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John M. Lewis and Robert A. Maddox

In an effort to encourage college students to consider careers in scientific research, NOAA's National Severe Storms Laboratory has instituted a Summer Employment Program. The program is centered around a scientific mentorship experience that matches each student with a laboratory scientist. During the nominal 12 weeks of the program, the scientist leads and directs a research project that is designed to be commensurate with the student's background. Along with the research experience, there is an educational component that encompasses both classroom work and experimentation. Additionally, students are introduced to a variety of research efforts in the laboratory through a continuing series of guest lectures by lab scientists.

The program has operated in 1987, 1989, and 1990, and has included 17 students, 12 of whom have come from under-represented groups in our society. We report on the evolution of the program and scrutinize the results after these three years of effort.

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Robert W. Pratt and John M. Wallace

Abstract

The general character of zonally propagating fluctuations in the mid-latitude troposphere is illustrated using the quadrature spectrum of the zonal sine and cosing Fourier coefficients. It is shown that for low frequencies (periods around 20 days) and low wavenumbers, fluctuations in the geopotential field tend to be westward, while those in the temperature field are eastward. Evidence is provided, by means of empirical orthogonal function analysis, that this situation is explained by the existence of two distinct modes of fluctuation, which together account for most of the large-scale, low-frequency variance. One mode has substantial surface amplitude, is nearly barotropic in structure, and propagates westward with respect to the earth. The other mode is more baroclinic and propagates eastward.

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John M. Leone Jr. and Robert L. Lee

Abstract

One of the objectives of the Atmospheric Studies in Complex Terrain (ASCOT) program is to develop numerical models that can be used to aid in the understanding and prediction of flow patterns observed over complex terrain. As part of this program, we have developed a three-dimensional dynamic model that uses a simplified finite element scheme combined with a variation of the forward Euler time integration scheme to solve the Boussinesq equations of motion over irregular terrain. We have used the model to simulate the development of nocturnal down-valley circulation in the area where the ASCOT field observations were made at Brush Creek, Colorado. The model and the simulation results are described in this paper.

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Robert A. Kirkman and John M. LeBedda

Abstract

The course of development of Army radio equipment at the Signal Corps Engineering Laboratories for the measurement of upper wind velocities is reviewed. It is indicated that this type of information is needed for the successful navigation of aircraft, for weather predictions, and for the determination of ballistic winds used in connection with artillery fire. Operation of the two latest types of direction-finders, for upper wind measurements up to 100,OO~O feet altitude, is described in some detail. Performance data are given. Sources of error in connection with accuracy measurements are discussed quantitatively. ,Standard angular errors of the apparatus considered are in the range from 0.05 to 0.15 degrees.

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John M. Wallace and Robert E. Dickinson

Abstract

Difficulties in using conventional cross-spectrum analysis to explore atmospheric wave disturbances have indicated the need for some extension of the usual technique. It is suggested here that the eigenvectors of the cross-spectrum matrix be used for interpreting such data. The method is analogous to the use of empirical orthogonal functions applied to band-pass filtered time series. However, the eigenvectors of the cross-spectrum matrix contain additional information concerning phase which is not available from the eigenvectors of the covariance matrix. It is possible to generate a new set of time series which are mutually uncorrelated within a pre-selected frequency interval and which have the same combined variance in the frequency interval as the original set of time series. These new series are obtained by applying the eigenvectors of the cross-spectrum matrix to a set of complex time series involving the original time series and their time derivatives. The application and physical interpretation of the technique are discussed. Examples of the technique applied to atmospheric data are given in a second part of this study.

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John M. Firda, Stephen M. Sekelsky, and Robert E. McIntosh

Abstract

Millimeter-wave Doppler spectra obtained from the dual-frequency Cloud Profiling Radar System (CPRS) are used to retrieve both the drop size distribution and the vertical air motion in rain. CPRS obtains collocated spectra at W and Ka bands through a single 1-m-diameter lens antenna. The vertical air motion is determined primarily from the 95-GHz Mie scattering from rain, whereas turbulence effects are minimized by correlating the drop size distributions measured at both the 95- and 33-GHz frequencies. The authors describe an iterative procedure that estimates the drop sizes and vertical motions with range and horizontal resolution of 60 m and temporal resolution of 2 s. Model drop size distributions are used to initiate the procedure, but the retrieved distributions and vertical air motions are seen to be independent of the particular model used.

Data were gathered to test the procedure during the Ground-Based Remote Sensing Intensive Observation Period (GBRS IOP) sponsored by the Department of Energy Atmospheric Radiation Measurement (DOE ARM) program. The measurements represent the first simultaneous Doppler spectra of rain at these frequencies. The experiment took place in April 1995 at the Cloud and Radiation Testbed (CART) site in Lamont, Oklahoma. Radiosonde and surface measurements of temperature and pressure were used in the retrieval algorithm. Rain events from stratiform and transition region (i.e., decaying from the convective region toward the stratiform region of a storm) clouds were observed and are analyzed in this paper. The rain rate for the stratiform rain case was relatively uniform with small amounts of vertical air motion. Variations of the vertical winds for the transition region case, however, were larger and more frequent and were accompanied by short intense downbursts. The algorithm’s results are best for rain rates higher than 1 mm h−1.

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Addison L. Sears-Collins, David M. Schultz, and Robert H. Johns
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John E. Kutzbach, Robert M. Chervin, and David D. Houghton

Abstract

Four numerical experiments are analyzed to determine the three-dimensional response of the NCAR general circulation model to large prescribed changes in mid-latitude North Pacific Ocean surface temperature. The ocean surface temperature (OST) boundary conditions are subjected to changes of opposite sign in the eastern and west-central portions of the North Pacific Ocean. The maximum amplitude of the temperature changes is either 12°C or 4°C. The model atmosphere response in the North Pacific sector includes changes in amplitude and vertical tilt of the long waves, an increased direct thermal circulation (i.e., warm air rises over the positive OST change and cold air sinks over the negative OST change), and locally enhanced westerlies to the north of the positive OST change. Cyclones form and/or intensify over the positive OST change and tend to be absent or weak over the negative OST change. The mid-tropospheric response extends downstream from the prescribed change region, and the response both over and downstream from the region depends strongly on the longitude of the prescribed changes. Many features of the response are statistically significant, although generally not over the continental United States. The amplitude and phase of the mid-tropospheric long waves (zonal wavenumbers 1–4) are also affected. The prescribed change response is largest and of greatest statistical significance when the prescribed change is very large (12°C maximum amplitude) but is also frequently detectable when the prescribed change is one-third as large (4°C maximum amplitude). A comparable experiment involving a prescribed North Atlantic OST change produces a similar mid-tropospheric response.

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Montie M. Orgill, John D. Kincheloe, and Robert A. Sutherland

Abstract

The mesoalpha-scale upper-level sounding network data collected during the 1984 ASCOT meteorological and tracer experiments provided a unique opportunity to analyze the nocturnal drainage wind in four different valleys in western Colorado, and to examine the effects of the synoptic-and mesoscale ambient conditions on these valley drainage winds. The six experimental periods, although biased because of a “fair weather” selection process, provided an additional opportunity to examine “good” and “poor” drainage scenarios. The results show that drainage winds fill all four valleys up to 80%–l00% of their valley depths under favorable nocturnal radiative longwave cooling (1.0°–1.5°C h−1) and light (<5 m s−1) ambient winds. Valley drainage, once established, is rather resistant to erosion from above because of the large source regions of these valleys, their large volume fluxes and inertia, and the persistent stable conditions inside these valleys. Wind erosion was observed on three nights when the drainage depth was reduced to less than half of the valley depth. The principal contributing factors to wind erosion processes were above-valley along-valley wind component opposing the drainage, valley stability, height of the 5 m s−1 isotach above the valley, and total above-valley wind acceleration. Generally, wind erosion processes appear to be especially active when above-valley wind speeds exceed 5 m s−1 and accelerations exceed 0.00040 m s−1. Other contributing factors that cause variable or terminated drainage depths are precipitation-evaporation effects causing nonradiative drainage events, wind shear above the valley, cloudiness, frontal passages, and synoptic winds directed in the down-valley direction.

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Captain Robert W. Lenhard Jr. and T/Sgt John M. Foard

A method is presented for determining the probability of occurrence of all combinations of crosswind and runway wind components at an airport. It is applied to Prestwick, Scotland for the existing instrument runway and for winds occurring with ceilings less than 1000 ft and/or visibilities less than three miles.

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