Numerical Simulations of a Case of Blocking: The Effects of Orography and Land–Sea Contrast

Abstract

The North Atlantic blocking case of 17?30 December 1978 is briefly described synoptically and the results from several 10-day integrations of the ECMWF global numerical model are investigated to assess the role played in both onset and maintenance of the block by the global orography, the Rocky Mountains and the Tibetan Plateau separately; and by the surface exchange (land-sea contrast) and the model's physical parameterizations. While the influence of land-sea contrast is confined to partially controlling the initiation of the process, the dominating effects of global orography (and of the Rocky Mountains in particular) in conditioning both onset and maintenance of the block are confirmed once more for this case. Despite the limitations of a single case study, some efforts are made to bridge the gap between oversimplified numerical models and observational diagnostic studies. The relative success of the control run in simulating onset and early maintenance of the blocking occurrence lends some confidence to the process of exporting the results from the model to the real atmosphere.