Haurwitz, Bernhard

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HAURWITZ, BERNHARD

(b. Glogau, Germany [later G≢ogów, Poland], 14 August 1905; d. Fort Collins, Colorado, 22 February 1986),

dynamic meteorology, atmospheric tides, wave motions.

Researcher, educator, and author Bernhard Haurwitz was an archetypical model of scientific success in the twentieth century. Educated in Europe, effectively displaced by the upheaval there in the 1930s, and eventually welcomed to the United States, Haurwitz witnessed and participated in the explosion of interest in the sciences that blossomed in the post–World War II era. Meteorology was no exception to this rule, and Haurwitz made use of the growing supply of data to learn more about the dynamics of the tides in the atmosphere and oceans that are affected and modulated by the Moon and the Sun.

Training . Bernhard Haurwitz was born the son of a merchant father, and had one sister. Having had an interest in science from a young age (particularly astronomy), Haurwitz pursued advanced training in the areas of physical science. He studied at the University of Göttingen, after a period at the University of Breslau, where he had matriculated in 1923. While at the University of Göttingen, his interest in meteorology developed while studying under such luminaries as Richard Courant. In 1925, Haurwitz moved to the Geophysical Institute at the University of Leipzig in order to work with Dr. Ludwig Weickmann, whose paper on atmospheric waves interested him.

While in Leipzig, Haurwitz used data from early radiosonde balloon flights to predict the existence of the level of nondivergence in a hydrostatic atmosphere. This finding was of particular importance in early numerical weather prediction efforts, and remains an important theoretical assumption in many meteorological research efforts to this day. While a doctoral student, Haurwitz paid visits to Oslo, Sweden, and Bergen, Norway, in order to study with scientists who were then the world leaders in theoretical and applied meteorology, respectively. During the 1920s the group at the Bergen School, as it has come to be known, were the progenitors of the Norwegian

Cyclone Model, a weather forecasting tool that described how low pressure systems developed and evolved, and dominated operational meteorology throughout the twentieth century. Yet his interests remained in the realm of theoretical meteorology, and while in Norway he worked with such notable contemporaries as Harald Sverdrup, Halvor Solberg, and Carl Størmer. Upon returning to Leipzig, Haurwitz completed his dissertation in 1927. He then focused his attention on wave motions in the atmosphere, in particular the dynamics of billow clouds. Haurwitz completed his second (habilitation) thesis in 1931.

Early Career . In 1932, Haurwitz traveled to the United States to work temporarily at the Massachusetts Institute of Technology (MIT) with Carl-Gustaf Rossby. While at MIT, he was also employed part time at the Blue Hill Observatory near Boston. It was during this time that Haurwitz investigated the height of the typical tropical cyclone. Until then, observations in tropical cyclones at levels above the surface were virtually nonexistent. Consequently, tropical cyclones were thought to be shallow phenomena, specifically in terms of the physical depth of the constituent clouds. Previous investigators had concluded that tropical cyclones must be relatively “thin,” based largely on the observed weakening behavior of hurricanes and typhoons near mountains and other topographical features. If tropical cyclones weaken so dramatically when encountering mountains, the reasoning went, then they must not be as deep as their midlatitude cyclone cousins.

However, Haurwitz also observed that many tropical cyclones regenerated after crossing a mountain range. By extension, he surmised then that the original circulation must have been much deeper than just a few kilometers, and that regeneration was aided by the remnants of the original circulation that persisted aloft. Indeed, it has long been known that warm-core low-pressure systems weaken with height. As such, it is reasonable to expect that at some level over the center of the cyclone, the flow “smoothes out”; this is known as the surface of pressure equalization. Although below the height of the top of the tropical cyclone, this surface of pressure equalization was considered a good index for the cloud depth. Haurwitz’s 1935 article “The Height of Tropical Cyclones and of the ‘Eye’ of the Storm” used the barometric equation to show convincingly that the surface of pressure equalization should be found at 10 kilometers in realistic tropical cyclones. Later observational evidence has shown that tropical cyclones extend over the depth of the troposphere.

In early 1933, Haurwitz had occasion to visit the California Institute of Technology (Caltech) to deliver a set of lectures. While in California, Haurwitz also visited the Scripps Institution of Oceanography at the behest of Horace Byers. It was around this same time that Adolf Hitler became chancellor of Germany, and Haurwitz’s ostensibly temporary travels outside his home country became permanent. Haurwitz returned from California and remained employed by MIT until September 1935, when he went to work at the University of Toronto.

While in Canada, Haurwitz did a great deal of teaching for the university, which also had an agreement with the Canadian Meteorological Service for the instruction of its staff. He taught several sections of dynamic meteorology (the subdiscipline that describes mathematically the fundamentals of atmospheric motion) during this period, and developed a collection of lecture notes on the topic that were quite thorough and complete. Given that a satisfactory text for an introduction to the subject did not exist at the time, Haurwitz penned one, and it was published in 1941, as Dynamic Meteorology.

The time spent in Canada was some of the most productive of Haurwitz’s career. Landmark papers by Haurwitz (1937, 1940a, b) on the motion of large-scale horizontal atmospheric waves emerged from this period. Together, these papers complemented Rossby’s work wherein a detailed theoretical description was provided for the motion of horizontal planetary waves in the atmosphere. The earlier (1937) work described the westward motion for planetary scale waves with life spans well in excess of one day. The later (1940a, b) works built upon the earlier paper and included the beta effect (how the coriolis force changes as a function of latitude), the treatment of the wavelength of the system, as well as the influence of friction on planetary waves moving across a rotating, spherical Earth. With the 1940 results, Haurwitz was able to show practical benefit from his work. Specifically he was able to account for much of the motion (or lack thereof) on the part of semipermanent pressure systems in the atmosphere (e.g., the subpolar Icelandic and Aleutian lows as well as the subtropical Pacific and Azore highs). This wave class has come to be known as Rossby-Haurwitz waves.

In addition to an improved understanding of Earth’s global circulation, these findings also had broader application to the daily practice of weather forecasting, especially in the early years of numerical weather prediction. Certainly, Haurwitz’s work helped to give operational weather forecasting a firm mathematical foundation. Although the Norwegian cyclone model was useful as a conceptual tool to describe the life cycle of extratropical cyclones in the midlatitudes, it was just that: a qualitative graphical description of extratropical cyclone evolution, devoid of mathematical rigor. Throughout the 1930s and 1940s, forecasts of the future state of a cyclone were based upon pattern recognition and extrapolation toward a future state suggested by the Norwegian model. Haurwitz’s work treated the atmosphere for what it was, namely, a three-dimensional fluid whose future state could be predicted using the Navier-Stokes equations and adequate initial conditions. His work in this area helped to provide the dynamical, mathematical underpinnings of meteorology in the latter half of the twentieth century.

A Leader in Meteorology . In 1941, Haurwitz returned to the United States to work again for MIT at the invitation of Sverre Petterssen, a towering figure in the subdiscipline of synoptic meteorology. Haurwitz’s primary teaching responsibility was the dynamic meteorology course (a cornerstone of the meteorology training of both navy and army air corps personnel at MIT) as well as one devoted to physical climatology, about which he coauthored a text with James Austin in 1944. Also while at MIT, Haurwitz served as the fourteenth president of the American Meteorological Society. These obligations, and the divorce from his wife (the former Eva Schick, with whom Haurwitz had one son, Frank) of twelve years in 1946, appear to have limited Haurwitz’s research productivity during his later period at MIT.

From 1947 to 1954 Haurwitz chaired the Department of Meteorology at New York University; this move was made at the behest of Athelstan Spilhaus. During this time, he helped to oversee the assembly of the Compendium of Meteorology (1951), a collection of papers and essays on the state of meteorology at the time authored by the world leaders in the field. Haurwitz’s contribution concerned perturbation equations in meteorology. This was not only a basic treatise on the mathematics of dynamic meteorology, but also a review of his own work and that of Rossby to describe quantitatively the motion of waves on a rotating, spherical platform (e.g., a planet).

In that same year, he returned briefly to the area of tropical meteorology with a paper that addressed the motion of binary tropical cyclones. Known as the Fuji-whara effect after the Japanese scientist who wrote the first paper on the phenomenon, the term refers to the tendency of two tropical cyclones in proximity to one another, and in a weak background flow, to tend to rotate about one another’s center in a counterclockwise fashion. Haurwitz extended the work of Fujiwhara by showing mathematically that the motion was a function of the cyclones’ intensities as well as the distance between their centers.

Later Pursuits . In the years beyond 1954, Haurwitz made his home in the western United States, enjoying affiliations with the University of Alaska, Colorado State University, and the University of Colorado. Haurwitz settled in Colorado after 1959 and was elected to the National Academy of Sciences in 1960, as well as the Deutsche Akademie der Naturforscher Leopoldina in 1964. Also in 1964, Haurwitz joined the National Center for Atmospheric Research and its Advanced Study Program, which he directed from 1966 to 1969. On the homefront, Haurwitz did remarry in 1961, to Marion Wood of Colorado.

During this period, Haurwitz returned to a topic that had interested him from earlier years, the subject of atmospheric tides. In addition to the daily oscillations in the atmospheric surface pressure due to the passage of transient cyclones and anticyclones, there are also longer-scale pressure oscillations that are ever present in the background. In particular, there is the diurnal pressure tide (known as S₁) and a semidiurnal pressure tide (known as S₂). Moreover, both tides were known to be thermally driven modes and thus clearly related to the Sun’s influence. What was troublesome to Haurwitz was the relatively small size of S₁ compared to S₂, and the inconsistency between the diurnal and semidiurnal temperature oscillations. Our own human experience tells us that there is generally only one maximum temperature and one minimum temperature at any given location in a given day. This is another kind of S₁, but for temperature, and it is typically much larger than any other shorter variation. Yet the same is not true for the S₁ of pressure; it is not the dominant mode.

Until the mid-twentieth century, the dominance of S ₂ over S₁ had been ascribed to resonance theory, which demanded a free oscillation in the atmosphere with a period of very nearly twelve hours. Yet another unsavory outcome of resonance theory demanded stratospheric temperatures well in excess of what are observed normally. Later, Haurwitz (1965) published a paper that used a spherical-harmonic analysis of a global dataset to document the behaviors of S₁ and S₂ and show that the magnitude of the latter was nearly double the former, thus making another step toward a viable replacement for resonance theory. What his analysis provided was evidence for the weaker signal in the S₁ pressure tide that was later corroborated and explained by contemporaries Richard S. Lindzen and S. Kato (a sequence of events described in detail by Platzman, 1996), who showed that the absorption of solar radiation by water vapor and ozone in the stratosphere excited the S₁ pressure tide.

About a decade later, Haurwitz effectively retired in 1976. For his achievements in meteorology and geophysics, Haurwitz had received the Carl-Gustaf Rossby Award for Extraordinary Scientific Achievement from the American Meteorological Society in 1962, and the Bowie Medal by the American Geophysical Union in 1972. Bernhard Haurwitz was a twentieth-century leader in meteorology, at the forefront of its establishment as a legitimate and distinct scientific discipline. His contributions to the subdiscipline of dynamic meteorology before, during, and immediately after World War II allowed the day-to-day task of operational weather forecasting to assume a more rigorous and quantitative approach, and thus a firmer scientific footing. Because of Haurwitz’s work, our understanding of large-scale, long-period, planetary motions in Earth’s atmosphere has been altered permanently and for the better.