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Neil B. Ward

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NEIL B. WARD

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Neil B. Ward

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Three characteristic features of tornadoes are simulated in a laboratory system and the associated flow is observed and discussed. These are (i) a characteristic surface pressure profile, (ii) a bulging deformation on the vortex core, and (iii) multiple vortices in a single convergence system. Vortex motion is very sensitive to the geometrical features of the larger flow in which it is imbedded. Only when the diameter of the updraft column exceeds the depth of the inflow layer can features (i) and (iii) be produced in the present model. When the updraft diameter is large compared to the depth of inflow, inertial effects associated with large changes in radial momentum produce significant convergent forces. When the updraft diameter is small compared to depth of inflow layer, the inflow speed is relatively small and related inertial effects are small. It is concluded that radial momentum flux is an important factor in the production of atmospheric vortices.

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Neil B. Ward

Experimental results are shown which indicate that one of the factors which differentiates between ordinary thunderstorm convectivity and tornado activity may be the formation or intensification of a restricting layer such as a temperature inversion. Evidence is presented to show that such a layer, penetrated by a convective column, can bring about horizontal convergence beneath the layer into an area sufficiently small that, if some rotary motion is present, a funnel is produced. Indications are that the diameter of the opening through which the converging air exhausts has an important effect on the formation, type and intensity of the funnel. Further, it is shown that the formation or intensification of a temperature inversion may restrict the diameter of an existing convective column in a manner similar to the experimental device.

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Asher B. Siebert and M. Neil Ward

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A statistical simulation framework is developed to explore the future frequencies of threshold-crossing events, focusing here on low seasonal rainfall totals. Global change (GC) is represented by a trend on the seasonal mean rainfall total. Natural decadal to multidecadal variability (MDV) is represented by an autoregressive process. Interannual variability (IV) of seasonal totals is represented by white noise with either a normal or skew normal distribution consistent with parameters observed in the historical record at the location being modeled. Monte Carlo simulations are undertaken for various combinations of the above components, and the authors evaluate the extent to which future event frequencies can be estimated from the statistics of previous years. The sample of four study locations used to illustrate the approach is drawn from the Millennium Villages Project in Africa, where the potential of index insurance as a development and adaptation tool has been considered, thereby bringing a targeted problem setting to the analyses. The simulations highlight a number of general principles. For example, it is shown that a 10% change in the mean rainfall can lead to a change of order times 2 in the number of threshold-crossing low seasonal rainfall totals, even without invoking any change in the characteristics of the IV. The magnitudes of change are also shown to be sensitive to the threshold studied, as well as to site-specific climate features (here, coefficient of variation and skewness). The framework developed permits quantification of how, especially in the near term (of order 30 years), MDV can strongly add to uncertainty about future event frequencies. Therefore, statistical treatment of estimated MDV magnitudes will often be a key input to optimal risk management, with further enhancements expected through explicit MDV forecasts. The results highlight the importance of finding optimal ways to update climate statistics such as event thresholds, in the presence of GC and MDV.

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R. P. Davies-Jones and N. B. Ward

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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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R. O. B. E. R. T. DE C. WARD

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Prof. R. O. B. E. R. T. DeC. WARD

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R. O. B. E. R. T. DE C. WARD

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