Accuracy of the Total Ozone Mapping Spectrometer Algorithm at Polar Latitudes.
Abstract
It has been noted that for large solar zenith angles (theta_circ > 75^ circ), 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 lookup table radiances were calculated by a radiative transfer algorithm using an approximate pseudospherical atmosphere. The pseudo spherical code calculates the primary scatter using spherical geometry but higher order scattering is computed for a planeparallel atmosphere. To test the accuracy of the pseudospherical 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 GaussSeidel iteration scheme to calculate a steady state solution including all significant orders of scattering. For the TOMS instrument which was on Nimbus7, large solar zenith angles corresponded primarily to high latitudes due to its sunsynchronous 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 GaussSeidel codes show that for solar zenith angles greater than 80^ circ the error introduced by not properly accounting for the sphericity can be significant. Large intensity and ozone differences (510%) are possible in the forward (phi = 0^circ) and backscatter (phi = 180^circ ) directions which are caused by the incorrect attenuation of the solar beam. Because of its particular viewing geometry (along phi = 90^circ), the error in the Nimbus7/TOMS ozone amount is generally less than 1%. However, when theta_ circ = 88^circ with a high surface reflectivity, ozone amounts reported for Nimbus7/TOMS may be overestimated by up to 8%. The TOMS instrument currently on Meteor3, 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.
 Publication:

Ph.D. Thesis
 Pub Date:
 1994
 Bibcode:
 1994PhDT.......201C
 Keywords:

 Physics: Atmospheric Science; Physics: Radiation; Remote Sensing