A radius–depthmodel for midlatitude cyclones in reanalysis data and simulations

Geometric properties of winter (DJF) and summer (JJA) midlatitude cyclones are analysed in reanalysis data, present-day and global warming simulations. Cyclone centres are identified by the minima of the 1000 hPa geopotential height. Fitting an azimuthally symmetric Gaussian function to the surrounding height field provides cyclone depth (difference between the cyclone centre and the synoptic environment), radius (standard deviation), geostrophic wind and vorticity. Analysing ERA-40 reanalysis data of different resolutions and Intergovernmental Panel on Climate Change (IPCC) scenario simulations by the coupled atmosphere–ocean general circulation model ECHAM5/MPI-OM yields mean radii of 300–500 km in winter and 300–400 km in summer. Depth maxima occur in the storm tracks (determined by the bandpass-filtered variance of the geopotential height), and the smallest radii characterize oceanic cyclogenesis regions. The geostrophic vorticity, derived from the fitted Gaussian model, agrees reasonably well with the observed relative vorticity. Future warmer climate scenarios exhibit smaller radii and weaker depths during winter and summer. An intense growth of the depth is found during the 2–10 day cyclone life cycles, while the radii reveal negligible growth. Compositing depths with respect to normalized total lifetime leads to rescaled depth life cycles, which collapse to a simple universal function, ã(1 − ã), for relative cyclone age ã. Copyright c