Accuracy of the Total Ozone Mapping Spectrometer algorithm at polar latitudes.
AuthorCaudill, Thomas Robert.
Committee ChairHerman, Benjamin M.
MetadataShow full item record
PublisherThe University of Arizona.
RightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
AbstractIt has been noted that for large solar zenith angles (θ₀ > 75°), there is some uncertainty in the retrieval scheme for determining the total column ozone amount using the Total Ozone Mapping Spectrometer (TOMS) instruments. This uncertainty arises because the current look-up table radiances were calculated by a radiative transfer algorithm using an approximate pseudo-spherical atmosphere. The pseudo-spherical code calculates the primary scatter using spherical geometry but higher order scattering is computed for a plane-parallel atmosphere. To test the accuracy of the pseudo-spherical approximation, a new method for numerically solving the equation of radiative transfer in a spherical shell atmosphere including polarization has been developed. This technique uses a Gauss-Seidel iteration scheme to calculate a steady state solution including all significant orders of scattering. For the TOMS instrument which was on Nimbus-7, large solar zenith angles corresponded primarily to high latitudes due to its sun-synchronous noon crossing orbit. Therefore, the accuracy of the algorithm at large solar zenith angles becomes a critical issue particularly for the precise measurement of ozone over polar regions. Comparisons between the pseudo-spherical and the spherical Gauss-Seidel codes show that for solar zenith angles greater than 80° the error introduced by not properly accounting for the sphericity can be significant. Large intensity and ozone differences (5-10%) are possible in the forward (φ = 0°) and backscatter (φ = 180°) directions which are caused by the incorrect attenuation of the solar beam. Because of its particular viewing geometry (along φ = 90°), the error in the Nimbus-7/TOMS ozone amount is generally less than 1%. However, when θ₀ = 88° with a high surface reflectivity, ozone amounts reported for Nimbus-7/TOMS may be overestimated by up to 8%. The TOMS instrument currently on Meteor-3, because of its less inclined orbit, has a much wider range of viewing geometries. Under extreme conditions, it appears that errors on the order of 10 to 30% may be possible.
Degree ProgramAtmospheric Sciences