Cloud radiative forcing ratio. An analytical model

The wintertime extratropical intraseasonal variability in the North Atlantic-European region has been analysed, and the ability of an atmospheric general circulation model to simulate it at different horizontal resolutions is discussed. Three runs of the French global spectral model (ARPEGE) are investigated: both medium (T42) and high (T106) resolution, and a conformally stretched T63 version with maximum resolution over the Mediterranean. Circulation features are validated against ECMWF analyses. The systematic errors in the simulated variability have been isolated using space-time spectral and complex principal component analyses. The time-mean structure of the 500 hPa geopotential height indicates that the model is too zonal, especially at high resolution, with a stronger than normal synoptic-scale activity. The Atlantic storm track penetrates too far eastward over Europe, is too far south and does not veer to the northeast at the exit of the jet. These features may be in part associated with too strong barotropic forcing of the mean flow by the synoptic-scale eddies on the equatorward side of the jet. Despite the overly strong zonal mean flow, the high resolution simulation has more realistic storm tracks than the other two versions. Low-frequency intraseasonal variance is underestimated at each resolution, mainly north of 50°N. Furthermore, the model lacks planetary-scale slow westward travelling waves and does not show realistic spatial modes of low-frequency variability at any resolution. Blocking frequency is underestimated and the blocking patterns are shifted eastward, following the excess of storm track penetration over Europe. However, these drawbacks are slightly improved at higher (T106) resolution, especially for blocking