ESA / ECSAT CCIWV.REP.022 Water Vapour Climate Change Initiative (WV_cci) - CCI+ Phase 2 D2.2 Figure 3-3: Example of bias-correction on MLS observation with reference to BBH profiles. Figure shows the water vapour horizontal distribution at pressure levels from top to bottom: 70 hPa, 100 hPa, 150 hPa, 200 hPa, 250 hPa, and 300 hPa. From left to right, the data are from: original satellite mean, bias-corrected satellite mean, ERA-5, and MERRA-2. The water vapour values are averaged from 2010 to 2014. 3.2.2 Bias-correction with ACE-FTS observations In Phase 2, the WV observations from ACE-FTS were proposed to be used as new reference data in the bias-correction procedure.
Same as the processing in Section 3.2.1 with BBH as the reference, satellite observations in the period of 2010-2014 are bias-corrected with ACE- FTS profiles obtained during 2004-2019. Figure 3-4 shows the example of bias-correction on MLS WV data with ACE-FTS profiles and further comparison with reanalyses. The bias- correction with ACE-FTS tends to reduce the WV in the upper troposphere, especially over the tropical region during summer, which is opposite to the bias-correction based on BBH profiles.
This confirms a low-bias found in ACE-FTS observations in the upper troposphere (RD-13) and that the ACE-FTS profiles are not suitable to improve the merging algorithm with reference to BBH profiles (at least in the upper troposphere). Thus, other new reference profiles should be explored or alternative merging techniques should be investigated. 18