Wind Profiling Radar and RASS

449 MHz wind profiler

WFIP 915 MHz wind profiling radar at Saint James, MN: Enclosure with blue edge contains the transmitter and antenna. White rectangular boxes are RASS loud-speakers. Portable trailer contains the computer data system and communication equipment.

Wind Profilers

A Wind Profiling Radar (WPR) is a weather observing device that uses electromagnetic signals to remotely detect wind speed and direction at various elevations above the ground.

WPRs transmit pulses of electromagnetic radiation vertically and in at least two slightly off-vertical (~75 degree elevation) directions to resolve the three-dimensional vector wind. A small amount of the energy transmitted in each direction is reflected or backscattered to the radar. The backscatter returns are Doppler shifted by the motion of the scattering media. The return signals are sampled in the receiver at discrete intervals called range gates.

Profilers receive backscatter returns from atmospheric features (turbulence, clouds, precipitation) and non-atmospheric features (insects, birds, trees, airplanes, radio frequency interference). The challenge in signal processing is to focus on the atmospheric returns. To do this, profilers integrate thousands of consecutive samples to boost the signal-to-noise ratio of the atmospheric returns, a process known as coherent integration.

A set of coherent integrations is processed to produce a single Doppler velocity spectrum, and a set of spectra are averaged together to improve the detectability of the spectral peak. The strongest peak in the spectrum is analyzed and assumed to be the peak resulting from atmospheric backscatter. The spectral peak is analyzed to produce a set of Doppler spectral moments, the first three of which correspond to the signal power, radial velocity, and spectral width. This is repeated for each range gate and for each of the three transmitted beam directions.

449 MHz wind profiler
The WFIP 449 MHz wind profiling radar located at Buffalo, ND.
The WFIP 449 MHz wind profiling radar located at Buffalo, ND.

A wind profile is constructed by geometrically transforming the radial velocities into the meteorological coordinate system. A single wind profile is produced over an observing period of 30 to 90 seconds. All of the wind profiles measured within a specified averaging period (15 min to 60 min) are averaged together using a consensus routine. Doppler radars can operate over a wide range of frequencies, in the VHF (30–300 MHz) and UHF (300–30,000 MHz). UHF radar wind profilers are specifically designed for boundary layer and lower-tropospheric studies. For the WFIP campaign, two types of WPRs were used: the first uses 915 MHz frequency microwaves (33 cm wavelength) while the second uses 449 MHz (67 cm wavelength) microwaves. The 915 MHz systems are frequently referred to as "boundary layer profilers" and have a typical lowest range gate near 100m, with a maximum detectable signal that varies with atmospheric conditions (higher in a moist atmosphere) but that typically ranges from 1.5 to 4 km above ground level. Typically two vertical sampling modes are interlaced in time, a 60 m high resolution mode and a coarser resolution 100m mode.

The 449 MHz WPR’s systems not only have a lower frequency but also more powerful transmitters. Both of these facts allow the 449 MHz WPR to observe a deeper layer of the atmosphere, often to 7 km AGL.

RASS

Wind Profiling Radars can include Radio Acoustic Sounding Systems (RASS) components for measuring temperature profiles. Both the 915 and 449 MHz wind profiling radars generally came equipped with RASS components. RASS measures the virtual temperature (the temperature that a completely dry parcel of air would have if it had the same density and pressure as a parcel of moist air) by emitting a vertically propagating acoustic signal from a loudspeaker near the side of the radar antennae, and tracking the speed of the acoustic signal with the Doppler radar beam. Since the speed of sound depends on the temperature of the air, the vertical profile of virtual temperature can be measured.

A time-height profile of the virtual temperature from the Buffalo WPR is shown in Figure 5. The height coverage of RASS for the 449 MHz systems was typically 1.0 km, and 0.6 km for the 915 MHz systems. RASS temperatures were measured and averaged over the last 5 minute period of each hour.