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W. Winters, S. Barnard, and A. Hogan

Abstract

A modern replica of the Aitken counter for detection of aerosol particles smaller in diameter than a half wavelength of visible light has been constructed using modern materials. The instrument employs photographic recording, rather than visual observations, of the cloud drops formed on these particles. This feature eliminates observer bias and provides a permanent record of the observation. Comparison of this instrument with a Pollak photoelectric nucleus counter indicated correspondence in the concentration sensed to well within experimental sampling error.

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A. Hogan, W. Winters, and G. Gardner

Abstract

A portable photoelectric nucleus counter, with similar sensitivity to the Pollak photoelectric nucleus counter with convergent light beam, has been developed and calibrated. This instrument has been incorporated into a packaged measurement system which allows the experimenter to determine the effective diffusion coefficient and fraction charged, of the natural aerosol, in uncontaminated areas. The photoelectric counter has comparable accuracy to the absolute (Aitken, Scholz) counters in the concentration range of interest, and is capable of determining the concentration once per minute.Field tests of the prototype instrument were conducted near sea level in Greenland. The concentration of natural aerosol in this area ranged from 150 to 200 particles cm−3. The instrumentation had sufficient sensitivity to detect a gradual increase in particle size at this low concentration.

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W. D. Smyth and K. B. Winters

Abstract

Motivated by the tendency of high-Prandtl-number fluids to form sharp density interfaces, the authors investigate the evolution of Holmboe waves in a stratified shear flow through direct numerical simulation. Like their better-known cousins, Kelvin–Helmholtz waves, Holmboe waves lead the flow to a turbulent state in which rapid irreversible mixing takes place. In both cases, significant mixing also takes place prior to the transition to turbulence. Although Holmboe waves grow more slowly than Kelvin–Helmholtz waves, the net amount of mixing is comparable. It is concluded that Holmboe instability represents a potentially important mechanism for mixing in the ocean.

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David M. Schultz, Lance F. Bosart, Brian A. Colle, Huw C. Davies, Christopher Dearden, Daniel Keyser, Olivia Martius, Paul J. Roebber, W. James Steenburgh, Hans Volkert, and Andrew C. Winters

Abstract

The year 1919 was important in meteorology, not only because it was the year that the American Meteorological Society was founded, but also for two other reasons. One of the foundational papers in extratropical cyclone structure by Jakob Bjerknes was published in 1919, leading to what is now known as the Norwegian cyclone model. Also that year, a series of meetings was held that led to the formation of organizations that promoted the international collaboration and scientific exchange required for extratropical cyclone research, which by necessity involves spatial scales spanning national borders. This chapter describes the history of scientific inquiry into the structure, evolution, and dynamics of extratropical cyclones, their constituent fronts, and their attendant jet streams and storm tracks. We refer to these phenomena collectively as the centerpiece of meteorology because of their central role in fostering meteorological research during this century. This extremely productive period in extratropical cyclone research has been possible because of 1) the need to address practical challenges of poor forecasts that had large socioeconomic consequences, 2) the intermingling of theory, observations, and diagnosis (including dynamical modeling) to provide improved physical understanding and conceptual models, and 3) strong international cooperation. Conceptual frameworks for cyclones arise from a desire to classify and understand cyclones; they include the Norwegian cyclone model and its sister the Shapiro–Keyser cyclone model. The challenge of understanding the dynamics of cyclones led to such theoretical frameworks as quasigeostrophy, baroclinic instability, semigeostrophy, and frontogenesis. The challenge of predicting explosive extratropical cyclones in particular led to new theoretical developments such as potential-vorticity thinking and downstream development. Deeper appreciation of the limits of predictability has resulted from an evolution from determinism to chaos. Last, observational insights led to detailed cyclone and frontal structure, storm tracks, and rainbands.

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D. Durnford, V. Fortin, G. C. Smith, B. Archambault, D. Deacu, F. Dupont, S. Dyck, Y. Martinez, E. Klyszejko, M. MacKay, L. Liu, P. Pellerin, A. Pietroniro, F. Roy, V. Vu, B. Winter, W. Yu, C. Spence, J. Bruxer, and J. Dickhout

Abstract

In this time of a changing climate, it is important to know whether lake levels will rise, potentially causing flooding, or river flows will dry up during abnormally dry weather. The Great Lakes region is the largest freshwater lake system in the world. Moreover, agriculture, industry, commerce, and shipping are active in this densely populated region. Environment and Climate Change Canada (ECCC) recently implemented the Water Cycle Prediction System (WCPS) over the Great Lakes and St. Lawrence River watershed (WCPS-GLS version 1.0) following a decade of research and development. WCPS, a network of linked models, simulates the complete water cycle, following water as it moves from the atmosphere to the surface, through the river network and into lakes, and back to the atmosphere. Information concerning the water cycle is passed between the models. WCPS is the first short-to-medium-range prediction system of the complete water cycle to be run on an operational basis anywhere. It currently produces two forecasts per day for the next three days. WCPS generally provides reliable results throughout the length of the forecast. The transmission of errors between the component models is reduced by data assimilation. Interactions between the environmental compartments are active. This ongoing intercommunication is valuable for extreme events such as rapid ice freeze-up and flooding or drought caused by abnormal amounts of precipitation. Products include precipitation; evaporation; lake water levels, temperatures, and currents; ice cover; and river flows. These products are of interest to a wide variety of governmental, commercial, and industrial groups, as well as the public.

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