ESA / ECSAT CCIWV.REP.022 Water Vapour Climate Change Initiative (WV_cci) - CCI+ Phase 2 D2.2 2.5 Assumptions and limitations The main assumption underlying the merging methodology is that the quality of the chemistry– climate model simulation is sufficient so as not to introduce spurious trends into the merged observational time series. This seems justified, since all the transfer function does is to shift a given instrument record by an absolute amount and does not affect the time series evolution in any other way.
We have so far used the Canadian Middle Atmosphere Model CMAM as transfer function, since its stratospheric transport has been tested rigorously and its representation of other trace gases has been shown to be promising (e.g., Shepherd et al., 2014 [RD-5]). This assumption will, however, need to be further tested in WV_cci Phase 2 by using different models as transfer function that are known to show somewhat different behaviour (e.g. Lossow et al., 2018 [RD-6]). The implementation of the optimal estimation approach in the final merging step was newly introduced in phase 1 of the project and underwent further testing during phase 2.
Overall, the OE method produces reasonable results as found in evaluations, which will be presented in an updated PVIR during year 2. However, the choice of using the mean bias-corrected differences as weighting in the optimal estimation equation, is found to be biasing the merged WV record towards the model concentrations and thus should be replaced with a different approach (to be decided). The solution to this problem will be investigated further in WV_cci Phase 2. 12