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Stephan B. Smith

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Stephan B. Smith and M. K. Yau

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An intercomparison of all 11 Limestone Mountain Experiment case days provided the basis for a conceptual model of severe convective outbreaks in Alberta. It is proposed that most severe convective events result when upper-level cooling, associated with an advancing, synoptic-scale trough, occurs in phase with strong surface heating over the Alberta foothills. The deep destabilization over the elevated topography acts to amplify the mountain-plain circulation and to generate mesoscale upslope moisture transport. Concurrently, the surface synoptic pressure gradient gives rise to east-northerly winds that advect the moisture-rich air of the eastern plains into the lower branch of the mountain-plain circulation. In this manner, the plains moisture is permitted to reach the convectively active foothills through underrunning of the capping lid. The end product is the initiation of well-organized, severe convective storms that move eastward with the westerly component of the midtropospheric winds. A statistical analysis based on archived hail data furnished additional evidence for the key synoptic-scale features of the conceptual model.

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Stephan B. Smith and M. K. Yau

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Mesoscale and synoptic-scale analyses were carried out for a severe convective outbreak and two nonsevere convective events in central Alberta. High-resolution upper-air and surface observations gathered during the Limestone Mountain Experiment (LIMEX-85) permitted a detailed diagnosis of the evolution of the atmosphere over the Alberta foothills. On the severe day, deep convection was triggered when upper-level cooling, associated with an advancing, synoptic-scale trough, occurred in phase with strong surface heating over the Alberta foothills from 0800 to 1200 local daylight time (LDT). The deep destabilization over the elevated topography acted to amplify the mountain-plain circulation and to generate mesoscale upslope moisture transport. Concurrently, the surface synoptic pressure gradient gave rise to northeasterly winds that advected a tongue of moist plains air into the lower branch of the mountain-plain circulation. The plains moisture was thus permitted to reach the foothills in time to reinforce the initial convection and effectuate a secondary destabilization. On the nonsevere days, the absence of such joint meso-synoptic-scale upslope moisture transport precluded the occurrence of severe convection.

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The Student Career Experience Program

A Door to a Career with the National Weather Service

Ward R. Seguin and Stephan B. Smith

Recent trends in U.S. undergraduate meteorology degree recipients and employment opportunities show that the American university system is producing many more graduates than traditional employers, such as the National Weather Service (NWS), can absorb. The selection process for vacancies is highly competitive. Having a large pool to draw on for filling the few vacancies that exist would normally be considered a good thing. However, for entry-level positions, where most applicants are coming straight out of university programs and possess little relevant job experience, distinguishing between the qualified candidates who will merely be able to do the work and those who will excel as NWS employees is challenging. One way that the NWS has been able to reduce its risk in this area is by taking advantage of the Student Career Experience Program (SCEP) to identify, train, and select promising future employees. This program allows the NWS to hire students with bachelor's, master's, and doctoral degrees and upon graduation to convert the students to permanent employees relatively quickly. The SCEP goes back many years under such names as the Student Trainee Program, and the Cooperative Education Student Program, and has enabled students to embark on NWS careers. For example, the Meteorological Development Laboratory has graduated more than 170 students from its program since the mid-1970s. This article discusses the use of the program at NWS field offices, regional headquarters, and laboratories and provides statistics on NWS job placements. It is shown that SCEP students fill a significant percentage of NWS's current need for entry-level meteorologists, physical scientists, and hydrologists. In addition, 85% of SCEP students go on to obtain permanent full-time employment with the NWS.

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John Weaver, James F. W. Purdom, and Stephan B. Smith

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Mamoudou Ba, Lingyan Xin, John Crockett, and Stephan B. Smith

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NCAR’s AutoNowCaster (ANC) was modified to run over a large domain that encompasses the air traffic management hubs of Chicago, Illinois; New York City, New York; and Atlanta, Georgia. ANC produces nowcasts of convective likelihood (CL), with higher values delineating areas where storms are likely to form and be sustained, and vice versa. This paper presents the results of verifying ANC’s 60-min nowcasts of CL over the study area using data collected from 11 June to 30 September 2012. To reduce the high sensitivity of statistical scores to small errors in location and timing, spatial and temporal relaxation techniques were explored. The results show that, at a spatial scale of roughly 50 km and with no temporal relaxation, a CL value of 0.6 is an optimum threshold for nowcasting the general areas both where new storms may initiate and where existing storms will be sustained. Moreover, at that same spatial scale and with temporal relaxation (45–90 min from the nowcast issuance time), a CL value of 0.7 is a good threshold for nowcasting convective initiation alone.

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Stephan B. Smith, James G. LaDue, and Donald R. MacGorman

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The relationship between cloud-to-ground (CG) lightning polarity and surface equivalent potential temperature (θ e) is examined for the 26 April 1991, Andover–Wichita, Kansas; the 13 March 1990, Hesston, Kansas; and the 28 August 1990, Plainfield, Illinois, tornadic storm events. The majority of thunderstorms whose CG lightning activity was dominated by negative flashes (labeled negative storms) formed in regions of weak θ e gradient and downstream of a θ e maximum. The majority of thunderstorms whose initial CG lightning activity was dominated by positive flashes formed in regions of strong θ e gradient, upstream of a θ e maximum. Some of these storms moved adjacent to the θ e maximum and were dominated by positive CG lightning throughout their lifetimes (labeled “positive storms”). The other initially positive storms moved through the θ e maximum where their updrafts appeared to undergo intensification. The storms’ dominant CG polarity switched from positive to negative after they crossed the θ e maximum (labeled reversal storms). Summary statistics based on this storm classification show that all the reversal storms examined for these three events were severe and half of them produced tornadoes of F3–F5 intensity. By comparison, only 58% of the negative storms produced severe weather and only 10% produced tornadoes of F3–F5 intensity. It is suggested that the CG lightning reversal process may be initiated by rapid updraft intensification brought about by an increase in the buoyancy of low-level inflow air as initially positive storms pass through mesoscale regions of high θ e. As these storms move out of a θ e maximum, massive precipitation fallout may occur when their updrafts weaken and can no longer support the mass of liquid water and ice aloft. The fallout may in turn cause a major redistribution of the electrical charge within the storm resulting in polarity reversal and/or downdraft-induced tornadogenesis.

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THE TERRAIN-INDUCED ROTOR EXPERIMENT

A Field Campaign Overview Including Observational Highlights

Vanda Grubišić, James D. Doyle, Joachim Kuettner, Stephen Mobbs, Ronald B. Smith, C. David Whiteman, Richard Dirks, Stanley Czyzyk, Stephen A. Cohn, Simon Vosper, Martin Weissmann, Samuel Haimov, Stephan F. J. De Wekker, Laura L. Pan, and Fotini Katopodes Chow

The Terrain-Induced Rotor Experiment (T-REX) is a coordinated international project, composed of an observational field campaign and a research program, focused on the investigation of atmospheric rotors and closely related phenomena in complex terrain. The T-REX field campaign took place during March and April 2006 in the lee of the southern Sierra Nevada in eastern California. Atmospheric rotors have been traditionally defined as quasi-two-dimensional atmospheric vortices that form parallel to and downwind of a mountain ridge under conditions conducive to the generation of large-amplitude mountain waves. Intermittency, high levels of turbulence, and complex small-scale internal structure characterize rotors, which are known hazards to general aviation. The objective of the T-REX field campaign was to provide an unprecedented comprehensive set of in situ and remotely sensed meteorological observations from the ground to UTLS altitudes for the documentation of the spatiotemporal characteristics and internal structure of a tightly coupled system consisting of an atmospheric rotor, terrain-induced internal gravity waves, and a complex terrain boundary layer. In addition, T-REX had several ancillary objectives including the studies of UTLS chemical distribution in the presence of mountain waves and complex-terrain boundary layer in the absence of waves and rotors. This overview provides a background of the project including the information on its science objectives, experimental design, and observational systems, along with highlights of key observations obtained during the field campaign.

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