Geoscience Reference
In-Depth Information
inhomogeneities caused by turbulence. In the lower troposphere the refractive index
inhomogeneities are mainly produced by humidity fluctuations. The clear air Doppler shift
provides a direct measurement of the mean radial velocity along the radar beam. Typically a
UHF wind profiler operates at 1290 MHz or 915 MHz with a peak power of 3.5 kW and a
beamwidth of 8.5 degrees.
Wavelength / frequency
5 cm (C-band) / 5430-5800 MHz
Pulse repetition frequency
250-1200 Hz selectable
Bandwidth single PRF
dual PRF
± 15.9 m s
-1
± 63.8 m s
-1
Sampling frequency - IF
60 MHz
Peak power
250 kW
Minimum Detectable Signal
-111 dBm
Beamwidth
1 degree
Down range resolution/
Maximum range
75 m
200 km
Transmitter type
Coaxial magnetron
Table 1. Typical parameters of a C-band radar (from Selex Gematronik)
Generally radars used for weather forecasting other than wind profilers have a resolution of
about 100 m with an antenna having a diameter of about 4 m. Some research radars, such as
the Chilbolton radar in the UK, provide measurements with a resolution at 100 km range
using a 0.25 degree beamwidth (25 m diameter antenna) of 0.4 km. These resolutions
certainly improve our understanding of the structure and behaviour of boundary layer
phenomena, and examples will be described in this chapter. Nevertheless, boundary layer
turbulent eddies may exist with characteristic length scales from tens of metres to fractions
of a metre close to the ground. Such length scales require different instrumentation. Doppler
lidar is an instrument providing high resolution, clear air measurements with resolutions of
around 30 m, albeit over much shorter ranges than available from radar systems.
3. Characteristics of lidar
Lidar has been developed which operate in various atmospheric windows, namely the 10, 2,
1.5 and 1 micron spectral regions. Hardesty et al. (1992) compared the transmission,
backscatter, refractive turbulence and Doppler estimation characteristics of a 2 and a 10
micron Doppler lidar system. Whilst backscatter at 2 microns in the free atmosphere is 4-10
times higher than for a 10 micron Doppler lidar, the effects of turbulence on the 2 micron
system beyond a few kilometres range are significant, with the signal to noise ratio being
reduced by about 6 dB at 5 km range. However, early equipment operated with CO
2
lasers
at 10.6 micron wavelength, but involved delicate optical systems (Post and Cupp, 1990;
Mayor et al., 1997; Pearson and Collier, 1999). The advent of fibre optic technology has
enabled compact, robust equipment to be developed and operated remotely at wavelengths
of 1.5 microns (Pearson et al., 2002, 2009). Table 2 shows the parameters of this type of
Doppler lidar. The range resolution of 30 m is considerably smaller than that used in CO
2
lidars of about 112 m.
