Creating the Atlas

Following the work by Catto et al. (2010) we applied an objective feature tracking algorithm (Hodges, 1995; 1999) to fields from the European Centre for Medium Range Weather Forecasts (ECMWF) ERA-Interim reanalysis for the winter period (December - February) 1989-2009 (Dee et al., 2011). The ERA-Interim reanalysis data from ECMWF can be found at Reanalysis data provides a spatially complete and coherent record of the global atmospheric circulation. Unlike archived weather analyses from operational forecasting systems, a reanalysis is produced with a single version of a data assimilation system and is therefore not affected by changes in method. The temporal resolution of the data is 6 hourly. Tracks are identified using the 850 hPa relative vorticity truncated to T42 resolution to emphasize the synoptic scales. The 850 hPa relative vorticity features have been filtered to remove weak, stationary or short-lived features that are not associated with extratropical cyclones. Figure 1 shows the 200 most intense, in terms of the T42 vorticity, winter cyclone tracks with maximum intensity in the north Atlantic (70-10 degrees W, 30-90 degrees N). The cyclone tracks are filtered using a minimum distance travelled threshold (1000km) and a minimum lifetime threshold (60 hrs). These filters remove stationary, short-lived relative vorticity features that are not associated with extratropical cyclones. There are 1050 north Atlantic cyclones in total and the most intense 200 cyclones are shown by the grey shading in figure 2. Thus the cyclone composites in the Extratropical cyclone database represent only the most extreme cyclones (~19% of the entire north Atlantic cyclone distribution).

The required fields (e.g. relative humidity, temperature, geopotential height) are extracted from the ERA-Interim dataset along the tracks of the selected cyclones within a 20 degree radius surrounding the identified cyclone position. The data is extracted on a radial grid at 1 degree grid spacing, both azimuthally and radially, on pressure levels or on isentropic levels. A full mathematical description is given in the appendix of Bengtsson et al. (2007). Following Catto et al. (2010), the fields are rotated according to the direction of travel of each cyclone such that the direction of travel becomes westerly. The direction of travel is calculated by averaging the directions computed from the location of the cyclone at the times before and after the time of interest. If the time of interest is the genesis time only two points, the location at the genesis time and 6 hours later, are used. The composites are produced by identifying the required offset time relative to the time of maximum intensity of each cyclone and the corresponding fields on the radial grid averaged. Note, that there are not always the same number of storms existing at all times in the composite so care is taken to average over the correct number of storms at each time. As this method assumes that the cyclones all intensify and decay at the same rate only the 200 most intense cyclones are included in the composite. Limiting the number of cyclones produces a more homogeneous group in terms of their evolution by will bias the mean fields to be typical of the most intense cyclones. Horizontal and vertical composites of cyclone structure are computed.

Tracks image

Figure 1: Tracks of the 200 most intense winter cyclone tracks with maximum intensity, in terms of T42 vorticity, in the north Atlantic (white lines). Google Earth.

Maxvor image

Figure 2: Maximum relative vorticity reached by all 1050 north Atlantic cyclones. The grey shading represents the part of the distribution that includes the 200 most intense cyclones.