Geoscience Reference
In-Depth Information
11
Synergy Between Doppler Radar and Lidar for
Atmospheric Boundary Layer Research
Chris G. Collier
National Centre for Atmospheric Science, University of Leeds
United Kingdom
1. Introduction
The principle of operation of radar and lidar is similar in that pulses of energy at
wavelengths ranging from millimetres to metres for radar and 0.5 to 10 microns for lidar are
transmitted into the atmosphere; the energy scattered back to the transceiver is collected and
measured as a time-resolved signal. From the time delay between each outgoing transmitted
pulse and the backscattered signal, the distance to the scatterer is inferred. The radial or line-
of-sight velocity of the scatterers is determined from the Doppler frequency shift of the
backscattered radiation. The systems use a heterodyne detection technique in which the
return signal is mixed with a reference beam (i.e. local oscillator) of known frequency. A
signal processing computer then determines the Doppler frequency shift from the spectra of
the heterodyne signal. The energy content of the Doppler spectra can also be used to
determine boundary layer eddy characteristics.
2. Characteristics of radar
The atmospheric boundary layer has been studied using weather radar extensively over the
last forty years or so, such that networks of radars comprise systems sometimes operating
unmanned in remote locations (see for example Atlas, 1990). Doppler radar operating at X,
C or S-band (3cm, 5cm and 10cm wavelength respectively) has provided the opportunity to
measure the reflectivity of target hydrometeors and the three dimensional wind structure of
the lower parts of the atmosphere inferred from their motion (see for example Doviak and
Zrnic, 1984). Typical parameters for a C-band radar are listed in Table 1. In addition, high
power radar systems, such as the Oklahoma University Polarimetric radar for Innovations
in Meteorology and Engineering (OU-PRIME) which operates at C-band and has a peak
power of 1000 kW and a beamwidth of 0.45 degree (Palmer et al., 2011), provide information
on the clear air structure of wind fields, and sometimes lower power systems may do the
same at close range to the radar site. The detailed principle of operation has been described
elsewhere in this topic.
Millimetre-wave cloud radars exploit the fact that the echo intensity of Rayleigh scatterers
increases with the inverse fourth power of the wavelength. These radars normally operate at
35 GHz and 94 GHz. UHF and VHF Doppler radar systems measure both wind speed and
direction by detecting small irregularities in back scattered signals due to refractive index
