Reanalysis intercomparisons of stratospheric polar processing diagnostics
Authors: Lawrence Zachary D. , Manney Gloria L. , Wargan Krzysztof 
Published: January 2018

Abstract. We compare herein polar processing diagnostics derived from the five most recent full-input reanalysis datasets: the National Centers for Environmental Prediction Climate Forecast System Reanalysis (CFSR), the European Centre for Medium-Range Weather Forecasts Interim Reanalysis (ERA-Interim), the Japanese Meteorological Agency's Japanese 55-year Reanalysis (JRA-55), and the National Aeronautics and Space Administration's Modern Era Retrospective-analysis for Research and Applications version 1 (MERRA) and version 2 (MERRA-2). We focus on diagnostics based on temperatures and potential vorticity (PV) in the lower to middle stratosphere that are related to formation of polar stratospheric clouds (PSCs), chlorine activation, and the strength, size, and longevity of the stratospheric polar vortex. Polar minimum temperatures (Tmin) and the area of regions having temperatures below PSC formation thresholds (APSC) show large persistent differences between the reanalyses, especially in the southern hemisphere (SH), for years prior to 1999. Average absolute differences between the reanalyses in Tmin are as large as 6 K at some levels in the SH (2 K in the NH), and absolute differences in APSC larger than 2 % of a hemisphere (1 % of a hemisphere in the NH). After 1999, there is a dramatic convergence toward better agreement between the reanalyses in both hemispheres throughout the lower stratosphere, with average Tmin differences generally less than 1 K in both hemispheres, and average APSC differences less than 0.5 % of a hemisphere. The comparisons of diagnostics based on isentropic PV for assessing polar vortex characteristics, including maximum PV gradients (MPVG) and the area of the vortex in sunlight (or sunlit vortex area, SVA), show more complex behavior: Some reanalyses show convergence toward better agreement in vortex diagnostics after 1999, while others show some persistent differences across all years. While the average differences are generally small for these vortex diagnostics, understanding such differences among the reanalyses is complicated by the need to use different methods to obtain vertically-resolved PV for the different reanalyses. We also evaluated other winter season summary diagnostics, including the number of days below PSC thresholds integrated over vertical levels, the winter mean volume of air below PSC thresholds, and vortex decay dates. For these summary diagnostics, the reanalyses generally agree best in the SH, where relatively small interannual variability has led to many winter seasons with similar polar processing potential and duration, and thus low sensitivity to differences in meteorological conditions among the reanalyses. In contrast, the large interannual variability of NH winters has given rise to many seasons with marginal conditions and high sensitivity to reanalysis differences. Our results indicate that the transition from the reanalyses assimilating Tiros Operational Vertical Sounder (TOVS) data to Advanced TOVS and other data around 1998–2000 data had a profound effect on the agreement of the temperature diagnostics presented, and to a lesser extent the agreement of the vortex diagnostics. Our results lead to several recommendations for usage of reanalyses in polar processing studies, particularly related to the sensitivity to changes in data inputs and assimilation. Because of these sensitivities, we urge great caution for studies aiming to assess trends derived from reanalysis temperatures. We also argue that one of the best ways to present the sensitivity of scientific results on polar processing is to use multiple reanalysis datasets.