Geoscience Reference
In-Depth Information
Wavelength 1.5 μm
Pulse repetition rate 20 kHz
Bandwidth ± 14 m s
-1
Sampling frequency 30 MHz
Points per range gate 6
Number of pulses averaged 20 000
Δr 18 m
Δp 30 m
Averaging time 1 s
Table 2. Parameters of 1.5 micron Doppler lidar. Range gate parameters: Δr relates to pulse
length and Δp is the down range extent of the range gate used in the signal processing (from
Pearson et al., 2009).
4. Advantages and disadvantages of radar and lidar
Both radars and lidars have advantages and disadvantages, and in this chapter a review of
the accuracy with which measurements of boundary layer winds from both instruments is
given. However both instruments offer complimentary information. Lidar backscatter is
from widely dispersed aerosol particles in the clear air, but their concentration decreases
away from the surface. Although thin clouds also provide backscatter, hydrometeors
strongly attenuate the lidar signal. Radar backscatter is generally from hydrometeors, and
therefore the operating range is much greater than that of lidar although this depends upon
the wavelength used as short wavelength signals are attenuated.
Topography strongly controls the flux of momentum and energy between the terrain surface
and the boundary layer. In-situ instruments have minimum spatial coverage, and radar
cannot make measurements very close to the ground due to beam side lobes which produce
ground clutter. With beamwidths of 0.1-1 m. rad., lidar transverse resolution is 20-200 times
finer than the one degree (17.5 m. rad.) of weather radars (Drechsel et al., 2009). Lidar
provides wide area coverage, although not as extensive as radar, but does not suffer from
ground clutter problems. Hence lidar offers the opportunity to improve our knowledge of
flow over complex terrain close to the ground surface (Barkwith and Collier, 2011).
However radar can provide detailed information in the boundary layer over very wide
areas, particularly related to the development of convective systems.
5. Wind profiling
5.1 Single instrument measurements
Measurements of the vertical profile of wind may be made using Doppler lidar and both
weather radars and UHF / VHF profilers. There are two modes that can be utilised for this.
The beam can be scanned in a cone at fixed elevation and the resulting data fitted to a sine
wave. This is known as the velocity-azimuth display (VAD) approach as described by
Browning and Wexler (1968). This has been the preferred technique for radar systems
although it has also been used with lidar systems. An alternative approach uses three fixed
line of sights from which a vector analysis provides the three components of the wind (u, v
and w) as described by Werner (2005).
