Serendipity, Scientific Discovery, and Project Cirrus Walter on Roberts Lecture

Serendipity, Scientific Discovery, and Project Cirrus Walter On Roberts Lecture

Duncan C. Blanchard Albany, New York

ABSTRACT

Irving Langmuir defined serendipity as the art of profiting from unexpected occurrences. Numerous such occurrences during World War II led Langmuir and Vincent Schaefer from research on gas masks through a variety of defense- related projects ending with the supercooling of clouds. Serendipity led Schaefer to discover that dry ice could nucleate ice formation in supercooled clouds and Bernard Vonnegut to discover ice nucleation by silver iodide. The government- sponsored Project Cirrus grew out of these discoveries. During Project Cirrus (1947-52), many serendipitous discoveries and inventions were made, opening up areas of research still being pursued today. There has been speculation on why the role of serendipity is seldom mentioned in reporting discoveries in technical journals. The aversion to it may be ego related, the feeling that chance or luck is not good science. Editors inadvertently discourage it by the straight-jacket requirements in the writing of papers. By being curious, persevering, widely read, and aware that many branches of knowledge must often be brought to bear on a problem, one can be prepared to experience serendipity when it occurs.

l.On the mountain of Mount Washington in the winter, so it is likely that he had asked this question before, if only to him- On a cold winter's day early in 1946, three men self. But this time, with Schaefer, his long-time as- climbed slowly up the Lion's Head Trail that led to sistant at his side, an animated discussion took place. the summit of New Hampshire's Mount Washington. Though Schaefer, like Langmuir, had long been fa- From time to time they stopped to rest and speculate miliar with the supercooling of clouds, it is possible over some interesting and curious observations they had that Langmuir's question caused him to pose one of made as they approached the base of a supercooled his own. What could stratus cloud that hung like a veil around the upper part be done to hasten the of the mountain. Two of the men, Irving Langmuir process of the produc- and Vincent Schaefer, were staff scientists at the Re- tion of ice crystals, a search Laboratory of the General Electric (GE) Com- prerequisite for the pany in Schenectady, New York. The other, Raymond Bergeron (1935) pre- Falconer, a member of the Mount Washington Obser- cipitation mechanism vatory, had on many occasions climbed this trail to (which works because the top. Years later Falconer (1993) recalled that as the vapor pressure of they stood looking up at the stratus cloud deck supercooled water is Langmuir said, "Vincent, why, with all this heavy slightly higher than overcast, is there just one snowflake falling here and that of ice) to get another over there?" started? During the This was not the first time Langmuir, winner of the next several months 1932 Nobel Prize in chemistry, had climbed to the top Schaefer carried out numerous experiments. He devised an elegantly ©1996 American Meteorological Society simple piece Of appara- Duncan C. Blanchard

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Unauthenticated | Downloaded 10/08/21 07:21 PM UTC tus for producing and maintaining a supercooled cloud that a definition similar to Langmuir's is used by in the laboratory. This, the now-classic Schaefer cold many dictionaries today. box, was destined to usher in a new era in experimen- The idea behind serendipity, the art of profiting tal meteorology. from unexpected occurrences, is so important in dis- Though both Langmuir and Schaefer were avid covery that some eminent scientists have given their hikers and skiers and, in general, liked the open air in own views on how discoveries are made. Louis Pasteur, all kinds of weather, that was not the primary reason in an oft-quoted statement, said that, "In the field of they were climbing Mount Washington. They had observation, chance favors only the prepared mind." been doing experiments on the top of the mountain No mention of serendipity here, but Pasteur's chance on aircraft icing. How did these two men, who just 6 and prepared mind are analogous, respectively, to years before were deeply involved in fundamental Langmuir's unexpected occurrences and art. The research on surface chemistry, end up studying air- discoverer of vitamin C, Nobel Laureate Albert craft icing in the supercooled clouds that often blan- Szent-Gyorgyi, may not have had serendipity in mind keted the top of Mount Washington in the winter? It when he said that discoveries are made by those who, was because of serendipity. "see what everybody else has seen, but think what nobody else has thought." Few, however, would quar- rel that he, Langmuir, and Pasteur were saying essen- 2. Serendipity: The word tially the same thing.

Langmuir—whose research spanned a wide spectrum of science, from gas-filled lamps, submarine de- 3. From gas masks to aircraft icing tection devices, and theories of atomic structure to plasmas (he coined the word) and surface chemistry— The chain of unexpected events that led Langmuir was forever fond of saying it was serendipity that led and Schaefer to the supercooled clouds on Mount him from one subject to another. He believed so Washington had its beginning in 1940 when the U.S. strongly in the role of serendipity in scientific discov- government, concerned about the increasing number ery that he altered the dictionary definition of the word of German military victories in Europe and Africa, by defining serendipity as the art of profiting from un- began making preparations for possible entry into the expected occurrences (Langmuir 1948). war. Afraid that the Germans might use poisonous The word originated with Horace Walpole, a man smokes against combat troops, the National Defense of letters and connoisseur of the arts in eighteenth- Research Committee asked Langmuir to look into the century England. In 1754, in a letter to a friend, he development of a mask to filter out such smokes. wrote about an interesting discovery he had made Conventional gas masks would not work. They were about a painting, a discovery he had not been seek- designed to filter out gases, not the particles that com- ing. He said this was an example of serendipity. He pose smokes. Langmuir accepted the challenge. coined the word after remembering that, "I once read It is important to note that though Langmuir and a silly fairy tale called The Three Princes ofSerendip; Schaefer had many things in common, especially their as their highnesses traveled, they were always mak- love for research, the outdoors, and the continual ques- ing discoveries, by accident and sagacity, of things tioning of nature, it is hard to imagine two men of they were not in quest of" (Austin 1978). In more different backgrounds and training. Schaefer had Walpole's definition of serendipity, the discovery dropped out of high school to help his parents sup- must be both accidental and not sought, while port a large family. About the time Langmuir won his Langmuir's makes no distinction whether the discov- Nobel Prize, Schaefer was a machinist at the General ery had been sought for or not. To Langmuir, it was Electric Research Laboratory. His skill at designing only the accidental or unexpected occurrence that and building laboratory apparatus was recognized by mattered. Though it has been argued that a definition Langmuir, who brought him out of the machine shop of serendipity like Langmuir's should be called and into his lab. During the 1930s these two men pur- pseudoserendipity (Roberts 1989), I intend to make sued a variety of problems in surface chemistry. It was no distinction between the two in discussing the ex- an extraordinary scientific symbiosis in which the amples of serendipity that follow. The reader can older Langmuir would make theoretical predictions decide which is which. It should be noted, however, for their latest scientific puzzle, while the younger

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Unauthenticated | Downloaded 10/08/21 07:21 PM UTC Schaefer approached the answer with deceptively simple most of the time, immediately becomes covered with experiments. By letting both theory and experiment ice .. . due to the presence of supercooled water drop- feed back into each other, they made rapid progress. lets. We had heard about rime, we had seen it, but we But before they could begin the development of did not know it occurred all the time at a place like smoke filters, they had to learn how to make smokes Mt. Washington. There are very few winter days with- with which to test the filters. Over many months out it. This puzzled us a lot." Langmuir theorized about the proper design of a filter, Langmuir and Schaefer eventually obtained some while Schaefer did the experiments. "That work lasted measurements on precipitation static, but the phenom- for about a year," said Langmuir (1948). "We obtained enon of supercooling kept haunting them. When fairly successful theoretical results and a better under- Langmuir found that the government was just as in- standing of how to build a good filter. But notice what terested in the problem of aircraft icing, here was an- we did incidentally; we acquired a great deal of detailed other "unexpected occurrence" that allowed him to knowledge as to how to make a smoke which would be leave the precipitation static problem and pursue one non-volatile, which would consist of very, very small related to supercooling. The two men measured the particles, far smaller than those of any ordinary smokes, rate at which icing built up on rotating cylinders of and we learned much about their optical properties." different sizes as they were being exposed to super- In the summer of 1941 there was an unexpected cooled clouds moving by at known speeds. From these occurrence. A form letter that probably had been sent data and collection efficiency equations Langmuir had to many laboratories arrived on Langmuir's desk. It developed, they could deduce both the cloud droplet said that the Germans were using light-colored smokes size spectrum and the droplet sizes responsible for to cover and hide their industrial plants. Does anyone most of the icing. And then the war ended. There were know how they make these smokes, and could we do no more government contracts, but Langmuir and the same to protect troops and landing ships? Schaefer continued to think about what it is that con- Langmuir knew that he and Schaefer could profit from verts a supercooled cloud to one of ice crystals. this, as they had learned much about smokes during the smoke filter work. They started work on the problem in the usual manner, Langmuir with the theory 4. The cold box and Schaefer doing the experiments. A smoke generator was eventually developed that was far better After returning from that early 1946 climb up than any in existence. Many thousands were built and Mount Washington, Schaefer approached the super- used during World War II. They protected ships from cooling problem with a beautifully simple experiment. kamikaze attacks in the Pacific, and their smoke He lined the insides of an ordinary home freezer with screens allowed Allied forces to land on beaches in black velvet to provide a dark background. By breath- Italy and to cross the Rhine River in Germany. ing into it, he produced a supercooled cloud. To de- Shortly after the smoke generator work was com- tect any ice crystals that might be present, he directed pleted, the government asked Langmuir to look into the narrow beam of a microscope lamp down through the question of why military aircraft were losing radio the cloud. That was it. Simple, yet very effective. If contact when they flew through snowstorms. "Well, we just a single ice crystal was present in the supercooled had no particular ideas on the subject," said Langmuir cloud of water droplets, it stood out like a bright star (1948), "except that it had to do with weather. It looked in a midnight sky. like a good opportunity to go up on Mt. Washington Schaefer spent much time over the next few months in the winter-time. . . . We thought that with all the trying to find a substance that would change the wa- drifting snow, all we would have to do would be to ter droplet cloud into one of ice crystals. An endless take different kinds of surfaces . . . expose them to the variety of materials was dusted into the cloud: sand, drifting snow ... and make measurements ... of the ground-up rock, dirt from fields and along roads, and electrical charges which develop from the impact of even substances from his own home like talcum pow- snow on the exposed surfaces." Schaefer took a lot of der and kitchen cleanser. Nothing worked very well equipment to the top of the mountain, but, Langmuir until he encountered serendipity. continues, "We found, much to our surprise, that any- On a hot, humid day on 12 July 1946, Schaefer thing exposed on the summit of Mt. Washington dur- arrived at his lab to find the air in his cold box not as ing the winter when clouds were there, as they are cold as usual. Eager to get on with his experiments,

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Unauthenticated | Downloaded 10/08/21 07:21 PM UTC he ran to another lab to get a large block of dry ice. Because the ice crystals were so small and numerous, His idea was to place it in the bottom of the box. At a Schaefer and Langmuir knew that they had to be pro- temperature of about -80°C, he expected that the dry duced primarily by a spontaneous generation process ice would rapidly cool the box to operating tempera- and not by the freezing of cloud droplets in the path ture. However, an astonishing thing happened. "The of the cold object. Many books and articles attribute instant the dry ice was lowered into the chamber," the crystal production to the latter process and seem not Schaefer (1993) wrote in an unpublished autobiogra- to realize that crystal production by homogeneous nucle- phy, "I saw to my delight that the supercooled cloud ation is orders of magnitude larger than that caused had been displaced by a strange bluish fog unlike any- by the freezing of cloud drops (Vonnegut 1981). thing heretofore seen. Quickly lifting the dry ice from Cloud physicists the world over are aware that sev- the chamber, I introduced moisture from my breath eral months after Schaefer's discovery of dry-ice and gradually decreased the density of the fine-particle cloud seeding, Bernard Vonnegut (1947) discovered blue fog until I could see the glinting of incredible that silver iodide particles made excellent nuclei for numbers of ice crystals." At that moment Schaefer had the formation of ice crystals. But are they aware that found the trigger that had eluded Bergeron (1935) 11 the discovery was in large part serendipitous? years before, a mechanism to convert a supercooled Probably not. During the months that Schaefer was cloud into one of ice crystals (Schaefer 1946). looking for an efficient way for the production of ice Schaefer and his cold box are shown in Fig. 1. crystals in supercooled clouds, Vonnegut, in another Before the day was over, he had found that when section of the General Electric Research Laboratory, the tiniest fragment of dry ice fell through a super- was investigating the supercooling of tin. Why tin? cooled cloud, it left behind a vertical thread of tens "Vince Schaefer was working on supercooled water," of thousands of ice crystals. It was not the chemical said Vonnegut (Blanchard 1985), "and I knew that su- composition of dry ice that brought about the phase percooling was a general phenomenon in physics and change in the cloud. It worked simply because it was chemistry, so I decided to work on tin." But after very cold. Schaefer later found that when he dipped Schaefer's discovery, Langmuir, anxious to have a common sewing needle in liquid air and swung it someone carry out additional research, asked through the supercooled cloud, it left behind a thin Vonnegut to come and work with his group. curtain of ice crystals. He soon established that any- Vonnegut pursued studies on ice crystal production thing colder than about -40°C (now often called the by dry ice but at the same time began to wonder about Schaefer point) could bring about a phase change. other ways to nucleate the formation of ice. Could it be possible, he asked himself, that there is a substance whose crystal structure is very close to that of ice? If so, then perhaps it will provide a nucleus for ice formation. He searched through the X-ray cry stallographic data in the Handbook of Chemistry and Physics (1995) to see what substances could be found that re- sembled ice in crystal structure, space group, and dimensions of the unit cell. He soon found that silver iodide and ice had unit cell dimensions that were the same within about one percent. Vonnegut tried some silver iodide on a supercooled cloud in Schaefer's cold box, but it didn't work at all. He might have given up at this point were it not for a series of unexpected occurrences that

FIG. 1. Vincent Schaefer working with his cold box at the Research Laboratory began a few days later. Schaefer put some of the General Electric Co. in Schenectady, New York (ca. 1946) (courtesy of iodine vapor in his supercooled the Hall of History Foundation, Schenectady, New York). cloud and found that it made some ice

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Unauthenticated | Downloaded 10/08/21 07:21 PM UTC crystals, but the effect wore off with time. Vonnegut continued cloud seeding experiments. However, re- was surprised that it worked at all, since its crystal alizing that exploration of this new branch of experi- structure was quite unlike that of ice. Shortly there- mental meteorology, as Schaefer called it, could have after, he made a metallic silver smoke by making a great significance for the country, the company en- spark between two silver coins used as electrodes. tered into an agreement with the government to con- Like Schaefer's iodine, it also worked, but again the tinue the work. A contract was signed with the U.S. effect soon wore off. The solution to this curious be- Army Signal Corps, but it had joint sponsorship with havior came when Vonnegut wondered if the silver the Office of Naval Research and the cooperation of had first worked by combining with residual iodine the U.S. Air Force, which provided aircraft and flight in the air, and then, when the iodine was used up, not personnel. This new endeavor, named Project Cirrus worked at all. "So I put a little iodine in," Vonnegut by Schaefer, ran for over 5 years, ending in 1952. said, "and, oh boy! [when the silver coins were The Project Cirrus scientific team consisted of sparked] it worked fine. I soon discovered that the Langmuir, Schaefer, Vonnegut, Falconer (who had silver iodide I first used was badly contaminated with been working in Langmuir's lab since he left the sodium nitrate, an anti-freeze, and that was the reason it Mount Washington Observatory late in 1946), and me did not work. So it soon became clear that silver iodide (I had joined the group late in 1947). During Project was actually a good ice nucleus" (Blanchard 1985). Cirrus numerous discoveries and inventions, many Yet why did Schaefer's iodine vapor work in the first serendipitous, were made. Some of these opened up place? The two men speculated that minute traces of silver in the laboratory atmosphere may have been produced intermittently (either in their own lab or one nearby) by sparks from electrical equipment using con- tacts of silver or copper that contains silver. Some of the invisible silver smoke from the electrodes had apparently drifted into Schaefer's lab and into his cold box, where it reacted with the iodine vapor to produce pure silver iodide. Had the silver smoke not drifted into the cold box, Schaefer would not have found any nucleation effect with iodine, Vonnegut would not have been motivated to spark the silver coins, and the discovery of the ice- nucleating ability of silver iodide might not have been made. Serendipity works its magic in wondrous ways! A photograph of Vonnegut and Langmuir watching Schaefer do a cold-box experiment is shown in Fig. 2. Three successive days in November of 1946 proved to be momentous for the explosion of cloud physics work around the world. On 13 November, Schaefer (1949) did the first dry-ice seeding of a supercooled cloud by flying over it and dropping about 3 pounds of crushed dry ice, thus converting much of the cloud to ice crystals. On 14 November, Vonnegut discovered the ice-nucleating ability of silver iodide, and on 15 November Schaefer's landmark paper on the discovery of the ice-nucleating ability of dry ice was published in Science.

5. Project Cirrus FIG. 2. Vincent Schaefer making a supercooled cloud in his cold box at the Research Laboratory of the General Electric Co. in Schenectady, New York. Irving Langmuir (left) and Bernard Early in 1947, the General Electric Company be- Vonnegut are interested observers (ca. 1946) (courtesy of the Hall came concerned that legal problems might arise from of History Foundation, Schenectady, New York).

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Unauthenticated | Downloaded 10/08/21 07:21 PM UTC areas of research that are still being pursued today. It is mysterious that there was no mention of the Among the accomplishments were the development role of serendipity in the announcement of the discov- of silver iodide smoke generators to nucleate ice crys- ery of dry-ice cloud seeding. This is especially curi- tals in supercooled clouds, the first seeding experi- ous since Irving Langmuir, the project head, thought ments in supercooled stratus and cumulus clouds, the so much of serendipity's role in discovery that he had first use of diffusion cloud chambers in atmospheric his own definition of the word. Willis Whitney, the science, the first measurements of the concentration first director of the GE Research Laboratory, was of ice-forming nuclei, and the first use of collection- equally aware of the magic of serendipity. Yet efficiency equations to determine raindrop growth in Schaefer (1946), in his landmark paper, does not say clouds. Other accomplishments were the development that he first thought of using dry ice simply to cool of continuously operating condensation nucleus down his cold box. But he did say it in later papers. counters, automatic sea salt particle counters, verti- Vonnegut, however, did tell his readers of the twists cal wind tunnels for the suspension of water drops, a and turns of the trail that led to his discovery that sil- new method for the measurement of raindrop size ver iodide smoke also caused ice nucleation. I distributions, and vortex thermometers to measure true (Blanchard 1949, 1967), like Schaefer, however, air temperature. made no mention of the unexpected occurrences when The pioneering work done under Project Cirrus reporting a more modest discovery of a new way to appears to be little known or remembered by many measure raindrop size. atmospheric scientists today. Its origin and achieve- The aversion by many scientists to discussing even ments are discussed in the detailed, annotated, "Early briefly the serendipitous aspects of their discoveries History of Cloud Seeding" (Havens et al. 1981), has been long recognized. The ego probably plays a and Schaefer (1968) gave a brief account of the role. The attitudes of some scientists suggest that Project Cirrus seeding experiments. The project ended mention of serendipity would detract from the rigid about the time Langmuir retired. Schaefer and logic of their arguments. In short, it would not be sci- Falconer eventually went into consulting work, entific. As Kauffman (1991) puts it, "serendipity is Vonnegut went to Arthur D. Little, Inc., in Boston, probably more prevalent than is generally realized, Massachusetts, and I, after a stint at graduate school, largely because scientists are reluctant to admit hav- started work at the Woods Hole Oceanographic In- ing benefited from chance, even though chance con- stitution in Woods Hole, Massachusetts. In the late tinually enters into the everyday activities of human 1960s, after Schaefer had founded the Atmospheric beings." Why this reluctance? Medawar (1969) be- Sciences Research Center (ASRC) at the State Uni- lieves, "scientists are usually too proud or too shy to versity of New York at Albany, Falconer, Vonnegut, speak about creativity and 'creative imagination'; they and I returned to New York to staff appointments at feel it to be incompatible with their conception of the ASRC to work once again with Schaefer on a themselves as 'men of facts' and rigorous inductive multitude of interesting research problems, some of judgments." Austin (1978) is even more adamant in which had been initiated so many years before at this belief: Project Cirrus. Science has its taboos. Scientists operate under strictures. It is never entirely in fashion to men- 6.Serendipity's lack of recognition tion luck in the same breath as science. By con- vention, the investigator is constrained to put his work in the foreground, himself in the back- Though most discoveries are made by serendipity, ground, avoid the personal pronoun at all costs. why is this so seldom mentioned in the scientific jour- Still in the firm grip of the Protestant or other nals where the discoveries are reported? Why is it such ethic, he is supposed to make his discoveries a well-kept secret? How many people, both inside and for rational reasons by virtue of his own intel- outside of science, know, for example, that serendip- lectual hard work, and he feels guilty if he does ity was responsible for the discoveries of penicillin, not live up to the code. Velcro, X rays, the ring structure of benzene, the structure of DNA, Teflon, rayon, nylon, safety glass, dy- Although a taboo against mentioning luck may help namite, and even the Dead Sea Scrolls? Very few, I to squeeze any remark about serendipity off the pages would venture to say. of a technical paper, I suspect the rigid format required

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Unauthenticated | Downloaded 10/08/21 07:21 PM UTC of these papers by most editors is equally if not more to what Kuettner (1963) said when he left the atmo- damaging. The impersonal nature of the writing and spheric sciences to work elsewhere. "Gentlemen, I the straight-jacket requirements that all writers quickly know of no better way to describe versatility than by follow a straight path from the introduction through the answer von Braun gave when he offered me the sections labeled "experimental design," "the experi- job some years ago of getting a man into space, and I ments," "results," "discussion," and ending with "con- expressed my doubts that I had enough experience. clusions," effectively gets across the message that He simply said, 'Neither has anybody else.'" those interesting side excursions from the main path I leave the last word to Vincent Schaefer. When are not to be tolerated. What a pity! Often they can asked how one could develop the ability to profit from be told in a sentence or two, or at most a short para- unexpected occurrences, he would say in four simple graph. This minor detour, the mention of the role of words: "Keep your eyes open." serendipity in a discovery, is nothing more than tell- ing the whole truth about how the discovery was Acknowledgments. I am indebted to my wife, Julie, who made. checked the manuscript for all things grammatical, and to my friend, Bernard Vonnegut, who checked it for all things scientific. 7. The art of serendipity

We have all done experiments that failed, and we References wonder why. Actually, it was not the experiment that went wrong; it was the hypothesis that may not have Austin, J. H., 1978: Chase, Chance, and Creativity. Columbia University Press, 237 pp. allowed for those unexpected occurrences that Langmuir Bergeron, T., 1935: On the physics of cloud and precipitation. spoke about. It is the recognition of these occurrences Proc. Fifth Assembly of the UGGI, Lisbon, Portugal, Union that can lead the way to a serendipitous discovery. Geodesique et Geophysique Internationale, 156-178. To Langmuir, this special ability to recognize and Blanchard, D. C., 1949: The use of sooted screens for determin- profit from unexpected occurrences was an art, while ing raindrop size and distribution. Project Cirrus Occasional Rep. 16, General Electric Research Laboratory, Schenectady, to Pasteur it was the prepared mind. How does one NY, 11 pp. [Available from Sally Marsh, Department of At- develop these abilities? There are some scientists who mospheric Sciences, The State University of New York at seem to have been born with them. They are endowed Albany, Albany, NY 12222.] with the spark of genius that enables them to see and , 1967: From Raindrops to Volcanoes. Doubleday and Co., follow a new path from experiments that have 180 pp. "failed." Yet, others without these special abilities can , 1985: Bernard Vonnegut, the gentle iconoclast. ASRC Re- port 1984-85, Atmospheric Sciences Research Center, State in part develop them. Being curious and persevering University of New York at Albany, 8-17. will help. This curiosity should not be limited to a Falconer, R. E., 1993: Vincent J. Schaefer (1906-1993). Mount particular research interest. Read widely. Get a feel- Washington Observatory News Bull., 34, 77-79. ing for some of the exciting developments not only Handbook of Chemistry and Physics, 1995: Handbook of Chem- in the atmospheric sciences but in disciplines like bi- istry and Physics. 76th ed., CRC Press, 2576 pp. ology, geology, and chemistry. Many puzzling aspects Havens, B. S., J. E. Jiusto, and B. Vonnegut, 1981: Early history of cloud seeding. J. Wea. Mod., 13, 14-88. of nature cannot be understood without bringing to Kauffman, G. B., 1991: Oops .. . Eureka!: Serendipity and sci- bear many branches of knowledge. One must, how- entific discovery. Yearbook of Science and the Future, ever, get over the idea that knowledge can be com- D. Calhoun and C. Ceglielski, Eds., Encyclopaedia Britannica, partmentalized into rigid boxlike compartments. That 224-239. is the doing of humans, not nature. I wrote in the fi- Kuettner, J. P., 1963: Evolution and mutation in the atmospheric sciences. Bull. Amer. Meteor. Soc., 44, 631-633. nal pages of my book From Raindrops to Volcanoes Langmuir, I., 1948: The growth of particles in smokes and clouds (1967) that, "Nature knows no such compartments. and the production of snow from supercooled clouds. Proc. Her realms cannot be separated completely, for part Amer. Phil. Soc., 92, 167-185. of one is part of the other. They, like the colors in a Medawar, P. B., 1969: Induction and intuition in scientific rainbow, merge smoothly and harmoniously." thought. Mem. Amer. Phil. Soc., 75, 1-62. Roberts, R. M., 1989: Serendipity: Accidental Discoveries in One can always develop the versatility to work in Science. John Wiley and Sons, 270 pp. several of nature's realms by taking the drastic but ex- Schaefer, V. J., 1946: The production of ice crystals in a cloud citing step of leaving one to work in another. Listen of supercooled water droplets. Science, 104, 457-459.

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Unauthenticated | Downloaded 10/08/21 07:21 PM UTC —, 1949: The formation of ice crystals in the laboratory and Papers, University Archives, University Libraries, State Uni- the atmosphere. Chem. Rev., 44, 291-320. versity of New York at Albany, 910 pp. —, 1968: The early history of weather modification. Bull. Vonnegut, B., 1947: The nucleation of ice formation by silver Amer. Meteor. Soc., 49, 337-342. iodide. J. Appl. Phys., 18, 593-595. —, 1993: Serendipity in Science: Twenty Years at Langmuir , 1981: Misconception about cloud seeding with dry ice. University. Unpublished autobiography, Vincent J. Schaefer J. Wea. Mod., 13, 9-10.

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The Representation of Cumulus Convection in Numerical Models

Meteorological Monograph No. 46

Cumulus convection is perhaps the most complex and perplexing subgrid-scale process that must be represented in numerical models of the atmosphere. It has been recognized that the water vapor content of large parts of the atmosphere is strongly controlled by cloud microphysical processes, yet scant attention has been paid to this problem in formulating most existing convection schemes. This monograph is the fruit of the labors of many of the leading specialists in convection and convective parameterization to discuss this and other issues. Its topics include: an overview of the problem; a review of "classical" convection schemes in widespread use; the special problems associated with the representation of convection in mesoscale and climate models; the parameterization of slantwise convection; and some recent efforts to use explicit numerical simulations of ensembles of convective clouds to test cumulus representations. American

(includes shipping and handling). Please send prepaid orders to: Order Department, American # Meteorological Society, 45 Beacon St., Boston, MA 02108-3693. SOCiefy

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Unauthenticated | Downloaded 10/08/21 07:21 PM UTC On the Existence and Strength of the Summer Subtropical Anticyclones*

Bernhard Haurwitz Memorial Lecture Brian Hoskins Department of Meteorology, University of Reading, Reading, United Kingdom

ABSTRACT

The subtropical anticyclones are usually related to radiative cooling and the descending arm of the Hadley Cell. This explanation works well for the winter anticyclone belt. However, the summer subtropical anticyclones are in most measures stronger, as well as being more longitudinally localized, than their winter counterparts. The nature of the circulations in the summer and winter subtropics, and the Tropics are compared. It is hypothesized that the basic cause of the summer anticyclones is the monsoonal latent heat release over the neighboring continents to the east. As this diabatic heating pushes poleward to near 25° latitude, so deep descent is induced poleward and westward of it. Orographic effects can act to localize this descent. Enhanced radiative cooling and suppressed convection are then viewed as am- plifiers of the descent. Under this descent is the equatorward moving air of an anticyclone, which is centered to the west. This wind acts to drive oceanic upwelling, leading to cold sea surface temperatures and reinforcement of the suppression of convection.

1. Introduction The first thing I have to do on the topic of my lecture is to persuade you that there is a question to be It is a great pleasure for me to give this third lec- answered. When we were both waiting for a bus to ture in the memory of Bernard Haurwitz. I frequently the airport after an IUGG (International Union of Ge- shared a morning coffee table with him when I was odesy and Geophysics) Assembly many years ago, in the Advanced Study Program of NCAR (National Ed Sarachek asked me why the summer subtropical Center for Atmospheric Research) in 1970-71, and I anticyclones were stronger than their winter counter- remember him with warmth. parts, given the usual explanation for them in terms In preparing for this lecture I have of course read of radiative cooling and the descending arm of the all the latest literature. At the Royal Society I found Hadley Cell. At the time I did not even know that it A Handbook of Renaissance Meteorology (Heninger was true, and even less could I think of an expla- 1960). In Renaissance times, meteorology was placed nation. All I could do alongside all other activities that tried to foretell the was hope that the bus future and was firmly in the hands of the astrologers. would come soon! The book references a work A Prognostication of Figure 1 shows av- Right Good Effect (Digges 1555), which clearly must erage mean sea level be an account of a particularly good weather forecast! pressure fields for 5 My favorite book title is, however, Knowledge of years of ECMWF (Eu- Things Unknown (Godfridus 1530). This is more for ropean Center for Me- the theoretician! dium-Range Weather Forecasts) analyses for December to Feb- ruary (DJF) and June *This speech was presented at the 10th Conference on Atmo- spheric and Oceanic Waves and Stability in Big Sky, Montana, to August (JJA). In on 7 June 1995. both winter subtropics [NH (Northern Hemi- Brian Hoskins ©1996 American Meteorological Society 1287 Bulletin of the American Meteorological Society Unauthenticated | Downloaded 10/08/21 07:21 PM UTC 2. Some features of the circulation in J J A 1994

The average 500-mb vertical velocity for JJA 1994 is shown in Fig. 2a. There is general descent in the winter subtropics, ascent over the convective regions of Southeast Asia down to the SPCZ (South Pacific convergence zone), Central America, and Africa, and along the oceanic ITCZs. There is descent in the eastern subtropical ocean basins of the NH, and there is intense descent over North Af- rica/eastern Mediterranean. This feature is discussed in Rodwell and Hoskins (1996, hereafter RH), and many of the arguments presented here are a development of those given in that paper. The diabatic heating for the same season, calculated as a residual in the thermodynamic equation using ECMWF data, is FIG. 1. Average mean sea level pressure fields for 5 years of ECMWF analysis for exhibited in Fig. 2b. The max- (a) December-February and (b) June-August. The contour interval is 2 mb, with the ima associated with the convec- 1000-mb contour dotted; contours at value greater than this are solid, and those at smaller values are dashed. tive regions listed previously are apparent. There is general cooling in the winter subtropics. sphere) in DJF, Fig. la, and SH (Southern Hemi- There is also cooling in the regions of equatorward sphere) in JJA, Fig. 1 b] there is a belt of anticyclones flow associated with the summer subtropical anticy- near latitude 30° under the descending arm of the clones. It might then be argued that the summer sub- Hadley Cell. The summer subtropical anticyclones tropical circulation is simply ascent where there is (SH in DJF, Fig. la, and NH in JJA, Fig. lb) are more heating and descent where there is cooling, there be- localized in the ocean basins, and in most cases may ing associated surface cyclones and anticyclones, re- be considered to be stronger than in winter. This is spectively. However, this would not explain the larger particularly true for the longitudinal pressure gradi- intensity of the NH summer anticyclones since the ents and associated equatorward-moving flows in the cooling is larger in the SH winter. It also begs the eastern ocean basins, which are strong features of all question of why the cooling is present in the eastern the summer anticyclones. It is useful to note here that ocean basins of the summer hemisphere. there is a similar equatorward flow in JJA in the east- A scale analysis of the thermodynamic energy ern Mediterranean. In 887-mb streamfunction this equation, as in Hoskins (1986), suggests that the bal- flow has an associated anticyclonic center over Tunisia. ance of terms depends on the Burger number B The aim of this lecture is to discuss the nature of - N2H2/f2L2. On the large scale in the extratropics, B the circulation in the summer subtropics in general and is smaller than one and to the first-order horizontal to address the strength of the anticyclones in particu- thermal advection balances diabatic heating. This is lar. The focus will be on the JJA season, but the ar- the advective limit of Smagorinsky (1953). In the guments will be general in nature and should apply Tropics where B is large, adiabatic cooling and warm- to the Southern Hemisphere in DJF. ing are associated with ascent and descent, respec-

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Unauthenticated | Downloaded 10/08/21 07:21 PM UTC tively, and tend to balance diabatic warming and cooling. The tendency for this sort of balance is apparent in Fig. 2. However, in the subtropics both horizontal and vertical advection of potential temperature are, in general, important. Equiva- lently, diabatic heating is associated with deviations from trajectories up- and downsloping isentropic surfaces. In RH an analysis of the terms in the thermodynamic equation in the midtroposphere, concentrating on the Asian monsoon-North African descent region, is presented. It is shown that the tropical balance of mean ascent balancing mean diabatic heating works well in the Asian monsoon region. However, the North Africa/eastern Mediterranean descent is mostly downsloping isentropic surfaces. As suggested by Fig. 2, the diabatic part of the descent FIG. 2. (a) The average 500-mb vertical velocity for June-August 1994. Contours are 2 _1 is relatively small. A similar drawn at ±1, ±3, ±5 . . . 10~ Pa s , and the region with values below the first negative contour is shaded, (b) The column- and time-averaged diabatic heating for June-August analysis for the entire summer 1994. Contours are drawn at ±25, ±75, ±125 W nr2, etc., and the region with values above subtropics, to be given elsewhere, the first positive contour is shaded. shows that the diabatic cooling seen in Fig. 2b is more important in the eastern oceanic basins. However, the tendency for The region near 10°N is one of ascent in the ITCZ, air to descend downsloping isentropic surfaces is still im- and near 20°S there is the descent in the winter portant, particularly in the lower to middle troposphere. subtropics. The predominant ascent near 15°-20°N To illustrate the different regimes at different lati- is evidence of the NH monsoons. Nearer 30°N there tudes and times of year, we can produce two zonal is almost equal ascent and descent, which is gener- averages: one for air which is ascending in the mean, ally at an angle which is slightly steeper then the and the other for air which is descending in the mean. mean isentropes. Again, this suggests that the sum- In this manner, the contributions to the mean meridi- mer subtropics is a region where air moving poleward onal circulation from air that is rising in the mean and and rising up the isentropes is diabatically heated air that is descending in the mean can be calculated and rises farther, and air that is moving equatorward and can be shown as vectors. The sum of the two vec- and descending down the isentropes is diabatically tors gives the usual Hadley Cell picture, but at the cost cooled. of tremendous reduction in content. The tropical- tropospheric part of this diagram for July 1994 is shown in Fig. 3. A discussion of the full diagram and 3. Some idealized model studies of other figures obtained in this manner will be made elsewhere (B. Hoskins and P. Berrisford 1996, unpub- In Hoskins and Rodwell (1995) a spherical primi- lished manuscript). Here I restrict myself to a few tive equation model with a prescribed zonal flow and comments relevant to the present discussion. linear damping of temperature and velocity was in-

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Unauthenticated | Downloaded 10/08/21 07:21 PM UTC the solution. In Rodwell and Hoskins (1995) this model was used to look at the dynamics of the lower-tropospheric Somali jet. In RH the localized descent feature over North Africa-eastern Mediterranean is discussed. Simpler forcings are used to illuminate the dynamics of this feature. Figure 4 shows some results from two of these simulations. In each case there is no orography, and the diabatic heating (switched on at t = 0) is nonzero only in an elliptical region mimicking the shape of the Asian monsoon heating in Fig. 2b. Its vertical structure is also representa- FIG. 3. The contributions to the zonally averaged meridional circulation from tive of deep convection, with a maxi- -1 points with mean ascent and mean descent for July 1994. The scale for the vectors mum of 5 K day at about 400 mb. is shown. For heating centered at 25 °N in a lin- earized model with a basic resting atmo- tegrated forward in time with orography raised over sphere, the response develops in the manner suggested 5 days and diabatic heating switched on at day 5. It by Gill (1990) and Heckley and Gill (1984). As seen was shown that, using earth orography and diabatic in the 325-K pressure field (Fig. 4a), the warm equa- heating deduced from analysis, a quite realistic glo- torial Rossby wave spreads to the west. Associated bal asymmetric flow and in particular Asian monsoon with it there is descent (Fig. 4b). When the lineariza- circulation was produced. It remained quite steady for tion is about the observed JJA zonally averaged flow, about a week before baroclinic instability dominated two effects act to enhance the local descent. The zon-

FIG. 4. Pressure and horizontal winds on the 325-K isentropic surface and vertical velocity, CO, on the 477-hPa pressure surface 16 days after the switch on of a heat source centered on 25°N, 90°E in a resting atmosphere, (a) and (b), respectively, and in a JJA zonal flow, (c) and (d), respectively. The contour intervals are 40 hPa and 0.5 hPa-1 for p and co, respectively.

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Unauthenticated | Downloaded 10/08/21 07:21 PM UTC ally averaged flow has slight descent near 30°N. More importantly, the poleward part of the Rossby wave interacts with the midlatitude westerly. The corre- sponding results for this case are shown in Figs. 4c and 4d. The warm Rossby wave is still present, though it is somewhat distorted. The westerlies flow down the isentropic surface to the lowest point near 75°E and then sharply ascend. The resulting vertical motion (Fig. 4d) shows a strong maximum in descent centered some 25° west and 10° north of the heating. The ascent region now extends poleward and east- ward. It is shown in RH that in a nonlinear integra- FIG. 5. A schematic diagram showing elements of the proposed tion the enhanced descent is present, though mechanism for summer subtropical anticyclones. somewhat weaker. (It is also shown that the localiza- tion of the descent over northern Africa and the eastern Mediterranean and over the Kyzylkum Desert in eastern ocean basins is directly related to the mon- Asia is due to the topography of the region.) The soon heating on the continent to the east. This hypoth- poleward displacement of the monsoon heating is cru- esis will be discussed in more detail in a subsequent cial for this response. No such localized descent re- paper (B. Hoskins and M. Rodwell 1996, unpublished gion occurs for a heating at 10°N. manuscript). It was shown in RH that the ascent in the monsoon heating region and the descent in the anticyclone cool- 4. A mechanism for summer subtropic ing region are not arms of a simple vertical circula- anticyclone existence and intensity tion cell. The tropical easterlies feed the ascending air, whereas the middle-latitude westerlies feed the Based on the observational diagnoses and the ide- descending air. alized integration, the following sequence is proposed (see Fig. 5). 5. Concluding comments 1) The monsoon heating moves poleward in each continent through spring into summer. As it does The distinction between the slanting decent of the so, the descent westward and poleward of it in- localized summer subtropical anticyclone and the de- tensifies, particularly when there is interaction scending arm of the Hadley Cell, manifested in Fig. 1 with the midlatitude westerlies. as a longitudinally extended winter subtropical anti- 2) The enhanced descent leads to a suppression of cyclone, is not in reality as definite as suggested here. convection and enhanced diabatic cooling asso- For example, the equatorial flanks of the summer an- ciated with radiative processes. This cooling acts ticyclones have the tropical thermodynamic balance as a positive feedback on the descent. and are closely coupled to the oceanic ITCZs. The 3) Below the maximum descent, consistent with the poleward portions of the eastern ocean basin winter- vorticity balance j3v = fdw/dz, there is strong time anticyclones still have an element of the slant- equatorward motion. This is equivalent to the an- wise descent. However, I believe that the proposed ticyclonic vorticity generated in the region hav- mechanism gives insight into the nature and intensity ing moved westwards under the action of the /3 of the summer subtropical anticyclones: I hope that I effect. might have contributed a little "knowledge of things 4) The equatorward wind drives an oceanic Ekman unknown!" drift away from the coast, leading to the upwelling of cold waters. This acts to reinforce the suppression of convection in the region. Acknowledgments. Much of this research has been performed in collaboration with Dr. Mark Rodwell. The zonally averaged circulation study illustrated in Fig. 3 is from joint research Therefore, it is proposed that the existence and in- with Dr. Paul Berrisford. Their contribution is gratefully tensity of the summer subtropical anticyclones in the acknowledged.

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Digges, Leonard the Elder, 1555: A Prognostication of Right Good Effect. Thomas Gemini. Gill, A. E., 1980: Some simple solutions for heat-induced tropical circulation. Quart. J. Roy. Meteor. Soc., 106, 447-462. Godfridus, 1530: Boke of Knowledge of Thynges Unknowen Apperteynynge to Astronomye. R. Wyer. Heckley, W., and Gill, A. E., 1984: Some simple analytical solutions to the problem of forced equatorial long waves. Quart. J. Roy. Meteor. Soc., 110, 203-217. Heninger, S. K., Jr., 1960: A Handbook of Renaissance Meteo- rology. Duke University Press, 269 pp. Hoskins, B. J., 1986: Diagnosis of forced and free variability in the atmosphere. Atmospheric and Oceanic Variability, H. Cattle, Ed., Royal Meteorological Society, 57-73. , and M. R. Rodwell, 1995: A model of the Asian summer monsoon. Part I: The global scale. J. Atmos. Sci., 52, 1330- 1340. Rodwell, M. R., and B. J. Hoskins, 1995: A model of the Asian summer monsoon. Part II: Cross-equatorial flow and PV be- havior. J. Atmos. Sci., 52, 1341-1356. , and , 1996: Monsoons and the dynamics of deserts. Ml Quart. J. Roy. Meteor. Soc., 122, in press. LOSSARY OF Smagorinsky, J., 1953: The dynamical influence of large- scale heat sources and sinks on the quasi-stationary mean HYDROLOGY motions of the atmosphere. Quart. J. Roy. Meteor. Soc., 79, 342-366. Second Edition

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