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 Cut-off lows (COLs) in the Southern Hemisphere. The climatology of COLs based on the minima of 300-hPa vorticity (xi_{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 xi_{300} and, with more COLs identified in xi_{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 xi_{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.
Highlights
In general the differences between National Centers for Environmental Prediction (NCEP)-CFSR and MERRA-2 are close to zero in the main Cut-off lows (COLs) region for both autumn and winter, to the comparison between ERA-Interim and JRA-55. These results suggest that the similarities between ERA-Interim and JRA-55 as well as between NCEP-CFSR and MERRA-2 are likely related to the way the data are assimilated in each reanalysis, since the best performances were achieved by comparing reanalyses produced with similar assimilation systems
An intercomparison between five different reanalyses have been made based on the and COLs, with the aim of determining the differences and similarities between the reanalyses and the impact of the improvements in the data assimilation and forecast models used in the newer datasets (ERA-Interim, NCEP-CFSR, MERRA-2 and JRA-55) over those used in the older JRA-25
In particular the smallest differences were found for the comparison between ERA-Interim and JRA-55 as well as between NCEP-CFSR and MERRA-2
Summary
The focus of attention has been on revealing the climatological aspects of COLs, such as their spatial distribution, seasonality, intensity, mean lifetime, and genesis and lysis statistics. The earliest objective climatology of SH COLs was carried out by Fuenzalida et al (2005) based on the Laplacian of 500-hPa geopotential of National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP-NCAR) reanalysis data (Kalnay et al 1996). Other studies have investigated the synoptic and climatological features of COLs in the SH, focusing on particular regions
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call