1956

The General Circulation of the Atmosphere: A Numerical Experiment

Norman A. Phillips

Observation and Context

In the mid-20th century, meteorologists gained a much better understanding of global weather patterns due to expanded upper-air observation networks established between 1935 and 1945. Scientists observed that the earth’s atmosphere acts as a highly inefficient engine, converting only a tiny fraction of incoming solar energy into kinetic energy (motion). They also observed that atmospheric winds create distinct global patterns, such as the strong westerly winds and high-altitude “jet streams” in middle latitudes, and easterly trade winds near the subtropics. Furthermore, research showed that large-scale weather disturbances, like cycling storms (eddies), play a critical role in moving heat and momentum across latitudes to balance global temperatures.

Hypothesis

Norman A. Phillips hypothesized that the gross features of the general circulation of the atmosphere could be accurately simulated and predicted using a simplified hydrodynamic model. He proposed that a numerical model could successfully generate realistic global wind patterns, temperature structures, and atmospheric disturbances starting from a state of total rest, by applying a prescribed distribution of solar heating and simple empirical friction laws.

Experiment and Methodology

To test this, Phillips developed a “two-level quasi-geostrophic model” that represented the upper and lower halves of the atmosphere using data points at the 250-mb and 750-mb pressure levels. The experiment was performed using a simplified rectangular grid on an electronic computer at the Institute for Advanced Study.

The methodology was divided into two phases:

1. Preliminary Phase (Days -130 to 0): The model was run without any horizontal wave variations to build up a basic temperature difference between the hot “equator” and cold “pole” using linear heating functions.
2. Main Phase (Days 0 to 31): After a realistic temperature gradient was established, a small random wind disturbance was introduced. The complete physical equations — incorporating lateral friction, surface skin friction, and solar radiation inputs — were solved using short time steps to observe how the atmosphere changed over a month-long period.

Results and Data

The numerical experiment was remarkably successful in mimicking the real atmosphere. Over the first three weeks, the small random variations organized into a massive, eastward-moving wave pattern that closely resembled a real weather system, eventually developing into an occluded storm center complete with cold and warm fronts.

Data gathered during the run showed the formation of a powerful, realistic jet stream in the center of the simulated region. At the surface, the initial uniform winds transformed into a distinct three-tiered system: westerly winds in the middle latitudes flanked by easterly winds to the far north and south. Finally, math tracking the model’s energy transformations proved that these large weather eddies directly generate the kinetic energy needed to maintain the planet’s prevailing zonal currents.

Conclusion and Climate Impact

Phillips concluded that a basic hydrodynamical model can successfully replicate the fundamental wind and storm patterns of Earth’s atmosphere. While the simulation ultimately crashed after 31 days due to computational blending errors (truncation errors), it proved that global weather patterns are driven dynamically by the conversion of potential energy (from solar heating) into kinetic energy via large-scale storms.

This experiment had a massive impact on climate science because it was the world’s first successful General Circulation Model (GCM). It laid the foundation for modern numerical weather prediction and proved that computers could simulate long-term climate patterns, paving the way for today’s complex models used to forecast global climate change.