Geoscience Reference
In-Depth Information
Recently, Le et al. (2010) proposed a technique that improves received signal power by
exploiting the temporal correlation difference between the desired signal and system noise.
Le et al. (2010) showed that the technique has better performance of radar echo production
than RIM in the low SNR regions.
2.2.2 Measurement results
In order to demonstrate that RIM is useful for improving range resolution, measurement
results are presented. Fig. 5 shows an example of RIM measurement. The brightness profiles
were produced with 6-m range intervals by applying RIM to received signals measured
with the 2-μs transmitted pulse and four transmitted frequencies (914.0, 914.33, 915.33, and
916 MHz). The bandwidth of the RIM measurement was 2.5-MHz (2 MHz for the actual
frequency spread and 0.5 MHz for the 2-μs transmitted pulse). During the experiment, the
RIM measurement and single frequency measurement using the 0.5-μs transmitted pulse
(i.e., corresponding to 75-m range resolution and 2-MHz bandwidth) were carried our
alternatively. Although the frequency bandwidth difference between the two observation
modes was as small as 500 kHz, it is clear that both the brightness produced by the Fourier-
based method and by the Capon method show finer height variations than the backscattered
power measured with the 75-m range resolution. Further, it is clear that the Capon method
attains finer range resolution than the Fourier-based method. By applying RIM to the same
dataset, Chilson et al. (2004) produced Doppler velocity with 15-m range intervals and
showed that the Doppler velocity produced by RIM agreed well with that measured with
the 0.5-μs transmitted pulse.
Fig. 6 shows an example of Kelvin-Helmholtz (KH) instability (i.e., shear instability) measured
by the MU radar operated with a RIM observation mode (Fukao et al., 2011). The
measurement was carried out using the new MU radar system upgraded in 2004 (Hassenpflug
et al., 2008). Structure of KH billows is clearly seen in the brightness around 1.5 km from 00:35
to 00:53 (Fig. 6a). The resemblance to the evolution of KH vortices measured in the laboratory
experiment (Patterson et al., 2006) is striking. Vertical wind velocity shows perturbations with
magnitudes of 1 m s
-1
or more. Because accurate high-resolution vertical wind measurement is
quite difficult for instruments other than clear-air Doppler radars (e.g., Fukao, 2007; Hocking
2011), measurements by wind profiling radars are indispensable to understand turbulence
processes in the atmosphere. RIM measurements have revealed a fine structure of KH billows
in the jet stream in the mid-latitudes (Luce et al., 2008) and upper-tropospheric easterly jet in
the tropical region (Mega et al., 2010). Fukao et al (2011) carried out a statistical analysis of KH
billows in order to quantify their occurrence frequency, spatial scales, energy dissipation rate,
and vertical eddy diffusivity.
2.2.3 Advantage of range resolution enhancement using multiple frequencies
In the section, advantages of RIM over other methods are described. Radar range resolution
is determined by the transmitted pulse width, and ranges typically a hundred to several
hundreds of meters. However, for UHF wind profilers, a range resolution down to
approximately 30 m is able be attained by transmitting shorter pulses (e.g., Wilson et al.,
2005). Although the range resolution is able to be improved by transmitting shorter pulses,
it requires not only wider bandwidth but also more transmitted power in order to keep the
