Abstract
Reanalysis data sets are widely used to understand atmospheric processes and past variability, and are often used to stand in as "observations" for comparisons with climate model output. Because of the central role of water vapor (WV) and ozone (O3) in climate change, it is important to understand how accurately and consistently these species are represented in existing global reanalyses. In this paper, we present the results of WV and O3 intercomparisons that have been performed as part of the SPARC (Stratosphere-troposphere Processes and their Role in Climate) Reanalysis Intercomparison Project (S-RIP). The comparisons cover a range of timescales and evaluate both inter-reanalysis and observation-reanalysis differences. We also provide a systematic documentation of the treatment of WV and O3 in current reanalyses to aid future research and guide the interpretation of differences amongst reanalysis fields. The assimilation of total column ozone (TCO) observations in newer reanalyses results in realistic representations of TCO in reanalyses except when data coverage is lacking, such as during polar night. The vertical distribution of ozone is also relatively well represented in the stratosphere in reanalyses, particularly given the relatively weak constraints on ozone vertical structure provided by most assimilated observations and the simplistic representations of ozone photochemical processes in most of the reanalysis forecast models. However, significant biases in the vertical distribution of ozone are found in the upper troposphere and lower stratosphere in all reanalyses. In contrast to O3, reanalysis estimates of stratospheric WV are not directly constrained by assimilated data. Observations of atmospheric humidity are typically used only in the troposphere, below a specified vertical level at or near the tropopause. The fidelity of reanalysis stratospheric WV products is therefore mainly dependent on the reanalyses' representation of the physical drivers that influence stratospheric WV, such as temperatures in the tropical tropopause layer, methane oxidation, and the stratospheric overturning circulation. The lack of assimilated observations and known deficiencies in the representation of stratospheric transport in reanalyses result in much poorer agreement amongst observational and reanalysis estimates of stratospheric WV. Hence, stratospheric WV products from the current generation of reanalyses should generally not be used in scientific studies.
Highlights
Ozone and water vapor are trace gases of fundamental importance to the radiative budget of the stratosphere
The pressure-level products provided by JRA-25 and JRA-55 do not include analyzed stratospheric water vapor fields, while Climate Forecast System Reanalysis (CFSR) produces a stratosphere that is much too dry
In contrast to other reanalyses, MERRA and MERRA2 extend up to the lower mesosphere, and, albeit with some limitations, they both capture the water vapor maximum found in the upper stratosphere (e.g., Hegglin et al, 2013), slightly underestimated compared to observations, consistent with the simple parameterization as a 3-day relaxation to a climatology (Sects. 2.6 and 2.7)
Summary
Ozone and water vapor are trace gases of fundamental importance to the radiative budget of the stratosphere Because of their impact on stratospheric temperatures, winds, and the circulation (e.g., Dee et al, 2011), ozone and water vapor are represented as prognostic variables in almost all current reanalysis systems. Water vapor is a prognostic assimilated variable in MERRA; unlike ozone, moisture fields in the stratosphere are relaxed to a 2-D monthly climatology with a relaxation time of 3 days. This climatology is derived from water vapor observations made by the UARS HALOE and Aura MLS instruments (e.g., Rienecker et al, 2011, and references therein). No attempt was made to account for methane oxidation or trends in stratospheric methane concentrations
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