ECMWF analyses and forecasts of 500 mb synoptic-scale activity during wintertime blocking

Abstract

An observational study of 500 mb atmospheric blocking is conducted based on an European Centre for Medium-Range Weather Forecasts (ECMWF) wintertime analysis and forecast dataset during dynamic extended range forecasting (DERF). Eight Atlantic and ten Pacific blocking cases are identified from objective criteria in the 10-year DERF dataset. Model changes during the DERF period positively impacted the blocking forecasts by reducing regional and overall northern hemispheric RMS height errors during blocking events; however, the reduction is commensurate with average detrended wintertime statistics. Initial-condition studies reveal that the blocking pattern develops more slowly in the forecasts than in the observational analyses. Synoptic-scale barotropic forcing on both the zonal-mean and time-mean flow and storm track positions are studied during periods of block genesis (day -4 to day +4 relative to blocking onset). Maxima in synoptic-scale eddy activity are displaced onto the northern flank of the block while upstream there is evidence of enhanced synoptic-scale activity throughout block genesis. The Pacific mature phase (day +4) exhibits a wave packet that propagates from the Pacific Ocean into a zonally elongated Atlantic storm track. During block genesis, Atlantic disturbances fail to propagate into the Pacific storm track. Barotropic height tendency forcing derived from synopticscale time-mean flow increases jet tendencies north and west of the blocking center. Maximum forcing is displaced one-quarter wavelength upstream for Pacific blocks; the forcing is stronger for, and in phase with, Atlantic blocks. Anomalously poor forecasts reveal weaker westerly forcing upstream of the respective blocking centers and more energetic wave trains propagating downstream from the western Pacific source region. For poorly forecasted Pacific blocks, the maximum barotropic forcing propagates downstream embedded in a stronger zonal current while, in poorly forecasted Atlantic blocks, the barotropic forcing moves onehalf to one- quarter wavelength upstream from the blocking center, balancing stronger zonal advection. ECMWF skillful forecasts require downgradient heat fluxes and/or mean vorticity advection to force the onset height tendency " pulse", especially in the Pacific events.