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J. E. Gaynor

Abstract

Root-mean-square error statistics were applied to tower wind data to find the optimum average times of the wind and azimuth to be used as predictors of a 2-min mean wind speed and direction, 0–10 min in the future. The data were divided into classes of stable and unstable boundary layers, and although the predictability was the same, the results showed the difference in the frequency distribution of the atmospheric wave structure for stable and unstable conditions. As opposed to the conclusion of previous studies, beyond the 1-min forecast interval, simple persistence is the best predictor of both the wind speed and direction at the three levels.

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J. E. Gaynor

Abstract

Acoustic echo sounder (echosonde) and meteorological tower measurements of the turbulent velocity structure parameters D(r) and Cv2 and the rate of dissipation of turbulent energy ε are compared. The two acoustic Doppler methods attempted, utilizing pulse differencing and Taylor hypothesis approaches, show good agreement. The small discrepancy in these parameters between the tower and echosonde is explained by the wind noise and ambient noise characteristics of the echosonde and by the effects of pulse volume averaging. Time-averaged, acoustically derived Cv2 values are compared with acoustic facsimile records in both stable and unstable atmospheric conditions. The temporal and (implicitly) the spatial variations of Cv2 were observed to be large, and correlated well with echosonde-detected waves, turbulent layers and thermal plumes. The hour-average vertical ε profiles for the two stability cases show reasonable comparison with those calculated by other investigators.

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L. Kristensen and J. E. Gaynor

Abstract

We present a theoretical derivation for errors in calculated second moments arising from the temporal and spatial separation between individual wind measurements obtained from three-axis colocated monostatic Doppler sodar systems. The derived relations require as input the sodar monostatic geometric parameters, pulse repetition rate, pulsing sequence, mean wind velocity and an estimate of the turbulence length scale. The model results indicate an optimum monostatic configuration that minimizes the bias due to temporal and spatial separation. A correction for pulse volume averaging is also derived which requires a turbulence length scale and an estimate of the pulse volume diameter. A monostatic technique, based on radar VAD methods, which also minimizes the bias caused by spatial and temporal separations in the sampling volumes, is proposed.

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J. C. Kaimal and J. E. Gaynor

Abstract

The Boulder Atmospheric Observatory (BAO) is a unique research facility for studying the planetary boundary layer and for testing and calibrating atmospheric sensors. The facility includes a 300 m tower instrumented with fast- and slow-response sensors, a variety of remote sensing systems, and a real-time processing and display capability that greatly reduces analysis time for scientists working with current or archived data. In the past four years of operation the BAO has been the site of several large cooperative experiments and numerous smaller ones. Details of the data acquisition, processing and archiving schemes are presented. Results of studies conducted and opportunities for future investigations are also described.

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J. E. Gaynor and P. A. Mandics

Abstract

An acoustic sounding system placed on the NOAA Ship Oceanographer during GATE provided a unique meteorological data set. Examples of three distinct boundary-layer situations are discussed as they appear on the facsimile records of backscattered acoustic intensity: 1) ubiquitous plume echoes associated with undisturbed conditions, 2) cool-air outflows (or wakes) from either squall-line or isolated cumulonimbus activity associated with disturbed conditions and 3) “hat”- or “hummock” -shaped echoes associated with low-level cumulus clouds usually occurring during weakly disturbed conditions. Profiles of potential temperature and mixing ratio from radiosonde flights launched from the Occonograph are compared with the acoustic data. Convective plumes observed during GATE were less vigorous than those seen over land. Bulk aerodynamic fluxes of surface sensible and latent heat varied in time with the passage of thermal plumes. This indicates a minimum averaging time for valid flux estimates of about 30 min. Outflows resulted in order-of-magnitude increases in surface sensible heat flux and large increases in surface stress, but only relatively small increases in latent heat flux. The increased stability following the outflows lasted from a few hours up to 16 h, depending on the intensity of the disturbance. The rate of dissipation of turbulent kinetic energy is calculated in the upper mixed layer for the three cases using an acoustic Doppler-differencing technique. These values are intercompared and compared with those from other studies. Evidence is presented indicating that hummocky echoes were associated with low-level clouds. Plumes underneath the hummocks were characterized by larger moisture content and surface beat fluxes when compared with plumes without hummocks.

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J. E. Gaynor and C. F. Ropelewski

Abstract

A boundary-layer categorization scheme based on GATE acoustic sounder data stratifies surface data and tethered balloon data from the NOAA research ship Oceanographer. The results indicate a clear increase in the sensible heat flux across the sea surface during disturbed conditions (the gravity current and storm wake of the disturbance) but no conclusive differences of latent heat and momentum fluxes. The tethered balloon profiles show a near disappearance of the mixed layer within the gravity current and, in the storm wake, a very shallow and cool mixed layer capped by a strong stable layer relative to the undisturbed category.

We calculate the vertical motion due to buoyancy-driven entrainment for a range on entrainment parameters after exploiting the categorized tethered balloon profiles to obtain mean gradients at the top of the mixed layer. Because we and others have observed that the shallow mixed-layer depth remains nearly constant with time in the storm wake, this calculated entrainment-induced vertical motion is balanced with an hypothesized mesoscale subsidence beneath the anvil in the wake. Even though our entrainment calculation ignores the possibly important but unknown effets of vertically propagating waves, breaking waves and wind shear, the integrated divergence derived from this subsidence agrees well with the range of mesoscale divergences in the storm wake presented by Zipser (1977).

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D. Skibin, J. C. Kaimal, and J. E. Gaynor

Abstract

This paper describes an effort to identify instrumental and flow-related contributions to the mean vertical velocity offsets observed by sonic anemometers on the Boulder Atmospheric Observatory's 300 m tower. A simple technique to remove these offsets, which typically range from 0.1 to 0.2 m s−1, is presented. The correction yields 20 min averaged vertical velocities with good temporal and spatial continuity. Corrections for the vertical velocity standard deviation and the temperature flux are negligible. For momentum flux, the correction could be important when that flux is very small.

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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.

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P. L. Finkelstein, J. C. Kaimal, J. E. Gaynor, M. E. Graves, and T. J. Lockhart

Abstract

Measurements of wind speed, wind direction, and the vertical component of turbulence, from four different commercially available Doppler sodars, are compared with similar measurements from in situ sensors on a 300 m instrumented tower. Results indicate that the four sodars measure wind speed and direction accurately and with reasonably high precision. The sodars tended to overestimate the vertical component of turbulence at night and to underestimate it during the day. Precision in those measurements was considerably poorer than for the averaged speeds and directions. Analysis of the vertical wind speed measurements from the sodars indicates that the measurement inaccuracies arise from a combination of aliasing and spatial averaging. Comparison of five in situ wind systems with a sonic anemometer are presented in Part I of this two part series of papers.

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P. L. Finkelstein, J. C. Kaimal, J. E. Gaynor, M. E. Graves, and T. J. Lockhart

Abstract

It has recently become clear through advances in both theoretical and experimental meteorology, that improvements in modeling the transport and dispersion of pollutants will require on-site measurements of the atmosphere. This requirement has in turn generated questions about 1) our ability to make such measurements both near the surface and through the first few hundred meters of the atmosphere and 2) the expected accuracy and precision of such measurements using current technology. To help answer these questions an experiment was conducted at the Boulder Atmospheric Observatory to assess the ability of in situ and remote sensors to measure the mean and turbulent properties of the lower atmosphere. Two categories of sensors were tested. One consisted of lightweight in situ sensors of types that have been frequently used in the recent past for boundary layer studies. The other category consisted of four commercially available Doppler sodars, with the capability to measure wind speed, wind direction, and vertical component of turbulence, at various heights above the ground. Part one of this two part study deals with comparisons of five in situ wind sensing systems with a three-axis sonic anemometer, all mounted on 10 m towers spaced approximately 5 m apart. Discussed in this paper are statistical measures of their accuracy, precision and spectral response to fluctuations in the wind.

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