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David M. Schultz

Abstract

Naud et al. constructed satellite-based composite analyses of clouds and precipitation across cold fronts. However, their approach does not exclude occluded fronts, does not separate anafronts from katafronts, does not separate frontlike phenomena primarily identified by thermal gradients from those primarily identified by wind changes, and smooths over alongfront variability. By lumping these disparate frontal structures together, the front-centered composite cross sections reveal forward-sloping structures and weak gradients across them, raising questions about how to interpret their composite cross sections.

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David M. Schultz

Based on a talk given at the sixth annual meeting of the Atmospheric Science Librarians International, this paper explores the author;s experiences performing reviews of the scientific literature as a tool to advancing meteorology and studying the history of science. Three phases of performing literature searches with varying degrees of interaction with a research librarian are considered: do it yourself, librarian assisted, and librarian as collaborator. Examples are given for each phase: occluded fronts, conditional symmetric instability, and static instability terminology, respectively. Electronic availability of information is changing the relationship between scientists and librarians. Yet, despite these changes, books on library shelves and knowledgeable human librarians remain essential to the scientific enterprise.

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David M. Schultz and Frederick Sanders

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Previous studies have shown that 500-hPa mobile trough births (or genesis) occur preferentially in northwesterly flow during upper-level frontogenesis, and that cold advection assists in, and is a product of, mobile trough intensification. This study focuses on the synoptic environments and thermal-advection patterns of upper-level fronts associated with mobile trough births in northwesterly flow. A climatology of 186 such events, derived from an earlier study by Sanders, shows that most births tend to occur within uniform or diffluent flow and that most tend to be associated with relatively weaker 500-hPa thermal advection. Most mobile trough births in diffluence, however, tend to be associated with increasing 500-hPa cold advection, typically indicated by a cyclonic rotation of isentropes, whereas, most mobile trough births in confluence tend to be associated with weaker 500-hPa thermal advection.

Two cases of upper-level frontogenesis associated with mobile trough genesis—one in diffluence and one in confluence—are compared to determine the processes acting to produce the differing thermal-advection patterns at 500 hPa. A thermal-advection tendency equation is developed and shows that the magnitude of the temperature advection can be changed by accelerating the advecting wind speed or by changing the temperature gradient (i.e., vector frontogenesis). The latter can be accomplished either by changing the magnitude of the temperature gradient (the frontogenetical component F n, also known as scalar frontogenesis) or by rotating the direction of the temperature gradient relative to the flow (the rotational component F s). The dominant processes acting on F n for the diffluence and confluence cases are tilting and deformation frontogenesis, respectively. The dominant process acting on F s for the diffluence case is rotation of the isentropes due to the vorticity term, whereas rotation of the isentropes due to the vorticity and tilting terms are both important for the confluence case. The rotational component of frontogenesis is cyclonic downstream of the vorticity maximum for both cases, favoring increasing cold advection downstream of the vorticity maximum. For both cases, the rate of rotation of the isentropes at a point due to horizontal advection is large and that due to vertical advection is negligible. Since advection can only transport the existing isentrope angle and cannot change the isentrope angle, the rotational component of frontogenesis normalized by the temperature gradient is the only term that can increase the isentrope angle following the flow. This term dominates in the diffluence case but is small in the confluence case. This diagnosis suggests the following reasoning. In diffluent flow, the vorticity associated with the incipient trough is compacted into a more circular shape and intensifies. The potent vorticity maximum leads to robust isentrope rotation. In confluent flow, however, the vorticity is deformed into an elongated maximum, inhibiting both strong isentrope rotation and increasing cold advection. Thus, the rotational frontogenesis component explains the rotation of the isentropes that is responsible for the differing thermal-advection patterns.

Diagnosis of these cases supports the results from the climatology indicating a strong relationship between the synoptic environment and the upper-tropospheric thermal-advection pattern. Nevertheless, current conceptual models of upper-level frontogenesis do not fully explain the variety of these features in the real atmosphere. In particular, mobile trough genesis and its associated upper-level frontogenesis can occur in weak 500-hPa thermal-advection patterns, in contrast to the confluence and cold advection that have been previously identified as important to upper-level frontal intensification. This result provides further support for the possibility that generation and intensification of mobile troughs can occur by barotropic processes.

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Vladimir Janković and David M. Schultz

Abstract

The potential and serious effects of anthropogenic climate change are often communicated through the soundbite that anthropogenic climate change will produce more extreme weather. This soundbite has become popular with scientists and the media to get the public and governments to act against further increases in global temperature and their associated effects through the communication of scary scenarios, what the authors term “atmosfear.” Underlying atmosfear’s appeal, however, are four premises. First, atmosfear reduces the complexity of climate change to an identifiable target in the form of anthropogenically forced weather extremes. Second, anthropogenically driven weather extremes mandate a responsibility to act to protect the planet and society from harmful and increased risk. Third, achieving these ethical goals is predicated on emissions policies. Fourth, the end result of these policies—a nonanthropogenic climate—is assumed to be more benign than an anthropogenically influenced one. Atmosfear oversimplifies and misstates the true state of the science and policy concerns in three ways. First, weather extremes are only one of the predicted effects of climate change and are best addressed by measures other than emission policies. Second, a preindustrial climate may remain a policy goal, but it is unachievable in reality. Third, the damages caused by any anthropogenically driven extremes may be overshadowed by the damages caused by increased exposure and vulnerability to the future risk. In reality, recent increases in damages and losses due to extreme weather events are due to societal factors. Thus, invoking atmosfear through such approaches as attribution science is not an effective means of either stimulating or legitimizing climate policies.

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David M. Schultz and Thomas Spengler

Abstract

In a recent article, Qian et al. introduced the quantities moist vorticity and moist divergence to diagnose locations of heavy rain. These quantities are constructed by multiplying the relative vorticity and divergence by relative humidity to the power k, where k = 10 in their article. Their approach is similar to that for the previously constructed quantity generalized moist potential vorticity. This comment critiques the approach of Qian et al., demonstrating that the moist vorticity, moist divergence, and by extension generalized moist potential vorticity are flawed mathematically and meteorologically. Raising relative humidity to the 10th power is poorly justified and is based on a single case study at a single time. No meteorological evidence is presented for why areas of moist vorticity and moist divergence should overlap with regions of 24-h accumulated rainfall. All three quantities have not been verified against the output of precipitation directly from the model nor is the approach of combining meteorological quantities into a single parameter appropriate in an ingredients-based forecasting approach. Researchers and forecasters are advised to plot the model precipitation directly and employ an ingredients-based approach, rather than rely on these flawed quantities.

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David M. Schultz and Vladimir Janković
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David M. Schultz and Geraint Vaughan

Traditionally, the formation of an occluded front during the occlusion process in extratropical cyclones has been viewed as the catch-up of a faster-moving cold front to a slower-moving warm front separating the warm-sector air from the low center, as first described in the Norwegian cyclone model over 90 yr ago. In this article, the conventional wisdom, or the commonly held beliefs originating from the Norwegian cyclone model, about occluded fronts and the occlusion process are critically examined. The following four tenets of this conventional wisdom are addressed. First, the occlusion process is better described not by catch-up, but by the wrapping up and lengthening of the warm-air tongue as a result of deformation and rotation around the low center. Second, the merger of the cold front and warm front does not result in the frontal zone with the warmer air ascending over the other frontal zone. Instead, the occluded frontal zone tilts over the more statically stable frontal zone. Because a warmfrontal zone tends to be more stable than a cold-frontal zone, this process usually produces a warm-type occlusion, confirming that cold-type occlusions are less common than warm-type occlusions. Third, occlusion does not mean that the cyclone has stopped deepening, because many cyclones continue to deepen 10–30 mb for 12–36 h after the formation of the occluded front. Fourth, clouds and precipitation associated with occluded fronts differ from their widespread stratiform depiction in textbooks. Embedded precipitation bands may be parallel to the front, and little relationship may exist between the fronts and the cloud mass. These four tenets help to explain anomalies in the Norwegian cyclone model, such as how occluded fronts that spiral around the low center do not require catch-up to form, how Shapiro–Keyser cyclones undergo occlusion, why some cyclones do not form occluded fronts, how some cyclones deepen after occlusion, why few cold-type occlusions have been observed, and why occluded cyclones are often associated with heavy precipitation. This reexamination of conventional wisdom leads to a new paradigm for occluded fronts and occluded cyclones.

A supplement to this article is available online:

DOI: 10.1175/2010BAMS3057.2

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Mary Golden and David M. Schultz

A survey of 310 reviewers for Monthly Weather Review addresses how much time and effort goes into the peer-review process and provides insight into how reviewers function. Using these data, the individual and collective contributions of volunteer peer reviewers to the peer-review process can be determined. Individually, respondents to the survey review an average of 2 manuscripts a year for Monthly Weather Review, 4 manuscripts a year for AMS journals, and 8 manuscripts a year in total, although some review more than 20 manuscripts a year. Each review takes an average of 9.6 h. Summing the individual contributions of the reviewers, respondents averaged 18 h yr−1 performing reviews for Monthly Weather Review, 36 h yr−1 for AMS journals, and 63 h yr−1 for all journals. The collective time that all of the reviewers put into the peer-review process for all manuscripts submitted to Monthly Weather Review for each year amounts to 362,179 h, or more than 4 years of voluntary labor valued at over $2.34 million. Nearly all respondents (95%) are comfortable with their current load, but only 30% said that they would be willing to perform more reviews. Because the number of submissions to journals has been increasing over time and is unlikely to decrease in the near future, this burden is anticipated to grow. Options for reducing the burden include using fewer reviewers per manuscript, increasing the number of unilateral decisions made by editors, and increasing the size of the reviewer pool (particularly from active retired and early-career scientists).

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David M. Schultz and Daniel Keyser

Abstract

Two widely accepted conceptual models of extratropical cyclone structure and evolution exist: the Norwegian and Shapiro–Keyser cyclone models. The Norwegian cyclone model was developed around 1920 by the Bergen School meteorologists. This model has come to feature an acute angle between the cold and warm fronts, with the reduction in the area of the warm sector during the evolution of the cyclone corresponding to the formation of an occluded front. The Shapiro–Keyser cyclone model was developed around 1990 and was motivated by the recognition of alternative frontal structures depicted in model simulations and observations of rapidly developing extratropical cyclones. This model features a right angle between the cold and warm fronts (T-bone), a weakening of the poleward portion of the cold front (frontal fracture), an extension of the warm or occluded front to the rear of and around the cyclone (bent-back front), and the wrapping around of the bent-back front to form a warm-core seclusion of post-cold-frontal air. Although the Norwegian cyclone model preceded the Shapiro–Keyser cyclone model by 70 years, antecedents of features of the Shapiro–Keyser cyclone model were apparent in observations, analyses, and conceptual models presented by the Bergen School meteorologists, their adherents, and their progeny. These “lost” antecedents are collected here for the first time to show that the Bergen School meteorologists were aware of them, although not all of the antecedents survived until their reintroduction into the Shapiro–Keyser cyclone model in 1990. Thus, the Shapiro–Keyser cyclone model can be viewed as a synthesis of various elements of cyclone structure and evolution recognized by the Bergen School meteorologists.

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Trevor Mitchell and David M. Schultz

Abstract

A dataset of drylines within a region of the southern Great Plains was constructed to investigate the large-scale environments associated with the initiation of deep moist convection. Drylines were identified using NOAA/NWS Weather Prediction Center surface analyses for all April, May, and June days 2006–15. Doppler radar and visible and infrared satellite imagery were used to identify convective drylines, where deep, moist convection was deemed to have been associated with the dryline circulation. Approximately 60% of drylines were convective, with initiation most frequently occurring between 2000 and 2100 UTC. Composite synoptic analyses were created of 179 convective and 104 nonconvective dryline days. The composites featured an upper-level long-wave trough to the west of the Rockies and a ridge extending across the northern and eastern United States. At the surface, the composites featured a broad surface cyclone over western Texas and southerly flow over the south-central states. Convective drylines featured more amplified upper-level flow, associated with a deeper trough in the western United States and a stronger downstream ridge than nonconvective drylines up to 5 days preceding a dryline event. By the day of a dryline event, the convective composite features greater low-level specific humidity and higher CAPE than the nonconvective composite. These results demonstrate that synoptic-scale processes over several days help create conditions conducive to deep, moist convection along the dryline.

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