- Radio acoustic sounding system
A radio acoustic sounding system (RASS) is a system for measuring the atmospheric
lapse rateusing backscattering of radiowaves from an acoustic wave front to measure the speed of soundat various heights above the ground. This is possible because the compression and rarefaction of air by an acoustic wave changes the dielectricproperties, producing partial reflection of the transmitted radarsignal. [cite book | last = Avouris | first = Nicholas | title = Environmental Informatics | publisher = Kluwer Academic | location = City | year = 1995 | isbn = 0792334450 ] From the speed of sound, the temperature of the air in the planetary boundary layercan be computed. [cite book
last = Raghavan
first = Soundararajan
title = Radar Meteorology
url = http://www.springer.com/west/home?SGWID=4-102-22-33611767-0
edition = Vol. 27
series = Atmospheric and Oceanographic Sciences Library
year = 2003
publisher = Springer
isbn = 978-1-4020-1604-2
pages = p. 442 ] The maximum
altituderange of RASS systems is typically 750 meters, although observations have been reported up to 1.2 km in moist air. [cite book | last = Kaimal | first = J. | title = Atmospheric Boundary Layer Flows | publisher = Oxford University Press | location = Oxford Oxfordshire | year = 1994 |pages = p. 245 | isbn = 0195062396 ]
The principle of operation behind RASS is as follows: Bragg scattering occurs when
acoustic energy (i.e., sound) is transmitted into the vertical beam of a
radarsuch that the wavelengthof the acoustic signal matches the half-wavelength of the radar. As the frequency ofthe acoustic signal is varied, strongly enhanced scattering of the radar signal occurs when theBragg match takes place. When this occurs, the Doppler shift of the radar signal produced by theBragg scattering can be determined, as well as the atmospheric vertical velocity. Thus, the speedof sound as a function of altitude can be measured, from which virtual temperature(Tv) profilescan be calculated with appropriate corrections for vertical air motion. The virtual temperature ofan air parcel is the temperature that dry air would have if its pressure and density were equal tothose of a sample of moist air. As a rule of thumb, an atmospheric vertical velocity of 1 m/s canalter a Tv observation by 1.6°C.
RASS can be added to a radar
wind profileror to a sodarsystem. In the former case, thenecessary acoustic subsystems must be added to the radar wind profiler to generate the soundsignals and to perform signal processing. When RASS is added to a radar profiler, three or fourvertically pointing acoustic sources (equivalent to high quality stereo loud speakers) are placedaround the radar wind profiler's antenna, and electronic subsystems are added that include theacoustic power amplifier and the signal generating circuit boards. The acoustic sources are usedonly to transmit sound into the vertical beam of the radar, and are usually encased in noisesuppression enclosures to minimize nuisance effects that may bother nearby neighbors or othersin the vicinity of the instrument.
When RASS is added to a sodar, the necessary radar subsystems are added to transmitand receive the radar signals and to process the radar reflectivity information. Since the winddata are obtained by the sodar, the radar only needs to sample along the vertical axis. The sodartransducers are used to transmit the acoustic signals that produce the Bragg scattering of the radarsignals, which allows the speed of sound to be measured by the radar.
The vertical resolution of RASS data is determined by the pulse length(s) used by theradar. RASS sampling is usually performed with a 60 to 100 meter pulse length. Because ofatmospheric attenuation of the acoustic signals at the RASS frequencies used by boundary layerradar wind profilers, the altitude range that can be sampled is usually 0.1 to 1.5 km, depending onatmospheric conditions (e.g., high wind velocities tend to limit RASS altitude coverage to a fewhundred meters because the acoustic signals are blown out of the radar beam). [cite book
last = Bailey
first = Desmond T.
others = John Irwin
title = Meteorological Monitoring Guidance for Regulatory Modeling Applications
origyear = 1987
origmonth = 6
url = http://www.epa.gov/scram001/guidance/met/mmgrma.pdf
year = 2000
month = 2
United States Environmental Protection Agency
location = Research Triangle Park, NC
id = EPA-454/R-99-005
pages = pp. 9-13 to 9-14
chapter = Upper-air Monitoring]
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