The Life and Contributions of Carl Gustaf Rossby
Meteorologist and Pioneer in Atmospheric Dynamics

ABSTRACT Carl Gustaf Rossby, a Swedish born mathematical physicist and later meteorologist is among the greatest contributors to modern-day meteorology. It was through his extensive research in the general circulations, that modern day weather forecasting has emerged to what it has become today. Rossby had a well-established career in the geosciences, with a broad educational background. It wasn’t until later in his life did he come to publish his greatest works. Rossby published three works in the late 1930’s and early 1940’s that involve potential vorticity, which would help establish the ground work for his most famous work on Rossby waves. The 1936 paper shows the conservation of potential vorticity in a hydrostatic, shallow fluid state, while the 1938 paper is an extension to the highly stratified hydrostatic flow, this time using isentropic coordinates. The term potential vorticity is actually never used until the 1940 paper where it is introduced. The 1939 work by Rossby is considered by some the most famous work contributed by Rossby, as he explains the semipermanent centers of action which would become the well-known Rossby waves.

INTRODUCTION When people think of meteorology, the first names to appear are John Dalton, Gabriel Fahrenheit, William Ferrel, and Anders Celsius. These men are the founders of modern meteorology and made tremendous contributions to the field. These names appear in many text books and are known throughout the world. However one such name that is only known in the science of Meteorology and not so much outside is that is Carl Gustaf Rossby. Rossby not only developed the foundations of atmospheric dynamics, but long range forecasting would not be possible without his exquisite work. Rossby’s work on the polar front theory, and his Rossby Waves, are a few of the major contributions he made toward meteorology. His findings and explanations in the general circulation of the atmosphere are extremely important to how science has progressed over the twentieth century.
Early Life and Education Carl-Gustaf Rossby, a Swedish born Meteorologist was born 28 December 1989, and was one of five children by Arvid and Alma Charlotta Rossby. His father, a construction engineer, was able to provide his children with good education in Sweden. Here he was able to succeed in his studies which is shown in progress reports that were saved by his family. Rossby first began his college education at the University of Stockholm with the intent of studying mathematical physics. At the age of 18, he did in fact receive a degree of Filosofie Kandidat, specializing in mathematics. After Stockholm, he engaged in more study at the Geophysical Institute in Norway. It has been said that his first interest in meteorology came from a lecture he sat through by Professor Vilhelm Bjerknes. He received an assistantship to support his studies at Bergen, and was soon after given the title Meteorologist. Lewis (1960) states that while it has been said, he had great ideas about Meteorology, he was too theoretical with those ideas, and lacked the mathematical physics required to be a successful research scientist in the field. It was then when Rossby returned to the University of Stocklohm to further enhance his education in mathematical physics. He took a year off to finance his studies, during which he took the positions of meteorologist at the Swedish Meteorologic-Hydrologic Service. His first published work, published in Geografiska Annaler in 1923 was a paper that identified the major short comings and problems of meteorology. During this short two years at Stockholm, Rossby obtained a Filosofie Licentiat (the equivalent of a Master of Science in North American universities) in Mathematical Physics. During his studies at Stockholm he was able to keep up with meteorological literature while also continuing his own research. His impatience to study meteorology led him to apply for a fellowship of the American Scandinavian Foundation to work at the U.S. Weather Bureau, with the intent “to study the application of the polar front theory to American Weather.” (Lewis 1960:252). Working with a Weather Bureau forecaster by the name of Richard H. Weightman, the two compiled and published a paper in the Monthly Weather Review called Applications of the Polar Front Theory to a Series of American Weather Maps (1926). The rest of his time in the Weather Bureau was spent with great hostility toward his idea of the polar front theory and air-mass analysis. During this unsettled period he was able to present a few manuscripts to the Monthly Weather Review, about atmospheric turbulence. (Rossby 1926, 1927) In the late 1920’s, Rossby began a new adventure with MIT, that would provide him with new students, extinguished professionals, and numerous opportunities. Rossby was made a full professor by 1928, and was given the opportunity to teach a new meteorology course. This marked the beginning of numerous opportunities that would lead him to becoming a famous scientist and changing the way meteorology is seen and ultimately used in forecasting. Rossby sent Hurd C. Willett, whom he had met and corresponded with on many occasions, to Bergen, Norway for study. The two formed a team that would work together to further amplify works in the polar front theory and air-mass analysis among other major ideas. Throughout the early 1930’s, Rossby with the help of his graduate student Willett, mainly focused on thermodynamics and its applications to air-mass analysis. He co-authored a paper in 1935 which further explained how an air parcel ascending through the atmosphere can be graphically and numerically explained by thermodynamics diagrams (Rossby, 1932, 1935). After this paper, he immediately began his previous research into turbulence in the friction layer. This work would eventually lead to other works in oceanography where he would publish a rather large paper discussing ocean circulations and how friction, turbulence, and mixing would affect their generation and maintenance. (Lewis, 1960) His work in Oceanography could be described as the key piece that lead to his ideas about atmospheric fluid dynamics and would eventually lead to his most famous works in Rossby waves. The studies allowed “him to realize that lateral mixing was an important process to consider in explaining the adjustment of current systems.” (Lewis, 1960) With the help of Sir Napier Shaw, he began to realize that the lateral mixing was also evident on isentropic surfaces. This not only was a great tool for synoptic meteorology and everyday forecasting but also showed evidence of large-scale mixing in the atmosphere. These brief years was a crucial time and resulted in the publication of a few paper from 1937-1938. It was not until the end of his years at MIT, did Rossby begin his ground breaking work in the general circulation. This work would lead to two of his most famous works and one that would ultimately leave his name engraved in history, Rossby waves. The work allowed the “concept of the conservation of the vertical component of the absolute vorticity (relative plus terrestrial) in currents going from one latitude to another to be evoked. He showed that in a simple linear system this effect could account for the perturbations in the upper westerlies.” (Lewis, 1960). The above results lead to the formulation of the Rossby equation for the long waves in the troposphere. These works finally gave him status of a world-wide recognized scientist. He celebrated by taking a leadership positions as assistant chief of the Weather Bureau where he was in charge of research and education. Leaving this position in 1941, he went to the University of Chicago as the chairman of the newly created Department of Meteorology. Rossby continued his works in the general circulation and atmospheric dynamics with the help of his numerous graduate students, producing more great works. This was also the time the second World War was beginning which Rossby devoted his time and research into as well, only through Meteorology however. He trained thousands of needed meteorologists in the Navy, Army, and Weather Bureau and even went on numerous trips to investigate the problems the military was having with weather forecasting. He continued his great works in the field of meteorology throughout the 1940’s developing ideas about the jet stream and perturbations in the atmosphere. Many of the works were published which he received great admiration from around the world.

PUBLISHED WORKS While Rossby published many outstanding and vital papers to the Meteorological community, a few stand out as possibly foundations to modern Meteorology. Dicksinson (1978) even suggest these papers “laid the foundation for modern dynamics Meteorology”. It would not be easy to write such a paper without including these works. It is with the intent of this paper to address some of the major works by Rossby, and explain the significance of each to the field. While this paper is not intended to provide any type of education on Rossbys’ work, it will try to describe his findings. Lewis (1960) considers the papers presented here are considered by some to be Rossby’s most important works.
1.) The Papers on Potential Vorticity
Rossby submitted two papers on the topic of the general circulation in 1936 and 1938 respectively that basically started a new era in modern meteorology and weather forecasting. The two papers Rossby submitted are as follows: Dynamics of steady Ocean Currents in the light of Experimental Fluid Dynamics (1936), and On the Mutual Adjustments of Pressure and Velocity Distributions in Certain Simple Current Systems (1938). Before this time, potential vorticity was never heard of, yet alone used in operational forecasting. In today’s age, a world without potential vorticity would be detrimental to accurate weather forecasts. Rossby must have realized how important his works are, after Hans Ertel submitted works using Rossbys research to explain non-hydrostatic and continuously stratified flows. The first paper in 1936 discusses the idea of conservation of potential vorticity in a hydrostatic, yet shallow water fluid. What is interesting is that since this paper uses methods of fluid mechanics, it can actually also be applied to oceanography which Rossby seems to address most of his audience too. Science knows that the atmosphere also works like a fluid, which is what makes his work so incredible, that it can be applied to two fields at the same time. Both papers contain mostly equations that describe and differentiate the equations of potential vorticity.

2.) Relation between Variations in the Intensity of the Zonal Circulation of the Atmosphere and the Displacements of the Semi-Permanent Centers of Action.
During his time at MIT, Rossby began his adventures looking into atmospheric dynamics, and synoptic Meteorology, specifically large-scale circulations in the atmosphere. The paper tries to address the significant and otherwise, non-significant results of the extensive studies. While not explicitly written for Meteorology, Rossby does mention the results would be of significance to oceanographers (1939). First, Rossby describes an observed “anticyclonic cellular structure” or eddy that consists of a dry current that stretches from the north and a moist current from the south. Further evidence suggests that geography plays an important role in the distribution of these eddys with the moist eddys appearing generally west of El Paso in New Mexico, and another appearing near western Florida drawing moisture northward into the southern United States. Rossby and his colleagues constructed daily weather maps in an effort to extend the time range of synoptic forecasts. The result was to try and remove perturbations in the pressure distributions associated with fast moving cyclones. Rossby noted that during winter months, there were five perturbations, the Icelandic and Aleutian lows, the Azores and Asiatic highs and the Pacific high, however noting that one or more of these perturbations actually break up into separate parts. These perturbations are known to not appear as closed isobaric system with vertical height into the troposphere. These perturbations also shift westward before splitting up into different parts. The third part of the analysis, Rossby constructed five-day mean isentropic and pressure charts. These pressure charts indicated a deep trough existing over the United States, with it remaining stationary for a few days or even up to a few weeks at a time. Rossby makes note that no explanation is given to why this synoptic placement is given. The significance of the isentropic chart shows that south-southwesterly gradient wind over the eastern part of the country coincides with a moisture channel extending from the Gulf northeast towards the middle Atlantic states. Finally, an attempt is made to create indices for the intensity of the general zonal circulation in the Northern hemisphere. Rossby describes his methods as using five-day means of the mean pressure on each latitude circle where they are computed and plotted weekly in the form of pressure profiles from equatorial to polar regions. Rossby notes that a key work in the understanding of the dynamics of the upper level trough over the United States is given by J. Bjerknes (1937) which offers an explanation for the location and movement of these perturbations plotted with the zonal pressure distributions. In the work, Rossby begins to describe through diagrams how a trough will move eastward given an understanding of horizontal divergence and how pressure responds to such changes. He also hints at the possible explanations of shortwave troughs that can be applied to operational forecasting. Rossby notes that given an increase in magnitude of the pressure changes with decreasing radius of curvature that the short wave trough must move more rapidly than the long wave trough given the same amplitude. Rossby (1939) states that one of the most interesting findings is that “the zonal circulation varies with the intensity of the solenoids in meridional flow while the maintenance is the permanent perturbations on this zonal circulation must be the result of solenoids contained in vertical plans parallel to the latitude circles” (Figure1). This paper describes where and why such permanent perturbations are located across the Northern hemisphere which will be crucial for the future of climate research.
3.) Planetary Flow Patterns in the Atmosphere
Rossby’s Planetary Flow Pattern in the Atmosphere (1940) is one of the best works that tie in forecasting with the theory of atmospheric dynamics. The idea behind this work was to describe the factors which contribute to the prevailing flow patterns in the atmosphere and show what conditions affect the displacement of these features. The 1940 work is an addition to the 1939 work presented above. Rossby notes that the 1939 work leaves behind some unanswered question such as “Do certain preferred flow patterns exist which are more readily established than others?”, and “When will an arbitrarily prescribed flow pattern tend to remain stationary and when will it change or more?” (1940: 69) Rossby states that the gradient wind equation is unable to help understand the above question because it is always possible to find the theoretical wind blowing along the isobars such that the speed is deflected and centrifugal forces balance the horizontal pressure gradient. If the gradient wind balance is however studied with the equation of continuity however, regions of horizontal convergence and divergence will indeed appear signifying movements of intensity and locations of the pressure centers. The 1940 paper is an extremely important work for modern-day operational weather forecasting. His work showed, through the gradient wind equations and equation of continuity, how large scale weather centers moved in location and intensity (Figure2). This would start a revolution in the accuracy of weather forecasting, and not having this information in today’s day and time seems almost impossible.

Summary Carl Rossby was a major contribution to the field of meteorology. His research made modern day forecasting and weather analysis what it is today. Rossby’s background in mathematical physics leads him to be arguably the most influential meteorologists. Rossby’s papers on potential vorticity were most influential to weather forecasting because it allowed forecasters to understand large scale motions in the atmosphere and how they move thereby allowing for more accurate forecasting. The 1939 work was significant because it shows the areas of semi-permanent centers of action. These centers of action are located in the following areas; the Icelandic and Aleutian lows, the Azores and Asiatic highs and the Pacific high. These regions would eventually be important for climate research. The 1939 paper also looked into several other ideas that would lead to his most famous discovery, Rossby waves. Rossby makes note of a deep trough over the United States that can remain stationary upwards of a few days to a week or more. While he gives no explanation as to why the initial placement is given he does attempt to create an index of the zonal circulation using his 5-day mean pressure maps. He would later go on to explain how the trough moves through divergence aloft. The 1940 paper is an extension of the 1939 paper, and attempts to answer some questions that were left from the 1939 paper. Such questions are in respect to the overall flow across the upper atmosphere (Figure 2) such as are there certain flow patterns that exist more frequently than others, and a subsequent question of how do these flow patterns move or propagate. Using theoretical aspects of meteorology he suggests that the gradient wind equation alone is unable to describe the movements of the flow patterns, and it would need to combine elements of the equation of continuity. The latter would finally describe how regions of horizontal divergence and convergence are responsible for the movement and displacement of such features. It is not difficult to understand how Carl Rossby could be the most influential meteorologist. His works not only changed the way we study and analyze weather but how we forecast it. Visualizing a world without Rossby’s discoveries and knowledge would be frightening considering how much humans rely on accurate weather forecasts.
FIGURES AND CHARTS Figure 1: Rossby, 1939. Image depicts the relation between positions of Aleutian Low and Zonal Circulation Intensity.

Figure 2: Rossby, 1940. Horizontal flow patterns in a barotropic atmosphere under a steady zonal circulation.

BIBLIOGRAPHY
Dickinson, R. E., 1978. Rossby Waves-Long-Period Oscillations of Oceans and
Atmospheres. Annual Review of Fluid Mechanics. 2: 159-95.
Lewis, J. M., 1960. Carl-Gustaf Arvid Rossby 1898-1957, A Biographical Memoir. National Academy of Sciences.
Rossby, C. G., Collaborators. 1939. Relation between Variations in the Intensity of the Zonal Circulation of the Atmosphere and the Displacements of the Semi-Permanent Centers of Action. Journal of Marine Research. 2: 38-55
Rossby, C. G. 1927. Convection in the Free Atmosphere and over a Heated Surface. Monthly Weather Review. 55:1-5.
Rossby, C. G. 1932. Thermodynamics Applied to Air Mass Analysis. A Generalization of the Theory of the Mixing Length with Application to Atmospheric and Oceanic Turbulence. Papers in Physical Oceanography And Meteorology. 3:
Rossby, C. G. 1936. Dynamics of Steady Ocean Currents in the Light of Experimental Fluid Mechanics. Papers in the Physical Oceanography and Meteorology 1. (Aug): 1-43.
Rossby, C.G. 1938. On the Mutual Adjustment of Pressure and Velocity Distributions in Certain Simple Current Systems, II. Journal of Marine Research. 5: 239-263
Rossby, C.G. 1940. Planetary Flow Patterns in the Atmosphere. Royal Meteorological Society. 66: 68-87
Rossby, C. G., Montgomery, R. B. 1935. The Layer of Frictional Influence in Wind and Ocean Currents. M.I.T. and W.H.O.J. Papers in Physical Oceanography, and Meteorology. 3: No. 3
Rossby, C.G., Weightman, R. H. 1926a. Applications of the Polar Front Theory to a Series of American Weather Maps. Monthly Weather Review. 54:485-96.
Rossby, C. G., Weightman, R. H. 1926b. The Vertical Distribution of Atmospheric Eddy Energy. Monthly Weather Review. 54:321-32.