Validation of GOMOS ozone precision estimates in the stratosphere.

Accurate information about uncertainties is required in nearly all data analyses (intercomparisons, data assimilation, combined use, etc.). Validation of precision estimates (viz., the random component of estimated uncertainty) is important for remote sensing measurements, which provide the information about atmospheric parameters via solving an inverse problem. For the Global Ozone Monitoring by Occultation of Stars (GOMOS) instrument, it is of a real challenge, due to dependence of signal-to-noise ratio (and thus precision estimates) on stellar properties, small number of self-collocated measurements, and uncertainty estimates growing with time due to instrument ageing. Estimated uncertainties of ozone retrievals are small in the stratosphere for bright stars, which results in additional complexity of detecting them on the background of natural ozone variability. In this paper, we discuss different methods for geophysical validation of precision estimates and their applicability to GOMOS data. We propose a simple method for validation of GOMOS precision estimates for ozone in the stratosphere. This method is based on comparisons of difference in sample variance with the difference in uncertainty estimates for measurements from different stars selected in a region of small natural variability. For GOMOS, the difference in sample variances for different stars at altitudes 25- 45 km is well explained by the difference in squared precisions, if stars are not dim. Since it is observed for several stars, and since normalized χ2 is close to 1 in these occultations in the stratosphere, we can conclude that GOMOS precision estimates are realistic in occultations of sufficiently bright stars. For dim stars, errors are overestimated due to improper accounting for the dark charge correction uncertainty in the error budget. The proposed method can also be applied to stratospheric ozone data from other instruments, including multi-instrument analyses. [ABSTRACT FROM AUTHOR]