On the operational predictability of blocking

Abstract

The entire 7-year archive of ECMWF operational anlaysis and forecast data is used to assess the skill of the Centre's model in short- and medium-range forecasting of atmospheric blocking. A slightly modified version of the Legenas and Okland objective zonal index is used to quantify both observed and forecast occurrence of blocking. The study is performed on 500 mPa geopotential height and on Euro-Atlantic and Pacific blocking separately. It is found that blocking frequency is severely underestimated in medium-range forecasts; the model is, on average, reasonably skilful if the initial conditions are blocked, but blocking onset is poorly represented if it occurs more than a few days into the forecast. -from Authors