Impact of horizontal resolution on seasonal integrations

The effects of changing horizontal resolution are studied using ensembles of seasonal simulations made by a relatively recent version of the ECMWF model. The model is integrated at T63, T(L)159 and T(L)319 spectral resolutions. The last corresponds approximately to a 0.56 degrees latitude-longitude grid and may be considered, by climate modelling standards, as very high resolution. Though, on average, no dramatically large differences in ensemble mean quantities are found between the three resolutions, some monotonic (systematic) differences with increasing resolution are evident. Whilst the better representation of orography with increasing resolution accounts for many differences between the resolutions considered, not all variations can be associated with changes in local orographic features. The existence of systematic changes with resolution indicates that the development of numerical models, especially physical parametrizations, should be carried out simultaneously at various resolutions. The study indicates that the highest model resolution does not always give the best results in terms of verification. In addition, reanalysis parameters that are mainly driven by model physics may not be optimal for the verification in such resolution studies.