OPERATIONAL PREDICTABILITY OF WINTER BLOCKING AT ECMWF - AN UPDATE

Seven winters of analyses and forecasts from the operational archives of the European Centre for Medium Range Weather Forecast had been prev iously analyzed to assess the performance of the model in forecasting blocking events. This work updates some of this previous diagnostic wo rk to the last five winters, from 1987/88 to 1991/92. The data sent th erefore covers all winter seasons (DJF) from 1980/81 to 1991/92 and co nsists of daily northern hemisphere 500 h Pa geopotential height analy ses and of the ten corresponding forecasts verifying on the same day ( ''Lorenz data''). Local blocking and sector blocking have been defined , using different modifications of the original Lejenas and Okland ind ex. The comparison between the first seven and the last five winters, within the restrictions imposed by limited length of the data set, sug gests a much improved situation as far as model climatology of blockin g is concerned, especially over the Euro-Atlantic region. Operational predictability of blocking as an initial value problem is also shown t o be measurably improved, in both Atlantic and Pacific sectors. All su ch improvements are shown to have taken place together with a consider able reduction of the model systematic error. Nevertheless, forecastin g blocking in the medium range remains a difficult task for the model. More work is needed to understand whether the improvements are to be ascribed to the increased model resolution or to better physical param etrisations.