Blocking prediction in an ensemble forecasting system

The predictability of the atmospheric blocking phenomenon is investigated using the output of the high-resolution ensemble prediction system of the European Centre for Medium-Range Weather Forecasts. The output from the model is analysed using an objective blocking index. This is compared with the theory of Charney and DeVore that blocking is a large-scale non-linear phenomenon. A consequence of the non-linearity is that in some cases multiple quasi-stationary atmospheric states can exist for the same set of boundary conditions. It is found that the model in general produces too few blocks. Good agreement is found between the models lacking ability to predict blocking frequency and the systematic errors of 500-hPa geopotential height. It is found that there exists a limit, in the middle of the medium range, beyond which forecasts of blocking onset should be considered as probabilistic rather than dynamical. Inspection of individual blocking events adds new support to the idea that atmospheric blocking can be explained using the Charney-DeVore model.