Atmospheric Turbulence Measurements Research Articles

Continuous measurement of upper atmospheric winds and turbulence using a VHF Doppler radar: preliminary results

We report here preliminary results of a VHF radar experiment designed to monitor atmospheric winds and turbulence continuously over periods of many days. The radar system used in this experiment is a prototype of a much larger system under development and uses an average transmitted power of about 200 W and a 100 m × 100 m dipole array. With this system it is possible to observe winds and turbulence parameters regularly to about 19 km using a height resolution of 2.2 km. Similar results are reported for mesospheric heights between about 73 km and 82 km.

Angular spectrum measurements of atmospheric turbulence

The unique property of the theoretical formula for the angular spectrum of a point source seen through a turbulent medium is that it is related in simple closed form to the integrated structure function of the turbulence. It therefore provides a reliable measure of the state of turbulence. Rudimentary measurements over a 5 km path show a remarkable confirmation, over a range 1000:1 in intensity, of the Kolmogoroff two-thirds law, and of the underlying theory of wave propagation through a turbulent medium.

An instrumentation system for the measurement of atmospheric turbulence

A method is described for the detailed multi-point measurement of the turbulent velocity components and temperature in the atmospheric surface layer. The TSI three-dimensional split-film anemometer system is used as sensor and a special designed data-acquisition and data-handling system is used for the sampling, digitizing and storing of the data on digital magnetic tapes. Further processing of the data is performed in a digital computer and programs are developed to allow efficient calculation of the statistical quantities such as, mean values, covariances, power- and cross-spectral density functions. At the present, six anemometer systems have been mounted on the 30-, 50-, 100-, 150-, 200- and 250-foot levels of the 250-foot Meteorological Tower at NASA, Wallops Flight Center. The tower is located at Wallops Island which is one of the barrier islands protecting the so-called “Delmarva” peninsula from the Atlantic Ocean. The island, which consists of a narrow strip of dunes approximately three meters above sea level, is situated in a northeast-southwest direction and is separated from the mainland by a three-mile wide tidal swamp.

Frequency response of hot wires used for atmospheric turbulence measurements in the marine environment

We have examined the frequency response of hot wire anemometers using 4.5-μ tungsten wires with laboratory-produced salt layers. The salt layers are produced by exposure of the heated wire to a NaCl–water aerosol and are intended to simulate salt layers produced on wires used in the marine environment. The frequency dependence was determined by measuring the response of the wires to a sinusoidal velocity field generated in a sound chamber. The relaxation frequency (or rolloff frequency) was found to vary as t−2, where t is the thickness of the salt layer, in agreement with the theory presented in this paper. In addition, the dc calibration parameters for thick coatings were investigated and compared with theory.

New low-speed anemometer with constant temperature thermistor

A new constant temperature thermistor anemometer for very low gas velocities is described. The principle of a constant temperature method and the operation of the instrument are outlined. With the electronic equipment used, the temperature of the probe can be fixed independently of both velocity and temperature of the gas flow. Good stabilisation of the resistance and hence of the temperature gives a short response time which is compatible with measurements of atmospheric turbulence. The nonlinear term in the expression for the thermistor's temperature coefficient can therefore be neglected. The experimental calibration curves agree well with King's linear law.

Interpretation of sodar measurements: relevant properties of the atmosphere's planetary boundary layer

Assuming a Kolmogorov spectrum of turbulence, the acoustic scattering cross section may be written as a function of incident wavelength, scattering angle, and the magnitude of the root-mean-square turbulent temperature and velocity fluctuations. In the “surface” layer SL (max ≃ 3–30 m height) of the atmosphere the statistical properties of turbulent velocity, temperature, humidity, and pressure fluctuations above many surface types are well understood for a wide variety of meteorological conditions. Hence, it is possible using sodar to derive unique, quantitative measurements of SL atmospheric turbulence. Detailed information about planetary boundary layer PBL (max ≃ 300 m–2 km height) turbulence is not required in order to make continuous sodar measurements of vertical profiles of turbulence related atmospheric structure and winds (from signal Doppler shifts). Sodar is, thus, now routinely used, e.g., for air pollution related, micrometeorological research, and wind-shear detection measurements. Our perception of many complicated PBL structural features and processes has been greatly expanded by the now familiar vertical-time section sodar records of various “layers” and thermal convection. However, except for “unstable” conditions of free convection, SL turbulence models are not generally applicable for the quantitative interpretation of PBL sodar data. In order to realize the full indirect sounding potential of sodar during “stable” atmospheric conditions, it is necessary to determine the relationship(s) between observed “macroscale” features such as multiple layers (with and without waves), intermittant “patches” of turbulence and “bursts” of vertical momentum or heat transport and the space-time dependent properties of the turbulence in such situations. This paper first reviews the application of contemporary SL and PBL models to the interpretation of sodar measurements. Functional relationships between sodar observed and model implicit parameters are summarized. Finally, the application of sodar for development and verification of selected atmospheric models is discussed. [Research supported by EPA Grant R800397.]

Use of resistance wires for atmospheric turbulence measurements in the marine environment

The dc calibration parameters of hot wires used for wind speed measurements and cold wires used for air temperature measurements have been investigated for wires used in the marine environment. Tungsten hot wires and platinum temperature wires have been calibrated both before, during, and after their use at sea. A scanning electron microscope has been used to determine the surface condition of the wires and the amount of sea salt that is deposited on them. The results show that sea salt is quickly deposited on the wires when they are used at sea but that the deposits are rarely of sufficient thickness to affect the dc calibration of the wires.

Measuring Atmospheric Turbulence with Airborne Hot-Film Anemometers

Extensive flight tests during GATE showed hot-film anemometry to be a useful tool for the airborne measurement of atmospheric turbulence in clear air and in subcloud rain, but not within clouds. Root-mean-square noise values lower than 0.08 ms−1 for velocity and 0.03°C for temperature were obtained over the scale range of 50 m to 4 cm at altitudes from 16 to 2000 m. Spectra of U′, W′ and θ were obtained over the same range with roughly 1 dB accuracy. Dissipation rates could be determined to within ±30%. Cross-component contamination was too large to permit reliable cross spectra to be obtained. It is suggested that an upgraded system could significantly reduce such contamination and improve the overall accuracy and signal-to-noise ratio.

The impact of space electric field research on atmospheric studies

Space measurements of electric fields have provided instrumentation for measuring atmospheric parameters and a better basis for understanding the electrical coupling between the magnetosphere and the atmosphere. Applications of an incoherent scatter radar (developed for ionospheric electric field research) to the measurement of atmospheric winds and turbulence and of Langmuir double probes (also developed for space research) for measurement of atmospheric electric fields are described. The increased knowledge of magnetospheric electric fields has focused attention on the electrical coupling between the magnetosphere and the atmosphere with conclusions that should considerably modify previous physical concepts in both domains.

The intermittent small-scale structure of turbulence: data-processing hazards

Velocity derivatives in turbulence at large Reynolds numbers exhibit probability densities with a large kurtosis. If the value of the kurtosis is about 40, as in many measurements of atmospheric turbulence, careful attention to the processing of such signals is required in order to obtain an acceptable level of accuracy. Signal-to-noise and integration-time limitations make measurements of moments higher than the fourth nearly impossible for signals of this kind.

A Laser Velocimeter for Remote Wind Sensing

A continuous wave carbon dioxide laser Doppler radar has been developed and applied to remote measurement of atmospheric wind velocity and turbulence. The carbon dioxide laser illuminates residual particulate matter in the atmosphere. Radiation scattered by these particles is homodyned with a local oscillator to provide the Doppler signal. The performance of the instrument is verified by comparison of wind velocity data recorded simultaneously by the laser Doppler system and a cup-anemometer-wind-vane system. Data were recorded, for 45 min intervals, of a single component of the horizontal wind velocity at an altitude of 10 m above the ground. The principle of the laser Doppler system and the theory of the system configuration are presented first, followed by the evaluation of the system. Sample data of a 20 min duration test run of a laser Doppler system are compared with conventional anemometer data. Further comparisons are given of the power spectral densities derived from the two time histories. All data comparisons indicate very close agreement of the two systems. Data inconsistencies are within the accuracy limitations of the conventional anemometer system. The range of the laser Doppler system during these tests was confined to approximately 30 m. Laser Doppler wind velocity data were observed at ranges exceeding 300 m; however, no conventional anemometer was set up at these ranges for data comparisons. Tests are planned for the near future, at which time comparison data at extended ranges will be recorded. These tests will also incorporate an on-line data processing system.

Optimal Choice of Parameters for the Measurement of Small-Scale Atmospheric Turbulence with an Airborne Hot-Wire Anemometer

The paper presents typical spectra of probe motions, turbulence, and electronic noise of an airborne hot-wire anemometer. Their relative amplitudes as a function of aircraft size and velocity, hot-wire length, and turbulence dissipation are discussed. The optimal choice of these parameters in the measurement of small-scale atmospheric turbulence with an airborne hot-wire anemometer is recommended.

The structure and intensity of turbulence in air over the sea

Measurements of atmospheric turbulence have been made from an aircraft at heights in excess of 30 m over the open sea in convective situations. They are related to height, wind speed, heat flux and stability. The variation of the r.m.s. vertical velocity with height is examined for two different scaling systems. From calculations of the power spectra a length scale is derived for the vertical velocity fluctuations which is found to increase slowly with height.

Comparison of acoustic instruments in an atmospheric turbulent flow over water

In the fall of 1968 an International Intercomparison Experiment was conducted to compare, among other things, acoustic anemometers designed for measurements of atmospheric turbulence. Excellent agreement was obtained in measurements of the vertical component, but an important discrepancy was revealed in measurements of the downwind velocity. The discrepancy proved far more significant in the cospectra than in the spectra themselves. It is evident that great care must be taken in the design of such instruments. An acoustic instrument was shown to have insufficient signal-to-noise ratio to serve as a thermometer under the near-neutral conditions often encountered over water.

Buildings in the wind

AbstractSome of the problems arising from the flow of the wind about buildings are selectively illustrated in terms of the aerodynamics of vortex shedding and the interaction between a bluff body and turbulence in the approaching stream. Examples chosen draw attention to the spanwise incoherence of vortex shedding from a rigid body and vortex‐capture by an oscillating body, the effects of geometrical taper and shear in the undisturbed stream and the pecularities exhibited by the wakes behind porous windbreaks. Galloping oscillations of cables together with the large amplitude motions that can be engaged in by stranded electrical conductors are also briefly mentioned.As a preliminary to discussing the effect of turbulence on the flow past a body, as well as the distortion of the turbulence produced by the body, observations are offered on the relation between the known properties of the micro‐spectrum of atmospheric turbulence and wind tunnel measurements made in turbulent boundary layers. An interpretation of the latter in terms of the ‘inactive’ component of the unsteady motion near a surface, contributed by eddies in the outer parts of the boundary layer, suggests that further meteorological data, so frequently demanded by architects and aerodynamicists, may fail to add systematic information about the behaviour of the u‐component of the turbulent velocity other than is already available.Modern techniques of wind tunnel testing, which attempt to represent mean and turbulent properties of the wind, are described and cautionary remarks are made about the prospect for significant further refinement.

The importance of instrumental tilt on measurements of atmospheric turbulence

This note discusses the equations that govern the dependence of observed turbulence characteristics on instrumental tilt and then considers the magnitudes of the effects within the constant stress layer.

The importance of instrumental tilt on measurements of atmospheric turbulence

AbstractThis note discusses the equations that govern the dependence of observed turbulence characteristics on instrumental tilt and then considers the magnitudes of the effects within the constant stress layer.

A Method for Radar Turbulence Detection

A method is described whereby a conventional non-Doppler radar may be used in the measurement and detection of atmospheric turbulence. The technique involves the measurement of the average echo power and the variance of the fluctuation spectrum in each of two separated pulse volumes, and corresponding measure ments of echo power and variance of the detected signals in a channel representing the predetection sum of the signals from those two volumes. It is shown that the variance of the sum channel is the power-weighted sum of the variances in the independent channels plus a term dependent upon the difference in mean radial velocity of the associated pulse volumes. Accordingly, the difference in mean radial velocity can be measured. When the spacing of the sampled volumes is adjusted to represent distance along an aircraft flight plan, the measurement is a measure of the turbulence to be experienced in traversing the space between those volumes. Specific systems for accomplishing the measure ment are discussed.

TEMPERATURES AND TURBULENCE AT TROPOPAUSE LEVELS OVER HURRICANE BEULAH (1967)

Horizontal and vertical temperature variations along with true gust velocity measurements of atmospheric turbulence were obtained above hurricane Beulah (1967) by an instrumented U–2. The U–2 flight was part of the U.S. Air Force High Altitude Clear Air Turbulence program. Pertinent findings include: (1) location of the tropopause just above the cloud tops at 54,000 ft (100 mb) with a temperature of −86°C, 12°C lower than the mean; (2) a vertical temperature rise of 11°C in a few hundred feet just above the tropopause; (3) horizontal temperature changes up to 7°C and smooth flight conditions in the stable layer above the cloud tops; and (4) small temperature fluctuations and generally turbulent conditions at cloud top level. Aircraft measured winds, although questionable as to their exact directions and speeds, indicated that the flow was weak and anticyclonic near Beulah's top, becoming increasingly anticyclonic above.

Remote sensing of winds and atmospheric turbulence by cross-correlation of passive optical signals

A new method is described for the remote measurement of winds and atmospheric turbulence by the cross-correlation of passive optical signals. If small local variations in atmospheric density, temperature or other parameters cause fluctuations in scattered or thermal radiation detected by a radiometer on the ground, then the cross-correlation of fluctuations detected by two radiometers with crossed fields of view can yield turbulence information pertaining to the region about this intersection point. When the fields of view are not quite crossed turbulent eddies will be convected through the fields of view sequentially, and the transit times of the eddies identified by the correlation procedure will yield wind information. The successful application of this technique, detecting fluctuations in scattered sunlight, has demonstrated both the potential, and the present limitations of the method, which are discussed. Results for the power spectrum of the fluctuations and for winds at an altitude of 61 meters are shown.