ESA / ECSAT CCIWV.REP.022 Water Vapour Climate Change Initiative (WV_cci) - CCI+ Phase 2 D2.2 4. SUMMARY AND CONCLUSIONS For CDR-3, the merging algorithm by Hegglin et al. (2014) [RD-1] has been further developed towards operational implementation. In particular, the optimal estimation theory as new and found robust (apart from the use of the weighting factors based on the mean-differences approach for the merging of the different satellite instrument inputs has been further evaluated from the model).
While this merging approach addresses many instrumental issues identified as potential problems in the merging process, others still need further implementation and will be the focus of continued improvements of CDR-3 in WV_cci Phase 2. However, the final quality of the dataset is limited mainly by the temporal and spatial coverage of the limb satellite input data, especially in the early years of the instrumental record as is detailed in the PVIR [RD-15]. Here, the quality of other historical limb satellite datasets is being explored to see whether they can be used to fill gaps and extend CDR-3 into the past back to 1978.
For CDR-4, a new prototype merging algorithm is under development that applies a novel bias-correction methodology (quantile mapping) to the input profiles before merging. This methodology accounts for biases in the limb and nadir satellite sounders across the UTLS region, including the smoothing resulting from averaging kernels that are too broad to resolve the vertical gradients found in WV across this region. While the merging algorithm has been found to be promising, limitations arise from a too-scarce spatio-temporal coverage in the reference datasets used for the bias-correction (BBH observation stations).
Alternative merging technique, such as the approach employed for data merging in CDR-3 (using a CCM as transfer function) is undergoing. Currently, the WV simulations from CMAM are investigated for the potential transfer function in the bias-correction procedure on the satellite observation for the production of CDR-4. 23