Assessing the Impact of Diurnal Variability in Stratospheric Ozone on the SBUV Merged Ozone Data Record

The Solar Backscatter Ultraviolet (SBUV and SBUV/2) series of instruments measures broadly-resolved vertical ozone profiles retrieved from backscattered UV radiances. The next generation Nadir Profiler (NP) sensor, part of the Ozone Monitoring and Profile Suite (OMPS) on the Suomi NPP satellite, continues a nearly unbroken record of SBUV-type measurements dating back to late 1978, with OMPS instruments slated to continue the record for decades to come. The SBUV Merged Ozone Dataset (MOD) combines data from the SBUV instrument series into a single coherent data record. Individual instruments are similar in design and measurements are processed using the same retrieval algorithm, which also incorporates an internal cross-calibration at the radiance level within the processing. Recently, NP measurements have also been cross-calibrated to the SBUV record for inclusion in MOD. However, despite careful cross-calibration, even small differences in radiance at certain wavelengths can induce ozone differences of several percent between instruments, complicating efforts to merge the records. One source of uncertainty in the calibration is the diurnal variation in ozone. Many of the SBUV instruments measure from satellites in slowly drifting orbits, such that measurements are made at slowly varying local solar times. This imparts true geophysical variation between the SBUV data sets, which to date has not been accounted for when merging the ozone records. Further, including realistic diurnal variations within the algorithm through the a priori may help reduce geophysical variability at the radiance level and improve cross-calibration efforts. In this work we analyze hourly output from the NASA Goddard Earth Observing System Chemistry-Climate Model (GEOSCCM) to investigate the diurnal variation as a function of latitude, altitude and season. We compare the model diurnal cycle with satellite measurements from UARS and AURA MLS, SMILES, and MIPAS, and with available ground-based microwave data. We will discuss the impacts of the diurnal variation on the SBUV instrument calibration and the MOD record, as well as on intercomparisons with instruments measuring at different times, including MIPAS, SAGE III and AURA MLS as it drifts into a later afternoon orbit.