An intercomparison of subtropical cut-off lows in the Southern Hemisphere using recent reanalyses: ERA-Interim, NCEP-CFRS, MERRA-2, JRA-55, and JRA-25

Abstract

Four recent reanalysis products ERA-Interim, NCEP-CFSR, MERRA-2 and JRA-55 are evaluated and compared to an older reanalysis JRA-25, to quantify their confidence in representing Cutoff lows (COLs) in the Southern Hemisphere. The climatology of COLs based on the minima of 300-hPa vorticity (300) and 300-hPa geopotential (Z 300) provides different perspectives of COLs and contributes to the understanding of the discrepancies observed in the literature regarding their numbers and seasonality. The COLs compare better among the newest reanalyses than compared to the older reanalysis JRA-25. The difference in number between the latest reanalyses are generally small for both 300 and, with more COLs identified in 300 than in Z 300 for all reanalyses. The spatial differences observed between the newest reanalyses are mainly due to differences in the track lengths, which is larger in ERA-Interim and JRA-55 than in NCEP-CFSR and MERRA-2, resulting in disparities in the track density. This is likely due to the difference in the assimilation data system used in each reanalysis product. The largest differences in intensities occur in the 300 , because this field is very sensitive to the reanalysis resolution. The mean separation distance of the COLs that match between the latest reanalyses are generally small, while the older JRA-25 has a broader distribution and larger number of matches with relatively large distances, indicating larger uncertainties in location of COLs. The results show significant improvements for the most recent reanalyses compared to the older JRA-25 reanalysis, indicating a progress in representing the COL properties.

Sensitivity of identifying cut-off lows in the Southern Hemisphere using multiple criteria: implications for numbers, seasonality and intensity

“…Finally, the comparison between the four-point and the eight-point filters has 244 matches yearly, which corresponds to 47% and 81% of the tracks observed in the four-point and the eight-point filters, respectively. Surprisingly, the number of matches does not increase for the most intense COLs for each filter, as is the case for the comparison of COLs between different reanalyses (Pinheiro et al 2019). This means the choice of the filtering method to detect cut-off lows does not affect the distribution of the COL intensities, i.e.…”

“…Thus, further study is needed to investigate the COL structure using a common set of analysis methods and applied to particular regions to see how methods would work better if one were able to detect regional and seasonal features. Finally, it is worth considering that, even if similar schemes were employed, the results could vary significantly depending on other aspects such as the level used to identify COLs, region, intrinsic variability, and uncertainties in datasets, such as reanalysis data (Pinheiro et al 2019). more appropriate for the observed COL motion, makes the tracks closer to the typical observed tracks by excluding possibly more "merged systems" as well as more mobile earlier or later stages of the COL lifecycle.…”

“…The tracking of SH COLs is performed using six hourly data based on the 300 and Z ′ 300 minima in order to present different perspectives. The data are also spectrally truncated to triangular truncation 42 (T42) on a Gaussian grid for relative vorticity, as this is a very noisy field, whilst T63 is used for the geopotential field, similar to previous studies (Pinheiro et al 2017(Pinheiro et al , 2019. The filtering focuses the identification on the same spatial scale for each data set which allows a fairer comparison between data with different resolutions.…”

“…with cold-core and PV restrictions. According to Pinheiro et al (2019), differences in number are also apparent between the modern reanalyses ERAI, NCEP Coupled Forecast System Reanalysis (NCEP CFSR), the second National Aeronautics and Space Administration Modern Era Retrospective Reanalysis for Research and Applications (NASA MERRA-2), and the Japanese 55-year Reanalysis (JRA-55), although there is a considerable improvement in the agreement between the new reanalyses compared to the older Japanese 25-year Reanalysis (JRA-25). These results confirm that the number of systems is very sensitive to the criteria and dataset used to identify COLs.…”

“…The location of the main areas of COL activity in the SH are generally consistent in the literature, but the numbers and seasonality differ considerably between studies. Therefore, one research question arises as to whether the discrepancies between studies are mainly due to uncertainties in using different reanalyses, as discussed in the recent study of Pinheiro et al (2019), or whether the method used to identify COLs also has important implications for the results. In relation to the latter, large differences were found between the statistics produced by a tracking algorithm applied to vorticity and geopotential fields, since these focus on different spatial scale features (Hoskins and Hodges 2005).…”