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R. B. Smith

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R. B. Smith

Abstract

The nonlinear effects in the flow of a Boussinesq stratified fluid over two-dimensional sinusoidal topography are examined in this paper using second-order perturbation theory. The waves generated by finite-amplitude topography are found to steepen in the forward sense in a way which is periodic with height. The nonlinear lower boundary condition has an important influence on the steepening as well as on wave breakdown and the production of severe downslope winds. These nonlinear effects are shown to be important for moderate and large mountain even if the surface slopes are small. Using the results of linear theory, other causes of steepening such as mountain asymmetry, vertical variation of wind, stability, density and mountain isolation can be estimated. The nonlinear effects associated with partial reflection from the tropopause are found to be very important in certain situations.

The theoretical ideas of wave steepening are in qualitative agreement with previous case studies of flow over large mountains and with seasonally averaged measurements of stratosphere turbulence. The theory suggests that for large mountains light wind conditions (as in the summer in mid-latitudes) will lead to wave breaking just above the mountain, while the stronger winds (e.g., winter-time) the mountain waves will propagate vertically through the troposphere—finally breaking down in the lower stratosphere near 17 km.

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R. B. SMITH, C. E.

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Steven B. Newman and David R. Smith

The Third International Conference on School and Popular Meteorological and Oceanographic Education was held 14–18 July 1993 in Toronto, Ontario, Canada. This conference was attended by approximately 150 educators, meteorologists, oceanographers, and government officials representing 12 countries. The themes of this conference were the role of meteorology and oceanography in the formal science education of students in grades K-12 and the enhancement of scientific literacy of the public in order to permit individuals to make better use of products and services provided by the national environmental services and the media. Sixty formal presentations plus two poster sessions and six workshops provided information on educational programs as well as a variety of classroom activities on meteorological and oceanographic topics.

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Neville R. Smith and C. B. Fandry

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A simple, frictional, linear model is used to study the motion of the Gull Stream over the continental shelf. It is found that the combination of frictional and topographic effects may provide a further mechanism by which the observed separation of the Gulf Stream may be achieved.

The model predicts separation in the form of a classical separated boundary layer and interrelates the slope of the bottom with the position of separation. Counter-circulations northwest of the stream and increased northward transport of the current are also predicted.

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Y. L. Lin and R. B. Smith

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The response of a stratified atmosphere to local heating is a common element in several problems in mesoscale dynamics. To investigate this response, a time-dependent linearized problem is solved analytically for an elevated, local heat source turned on as a pulse in a stratified, moving fluid. The thermally induced circulation in the vicinity of the drifting disturbance is qualitatively similar to that of a cumulus cloud in mean wind. The updraft at the center of this cloud is surrounded by the compensating downdrafts at early times even if that air has also been heated. Once the updraft at the drifting center weakens, upward motion begins in the adjacent regions. An integration of the pulse solution yields the response to steady heating, turned on at t = 0. As steady state is approached, this solution exhibits a region of positive displacement moving downstream while negative displacement develop near the stationary heat source. The solution offers an explanation to a curious negative phase relationship between heating and displacement and the lack of a true steady state noted by other authors. It is suggested that the nature of this response may help to explain three problems in mesoscale dynamics: cloud interaction, heat island/orographic rain, and the squall line.

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J. P. Evans and R. B. Smith

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The study presented here attempts to quantify the significance of southerly water vapor fluxes on precipitation occurring in the eastern Fertile Crescent region. The water vapor fluxes were investigated at high temporal and spatial resolution by using a Regional Climate Model [fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5)–Noah land surface model] to downscale the NCEP–NCAR reanalysis. Using the Iterative Self-Organizing Data Analysis Techniques (ISODATA) clustering algorithm, the 200 largest precipitation events, occurring from 1990 through 1994, were grouped into classes based on the similarity of their water vapor fluxes. Results indicate that, while southerly fluxes were dominant in 24% of tested events, these events produced 43% of the total precipitation produced by the 200 largest events. Thus, while the majority of precipitation events occurring in the Fertile Crescent involve significant water vapor advected from the west, those events that included southerly fluxes produced much larger precipitation totals. This suggests that changes that affect these southerly fluxes more than the westerly fluxes (e.g., changes in the Indian monsoon, movement of the head of the Persian Gulf, etc.) may have a relatively strong affect on the total precipitation falling in the Fertile Crescent even though they affect relatively few precipitation events. To obtain a clearer view of the precipitation mechanisms, the authors used a linear model, along with the estimated water vapor fluxes, to downscale from 25 to 1 km. The result shows a spectrum of mountain scales not seen in the regional model, exerting tight control on the precipitation pattern.

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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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S. P. Ballard, B. W. Golding, and R. N. B. Smith

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A mesoscale model is used to simulate the diurnal evolution of sea fog off the northeast Scottish coast observed on 27 April 1984. It is shown that the accuracy of the early part of the forecast is very dependent on the specification of the initial conditions. If the initial description of the fog is sufficiently good the model can accurately erode it during the day and reform it in the following evening. The dependence of the accuracy of the forecasts on vertical resolution is also discussed.

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D. Lynch, K. Smith, B. Blanton, R. Luettich, and F. Werner

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This paper addresses shelf-scale simulation with dominant open-water boundary conditions obtained by inversion of interior data. Important, established operational data streams are located along the shore of the study area, in areas influenced strongly by the local geometry. Failure to properly resolve the modeled near field surrounding these data results in their incorrect interpretation, causing invalid inversions and erroneous field estimates far across the shelf. Specifically, improving the model fit to the unresolved data leads to skill degeneration farther offshore and generally unacceptable field estimates remarkably far from shore. Proper near-field resolution leads to valid interpretation and inversion of the same data, with high inverse skill apparent across the shelf. The resolution required is within reach of today's technology.

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