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The Life and Science Ol Tor Bergeron The Life and Science ol Tor Bergeron Editor's note: When a prominent scientist in our profession dies, his working notes are often inaccessible to the general scientific community and the public. In the case of Tor Bergeron, however, we are fortunate to have some of his personal notes on the ice nucleus theory. The Notes were submitted by Duncan C. Blanchard of the State Uni- versity of New York at Albany, who has also prepared an introduction describing how the Notes came about. We are also pleased that one of Bergeron's colleagues, Arnt Eliassen of the University of Oslo, has prepared a necrology of Bergeron, which appears first in the series that follows. In August, a Bergeron Memorial Lecture will be given by Helmut K. Weickmann of ERL/NOAA at the AMS Conference on Cloud Physics and Atmospheric Electricity being held in Issaquah, Wash.; the lecture will appear in the Preprint Volume. Tor Bergeron, 1891-1977 With the death of Tor Bergeron on 13 June 1977 the science of meteorology has lost one of its most creative and original minds. Bergeron's contributions cover a wide range of sub- jects, and, to a greater extent than is realized today, his ideas are behind much of what is now considered basic meterological knowledge. He probably did more than anybody to bring order and a system to the seemingly chaotic realm of weather phenomena. Bergeron was born in 1891 in England, but he went to school in Sweden, and received his B.Sc. in Stockholm in 1916. From 1919 to 1922 he was employed as a Me- teorologist at the Swedish Weather Service. However, much of the year 1919 he spent in Bergen, assisting Vil- helm and Jack Bjerknes with their newly organized fore- casting service. Tor Bergeron with the Pluvius rain gage. The year before, J. Bjerknes had presented his famous cyclone model. So far, however, this model was static, mann; there he worked together with G. Swoboda and and it came to life only after two additional discoveries G. Schinze. Together with Swoboda, he published in had been made, on the basis of careful synoptic analyses: 1924 a paper that demonstrates the application of the Solberg showed that cyclones may form as small, amplify- Bergen methods in the analysis of a particular weather ing waves on a preexisting polar front, and Bergeron situation over Europe. discovered the complicated occlusion phenomenon, lead- In 1928 came the first part of Bergeron's impressive ing to the final stage in the life cycle of cyclones. Thus work, "Uber die dreidimensional verkniipfende Wetter- the cyclone model of the Bergen School was extended analyse" (in Geofysiske Publikasjoner)1 for which he from an instantaneous picture into a dynamic process. was given the Ph.D. from the University of Oslo. The Today the occlusion process is a habitual part of our paper is a tremendous effort to classify and systematize basic meteorological knowledge, and is displayed on the physical processes leading to the changing weather. every weather map. To appreciate Bergeron's achieve- It gives ideas and conclusions over a wide spectrum of ment, however, one must consider the fact that, at the subjects, although it has two main themes: air mass time, the idea was completely unknown, although me- analysis and frontogenesis. In addition, the paper con- teorologists had been looking at weather maps for more tains an early version of Bergeron's ice crystal precipita- than 50 years. tion theory. In 1922 Bergeron returned to Bergen as a Meteorologist The concept of air masses goes back to Dove in the at the Bergen Weather Service, a position he held until middle of the 19th century and was used by many other 1929. Part of this time (1923-25) he spent at the Geo- physical Institute in Leipzig, headed by Prof. L. Weick- i Now Geophysica Nowegica. Bulletin American Meteorological Society 387 Unauthenticated | Downloaded 09/28/21 05:41 PM UTC 388 Vol. 59, No. 4, April 1978 authors before the Bergen School. It was inherent in the At the International Union of Geodesy and Geophysics polar front concept; J. Bjerknes and Solberg, in their assembly in Lisbon in 1933, Bergeron presented a paper: papers from 1921 and 1922, state that the warm air "On the Physics of Cloud and Precipitation," which is south of the front originates from the subtropical high probably his most important contribution. Here he dis- pressure belt and the cold air north of the front, from cussed several conceivable mechanisms for precipitation polar latitudes. However, it was Bergeron who, in his release in clouds and advanced his ice crystal theory. 1928 paper, first took up a systematic study of air masses, Alfred Wegener had pointed out already in 1911 that their source regions, and transformations. He arrived at in a mixed cloud at temperatures below freezing, con- a rather elaborate classification based on the four main taining both undercooled droplets and ice crystals, the types: equatorial, tropical, polar, and arctic air. In par- droplets would evaporate, their mass being transferred ticular, he was looking for quasi-conservative air mass by diffusion toward the ice crystals, which would grow properties, and as one of these (besides potential tem- correspondingly. But this process does not lead per se to perature) he introduced a measure of aerosol content release of precipitation. The important point, which based on visibility. apparently Bergeron was the first to recognize, is the In his discussion of air mass transformations, he intro- condition that the number density of ice particles must duced the important notion of air masses that are cold be much smaller than that of the droplets (but not too or warm, respectively, relative to their underlying surface small). Only then will the ice particles grow to a size and discussed their characteristic temperature structure, sufficient to release precipitation. cloud and precipitation forms, visibility, and turbulence Bergeron's paper was the beginning of a rapid de- activity. This notion was not only a valuable forecasting velopment of cloud physics. It forms the basis for the tool but could also be used diagnostically, leading to methods of artificial precipitation, as well as cloud or what Bergeron termed indirect aerology: from surface fog dissipation, by means of cloud seeding. Although observations of cloud, precipitation forms, and other we know that precipitation can be released also without weather characteristics, it is possible to draw conclusions the presence of ice particles, Bergeron's ice crystal theory concerning the vertical thermal structure. still represents one of the cornerstones of cloud physics. The other important contribution contained in Ber- From 1929 to 1935 Bergeron worked as a Meteorologist geron's 1928 paper was the concept that fronts form in and consultant at the Norwegian Meteorological Insti- the vicinity of hyperbolic points in the large-scale hori- tute in Oslo. In 1936 he returned to Sweden as Senior zontal motion field, where, as a result of advection, Meteorologist and later became Scientific Chief of isotherms become crowded and oriented along the di- Weather Service at the Swedish Meteorological and Hy- latation axis. This idea enabled Bergeron to determine drological Institute, a position he held until 1947. During the location of the principal frontogenetic zones be- these years he did much to improve weather analysis tween the large-scale semipermanent low and high pres- and forecasting in the weather services of Norway and sure systems and thus give an explanation of the exis- Sweden. He was also a very active member of the Com- tence of the main frontal systems on the globe. He mission of Synoptic Meteorology of the World Mete- developed this idea even more convincingly in his paper orological Organization and its predecessor, and had a "Richtlinien einer dynamischen Klimatologie" (in Me- great influence on the internationally adopted terminol- teorologische Zeitschrift, 1930). ogy and classification of clouds and hydrometeors. Of the Bergen School team, Bergeron is distinguished In 1947 Bergeron became Professor of Synoptic Mete- by his intimate knowledge of earlier meteorological ideas. orology at the University of Uppsala. He now returned His papers are filled with references and comments to to his work on cloud physics. In a series of papers, he German, British, and particularly Austrian authors and discussed the precipitation release mechanisms in various thus form a good starting point for historical studies of cloud forms: cumulonimbus, warm front clouds, and pre-Bergen meteorology in Europe. orographic clouds. He introduced the useful notion of During the winter of 1928-29 Bergeron worked at the an upper "seeder" cloud, which provides the necessary Meteorological Office in Malta, acting as a missionary for ice crystals, and a lower "feeder" cloud, which supplies the methods of the Bergen School. In 1930 and 1932 he most of the water mass to the falling particles. He dis- lectured in Moscow on the polar front meteorology and cussed the prospects of cloud seeding and came to the air mass analysis. Part two of his 1928 paper, dealing with important conclusion that the possibilities of attaining the physics of atmospheric fronts and their perturbations, a significant increase of precipitation in this way are was published in Moscow in 1934. In the preparation of particularly good in orographic clouds lacking an effi- cient release mechanism. his paper, he was assisted by his Russian student Vera Romanovskaja, who became his wife. Bergeron's lectures In 1953 Bergeron started a research project on meso- in Moscow had considerable impact on the progress of scale rainfall distribution, which he named Project meteorology in the Soviet Union. Thu9 the well-known Pluvius. Realizing that existing rainfall-measuring net- textbook by Khromov, which was translated into German works are much too coarse to reveal the detailed distri- by Swoboda (Einfuhriing in die synoptische Wetter- bution, he organized experimental rainfall networks of analyse, 1940, Springer), was to a large extent based on very high resolution in several regions in Sweden.
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