9.1.5 RASS
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 radar such that the wavelength of the
acoustic signal matches the half-wavelength of the radar. As the frequency
of the acoustic signal is varied,
strongly enhanced scattering of the radar signal occurs when the Bragg
match takes place. When this occurs, the Doppler shift of the radar signal
produced by the Bragg scattering can be
determined, as well as the atmospheric vertical velocity. Thus, the speed of sound as a function of altitude can be
measured, from which virtual temperature (Tv ) profiles can
be calculated with appropriate corrections for vertical air motion. The
virtual temperature of an air parcel is
the temperature that dry air would have if its pressure and density were
equal to those of a sample of moist air.
As a rule of thumb, an atmospheric vertical velocity of 1 ms -1 can alter
a Tv observation by 1.6°C.
RASS
can be added to a radar wind profiler or to a sodar system. In the former
case, the necessary acoustic subsystems
must be added to the radar wind profiler to generate the sound signals
and to perform signal processing. When RASS is added to a radar profiler,
three or four vertically pointing
acoustic sources (equivalent to high quality stereo loud speakers) are
placed around the radar wind profiler's
antenna, and electronic subsystems are added that include the acoustic
power amplifier and the signal generating circuit boards. The acoustic
sources are used only to transmit sound
into the vertical beam of the radar, and are usually encased in noise suppression enclosures to minimize nuisance
effects that may bother nearby neighbors or others in
the vicinity of the instrument.
When
RASS is added to a sodar, the necessary radar subsystems are added to
transmit and receive the radar signals and to process the radar reflectivity
information. Since the wind data are
obtained by the sodar, the radar only needs to sample along the vertical
axis. The sodar transducers are used to
transmit the acoustic signals that produce the Bragg scattering of the radar signals, 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 the radar. RASS sampling is usually performed with a 60- to 100-m pulse
length. Because of atmospheric
attenuation of the acoustic signals at the RASS frequencies used by boundary
layer radar wind profilers, the altitude
range that can be sampled is usually 0.1 to 1.5 km, depending on atmospheric
conditions (e.g., high wind velocities tend to limit RASS altitude coverage
to a few hundred meters because the
acoustic signals are blown out of the radar beam).
9. UPPER-AIR MONITORING
9.1 Fundamentals
9.1.1 Upper-Air Meteorological Variables
9.1.2 Radiosonde Sounding System
9.1.3 Doppler Sodar
9.1.4 Radar Wind Profiler
9.1.5 RASS
9.2 Performance Characteristics
9.2.1 Definition of Performance Specifications
9.2.2 Performance Characteristics of Radiosonde Sounding Systems
9.2.3 Performance Characteristics of Remote Sensing Systems
9.3 Monitoring Objectives and Goals
9.3.1 Data Quality Objectives
9.4 Siting and Exposure
9.5 Installation and Acceptance Testing
9.6 Quality Assurance and Quality Control
9.6.1 Calibration Methods
9.6.2 System and Performance Audits
9.6.3 Standard Operating Procedures
9.6.4 Operational Checks and Preventive Maintenance
9.6.5 Corrective Action and Reporting
9.6.6 Common Problems Encountered in Upper-Air Data Collection
9.7 Data Processing and Management (DP&M)
9.7.1 Overview of Data Products
9.7.2 Steps in DP&M
9.7.3 Data Archiving
9.8 Recommendations for Upper-Air Data Collection