Sensitivity of extratropical cyclone characteristics to horizontal resolution in the ECMWF model

The sensitivity to horizontal resolution of northern hemisphere extratropical cyclone characteristics during the wintertime (December-March) is investigated using a set of seasonal forecasts (1982-2001) with the European Centre for Medium-Range Weather Forecasts (ECMWF) model. Three different horizontal resolutions (T(L)95, T(L)159 and T(L)255) are employed. In order to test the realism of the simulations, the model results are compared with those obtained from ERA-40 re-analysis data. The cyclone tracking is accomplished by applying an automatic tracking scheme to six-hourly mean-sea-level pressure data. It is shown that many of the key characteristics of extratropical cyclones in the ECMWF model are highly sensitive to horizontal resolution, with the low-resolution version (T(L)95), for example, simulating only about 60% of the re-analysed total number of extratropical cyclones. Regions found to be particularly sensitive include the northern Pacific, the Arctic, Baffin Bay and the Labrador Sea, as well as the Mediterranean Sea. For the latter region it is shown that even the relatively high-resolution version of the model (T(L)255) significantly underestimates the number of cases of Genoa cyclogenesis. Furthermore, it is shown that in some regions, such as the entrance regions of the major northern hemisphere storm tracks, model deficits are insensitive to increases in horizontal resolution. The same analysis has been repeated for the high-resolution operational ECMWF analysis (2000-2004), truncated at different total wave numbers (T(L)95, T(L)159, T(L)255 and T(L)511) in order to separate dynamical effects of differences in resolution from those due to pure spectral truncation. It is found that the dynamical effect of changing horizontal resolution dominates over the truncation effect for intense cyclones, whereas the truncation effect dominates for shallow cyclones.