Search Results

You are looking at 1 - 10 of 13 items for

  • Author or Editor: W. D. Neff x
  • Refine by Access: All Content x
Clear All Modify Search
W. D. Neff

Abstract

An “excess attenuation” experiment is described that uses an acoustic sounder (or echosonde) operating at simultaneous frequencies of 1250 and 2500 Hz. The relative error in the received powers at these two frequencies is shown to be a beam-width-dependent effect. The magnitude of this effect at 150 m ranged typically from 50% in free-convection conditions to 400% with wind speeds of 10 m s−1, consistent with a number of previous comparison experiments. The results of this experiment are interpreted in terms of models of the broadening of the beam by small-scale turbulence and the refraction of the beam by a transverse wind.

Full access
W. D. Neff and C. W. King

Abstract

We describe a sequence of tethersonde and solar measurements showing the effects of the pooling of cold air drainages in a basin located along the Colorado River below the Brush drainage. Results obtained during periods of weak ambient winds show that the basin fills over a period of several hours, then eventually overflows. The depth of the pool is such as to affect tributary drainages, such as that of Brush Creek, and to cause the accumulating drainage jets to become elevated as they flow down the larger drainage channels into the basin.

Full access
F. F. Hall Jr. and W. D. Neff

Abstract

Full access
M. J. Post and W. D. Neff

During September and October of 1984, the Wave Propagation Laboratory of NOAA used its pulsed infrared Doppler lidar in support of the U.S. Department of Energy's Atmospheric Studies in Complex Terrain (ASCOT) program. The lidar measured winds channeled within a narrow mountain valley on seven experiment nights, between 2300 and 1100 MST. We were able to quantitatively define the structure of the nocturnal drainage winds, monitor their decay in the morning, and sense the formation of a thermally driven up-valley flow later in the day.

Full access
F. F. Hall Jr., J. G. Edinger, and W. D. Neff

Abstract

The turbulent temperature structure and winds in thermal convective plumes over prairie grassland have been investigated with an acoustic echo sounder system. Three spaced acoustic antennas, with two inclined at 45° elevation, were used to provide plume shape information and Doppler-derived total wind-vector patterns between heights of 70 and 500 m. Supporting in situ measurements were made on a 15 m tower, with a tethered balloon-supported Boundary Layer Profiler, and from a light aircraft. The most probable orientation of the plumes was nearly vertical, but frequent upwind and downwind tilts were also observed. Maximum positive vertical velocities in the plumes at midday were near 2 m s−1, while maximum downward currents were one-half this value. Acoustic echoes from regions above the mixed layer, corresponding in height to an elevated temperature inversion, correlate well with regions of maximum wind shear.

Full access
B. Van Dam, D. Helmig, W. Neff, and L. Kramer

Abstract

Boundary layer conditions in polar regions have been shown to have a significant impact on the levels of trace gases in the lower atmosphere. The ability to properly describe boundary layer characteristics (e.g., stability, depth, and variations on diurnal and seasonal scales) is essential to understanding the processes that control chemical budgets and surface fluxes in these regions. Surface turbulence data measured from 3D sonic anemometers on an 8-m tower at Summit Station, Greenland, were used for estimating boundary layer depths (BLD) in stable to weakly stable conditions. The turbulence-derived BLD estimates were evaluated for June 2010 using direct BLD measurements from an acoustic sounder located approximately 50 m away from the tower. BLDs during this period varied diurnally; minimum values were less than 10 m, and maximum values were greater than 150 m. BLD estimates provided a better comparison with sodar observations during stable conditions. Ozone and nitrogen oxides were also measured at the meteorological tower and investigated for their dependency on boundary layer structure. These analyses, in contrast to observations from South Pole, Antarctica, did not show a clear relation between surface-layer atmospheric trace-gas levels and the stable boundary layer.

Full access
P. H. Gudiksen, J. M. Leone Jr., C. W. King, D. Ruffieux, and W. D. Neff

Abstract

An experimental and modeling investigation of nocturnal drainage flows within the Mesa Creek valley in western Colorado revealed their wind and temperature characteristics and the effects of the ambient meteorology on their development. The valley, located about 30 km east of Grand Junction, is situated on the north slopes of the Grand Mesa. It is surrounded by ridges on three sides with low terrain toward the north. The terrain at the higher elevations is characterized by steep slopes that become shallower at the lower elevations. A network of seven meteorological towers and a monostatic solar collected data within the study area from December 1988 through November 1989. Analysis of the experimental data indicated that shallow drainage flows generated over the many individual slopes at the higher elevations converge at the lower elevations to form deeper flows that join with those generated within adjacent drainage areas. The characteristics of the flows generally deviated from those displayed by idealized slope flows due to both internal circulations within the valley and external influences. During the summer, the depths of the flows were typically a few tens of meters along the upper slopes and about 100 m over the upper part of the lower slopes while during the winter, the depths decreased to about 10 and 60 m, respectively. Their frequency of occurrence was highest during the summer or fall, about 50%, when the synoptic-scale influences were minimal. The flows along the upper slopes were particularly susceptible to influences by the ambient meteorology due to minimal terrain shielding. When the larger-scale ambient flows over the Grand Mesa were greater than about 5 m s−1, the surface cooling along the slopes was unable to develop and maintain the surface temperature inversion needed to generate strong drainage flows. The radiative cooling rates of the sloped surfaces, as characterized by net radiation measurements, were correlated with the downslope wind speeds observed along the upper slopes. Thus, a decrease in the observed net radiation level will produce a corresponding decrease in the downslope wind speed. Since temporal changes in net radiation levels are primarily governed by variations in atmospheric moisture, the effect of increased atmospheric moisture is to retard the development of the drainage flows.

In order to place the observations in proper perspective, it was necessary to employ numerical models that account for the physical processes governing the dynamics of the flows. The general features of the wind and temperature characteristics of the valley circulations and the influence of strong ambient winds and atmospheric moisture on the drainage flows over the upper slopes could be accounted for by numerical modeling techniques based on solving the equations of momentum, continuity, and energy coupled with a surface energy budget and a radiation module.

Full access
E. E. Gossard, J. E. Gaynor, R. J. Zamora, and W. D. Neff

Abstract

A study of the finestructure within elevated stable atmospheric layers is described. The observational program consisted of measurements made with fast-response turbulence sensors on a carriage traversing a 300 m tower and comparison of the carriage data with data from acoustic and radar echo sounders. Some supporting observations using a free balloon-borne sensor of the temperature structure parameter are also shown. The layers studied were found to be composed of sheets and layers in temperature, humidity and wind reminiscent of the sheet and layer structures often reported in lakes, estuaries and the oceans. Finestructure in the profiles of temperature and humidity are very highly correlated within elevated stable layers. The sheets are generally accompanied by thin zones of very large temperature and humidity structure parameter, apparently the result of Kelvin-Helmholtz instability, that account for the strong returns from these zones recorded by short wavelength radar and acoustic sounders. The distributions of turbulence properties through the layered structures are described, and some implications for models are discussed. A quite general ratio of sheet-to-layer thickness is proposed toward which the process of step formation proceeds. Measured profiles of short term averages of wT′ show thin zones of apparently strong upward flux imbedded within generally stable regions of weak downward flux. These layers of positive flux are associated with thin superadiabatic zones in the temperature profile and suggest a much more complicated process of heat and momentum transport within stable, elevated regions than process suggested by classical turbulence theory.

Full access
J. C. Doran, M. L. Wesely, R. T. McMillen, and W. D. Neff

Abstract

Measurements of heat and momentum fluxes along the valley floor of Brush Creek in Colorado are described. The measurements were taken in the fall of 1984 as part of the Department of Energy's Atmospheric Studies in Complex Terrain field program. The sensible heat flux to ground decreased from approximately 40–60 W m−2 prior to midnight to about 10–25 W m−2 in the morning hours. Surface friction velocities u * ranged from approximately 20–15 cm s−1 during the corresponding time periods. Considerable site-to-site variability in flux values was found, and disturbances of the upwind flow appear to be a significant contributing cause.

Full access
E. E. Gossard, R. B. Chadwick, W. D. Neff, and K. P. Moran

Abstract

The use of ground-based clear-air Doppler radars to observe the structure of elevated atmospheric layers and associated flux quantities is described. Case studies in which radar and balloon data were available are analyzed. Doppler second-moment (velocity variance) data are used to calculate turbulent kinetic energy dissipation rate ε. Velocity variance, refractive index structure parameter and wind shear are used to estimate the refractive index gradient across elevated weather-frontal interfaces. A case is analyzed in which both acoustic-sounder and radar-sounder data are available, so profiles of structure parameter of both temperature and humidity can be deduced and used to calculate the fluxes of heat and moisture within the frontal interface. The fluxes deduced from radar data are compared with corresponding in situ measurements made by aircraft in other geographical regions. The relationship between the turbulent Prandtl number and the Richardson number emerges as very important to the generalization of the technique to the whole stable atmosphere.

Full access