OMDOAO3 README file
Pepijn Veefkind, KNMILast update: 29 May 2006
Overview
This document provides a brief description of the OMDOAO3 data product. The OMDOAO3 Level 2 data product contains total ozone and ancillary information produced by the OMI DOAS algorithm. Each 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 OMDOAO3 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.
You may refer to release specific information about OMDOA3 for details about software versions and known problems.
Algorithm Description
The OMI DOAS algorithm is described in the OMI Algorithm Theoretical Basis Document (ATBD). In addition, a paper that contains a description of the algorithm, including the latest modifications, has been published in IEEE Transactions on Geoscience and Remote Sensing (IEEE TGRS, Vol 44, No. 5, 2006, doi: 10.1109/TGRS.2006.871204). That paper also contains an initial validation of the accuracy.
In the DOAS (Differential Optical Absorption Spectroscopy) implementation for OMI the ozone vertical column is determined in three steps. In the first step the actual DOAS fitting is performed, resulting in the so-called slant column density, which is the amount of ozone along an average photon path from the Sun -through the atmosphere- to the satellite. In the second step the air mass factor is determined, which is needed to convert the slant column density into a vertical column. In the last step a correction is performed for clouds. All the results from intermediate steps are also contained in the product.
The OMI DOAS spectral fitting uses a 5 nm wide fit window centered around 334.1 nm. This window has been selected based on the very low temperature sensitivity for this window. In the spectral fitting, the inelastic rotational Raman scattering, as well as the effective ozone temperature are explicitly accounted for. The air mass factor is determined off-line, by simulating OMI spectra using a radiative transfer model. The cloud information needed to derive accurate air mass factors and for correction factors for cloudy and partly cloudy conditions, is obtained from the OMCLDO2 cloud product. This product derives the cloud fraction and pressure from the O2-O2 absorption band around 477 nm. The OMI Algorithm Theoretical Basis Document (ATBD) and the above mentioned paper in IEEE TGRS contain a complete description of the algorithm.
This algorithm is one of the two algorithms that will be used to derive total ozone values from OMI. The other is an algorithm based on the well-known TOMS method (OMTO3 data product). Initial comparisons show good overall agreement between the two algorithms, but there are noticeable differences over clouds and snow/ice, as well as at large solar zenith angles.
Data Quality Assessment
Validation was performed by comparing OMDOAO3 total ozone data with collocated Brewer data for the Northern hemisphere, for November 2004 (see above mentioned paper in IEEE TGRS) as well as for the period between March 28 and June 11, 2005. On average, the retrieved ozone column is biased by 2 % with respect to the Brewers ([OMI-Brewer]/Brewer), the standard deviation on this number is 3.5%. The validation results show that the errors increase with increasing Solar zenith angle. This is under investigation.
Product Description
A single OMDOAO3 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 OMDOAO3 data products is provided here. A more condensed listing of the data fields is available in the ProductFormat Specification.
The OMDOAO3 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.
Data Availability
The OMDOAO3 product can be obtained from the Goddard Earth Sciences (GES) Distributed Active Archive Center (DAAC) website.Full OMDOAO3 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 OMDOAO3 dataset we are developing several gridded products. In addition, we intend to make OMDOAO3 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 Goddard Earth Sciences (GES) Distributed Active Archive Center (DAAC) website for current information on these products.