The SPARC Reanalysis Intercomparison Project (S-RIP): Comparisons of Water Vapor and Ozone in Reanalyses

Abstract:

Reanalysis datasets are widely used to understand atmospheric processes and past variability, and are often used as “observations” for comparison with climate model output. Because of the central role of water vapour (WV) and ozone (O₃) in climate change, it is important to understand how accurately these species are represented in reanalyses, and whether or not significant differences exist among the eight existing global reanalyses.

The SPARC (Stratosphere-troposphere Processes and their Role in Climate) Reanalysis Intercomparison Project (S-RIP) is aimed at improving understanding of reanalysis products and ultimately contributing to future reanalysis developments. In this presentation, we present initial results from the WV and O₃component of S-RIP. Comparisons are made over a range of timescales between the different reanalyses, and between reanalyses and independent observational data from sondes, aircraft, and satellites.

Because stratospheric O₃ is assimilated in the newer reanalyses, the reanalysis O₃ fields are evaluated using common error statistics (e.g., observation-background departures) and comparison with independent non-assimilated data. On the other hand, since stratospheric WV data are not currently assimilated, the fidelity of reanalysis WV is sensitive to how accurately the fundamental drivers of stratospheric WV are represented in the reanalyses. We therefore attempt to relate differences in reanalysis WV to processes known to affect WV such as tropical tropopause layer temperatures, tape recorder speed of propagation, amplitude of the QBO response, and extent of polar dehydration. Ensemble trajectory runs coupled with a dehydration model are used to analyse WV variability related to these processes, and to assess the self-consistency of the reanalysis WV fields in the stratosphere.