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

Abstract

SUMMARYThe sensitivity to horizontal resolution of northern hemisphere extratropical cyclone characteristics during the wintertime (December-March) is investigated using a set of seasonal forecasts (1982)(1983)(1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(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)(2001)(2002)(2003)(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.

Can we trust climate models to realistically represent severe European windstorms?

“…The reference tracks are determined from data in the original resolution of T255 (about 0.47 • or 50 km grid spacing) and from the same data coarse-grained to T63 in order to match the climate model data. This was done to demonstrate the truncation effect discussed by Jung et al (2006). For Xynthia and Jeanette, the coarse-graining has very little effect on the track (Fig.…”

“…Model resolution is most commonly employed to explain differences between global models and (re-)analyses in the frequency of intense cyclones (Knippertz et al 2000;Pinto et al 2006;Leckebusch et al 2008;Zappa et al 2013a). Jung et al (2006) distinguish dynamical effects, related to the supposedly worse representation of crucial physical processes and the reduced height of orographic barriers such as Greenland and the Alps in coarser-resolution models, and truncation effects to do with changing the chances of detecting small-scale, short-lived cyclones with automatic tracking routines. They find that the effect of dynamics, physics and orography dominates over the truncation effect for intense cyclones, whereas the truncation effect dominates for shallow cyclones.…”

Interannual variations and future change of wintertime extratropical cyclone activity over North America in CCSM3

“…More complicated methods also inspect SLP gradients, length of the life cycle, and/or minimum SLP during lifetime. There are other studies that investigate map projection problems (Zolina and Gulev 2002), resolution issues (Blender and Schubert 2000;Jung et al 2006), and vorticity tracking (Sinclair 1997;Simmonds and Keay 2000;Rudeva and Gulev 2007;Greeves et al 2007). Comparison of the different algorithms is generally lacking.…”

Regional model simulation of North Atlantic cyclones: Present climate and idealized response to increased sea surface temperature

“…It is very important to reduce the sources of uncertainties to achieve a better understanding of cyclone characteristics. Jung et al [2006] proposed a high horizontal resolution to realistically simulate the frequency of extratropical cyclones. A sophisticated cyclone identification method, a review of the methods and the results in terms of trends in the frequency and intensity of extratropical cyclones of the past are presented by Benestad and Chen [2006].…”