The Bergen School of Dynamic Meteorology and Its Dissemination
The Bergen School of Dynamic Meteorology and Its Dissemination
Meteorology as a modern science evolved after the beginning of the twentieth century, from nineteenth-century origins as a subdiscipline of geography, unsystematically defined by subjective rules of weather forecasting. Defining the motion and phenomena of the atmosphere by the mathematics of hydro- and thermodynamics and capable of being predicted by systematic methods of data analysis and forecasting was the groundwork of Norwegian theoretical physicist Vilhelm Bjerknes, assisted by his young colleagues in founding the Bergen School of meteorology.
By the middle of the nineteenth century the study of meteorology, the motion and phenomena of the Earth's atmosphere, had progressed from isolated attempts at practical forecasting of the direction of weather patterns by unsystematic observational techniques and dependence on climatological data to focus on low pressure centers. Any mathematically grounded theory of atmospheric motion was limited to basic understanding of the general flow of the atmosphere on a rotating Earth, as in the work of American William Ferrel (1817-1891), followed by frictional considerations of Norwegian Henrik Mohn (1835-1916), and some others. Of fundamental importance to future atmospheric understanding was the contemporary hydrodynamic study of vortex or circular motion and its remarkable stability and conservation of its rotational flow, particularly worked out mathematically by Hermann von Helmholtz (1821-1894).
Intrigued by the possible applications of vortex theory by way of his own research with his father in hydrodynamic theory, a young Norwegian theoretical physicist, Vilhelm Bjerknes (1862-1951), had integrated his recent research in mechanics with the vortex theory by 1898. His modifications of the theory took in consideration the Sun's heating energy and frictional energy on the two largest systems on Earth behaving as fluids: the atmosphere and the oceans. These factors brought thermodynamic as well as hydrodynamic considerations into interpreting these fluid systems, and, as a result, Bjerknes developed the theory of "physical hydrodynamics," integrating relevant aspects of these sciences to explain atmospheric and oceanic motion. Most importantly, since atmospheric motion brought about weather changes, he considered the rigorous mathematical definition of atmospheric dynamics as promising a more fundamentally accurate means of predicting weather changes, given detailed analysis of atmospheric conditions.
With both theoretical and practical goals in hand, Bjerknes obtained financial support from the Carnegie Institute in Washington, D.C. (1905) to begin his research. He attracted promising students at the University of Kristiania (Oslo) from about 1907, with the basic task to develop comprehensive observational techniques entailing the three-dimensional atmosphere (particularly, using the relatively new technique of sounding balloons) to obtain an accurate meteorological database. Further opportunities offered at the University of Leipzig in Germany between 1912 and 1917 led back to Norway with the progression of World War I. At the new university at Bergen, Bjerknes was able to found a geophysical institute, where his core of researchers included his son Jacob (1897-1975), Halvor Solberg (1895-1974), one of his earlier important collaborators Harald Sverdrup (1888-1957), and soon the Swedes Tor Bergeron (1891-1977), and Carl Gustav Rossby (1898 -1957).
Bjerknes set up the first cramped location of the geophysical institute in the Bergen Museum, where, nonetheless, he continued an intensive program based on his dynamic meteorology theory. This involved systematic daily observations of and map plotting of meteorological parameters (temperature, pressure, humidity, winds, etc.), applications of both a graphical and simple mathematical formula analysis, followed by progressively more timely and extended forecasts. With the Germans cutting off Norway and the rest of Scandinavia from continental European weather communications, Bjerknes was able to convince the Norwegian government of the importance of his synoptic meteorology and forecasting to Norway's interests, thus obtaining the cooperation he needed to organize a countrywide network of observation stations for practical forecasting, as well as for continuing his research. In July of 1918 the experimental West Norwegian Meteorological Service at Bergen (in the attic of the Bjerknes home, actually) was inaugurated.
As other contemporary meteorological investigators, the Bergen researchers had been concentrating on analysis of storm movements, posing the most practical application of accurate forecasting. German research under military authority had been emphasizing this application, especially in regard to new air warfare. Large-scale storm movement usually entailed a developing low pressure center (counterclockwise or cyclonic-upward-converging movement of air in the northern hemisphere, opposite in the southern), but a large cloud band, labeled the "squall line," brought the precipitation. A lack of comprehensive data analysis kept other researchers in Germany and England from regarding these two phenomena as separate.
Bjerknes' son Jacob had been carrying out extensive surface analysis of the atmospheric parameters of wind, temperature, and precipitation along the squall line from 1917 and had discovered that temperature gradient (change of temperature) was large across the line and that wind converged along it to coincide with the clouds and weather patterns. Jacob realized that here was a key to predicting a weather pattern's movement by analyzing wind convergence with its direction and speed and precipitation. This new development was adopted by his father as a means of validating the Bergen team's forecasts of the future movement of such storms. The density of observations obtained had enabled the researchers to gain a much more detailed picture of the mechanics of storm movement. And Jacob also discovered from his surface analyses of convergence that a low pressure center developed out of the converging squall line, along with other lines of convergence. This development of squall line and low pressure into a cyclone was termed "cyclogenesis." Jacob called this atmospheric formation the "extratropical cyclone" and published a paper on it as a model of storm formation, On the Structure of Moving Cyclones (1919). The squall line and developing low pressure were integrated phenomena. Here was a recurring, large-scale system of the atmosphere, revealing a definite progression of weather patterns and an efficient means upon which to base a weather forecast. Using this model Jacob was able to extend forecasts of its movement to as much as a week in advance.
The other Bergen researchers were also finding things of note in the more detailed surface and upper level data being recorded and analyzed. Halvor Solberg traced squall line patterns over time and discovered that as a squall line developed into a cyclonic wave and evolved into a low pressure center and then dissipated, a new-like formation began to the rear, and so on in sets of four, labeled the "cyclone family." Usually, the last of the four waves in the cyclone family broke loose as an outbreak of cold air southward. Tor Bergeron studied the three-dimensional structure of the new extratropical cyclone and found that in its dissipation stages it could ride over colder air in front of it or be pushed up by colder air behind. This new step in its cycle was called seclusion and "occlusion."
The basic convergent squall line separated cold air to the north from warmer air to the south, and it was actually of hemispheric proportions. Jacob and his colleagues likened this to a battle line, and lifting the term "front" from the recent world war, they renamed the squall line the "polar front" in late 1919. Solberg made exhaustive analysis of the basic lines of convergence formed as the cyclone developed, a "steering line" pushing warm air was renamed the "warm front," while the squall line proper leading the cold air from the rear was called the "cold front." The formation of these frontal boundaries was called "frontogenesis."
About mid-1918 Vilhelm Bjerknes was already in the process of spreading the Bergen theory and practice of modern meteorology. Son Jacob, who became chief of the West Norwegian Weather Service and Solberg, who was sent to Oslo to set up an eastern branch as government demand for forecasts continued, was the first to inaugurate the dissemination of the practical application of the work accomplished by going to Sweden. The Swedish scientific community, as that in Germany and England, remained unimpressed with the Bergen researchers' claimed success in weather forecasting, unfairly calling it "humbug." There was a strong sense of nationalistic pride complicating weather research and forecasting theory at this time. Both German (M. Margules, 1856-1920) and English meteorologists on similar tracks of defining airmass boundaries, especially British meteorologist William Napier Shaw (1854-1945), criticized the Norwegians for not giving credit to others as precursors to the Bergen theory. Part of Vilhelm Bjerknes's dissemination strategy was to placate the competitors, the influential Shaw in particular, and urged acceptance of the Bergen model and forecasting techniques.
Into the 1920s the Bergen group's forecasting success could hardly be ignored, and converted British meteorologists began to credit the Norwegians with breakthrough theory and practice in analyzing and forecasting the weather with validation of the Bergen techniques. The Norwegian weather forecasters showed the way to further practical applications by issuing special forecasts tailored to such commercial interests as Norwegian fishermen and farmers. Vilhelm Bjerknes left the geophysical institute at Bergen, which he had founded, to return to theoretical physics pursuits at the University of Oslo in 1926 but was a avid lecturer on the dynamic and synoptic meteorology he had initiated until his death.
Jacob and Solberg completed the mature efforts of their research with the publication of the 1922 paper "The Life Cycle of Cyclones and the Polar Front Theory of Atmospheric Circulation." They also applied results on local vertical development of instability in the atmosphere and changes in other parameters indicating weather changes (such as humidity) to the new aviation industry. Jacob remained head of the weather service until 1931, then, as a professor of meteorology at the geophysical institute, continued his research into the influences of what appeared in upper air wind data as an upper wave above frontal development . Rossby and Solberg independently studied this phenomenon on a mathematical basis. Solberg looked at a more general mathematics of atmospheric waves out of phase but superimposed on one another at different layers in the atmosphere as reflecting Jacob's observed data. But Rossby would go further.
In 1928 Rossby came to the United States to lecture at MIT on the Norwegian meteorology theory and was given the opportunity to found the first modern department of meteorology in the U.S. Rossby was a forceful proponent of the theory and its potential, and in this gained the attention of the United States Weather Bureau, which he roundly characterized as more given to bureaucracy than to a scientific approach to meteorology. He became the head of the Weather Bureau and was responsible for its complete revamping on the Norwegian weather service model. Rossby would also develop a meteorology program at the University of Chicago in the 1930s and continued to pursue data gathering and analysis of the upper troposphere. Using an automatic recording instrument package called a meteorograph which could be raised by balloon, Rossby was able to compile much data to a height of 65,000 ft (19,812 m), particularly noting a band of unusually strong winds with core speeds as much as 300 mi (483 km) per hour. It was not until late in World War II that B-29 bomber pilots verified Rossby's tracking of these winds, flying at altitudes above 20,000 ft (6,096 m). These winds were to be called the "jet stream." Rossby also defined the upper wave (first noted by Jacob and Solberg and later called the Rossby Wave) above a surface cyclone wave as integrated with this band of strong winds. Together they developed and steered the surface cyclone and its frontal development.
Jacob came to lecture at MIT in 1939 but with the German invasion of Norway in early 1940, he decided to accept an offer from Joseph Kaplan of the UCLA physics department to head a meteorology annex. By the beginning of the war it was realized that weather forecasters would be essential to a worldwide conflict, particularly in regard to the high technical level of aerial warfare. Kaplan wanted UCLA to be one of the universities (MIT was one) chosen for military forecaster training, and enticing Jacob was an important selling point. Before war's end Jacob would be asked to found a full department of meteorology at UCLA, rivaling MIT as a center of modern synoptic meteorology and forecasting. With another of Vilhelm Bjerknes collaborators Jorgen Holmboe (1902-1979), Jacob finalized research on the integration of the upper and lower atmosphere in the development of the extratropical cyclone in their joint paper, "On the Theory of Cyclones" (1944). By the end of the war, the polar front theory of the Bergen School was widely accepted.
The immensity and changeableness of the atmosphere had long foiled attempts at realistic (time-valued), mathematically driven forecasts of weather simply because the number of equations could not be solved fast enough to be of value. But progress with the high speed digital computer resulted in, among late 1940 applications, a program of research applied to weather forecasting headed by mathematician John von Neumann in 1946. Intrigued by the challenges of weather forecasting posed by U.S. Weather Bureau meteorologists, von Neumann focused his computer's (Mathematical Analyzer, Numerical Integrator and Computer or MANIAC) initial tasks on solving equations of atmospheric flow over time. In putting together a team of scientists for the program, he turned to a young MIT scientist for important meteorological expertise. This was Jacob Bjerknes's first doctoral recipient in meteorology, Jule Gregory Charney (1917-1981). Charney had worked out mathematical equations of instability in the atmosphere in association with the upper level wave both under Bjerknes and Rossby and integrated his results into the program for MANIAC. The first successful forecast from MANIAC came in 1952—forecast of a snowstorm of 1950. By 1955 the Weather Bureau incorporated the computer into the process of analyzing and forecasting the weather. The foundation of that and a modern meteorology of airmass movement and major weather phenomena boundaries via the frontal theory was the legacy of the Bergen School initiated by Vilhelm Bjerknes.
WILLIAM J. MCPEAK
Editors of Time-Life. Weather. New York: Time-Life Books, 1968.
Holmboe, Jorgen, George Forsythe, and William Gustin. Dynamic Meteorology. New York: Wiley, 1945.
Wurtele, Morton G., ed. Selected Papers of Jacob Aall Bonnevie Bjerknes. Los Angeles: Western Periodicals Co., 1975.
THE JET STREAM CURRENT
The conservation of momentum analogy that increasing speed of a weight on the end of a string being wound on a pencil brought up the question of why the same thing did not happen in Earth's atmosphere. Since Earth rotates and air rising over the equator tends toward the poles, the air at the poles should be moving at speeds far greater than at the equator. American meteorological theorist William Ferrel (1817-1891) noted that surface friction kept such winds from developing. But how such a building of momentum was balanced continued to be questioned. With the development of frontal theory and emphasis on atmospheric instrument sounding, the dynamic structure of the upper troposphere became an important area of research in the 1920s.
Winds of high velocity above about 20,000 feet (6,096 m) were recorded and particularly researched by Swedish meteorologist Carl-Gustav Rossby (1898-1957). Using balloon-borne automatic recording instruments (meteorographs) in the 1930s, he predicted a high wind belt based on his theory (1939-40) that such wind was the balance conserving the angular momentum of the atmosphere. The validation of this came in 1944 toward the end of World War II when American B-29 Superfortresses, the first planes to fly at 30,000 feet (9,144 m) with speeds of up to 350 miles (563 km) per hour, encountered headwinds of 200 or more miles (322 km) per hour slowing their bombing runs westward toward Japanese targets. Planes flying eastward found the opposite—flying at assisted speeds totaling 500 to 600 miles (805 to 965 km) per hour—arriving at destinations hours earlier. These maximum wind belts were coined "jet stream" winds, and meteorologists would find that they influenced the intensity and direction of lower level weather system development, such as frontal cyclones, tornadoes, and the Indian monsoon bursts.
WILLIAM J. MCPEAK