Forecast drift in ERA-Interim

Forecast drift in ERA-Interim
Date Published
ERA Report Series
P.W. Kållberg, 
AbstractIn the ECMWF reanalyses, ERA-15, ERA-40 and ERA-Interim, model-generated estimates of the surface and top-of-atmosphere radiation fluxes, the turbulent fluxes and the hydrological fluxes at the surface were extracted from short-range forecasts. These estimates vary with the forecast length. This drift in the short-range forecasts can be a problem for users of monthly and climatic means. It is often referred to as 'spin-up' or 'spin-down'. Here the spin-up/spin-down properties of ERA-Interim will be summarized and compared to ERA-40. Since the comparisons are best seen directly in the maps and diagrams, the discussions will be brief. It will be seen that the conflicting requirements of 'stable forecasts' and 'close to the analysis time' can not always be met.