ESA / ECSAT CCIWV.REP.022 Water Vapour Climate Change Initiative (WV_cci) - CCI+ Phase 2 D2.2 2. ALGORITHM DEFINITION CDR-3 2.1 Introduction The ESA WV_cci CDR-3 features a long-term zonal monthly mean dataset of vertically resolved water vapour (VRWV) that consists of a merger between the satellite limb sounders SAGE II, UARS-MLS, HALOE, POAM III, SMR, SAGE III, SCIAMACHY, MIPAS, ACE-FTS, ACE-MAESTRO, Aura-MLS and SAGE III/ISS. A main goal of the construction of this CDR was the focus on correcting for spatio-temporal sampling differences and biases between input datasets.
2.2 Heritage The methodological approach that has been used as basis for the WV_cci CDR-3 merging stratospheric water vapour fields of a chemistry–climate model nudged to observed algorithm had been developed by Hegglin et al. (2014) [RD-1]. This merging method uses the meteorology as a transfer function between different satellite instruments. This approach has been shown to overcome issues arising from short overlap periods and instrument drifts, which are more likely towards the end of mission lifetimes when satellite instruments can suffer degradation and satellite orbits start to drift.
2.3 SPARC Data Initiative climatology construction As input data to CDR-3, the SPARC Data Initiative monthly zonal mean climatologies are being used. More instrument-specific information on these input datasets can be found in SPARC (2017) [RD-2], Hegglin et al. (2013) [RD-3], and Hegglin et al. (2021) [RD-4], but we here give a short general summary of how the climatologies are constructed.
Monthly zonal mean time series of water vapour have been calculated for fields on the SPARC Data Initiative climatology grid, using 5° latitude bins (with mid-points at -87.5°, -82.5°, -77.5°, …, 87.5°) and 28 pressure levels (300, 250, 200, 170, 150, 130, 115, 100, 90, 80, 70, 50, 30, 20, 15, 10, 7, 5, 3, 2, 1.5, 1, 0.7, 0.5, 0.3, 0.2, 0.15 and 0.1 hPa). Water vapour is reported as volume mixing ratios (VMR) along with the 1 - standard deviation, the number of averaged data values given for each month, latitude bin, and pressure level. In addition, the mean, minimum, and maximum local solar time (LST), average day of the month, and average Figure 2-1).
latitude of the data within each bin for one selected pressure level are also provided (see 8