Weather Prediction by Numerical Process

Accurate weather forecasts based on computer simulation are now produced as a routine, and have reached such a level of reliability that the rare forecast failures evoke a strong reaction in the media and amongst users. Numerical simulation of an ever-increasing range of geophysical phenomena is adding enormously to our understanding of complex processes in the Earth system. The consequences for mankind of ongoing climate change will be far-reaching. Earth System Models are capable of replicating climate regimes of past millennia and are the best means we have of predicting the future of our climate. The basic ideas of numerical forecasting and climate modelling date from long before the first electronic computer was constructed. These techniques were first developed by Lewis Fry Richardson about a century ago, and set down in this book. Richardson was concerned with establishing a scientific method of predicting the weather. Since he was not aware of the dominant role of dynamics in the short-term, he gave as much weight to small-scale physical processes as to large-scale dynamics. As a result, the algorithm he produced amounts, in essence, to a general circulation model of the atmosphere, capable of describing both weather and climate. The first explicit analysis of the weather prediction problem from a scientific viewpoint was undertaken by the Norwegian scientist Vilhelm Bjerknes. Richardson’s forecasting scheme amounts to a precise and detailed implementation of Bjerknes’ programme. Richardson had developed a versatile technique for calculating approximate solutions of nonlinear partial differential equations by numerical approximation. Realizing that it could be applied to the evolution of atmospheric flows, he laid out the principles of scientific weather prediction in this book. He constructed a systematic algorithm for generating the numerical solution of the governing equations, and he applied it to a real-life case, calculating the initial changes in pressure and wind. Although mathematically correct, Richardson’s prediction was physically unrealistic. The essence of the problem is that a delicate dynamical balance between the fields of mass and motion prevails in the atmosphere. This was absent from the initial data used by Richardson; only later did he come to understand this problem. The consequence of the imbalance was the contamination of the forecast by spurious noise. As a result, his ‘forecast’ was a failure. The significance of Richardson’s work was not therefore immediately evident, and his