Northern hemisphere extratropical cyclones: A comparison of detection and tracking methods and different reanalyses

The applicability of three different cyclone detection and tracking schemes is investigated with reanalysis datasets. First, cyclone climatologies and cyclone characteristics of the 40-yr ECMWF Re-Analysis (ERA-40) are compared with the NCEP-NCAR dataset using one method. ERA-40 shows systematically more cyclones, and therefore a higher cyclone center density, than the NCEP-NCAR reanalysis dataset. Geostrophically adjusted geopotential height gradients around cyclone centers, a measure of cyclone intensity, are enhanced in ERA-40 compared with the NCEP-NCAR reanalysis dataset. The variability of the number of cyclones per season is significantly correlated between the two reanalysis datasets, but time series of the extreme cyclone intensity exhibit a higher correlation. This suggests that the cyclone intensity is a more robust measure of variability than the number of cyclones. Second, three cyclone detection and tracking schemes are compared, based on the ERA-40 dataset. In general the schemes show a good correspondence. The approaches differ in technical aspects associated with the cyclone identification and the tracking procedure, leading to deviations in cyclone track length. However, it is often not clear which scheme is correct or incorrect. With the application of lifetime thresholds, some of the cyclone tracks are too short to be included in statistical measures of cyclones. Nevertheless, consequences of these differences in mean cyclone characteristics are minor, but for specific research questions - for example, what is the cyclone activity in the Mediterranean in winter - the users should be aware of these potential differences and adjust their scheme if necessary. A trend analysis of cyclone characteristics shows that results appear to be sensitive to both the choice of cyclone detection and tracking schemes and the reanalysis dataset.