. In the example below I remapped the full-resolution SeaWiFS Albers-projection
images to a smaller Mercator projection and applied another compositing step (wam_composite_2x). A sample resulting
image is shown below.
· The annual mean for 2003 SeaWiFS data is shown below
· An animated GIF of the whole series is below:
