NORMAN PHILLIPs is a theoretical meteorologist who pioneered the use of numerical methods for the prediction of weather and climate changes. His influential studies led to the first computer models of weather and climate, as well as to an understanding of the general circulation of the atmosphere, including the transports of heat and moisture that determine the Earth's climate. His 1955 model is generally regarded as a ground-breaking device that helped to win scientific skepticism in reproducing the patterns of wind and pressure of the entire atmosphere within a computer model.

Phillips received his B.S. from the University of Chicago in 1947 and his Ph.D. in 1951. He was the first to show, with a simple general circulation model, that weather prediction with numerical models was possible. The advent of numerical weather predictions in the 1950s also marked the transformation of weather forecasting from a highly individualistic effort to a cooperative task in which teams of experts developed complex computer programs. With the first digital computer in the 1950s, scientists tried to represent the complexity of the atmosphere and its circulation in numerical equations. Nineteenth- and early 20th-century mathematicians such as Vilhelm Bjerknes and Lewis Fry Richardson had failed to come up with adequate mathematical models. Through the 1950s, some leading meteorologists tried to replace Bjerknes and Richardson's numerical approach with methods based on mathematical functions, working with simplified forms of the physics equations that described the entire global atmosphere. They succeeded in getting only partial mathematical models. These reproduced some features of atmospheric layers, but they could not show persuasively the features of the general circulation. Their suggested solutions contained instabilities as they could not account for eddies and other crucial features. Discouraged by such failures, scientists began to think that the real atmosphere was too complex to be described by a few lines of mathematics. The comment of such a leading climatologist as Bert Bolin is revealing of this skepticism. In 1952 Bolin argued that there was very little hope for the possibility of deducing a theory for the general circulation of the atmosphere from the complete hydrodynamic and thermodynamic equations. Yet, computers opened up new possibilities in the field, although the first digital specimens were extremely slow and often broke down.

Jule Charney was the first to devise a two-dimensional weather simulation. Dividing North America into a grid of cells, the computer started with real weather data for a particular day and then solved all the equations, working out how the air should respond to the differences in conditions between each pair of adjacent cells. It then stepped forward using a three-hour step and computed all the cells again. The system was slow to operate and it had imperfections, yet its completion paved the way for more researches to be carried out. It was Norman Phillips who sought to address the problems in Charney's model. The challenge for meteorologists now became the computation of the unchanging average of the weather given a set of unchanging conditions such as the physics of air and sunlight and the geography of mountains and oceans. This was a "boundary problem." A parallel problem that they had to face was that of the "initial value," where the operation of calculating how the system evolves from a particular set of conditions found at one moment becomes less accurate as the prediction moves forward in time.

Phillips was inspired by "dishpan" experiments carried out in Chicago, where patterns resembling weather had been modeled in a rotating pan of water that was heated at the edge. For Phillips this showed that "at least the gross features of the general circulation of the atmosphere can be predicted without having to specify the heating and cooling in great detail."

Phillips argued that if such an elementary laboratory system could effectively model a hemisphere of the atmosphere, a more advanced tool such as a computer should be able to do it as well. Although certainly more advanced than a dishpan, Phillips's computer was still quite primitive. Thus, his model had to be extremely simple. By mid-1955 Phillips had devised improved equations for a two-layer atmosphere. To avoid mathematical difficulties, his grid covered not a hemisphere but a cylinder, 17 cells high and 16 in. in circumference. The calculations allowed the representation of a plausible jet stream and the evolution of a realistic-looking weather disturbance over a period of a month.

This settled an old controversy over what procedures set up the pattern of circulation. The simulation-based approach became the generally accepted method to devise circulation models. For the first time scientists could visualize how giant eddies spinning through the atmosphere played a key role in moving energy from place to place. Phillips's model was quickly hailed as a "classic experiment," the first true general circulation model (GCM). Phillips used only six basic equations (PDEs) which have been since described as the "primitive equations". They are generally conceived of as the physical basis of climatology. These equations represent well-known physics of hydrodynamics. The model was able to reproduce the global flow patterns of the real atmosphere. Phillips was awarded the Benjamin Franklin Award in 2003.

sEE ALsO: Atmospheric General Circulation Models; Bolin, Bert; Computer Models; Richardson, Lewis Fry.

bibliography. Edward N. Lorenz, The Global Circulation of the Atmosphere (Princeton University Press, 2007); Spencer Weart, The Discovery of Global Warming (Harvard University Press, 2004).

Luca Prono University of Nottingham

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