Pepijn Veefkind
Last update 16 March 2007
This document provides a brief description of the OMCLDO2
data product. The OMCLDO2 Level 2 data product contains the cloud fraction
and cloud pressure and ancillary information produced by the OMI Cloud O2-O2
algorithm. The main objective of the OMCLDO2 product is to support other OMI L2
algorithms by providing these with cloud information. Each OMCLDO2 file
contains the sunlit part of an OMI orbit, from the South Pole to the North
Pole. In the so-called global observation mode, the OMI swath is approximately
2600 wide, providing daily global coverage.
The OMCLDO2 is produced as a standard product as well as in near-real-time
(within three hours of observation) and as a very fast delivery product (within
20 minutes of observation). Although it is expected that the differences will
be very small, this document describes the standard data product. When the
near-real-time data becomes operational, information on the near-real-time data
will be posted on the KNMI OMI website.
Information on the very fast delivery product will be posted on the FMI OMI website.
Currently the OMCLDO2 product has been released for ECS Collection 2 (filename
contains v002) and Collection 3 (filename contains v003). The release specific
information about OMCLDO2 for ECS
Collection 2 and for Collection
3 contains details about software versions and known problems.
The OMI DOAS algorithm is described in the OMI
Algorithm Theoretical Basis Document (ATBD). In addition, a paper
describing the algorithm and applying it to the GOME data has been published in
the Journal of Geophysical Research (J. R. Acarreta,
J. F. de Haan and P. Stammes,
“Cloud pressure retrieval using the O2-O2 absorption band at 477
nm”, JGR, Vol 109, D05204, doi:
10.1029/2003JD003915, 2004).
The OMCLDO2 algorithm applies a DOAS type fit to the O2-O2 absorption band
around 477 nm. From this fit the O2-O2 slant column density and the reflectance
of the continuum reflectance are used to derive the effective cloud fraction
and the cloud pressure. The cloud model that is used represents a cloud by a Lambertian reflector with an albedo
of 0.8, which roughly corresponds to a cloud optical thickness of 30. As a
consequence thin clouds that fully cover an OMI pixel (e.g. cirrus clouds) are
represented by a small effective cloud fraction. Therefore, direct comparison
of the effective cloud fraction with a geometrical cloud fraction, or with a
cloud fraction derived in the infrared (IR), should be performed with great
caution, because these are different physical quantitues.
This also holds for direct comparisons of the cloud pressure to cloud top
heights derived using IR sensors. IR sensors tend to be sensitive to the top of
thin clouds, whereas in the visible the sensitivity is more a few kilometers
into moderately thick and thick clouds.
This algorithm is one of the two algorithms that derive cloud information from
OMI data. The other algorithm uses Raman scattering determine the cloud
pressure (OMCLDRR). The comparison of these two algorithms is ongoing work.
The OMCLDO2 effective cloud fraction and cloud pressure have been compared to MODIS Aqua, which flies 15 minutes in front of OMI. This comparison is described in the proceeding for the ESA Atmos Conference (Frascati, 9-12 May 2006). The most important conclusions are:
Further investigations on the accuracy of the OMCLDO2 product are planned. For example, comparisons to airborne and spaceborne LIDARs should give more information on the accuracy of the derived cloud pressure.
A single OMCLDO2 product file contains all OMI measurements
on the sunlit portion of the Earth, for one Aura orbit. During one orbit OMI
performs approximately 1650 measurements, which take 2 seconds each. In the
global observation mode, 60 across track ground pixels are measured
simultaneously during a measurement. These 60 measurements cover a swath of
approximately 2600 km wide. During so-called zoom-in measurements the swath
width maybe reduced. The operational baseline includes zoom-in measurements for
1 day a month. In case of zoom-in measurements 30 of the 60 across track pixels
contain data, the other 30 contain fill values.
A complete description of the format of the OMCLDO2 data products is provided here.
A more condensed listing of the data fields is available in the Product Format
Specification.
The OMCLDO2 product is written as an HDF-EOS5 swath file. For a list of tools
that read HDF-EOS5 data files, please visit this link:
http://disc.gsfc.nasa.gov/Aura/tools.shtml.
The OMCLDO2 product can be obtained from the Goddard Earth Sciences (GES) Distributed
Active Archive Center (DAAC) website.
Full OMCLDO2 data, as well as subsets of these data over many ground stations
and along Aura validation aircraft flights paths are also available through the
Aura Validation Data Center (AVDC) website
to those investigators who are associated with the various Aura science teams. Bojan Bojkov is the point of contact at the AVDC.
For users not interested in the detailed information provided on OMCLDO2
dataset we are developing several gridded products.
In addition, we intend to make OMCLDO2 data available in a geographically
ordered (rather than time-ordered) format that can be more easily subsetted and manipulated on-line prior to ordering. Please
check the Atmospheric
Composition Data Information Services Center website for current
information on these products.
Questions related to the OMCLDO2 dataset should be directed to the GES DAAC. For questions and comments related to the OMCLDO2 algorithm and data quality please send mail to contact omcldo2@ltpmail.gsfc.nasa.gov.