OMUVB README FILE

April 20, 2007

Overview

This document describes the OMUVB data product. OMUVB contains surface UV irradiance data along with ancillary information generated using the OMI global mode measurements. In this mode each file contains the sunlit portion of a single orbit from pole-to-pole, with an approximately 2600 km wide swath composed of 60 ground pixels. The OMI measurements are used to estimate the ultraviolet (UV) radiation reaching the Earth's surface. The OMUVB product contains erythemally weighted daily dose, and erythemal dose rate at local solar noon. Additionally, the product includes spectral irradiances at 305.1, 310.1, 324.1, and 380.1 nm also corresponding to the local solar noon. These spectral irradiances assume triangular slit function with full width half maximum of 0.55 nm. The algorithm is based on use of a radiative transfer model whose input parameters are derived from the OMI measurements using the OMTO3 total ozone data product as input data. You may refer to release specific information about OMUVB for details about software versions. Surface UV irradiance data is also produced by the Sodankylä Direct Broadcast processing system. However, Direct Broadcast processing provides only local data in form of images and different data is used as an input (ozone from OMDOAO3 and radiances from Level-1B).

Algorithm Description

The OMUVB algorithm is documented in the Algorithm Theoretical Basis Document (ATBD) on http://eospso.gsfc.nasa.gov/eos_homepage/for_scientists/atbd/viewInstrument.php?instrument=13 . Contrary to ATBD, the current surface UV algorithm does not use the OMTO3 aerosol index to correct for absorbing aerosols in the free troposphere. The algorithm is similar to the TOMS UV algorithm developed by NASA. It first estimates the clear-sky surface irradiance using measured total ozone and surface albedo from the dataset described in http://promote.fmi.fi/MTW_www/MTW.html . The clear-sky value is then adjusted by a transmittance factor that accounts for the attenuation of UV radiation by clouds and aerosols. The attenutation factor is derived from the ratio of measured backscatter radiances and solar irradiances at 360 nm assuming that clouds and aerosols are non-absorbing at this wavelength. This leads to an overestimation of surface UV irradiance when UV-absorbing aerosols such as smoke or desert dust are present. Additionally, boundary layer pollution aerosols cause overestimation of surface UV irradiance in urban areas. Future version of the algorithm may correct for these effects.

Data Quality Assessment

The radiative transfer model assumes that clouds are plane parallel and homogeneous, i.e., it doesn't account for broken, multi-layer or mixed phase clouds. This error is the principal source of noise in comparing satellite measurements with ground-based instruments. The OMI surface UV irradiance represents the spatial average over the OMI footprint. OMI measurements are made once a day around 1:45 p.m. local time. No correction is made for the change in cloudiness, ozone and aerosols between local noon and satellite overpass time, or for their diurnal variability. Previous validation studies with TOMS data suggest that OMI UV irradiance estimates are on the average 0-30% larger than the ground-based reference data. The OMI surface UV data were compared with spectral ground-based measurement data of Jokioinen (60.8N,23.5E), Sodankyla (67.4N,26.6E), Toronto (43.8N,79.5W), San Diego (32.8N,117.2W), Ushuaia (54.8S,68.3W), and Barrow (71.3N,156.7W). The validation results were presented in the AGU fall meeting in December 2005 , and they imply similar results as the previous validation studies with TOMS surface UV data. The systematic bias can be attributed to absorbing aerosols from natural and anthropogenic sources. Since the soot content of the urban aerosols tend to be highly localized, these errors presumably are also localized and do not necessarily represent the error in regional estimate of surface UV made by OMI. Snow and ice further complicate estimation of the surface UV since clouds cannot be distinguished from them. Therefore, in regions with temporary snow or ice or highly heterogeneous surface albedo the OMI UV irradiance estimates have much higher uncertainty. Future version of the algorithm may use snow cover information to reduce this uncertainty.

Product Description

An OMUVB product file contains the sun-lit portion of a single Aura orbit or some 1650 measurements. Each measurement consists of a 2600 km wide OMI scan of 60 pixels. Due to optical aberrations and small asymmetry between the instrument optic axis with the spacecraft nadir, the pixels on the swath are not symmetrically aligned on the line perpendicular to the orbital plane. The latitude and longitude provided with each pixel represent the center of each pixel on ground. The OMUVB product is written as an HDF-EOS5 swath file. Software tools that read HDF-EOS5 data files are available at http://disc.gsfc.nasa.gov/Aura/tools.shtml . The data are ordered in time. The information provided in these files includes latitude, longitude, solar zenith angle, and a large number of ancillary parameters that provide information to assess data quality. The most important of these parameters is the OMUVBQuality field, that describes the quality of the data and can be used for filtering of the data. For a complete list of the parameters, please read the OMUVB format specification document . Questions and comments related to the OMUVB dataset, the OMI Surface UV algorithm, or data quality should be directed to Aapo Tanskanen .

Data Access

The Level 2 OMUVB data can be downloaded from the following ftp site

Site: ftp://ftp.fmi.fi/OMUVBv002/Level2

Username: omuvb

Password: WHizzRD0

The OMUVB data are available in form of monthly, compressed tar files (*.tgz). The text files (*.txt) list the contents of each tar file. The size of a tar file corresponding to a full month of Level 2 OMUVB data is of the order of 2 gigabytes.