|Title||Diagnosing the origin of extended-range forecast error|
|Publication Type||Technical memorandum|
|Secondary Title||ECMWF Technical Memoranda|
|Authors||Jung, T, Miller, MJ, Palmer, TN|
Experiments with the ECMWF model are carried out to study the influence that a correct representation of the lower boundary conditions, the tropical atmosphere and the Northern Hemisphere stratosphere would have on extended-range forecast skill of the extratropical Northern Hemisphere troposphere during boreal winter. Generation of forecast error during the course of the integration is artificially reduced by relaxing the ECMWF model towards the ERA-40 reanalysis in certain regions. Prescribing rather than persisting sea surface temperature and sea ice fields leads to a modest forecast error reduction in the extended-range, especially over the North Pacific and North America; no influence is found in the medium-range. Relaxation of the tropical troposphere leads to reduced extended-range forecast errors especially over the North Pacific, North America and the North Atlantic. It is shown that a better representation of the Madden-Julian Oscillation is of secondary importance for explaining the results of the tropical relaxation experiments. The influence from the tropical stratosphere is negligible. Relaxation of the Northern Hemisphere stratosphere leads to forecast error reduction primarily in high latitudes and over Europe. However, given the strong influence from the troposphere onto the Northern Hemisphere stratosphere it is argued that stratospherically forced experiments are very difficult to interpret in terms of their implications for extended-range predictability of the tropospheric flow. The results are discussed in the context of future forecasting system development.