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John A. Knox

Abstract

The connections between the concept of nonlinear balance and the classical criterion of inertial stability are explored in the context of historical work on this subject. New analytic results are derived establishing that ellipticity and inertial stability are, in general, separate and distinct measures of balanced flows, even in the case of gradient flow. In particular, nonlinear balance is violated more for weaker anticyclonic flows than is inertial balance. These conclusions are supported by analysis of observational data.

A hierarchy of nonlinear balance criteria is constructed, which ranges from ellipticity to “realizability” conditions first obtained by Petterssen and Kasahara. Expressions interrelating all the nonlinear balance criteria and inertial stability are derived, clarifying the relationship between the Petterssen criterion, Kasahara’s realizability, and inertial stability. The balance-criteria hierarchy is tested for cyclonic and anticyclonic conditions using a nonlinear inviscid f-plane trajectory model. The modeling results confirm the analytical ellipticity–inertial stability relationship. In addition, an intercomparison of balance criteria reveals that Petterssen’s realizability (in the form derived here) is the most general and most physically interpretable balance criterion. The implications of this work for generalizations of inertial instability theory are briefly explored.

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John A. Knox

Using data derived from the American Meteorological Society-University Corporation for Atmospheric Research Curricula and U.S. Department of Education statistics, it is found that the number of meteorology bachelor's degree recipients in the United States has reached a level unprecedented in at least the past 40 years: from 600 to possibly 1,000 graduates per year. Furthermore, this number is increasing at a rate of approximately 8%–11% per year. The number of meteorology majors has also increased up to 10% per year since the late 1990s. The number of meteorology bachelor's degree recipients is projected to increase at a rate of approximately 5%–12% per year through 2011. This simultaneous combination of record numbers and rapid recent increases is not mirrored in other related fields or in the American college population as a whole, suggesting a meteorology-specific cause for the increase in undergraduates. These graduation and enrollment trends are compared to data on the employment of meteorology bachelor's degree holders. The number of entry-level meteorology positions in the United States available each year appears to be no more than about half the number of new degreed meteorologists. According to data from the U.S. Bureau of Labor Statistics, growth in meteorology employment has averaged 1.2% per year from 1994–2004 and is expected to be no more than 1.6% per year through 2014. These numbers and trends portend an increasing oversupply of meteorology graduates versus meteorology employment opportunities if current enrollment and employment trends continue. Possible responses of the meteorology community are explored.

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John A. Knox

Abstract

The forecasting of clear-air turbulence (CAT) continues to be a challenging problem despite progress made in the understanding of vertical shear (Kelvin–Helmholtz) instabilities. The possible connections between horizontal anticyclonic flows and CAT are addressed. Analytical expressions are derived to show that current CAT diagnostics do not correctly account for the dynamics of strongly anticyclonic situations. In gradient-balanced anticyclonic flows, nonfrontogenetical enhancement of vertical shear may lead to CAT. A review of observations, theory, and modeling is presented to support the claim that strong anticyclonic relative vorticity can also lead to CAT through the generation of gravity wave activity by geostrophic adjustment and inertial instability. CAT diagnostics are then discussed in light of these claims. Observational work is in progress to investigate the possibility of inertial instability-triggered CAT.

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Ellen Kohl and John A. Knox

Abstract

This paper explores the relationship between scientific operationalizations of drought and the politics of water management during times of drought. Drawing on a case study of the 2007–09 drought in Georgia in the southeastern United States, this paper examines how multiple ways of knowing drought were produced, circulated, and utilized by stakeholders. Moreover, this paper explores the policy implications of these multiple ways of knowing drought. Data were drawn from archival research, direct observation, and semistructured interviews with members of the green industry (self-identified members of the urban agricultural sector); state environmental regulators; and local governmental officials. Data were analyzed to examine the interplay between science and politics. This paper highlights the intersections of drought management policy and 1) scale and operationalization of drought; 2) how stakeholders know drought; and 3) societal context within which knowledge of drought is produced, circulated, and utilized. This research demonstrates how stakeholders can leverage the complexity of drought to pursue their political goals and change the way water is managed during times of drought. Even in instances where there are different knowledges of drought, stakeholders can still change the societal context, as the green industry did in Georgia in 2009. This paper argues that scientists and policymakers who work on drought management need to consider how knowledges of drought are coconstituted through interactions between science, nature, and society.

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John A. Knox and Steven A . Ackerman

During 2002 and 2003, surveys of introductory meteorology students were conducted at the University of Georgia and the University of Wisconsin—Madison. These surveys asked which one question about weather and climate each student would most like to have answered in the class, as well as other demographic and educational information. The more than 750 responses that were obtained ran the gamut of meteorology and were not overwhelmingly focused on any one topic, including severe weather. Results from the two universities are nearly identical, with the exception of a greater awareness of climate issues at Wisconsin. Several topics that are most commonly noted by students, such as weather forecasting and atmospheric optics, are given inadequate treatment in many introductory meteorology textbooks and classes. The results of the surveys suggest that an instructor could use students' first-day responses to this kind of question to shape a syllabus that would incorporate student interests, while retaining educational integrity.

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Jared A. Rackley and John A. Knox

Abstract

A 30-yr climatology (1981–2010) of cold-air damming (CAD) events in the southern Appalachians was conducted using hourly surface observations and North American Regional Reanalysis (NARR) data. Analysis of the spatial distribution and frequency of these events reveals that some part of the Southeast is affected by CAD on 50 days out of each year, and even the northern Florida panhandle and much of Alabama experience CAD conditions on about 30 days annually. Spatially, different CAD types tend to exhibit one of two patterns in the southernmost extent of the cold-air dome: a more southerly dome with a ridge axis oriented from north-northeast to south-southwest or a more westerly dome with a ridge axis in a northeast to west-southwest orientation. These patterns may be the result of both splitting around the region of higher terrain in east-central Alabama and Coriolis forcing in stronger CAD types with higher wind speeds. Analysis of the frequency of CAD by type on a month-by-month and year-by-year basis confirms previous work that CAD is much more frequent during the cold season versus the warm season, with CAD occurring on 6.8 days month−1 during December and only 1.3 days month−1 during July. Analysis was also stratified by CAD type, revealing that weak/dry events were the most common. Classical type events with stronger and more favorably positioned parent highs exhibited the longest average duration, nearly 45 h, while other CAD types averaged approximately half as long.

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John A. Knox, David S. Nevius, and Pamela N. Knox

Abstract

The wet-bulb temperature is a widely used moist thermodynamic variable. The relationship between the wet-bulb temperature, the dry-bulb temperature, and the dewpoint temperature is nonlinear. Most atmospheric thermodynamics textbooks indicate or imply that no simple and accurate approximation relating these three meteorological variables exists. This article provides theoretical justifications for, and real-life applications of, two different simple linear approximations for the wet-bulb temperature. These two approximations are 1) an arithmetic mean of dry-bulb and dewpoint temperatures and 2) a weighted mean of dry-bulb and dewpoint temperatures known as the “one-third rule.” These approximations are highly accurate in two contiguous temperature and moisture regimes: the arithmetic-mean rule outperforms other approximations for relatively moist (average relative humidity = 61%) situations with dry-bulb temperatures bracketing 13°C, and the one-third rule outperforms other approximations for relatively moist (average relative humidity = 50%) situations with dry-bulb temperatures bracketing 4°C. The one-third rule is especially useful because its domain of maximum accuracy includes the phase change for water from solid to liquid and vice versa. Examples of the application of the one-third rule to precipitation-type forecasting and to agricultural practices to prevent frost damage are presented.

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David M. Schultz and John A. Knox

Abstract

Several east–west-oriented bands of clouds and light rain formed on 20 July 2005 over eastern Montana and the Dakotas. The cloud bands were spaced about 150 km apart, and the most intense band was about 20 km wide and 300 km long, featuring areas of maximum radar reflectivity factor of about 50 dBZ. The cloud bands formed poleward of an area of lower-tropospheric frontogenesis, where air of modest convective available potential energy was being lifted. During initiation and maintenance of the bands, mesoscale regions of dry symmetric and inertial instability were present in the region of the bands, suggesting a possible mechanism for the banding. Interpretation of the extant instabilities in the region of the bands was sensitive to the methodology to assess the instability. The release of these instabilities produced circulations with enough vertical motion to lift parcels to their lifting condensation level, resulting in the observed cloud bands. A high-resolution, numerical weather prediction model demonstrated that forecasting these types of events in such real-time models is possible, although the timing, evolution, and spacing of the bands were not faithfully reproduced. This case is compared to two previous cases in the literature where banded convection was associated with a combination of conditional, symmetric, and inertial instability.

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Gary P. Ellrod and John A. Knox

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An operational clear-air turbulence (CAT) diagnostic index has been modified to improve its performance. The Ellrod–Knapp turbulence index (TI) was developed in the early 1990s and is in use at many aviation forecasting facilities worldwide. It has been recognized, however, that TI often does not sufficiently account for situations where anticyclonic shear or curvature is present. The proposed modification to TI is based on the addition of a proxy term for divergence tendency, appropriate for both anticyclonic flow and gravity wave generation in cyclonic regions. Examples show how the modified index [referred to as the divergence-modified turbulence index (DTI)] leads to better anticipation of significant CAT for two scenarios where rapid divergence changes were occurring. Preliminary objective evaluation of the 6-h forecast DTI derived from the Rapid Update Cycle-2 (RUC-2) was completed for 2 months in 2007 (using more than 1100 pilot reports). Results showed significant improvements over TI, based on verification metrics such as the probability of detection of turbulence (PODy) and the true skill statistic (TSS). Further evaluation is planned using a larger database of pilot reports, as well as forecast data from additional state-of-the-art prediction models, altitude ranges, and forecast times.

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John A. Knox and Paul J. Croft

The current explosion of scientific information available to science educators puts increasing pressure on conventional educational approaches. One educational technique that (a) facilitates the communication of essential knowledge, (b) is supported by cognitive science theory, and (c) is easily implemented in the atmospheric science classroom is the reformulating of lectures into stories. “Storytelling” here is understood to describe the oral or written communication of a “connected narrative of important events.” Stories differ from other pedagogical approaches, such as the traditional fact-laden lecture, through the network of multiple linkages between different characters, events, and facts in a story. Facts in a lecture may simply follow one after another; events in a story, by contrast, must follow from previous facts and the logic in the story itself.

An account is given of the lead author's use of storytelling in an atmospheric dynamics course at the University of Wisconsin—Madison. In the 2-hour-per-week laboratory, the course material was cast in the form of stories—stories that framed the basic knowledge, conveyed key concepts, and related key topics to one another. Stories were delivered orally in class and through an informal laboratory workbook. The rationale for this approach, the stories told, and the students' reactions are described. An example of storytelling in a global climate change course is also provided to illustrate the usefulness of storytelling in a wide range of meteorology courses.

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