SODAR

SEDA ŞAHİN

110020230

BRIEFLY…

• Sodar (sonic detection and ranging) systems are used to remotely measure the vertical turbulence structure and the wind profile of the lower layer of the atmosphere.  Sodar systems are like radar (radio detection and ranging) systems except that sound waves rather than radio waves are used for detection.  Other names used for sodar systems include sounder, echosounder and acoustic radar.  A more familiar related term may be sonar, which stands for sound navigation ranging.  Sonar systems detect the presence and location of objects submerged in water (e.g., submarines) by means of sonic waves reflected back to the source.  Sodar systems are similar except the medium is air instead of water and reflection is due to the scattering of sound by atmospheric turbulence.

BRIEFLY…• Most sodar systems operate by issuing an acoustic pulse

and then listen for the return signal for a short period of time.  Generally, both the intensity  and the Doppler (frequency) shift of the return signal are analyzed to determine the wind speed, wind direction and turbulent character of the atmosphere.  A profile of the atmosphere as a function of height can be obtained by analyzing the return signal at a series of times following the transmission of each pulse.  The return signal recorded at any particular delay time provides atmospheric data for a height that can be calculated based on the speed of sound.  Sodar systems typically have maximum ranges varying from a few hundred meters up to several hundred meters or higher.  Maximum range is typically achieved at locations that have low ambient noise and moderate to high relative humidity.  At desert locations, sodar systems tend to have reduced altitude performance because sound attenuates more rapidly in dry air.

BRIEFLY…

• Sodar systems can be used in any application where the winds aloft or the atmospheric stability must be determined, particularly in cases where time and cost are of the essence.  Some typical applications include: atmospheric dispersion studies, wind energy siting, wind shear warning, emergency response wind monitoring, sound transmission analyses, microwave communications assessments and aircraft vortex monitoring.

BRIEFLY…• Some of the advantages of sodar systems are obvious compared to

erecting tall towers with in-situ wind and temperature sensors.  First, a sodar system can generally be installed in a small fraction of the time it takes to erect a tall tower.  And when all of the costs are considered, a sodar system will generally offer a very attractive alternative.  Also, the practical height limit for meteorological towers is about 150 m (500 ft).  Most sodar systems will obtain reliable data well beyond this altitude.  Using a sodar system instead of a tall tower will also avoid many liability issues.  Sodar systems do have some drawbacks compared to tall towers fitted with in-situ wind sensors.  Perhaps the most significant is the fact that sodar systems generally do not report valid data during periods of heavy precipitation.  Another consideration is that sodar systems primarily provide measurements of mean wind.  Other wind parameters, such as wind speed standard deviation, wind direction standard deviation and wind gust, are usually either not available or not reliable.  This is because to obtain a wind measurement sodar systems sample over a volume and at multiple points in space and time, whereas an in-situ wind sensor on a tall tower samples instantaneously at a point in space and time.

SOME SODAR HISTORY…• Sound propagation in the atmosphere has been studied for at least 200 years, but it has

only been in the last 50 years that acoustic scattering has been used as a means to study the structure of the lower atmosphere. 

• In the United States during World War II, acoustic backscatter in the atmosphere was used to examine low-level temperature inversions as they affected propagation in microwave communication links. 

• During the late 1950's, acoustic scattering from the atmosphere was investigated both experimentally and theoretically in the Soviet Union, and researchers in Australia showed that atmospheric echoes could reliably be obtained to heights of several hundred meters. 

• Beginning in the late 1960's and early 1970's, scientists at the U.S. National Oceanic and Atmospheric Administration (NOAA) demonstrated the practical feasibility of using acoustic sounders to measure winds in the atmosphere by means of the Doppler shift and to monitor the structure of temperature inversions.

• During the 1970's, the engineering design of acoustic sounders was seriously pursued by several groups of researchers in the United States.  One of the earliest commercial systems was the Model 300 developed by AeroVironment, Inc. in California.

• In 1974, NOAA developed the Mark VII which was a portable system that was called an acoustic echosounder.  Both the Model 300 and the Mark VII were designed around a single 1.2-meter (4-foot) diameter parabolic dish, and a facsimile recorder was used to provide an analog record of backscatter data.

• During the early 1980's, Radian Corporation used the SES Echosonde as the basis for developing a microcomputer-based three-axis Doppler sodar system.

• Phased-array sodar systems were developed in the United States during the late 1980's and early 1990's by Xonics, Radian Corporation and AeroVironment, among others. 

SODAR THEORY OF OPERATION

SODAR THEORY OF OPERATION• The motion of the atmosphere is the result of general wind flow and

turbulence (the irregular fluctuations of small-scale horizontal and vertical wind currents). Atmospheric turbulence is generated by both thermal and mechanical forces. Thermal turbulence results from temperature differences, or gradients, in the atmosphere. Mechanical turbulence is caused by air movement over the natural or man-made obstacles that produce the “roughness” of the earth's surface. Turbulence from either source results in turbulent air parcels or eddies of varying sizes.

• When an acoustic (sound) pulse transmitted through the atmosphere meets an eddy, its energy is scattered in all directions. Although different scattering patterns result from thermal and mechanical turbulence, some of the acoustic energy is always reflected back towards the sound source. That backscattered energy (atmospheric echo) can be measured using a monostatic sodar system.  A monostatic sodar system is one in which the transmitting and receiving antennas are collocated, and thus the scattering angle between the target eddies and the sodar antenna is 180 degrees.  The backscattered energy is caused by thermally-induced turbulence only.

SODAR THEORY OF OPERATION• In a bistatic sodar system, the transmitting and receiving

antennas are at different locations, and hence scattering angles other than 180 degrees are relevant.  At a scattering angle other than 180 degrees, both thermal and mechanical turbulence come into play.  In principle, this provides for a stronger and more continuous signal, but nearly all commercial sodar systems are monostatic because their design is simpler and more practical.

• Much information about the atmosphere can be derived from monostatic sodar systems. The intensity or amplitude of the returned energy is proportional to the CT

2 function, which, in turn, is related to the thermal structure and stability of the atmosphere. CT

2 has characteristic patterns during ground-based radiation inversions, within elevated inversion layers, at the periphery of convective columns or thermals, in sea breeze/land breeze frontal boundaries, and at any interface between air masses of different temperatures.

SODAR THEORY OF OPERATION• Due to the Doppler effect, measuring the shift in the frequency of

the returned signal relative to the frequency of the transmitted signal provides a measure of air movement at the position of the scattering eddy. When the target (a reflecting turbulent eddy) is moving toward the sodar antenna, the frequency of the backscattered return signal will be higher than the frequency of the transmitted signal. Conversely, when the target is moving away from the antenna, the frequency of the returned signal will be lower. This is the physical characteristic that is used by Doppler sodar systems to measure atmospheric winds and turbulence.

• By measuring the intensity and the frequency of the returned signal as a function of time after the transmitted pulse, the thermal structure and radial velocity of the atmosphere at varying distances from the transmission antenna can be determined. Additional information can be obtained by transmitting consecutive pulses in the vertical direction and in two or more orthogonal directions tilted slightly from the vertical. Geometric calculations can then be used to obtain vertical profiles of the horizontal wind direction and both horizontal and vertical wind speeds.

SODAR THEORY OF OPERATION• A sodar system transmits and receives acoustic signals within a

specific frequency band. Any background noise within this frequency band can affect signal reception. Since the return signal strength usually varies inversely with target height, the weaker signals from greater heights are more readily lost in the background noise. Thus high levels of background noise may reduce the maximum reporting height to a level below that obtainable in the absence of noise.  Certain noise sources can also bias the sodar data. Thus, it is important to identify potential noise sources and estimate the background noise level when evaluating a candidate site for a sodar system.

• One of the other principle problems with sodar systems is ground clutter.  Interference from ground clutter occurs when side-lobe energy radiating from a sodar antenna on transmit is reflected back to the antenna by nearby objects such as buildings, trees, smokestacks or towers.  This reflected side-lobe energy can overwhelm the atmospheric return signal and cause the component wind speeds reported by a sodar system to be zero-biased.  Thus, sodar systems must either be located in areas with wide-open wind fetches (i.e., areas with no reflecting objects), or they must be designed to substantially eliminate side-lobe energy.

SOME APPLICATIONS…

• Example of SODAR wind profile…

SOME APPLICATIONS…

• Example of SODAR derived vertical velocities…

BACKSCATTER SIGNAL

• The sharp increase of the detected signal at 1954 UTC is caused by strong acoustical noise generated by the gust front arriving at the SODAR site. Between 2010 UTC and 2240 UTC noise was added from the precipitation. Acoustical sounding is strongly disturbed by this noise. Therefor, also wind detection is unpossible. If the noise is removed from the data, the remaining signal actually contains no useful backscatter information.

INVERSION STUDIES

• Because routine measurement of the structure and the dynamics of temperature inversions are not available, SODAR (SOnic Detection And Ranging) is used for the monitoring of inversion dynamics. Time series of several years are examined in order to derive a climatology of inversion structure (thickness and stability). Procedures have been developed to determine inversion structure from acoustic sounding in combination with vertical profiles of automatic meteorological observations.

AIR POLLUTION

• The health and environmental administration in the community of Stockholm has been using SODAR for more than 20 years as a tool for giving warnings to the public.

• The pictures show a typical situation with an increased air pollution concentration and causing a warning to the public. The SODAR is a key verification tool for the warning system. The SODAR data is obtained from a system running in the center of Stockholm City.

• The SODAR diagram is showing an increased stability (red color) in connection with a ground based temperature inversion lifting and disappearing during the day. We can also see a very good aggrement with the prediction of the expected air pollution concentration (red color, increased concentrations) made by the health administration.

SODAR IN ANTARCTICA

• it's a multimode Sodar made of 3 antennas in different directions allowing for the computation of the 3-dimensional wind vector at various heights.

• Left: The 3 Sodar antennas, next to each other, with Concordia in the background.

• Right: The Sodar acquisition system inside the container: the electronics and amplifier (blue and black boxes), the PC (on the ground) and the monitor. On the floor the blue boxes are the preamplifiers connected directly to the antennas.

SODAR IN ANTARCTICA

SODAR & WIND ENERGY• Knowledge of the boundary layer

at the heights of today’s large wind turbines can significantly impact turbine selection, predictions of energy production, wind plant maintenance, and proper site selection. In addition to providing high-resolution wind speed and direction data to significant heights, SODAR can also:

• Quantify the individual horizontal and vertical wind flow components

• Measure turbulence levels• Identify flow discontinuities that

fixed towers miss• Measure wind speed in a volume

of air, not just at one point• Confirm or revise the wind shear

aloft defined by on-site fixed towers

• Reduce the number of conventional met towers needed to qualify a site.

AVIATION APPLICATIONS

• The wind information given to the pilots, at take of and landing, normally include information of head and tail wind components together with the side wind component at the surface. A SODAR with a software package can calculate this information for all height intervals.

SODAR & POWER PLANTS

• SODARs with meteorological instruments nearby nuclear power stations ultimately provides emergency responders with a valuable picture of how and where

accidental releases may be transported from the sites.

SOME MODELS…

SOME SYSTEMS…

• Phased Array SODAR DSDPA.90-xx

• Accidental release of pollutant • Air pollution studies and

forecasting • Routine operation in

monitoring networks • Observation of inversion

layers • Airport shear wind warning • Observation of frontal

passages • Atmospheric research

• Technical Specifications of DSDPA.90-24 • Frequency:1000 ... 3000 Hz,

2200 ... 2500 Hz recommended Wind speed: 0 - 50 m/sWind direction: 0 - 360 degree Vertical wind speed: > +- 10 m/s Operating temperature: - 30 ° C to + 55 ° C (all without pos. 3) + 5 ° C to + 45 ° C (indoor components, pos. 3) Operating humidity: 10 - 100 % (outdoor), 20 – 80 % (indoor) Integration time: 10 seconds or more or instantaneous according to the signal repetition, increment 1 sec; for wind speed and wind direction, standard deviations of u-, v-, w-component 10 minutes or more are recommended Number of gates: adjustable, 1- 50 Minimum measuring height adjustable, ≥ 15 m, increment ≥ 1mHeight resolution: > 5 m, < 500 m, adjustable in 1 m – increments values of more than 100 m are not very informative, typical values are 10 - 30 m; Typical measuring height depends on atmospheric and site conditions, we define: 70 % availability (for wind speed and direction,30 m, 900 s, 50 dB stationary noise level, cluster algorithm for data evaluation): 350 m Maximum measuring height > 1000 m; Transmission frequency: adjustable within 1700 - 3000 Hz;(2200 … 2500 Hz recommended) Signal power: max. 800 W (elect.), automatically adjusted Antenna gain: typ. 20 dB, dependant on frequencySensitivity of receiver: 10-6 N/m2, dependant on frequency Beam width: typ. 7 -12 °, dependant on frequency Qualifying: according to german DIN 3786 (11), KTA1508 (nuclear power regularity) Power consumption: depends on pulse repitition rate 250 W

• Complete sets of operation parameter can be stored under up to 40 different user generated parameter set name. A complete parameter set is activated by entering such parameter set name. Various parameter set names can be entered also into a parameter name list of max. 40 names which will sequently activate the corresponding parameters sets. This list will be repeated after the last entry has been processed.

Phased Array SODAR DSDPA.90-xx

Phased Array SODAR DSDPA.90-xx

• SODAR PC (Midi tower type), Indoor • Minimum configuration:

• 600 MHz Celeron • 10 GB hard disc, 64 MB RAM, • 1.44 MB floppy disc drive, CD-ROM 24 x, zip-drive 100 MB • 17“-colour screen (1280 x 1024) • keyboard, compatible type WINDOWS 98, 104 keys, mouse and mousepad • ink jet colour printer, 600 x 600 dpi, A4, 6 pages/minute (b/w mode) • Ethernet port • MODEM unit for remote system access, Hayes AT compatible, supports

V.90 • WINDOWS 2000/NT english installed • Y2K compliance for hard and software • implementation of cluster algorithms for derivation of wind speed and

direction

Phased Array SODAR DSDPA.90-xx• SODAR PC Control and Data Visualization Software • Operating system WINDOWS 2000; • Control subsystem "sodar control" run time license (3 x)

• offers access to and control of all system parameters, measuring variables, port selections; • offers remote system access for system control and system testing (e.g. via cellular Modem); • stores data and handles data files automatically in a tree structured file system; • data sets are ASCII coded files, optional the structures can be defined according to the needs of the customer; • To set up a remote control with a modem the customer needs a second pc-station where the WINDOWS NT

software "sodar control" is installed. If enabled you will connect via modem to the SODAR pc, independent of the type of the MODEM (line or GSM).

• Graphic subsystem „METEK grafik“ run time license (3 x)

• offers a variety of powerful data presentation tools; • profiles, time series, vector plot, contour plot (smooth or raw); • time intervals (day, week, month, special) and height ranges selectable or automatically scaled); • time increments selectable; • off-line presentation (also batch mode for long term data evaluation) or on-line display function with automatic real

time refresh; • 1 or 4 or 9 plots in one frame; • presentation of all measuring variables as time series, profiles, vector plots; • selectable plausibility check validity/data acceptance; • selectable smoothing weighting function for all data with export feature; • SODARgramm display with selectable resolution; • statistics (relative and absolute histogramms and wind roses in adjustable classes); • automatic or user selectable ordinate and abscissa scaling; • indication of numerical values depending on the pointer position; • zoom-function and smooth function for all data; • manual invalidation procedure; • WINDOWS supported global print routine; • various export formats fpr plots(Windows bitmap, Windows metafile, agfa SCODL, HPGL, LOTUS 1-2-3, PIC,

GIF, TIFF, GEM, Encapsulated Postscript, CGM)

MODOS Mobile Doppler SODAR

• Preferred Applications • Remote sensing of wind and

turbulence in the atmospheric boundary layer

• Easy transportation, quick set up and prompt measurements

• Reliable unattended operation even under severe conditions

• Very low minimal measuring height, very fine height resolution

• Windows NT graphic package, LAN integration for raw spectra output

• Accidental release of pollutant • Air pollution studies and

forecasting • Routine operation in monitoring

networks • Observation of inversion layers • Airport shear wind warning • Observation of frontal passages • Atmospheric research

MODOS Mobile Doppler SODAR

• Features • Effective antenna shields allow measurements even under noisy

conditions • Individual alignment in zenith/azimuth for optimized system

performance at difficult sites • High system availability due to strict redudancy concept for critical

components • All operational parameters adjustable • Very low minimal measuring height • Very fine height resolution • Effective antenna shielding • Automatic system function monitoring • Automatic restart after power failure • Automatic antenna deicing (option) • Automatic control of signal power • Transmit frequency adjustable (1500 ... 3000 Hz) • Reliable data validation algorithm

MODOS Mobile Doppler SODAR

• Measuring Ranges • Mobile Doppler SODAR – MODOS• Wind velocity 0 ... 35 m/s • Direction 0 ... 360 ° • Standard deviation of radial wind components 0 ... 3 m/s • Finest height resolution >=10m/s • Minimum measuring height Standard 30m Optionally 10m Maximum

measuring height (10 min averages)¹ 90% 200m 80% 300m 70% 400m 60% 500m

• Measuring Accuracy (10 min averages) • Wind velocity for 0.0 ... 5.0 m/s ±0.5 m/s for 5.0 ... 35 m/s ±10 % • Wind direction for 0.8 ... 35 m/s ±5 ° • Radial wind components

±0.1 m/s• Standard deviation of radial wind components

±0.15 m/s

MODOS Mobile Doppler SODAR• System Specifications

• Antenna 3x7 exponential horns Aperture 1 m2

• Transmit Power 1 kW (electric)

• Wind velocity

• Vertical

• North/South, typical 20° tilt West/East, typical 20° tilt

MODOS Mobile Doppler SODAR• Available options include: • System operation with comfortable graphical user interface on a PC under

Windows NT. • Processing of quality flagged data on a PC under Windows NT for

comfortable graphical presentation: – Statistics – Time series – Time height cross section using

• profile series, • contours or • vector plots

• Expansion with RASS for simultaneous temperature profiling. • Ingestion and display of simultaneously measured data from USA-1 sonic

anemometers and additional standard surface sensors. • Integration to upper level networks and implementation of further features

on request.

CONCLUSION SODAR • TOOL FOR• Meteorologists• Atmospheric

physicist• Health and

environmental protection authorities

• Power plant industry

• APPLICATIONS• Predict dispersion of air

pollution• Elevated temperature

inversions• Atmospheric stability• Mountain/valley flow• Difuusion in complex

terrain• Plume dispersion

monitoring• Sea and land breeze• Weather forecasts• Climate research

SOURCES…• http://www.awstruewind.com/inner/services/meteorology/

sodar.htm • http://www.sodar.com• http://www.metek.de/• http://www.slb.mf.stockholm.se/e/weather_now.htm • http://www.gdargaud.net/Antarctica/WinterDC3.html

• http://www.pa.op.dlr.de/cleocd/sodar/q.htm • eflum.epfl.ch/research/ sodar-rass.en.php • http://www.aqs.se/Pages/airpollution.htm • http://lcrs.geographie.uni-marburg.de/ index.php?id=32 • http://apollo.lsc.vsc.edu/.../ remote/image_gallery.html