Bjerknes, Jacob Aall Bonnevie
BJERKNES, JACOB AALL BONNEVIE
(b. Stockholm, Sweden, 2 November 1897; d. Los Angeles, California, 7 July 1975)
Bjerkness father, Vilhelm. has often been called the father of modern meteorology. This title is doubly appropriate, for not only did Vilhelm’s vision and research programs lead to establishing new directions, methods, and the conceptual foundation for the science, but Vilhelm’s son also played a fundamental role in creating this new era in atmospheric science. Science was very much part of the world into which the younger Bjerknes was born. His father was professor of mechanics and mathematical physics at the Stockholm Högskola (later University of Stockholm); his father’s father, Carl Anton, was professor of pure mathematics at the Royal Frederick University (later University of Christiana [Oslo]); his mother, Honoria Bonnevie. studied natural science; and an aunt, Kristinc Bonnevie, zoologist and embryologist, became Norway, s first woman professor.
Bjerknes was a reserved child. At home he had to compete for his parents’ attention with a sickly older brother and twin younger brothers; at school he was harassed by Swedish classmates, especially when Norway broke away from the union with Sweden in 1905. The family moved home to Norway in 1907 after Vilhelm received a personal professorship in mechanics and mathematical physics at the University of Christiania. His research, which eventually proved crucial for Jacob, entailed trying to establish a physics of atmospheric change. When Vilhelm moved to the University of Leipzig in 1913 to head its new Geophysical Institute, Jacob first continued his schooling in Norway, but in 1916 enrolled at Leipzig. Here he began research as one of his father’s two assistants, funded by the Carnegie Institution. In 1917 the family moved to Bergen, where as part of a plan to expand the Bergen Museum into a university, Vilhelm became professor at a new geophysical institute. Although a university was not established until 1948, Vilhelm Bjerknes established a weather forecasting service in Bergen and Jacob became its head.
As chief forecaster at Bergen (1918–1931). Jacob. along with other young Norwegian and Swedish scientists there, effected a major transformation in meteorological practice and theory. In 1931 he assumed the professorship of meteorology at the Bergen Museum. Now able to devote considerably more time to research and also to rely on increasingly sophisticated technologies for obtaining observations in the upper troposphere, he began path-breaking studies relating lower atmospheric weather phenomena to wave patterns in air currents higher up. During the time spent at Bergen, on 11 July 1928, Bjerknes married Hedvig Borthen I hey had a son and a daughter.
When the Germans invaded Norway in April 1940. Bjerknes was in the United States, where he was helping to introduce the Norwegian forecasting methods and research programs. Not able to return home, he accepted a professorship of meteorology at the University of California at Los Angeles, where Jø Holmhoe, a former assistant to his father, was already working. The two developed a major academic department of meteorology and during the war trained a large number of military meteorologists. Bjerknes initiated several pioneering research programs related to upper atmospheric patterns, atmospheric general circulation, and interactions between the atmosphere and the oceans. Although he remained in Los Angeles until his death, he kept close contact with Norway and Norwegian meteorology, both by visiting there and by inviting Norwegians to come as guest researchers to the new “Bergen” on the Pacific coast.
Jacob Bjerknes is said to have discovered in 1918 that cyclones are composed of weather fronts, that is, three-dimensional surfaces of discontinuity separating air masses of different origin and physical characteristics. Although use of the term “discovered” enabled the Bergen scientists to legitimize their findings by imputing that they were in nature, waiting to he seen, the actual process of constituting new concepts and models was more complex and reveals considerableingenuity and insight. At Leipzig, Jacob had inherited the research of Herbert PetzokL the Geophysical Institute’s first doctoral student. Before being called into the army, Petzold was given the problem of studying the kinematics and dynamics of line squalls, which are long, narrow bands of thunderstorms and showers accompanied by strong winds.
These puzzling storms had become increasingly significant because of the danger they posed \o aeronautical operations. Vilhelm Bjerknes understood that aviation held the greatest potential for meteorology’s growth and renewal as a discipline; the Leipzig institute’s origin and mission was bound to German aeronautical development. His research program at the time entailed finding graphic methods for directly applying known hydrodynamic and thermodynamic principles to the atmosphere and oceans. Jacob began studying line squalls by relating them to the kinematics of wind How in proximity to so-called lines of convergence, in which, on a horizontal plane, wind converges from two directions.
In Bergen, Vilhelm Bjerknes sought to establish resources and markets for his meteorology. faking a cue from the massive buildup of meteorological services among the warring nations, he first tried to interest the military in creating a field weather service for Norway’s growing air force. When a threat of famine confronted Norway in 1917 and 1918, government intervention in all aspects of agricultural production prompted Bjerknes to propose implementing, during the summer of 1918, an experimental weather service for western Norway and an expansion of the existing Norwegian Meteorological Institute in Christians.
Without any weather observations from Britain and Iceland during the war, forecasting for western Norway seemed hopeless if one used traditional methods based on delineating patterns of atmospheric pressure, In order to overcome the lack of observations and to experiment with what he called rational precalculations of atmospheric changes, Vilhelm adapted the structure of ihe German military field weather service: an extremely tight network of observation stations using telephone and wireless telegraphy for rapid exchanges of detailed weather data. Jacob’s study of lines of convergence in the wind field offered a possible cognitive foundation for forecasting, in that these lines were associated with rain patterns. It was hoped that a dense network of observation stations on the many skeiries and islands off the coast might allow the early detection of lines of convergence, which in turn might permit calculation of their motion for the next lew hours according to a formula developed by Jacob while he was at Leipzig.
Although Jacob found that the data did not permit calculations, during the course of the forecasting he concluded that moving cyclones (midlatitude, low-pressure systems) are composed of two such lines of convergence that separate a tongue of warm air from surrounding colder air. The cyclones rain pattern and other physical characteristics seemed to be linked to these lines in the horizontal wind field. During the next several months Jacob and Vilhelm Bjerknes, along with Halvor Solbcrg, modified the model of 1918 several times. They attained a satisfactory version when they abandoned the original kinematic analysis of wind flow and replaced it with a more physical approach stressing the actual weather. Consequently, the geometric two-dimensional lines of convergence were replaced in their thinking and model by three-dimensional surfaces of discontinuity separating air masses of differing physical properties.
Although rapid transitions or discontinuities in temperature, pressure, and wind velocity had been noted in Jacob’s first model—and even by nineteenthcentury meteorologists—and although such discontinuities had been theoretically postulated, the 1919 Bergen cyclone model represented the introduction of discontinuities into meteorology as the major focus for practice and, eventually, theory. These “fronts,” as they later were termed, were endowed with scientific reality through the introduction of major changes in forecasting practice. In Norway, and especially abroad, the advent of commercial aviation prompted revolutionary changes in the types of weather data and the frequency of observation; when analyzed according to the methods devised in Bergen, the data allowed the regular reproduction of fronts in weather forecasting practice.
The cyclone model based on fronts entailed the ability to specify in time and three-dimensional space important weather phenomena associated with cyclones to a much greater degree than earlier models could achieve. During the next few years the Bergen group of Vilhelm and Jacob Bjerknes, Ernst Calwagen, Halvor Solberg, and Tor Bergeron, among others, expanded this preliminary cyclone model and accompanying forecasting methods to a comprehensive system for explaining midlatitude weather changes. The polar front, the occlusion process, an evolving cyclone model, and air mass analysis were the conceptual foundation for a new era in meteorology.
Although Jacob’s cyclone model was three-dimensional, the vertical structure of the fronts was inferred indirectly from cloud observations and from Max Margules’ earlier theoretical model of the character of boundary surfaces between air masses’ Observations from the upper air were still too sporadic and sparse to reveal the exact nature of the upper portions of fronts. From 1922 to 1923 Jacob spent a year in Zurich, where, in addition to trying to introduce the Bergen methods into the Swiss weather service, he studied weather data from Alpine observatories (some up to 4, 000 meters) to confirm the sloped character of the frontal surfaces, This work resulted in his doctoral dissertation at the University of Christiania (1924).
In the late 1920’s and 1930’s improvements in aerological instrumentation permitted better upper-air observations. Using the 1928 observations obtained with balloons and recording instruments by the Belgian P. Jeaumotte, Jacob proposed in 1932 a number of extraordinarily insightful ideas on the relation between upper-air currents and cyclonic and anticyclonic (high-pressure systems) activity in the lower atmosphere. He analyzed the flow of air over and under the sloping frontal surfaces in great detail, showing how the surface of discontinuity of a cold front eventually extends over and behind the cold air mass, where it becomes a warm front. By pointing out the wavelike character of the westerly air current in the upper atmosphere above the frontal surfaces, he had the beginnings of a theory associating the upper- and lower-level phenomena that also accounted for the wavelike pattern of the upper wind current.
After publishing in the 1930s a series of articles with the Finnish meteorologist Erik Palmén on the three-dimensional structure of clones, Jacob returned to the problem o( the theory of waves in the upper westerly wind current, now including as a dynamic factor the so-called beta-effect, the Coriolis parameter’s gradient. Using minimal mathematics and theoretical apparatus, but with extraordinary physical insight and ability to simplify in order to clarify, Jacob laid down the foundational ideas upon which the next decade’s theoreticians, such as Carl-Gustaf Rossby. would develop a new perspective on atmospheric motions. In 1944 he and Holmboe presented a theory linking cyclonic development to pressure and vorticity changes arising from horizontal divergence in a baroclinic air current. When in 1947 Jule Charney, who had been awarded the first UCLA Ph.D. in meteorology in 1946, added boundary conditions to this model, a mature theory of cyclogenesis arising from baroclinic instability was completed.
In the 1950’s Jacob Bjerknes continued research on upper-level air currents, including the newly discovered jet stream. Toward the end o( the decade he began investigating a subject that held his attention for the rest of his life: interactions between the ocean and the atmosphere, and their impact on weather, climate, and ocean currents. His work included an explanation of the “Niño” effect in the Pacific Ocean, More important, his efforts provided a stimulus to studying the two geophysical fluids— atmosphere and ocean—as a single system, a project that has become increasingly significant for science and society.
I. Original Works. Bjerknes’s publications are listed in PoggcndorfF. Vllb 404–405. His most important articles are in Selected Papers of Jacob Aall Bonnes ic Bjerknes. M. G, Wurtele. ed. (North Hollywood. Calif. 1975). Unpublished papers and correspondence have been deposited in the University of Oslow Library and the District State Archive (Statsarkiv) in Bergen.
II. Secondary Literature. An obituary by Arnt Eliassen is “Jacob Bjerknes og hans livsverk. Minneforelesning holdt ved Universilet i Oslo, 25 September 1975”. in Det Norske vdenskapsakademi. Arbok 1976. 142–156. Bjerknes, s Leipzig and early Bergen work is analyzed in Robert Mare Friedman, Appropriating the Weather: Vilhelm Bjerknes and the Constriuciton of a Modern Meteonlogy (Ithaca, N.Y., 1989). Discussions by meteorologists on various aspects of Bjerknes’s work are in his Selected Papers.
Robert Marc Friedman
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