ESA / ECSAT CCIWV.REP.022 Water Vapour Climate Change Initiative (WV_cci) - CCI+ Phase 2 D2.2 3. ALGORITHM DEFINITION CDR-4 3.1 Introduction The objective of ESA WV_cci CDR-4 is to construct a prototype three-dimensional vertically resolved water vapour dataset in the upper troposphere and lower stratosphere (UTLS) region by merging both limb- and nadir-viewing satellite observations from Aura-MLS, MIPAS and IMS. The nadir-viewing geometry delivers reliable VRWV profiles with high precision mainly in the troposphere and the limb-viewing geometry mainly covers VRWV profiles in the stratosphere and above.
In the UTLS region, due to the very strong vertical gradients in water vapour concentrations, both types of viewing geometries lead to large uncertainties. The merged prototype product CDR-4 takes advantages from these two types of viewing geometry to improve the quality of the VRWV profile data in this region. The methodology used for the merging of observations from the two viewing geometries developed in Phase 1 is applying a bias-correction relative to in situ VRWV profiles from balloon-borne hygrometer (BBH, same as FPH/CFH) observations.
All the input data are bias- corrected via a quantile-mapping technique (Maraun, 2013 [RD-7]) and turned into a harmonised limb/nadir data record of VRWV in the UTLS region. These harmonised VRWV data from multiple satellite instruments are then merged into a three-dimensional VRWV product CDR-4. In project Phase 1, the prototype merged CDR-4 data was produced for the period of 2010- 2014. As planned, the merged CDR-4 will be further extended backwards and forwards to cover the period of 2007-2023 with the merging algorithm described above.
This merging algorithm was, however, to be improved by adopting new reference profiles to compensate for the limited representativity of the BBH profiles. However, the proposed ACE-FTS and ACE- MAESTRO profiles were found to be too dry in the UTLS as a reference to bias-correct other satellite observations, hence the ACE profiles are deemed not suitable in the bias-correction procedure, similarly to the balloon-borne hygrometer profiles. Other new potential reference profiles will be investigated further in Phase 2. As proposed in Phase 2, alternative merging techniques will be additionally investigated for the merging of the CDR-4 product.
As an alternative merging technique, the approach employed for data merging in CDR-3 (using a CCM as transfer function) is investigated based on CMAM simulations. Currently, the WV simulations from CMAM are compared to global WV 13