Geoscience Reference
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On the other hand, there are two small regions near the radar at the range of 6 km where
EDR is also high. But in this region there is relatively strong horizontal shear of the radial
wind component (Figure 9(f); green color identifies the wind has a component toward the
radar and red color indicates wind is away from the radar). Furthermore, the reflectivity is
about 10 dBZ and SNR is around 35 dB. Because this region is on the downwind side of
Lantau Island, the ambient flow (green in Figure 9(f)) is blocked by the Island and back flow
(red in Figure 9(f)) is induced. The wind shear contributions, computed using Eq. (8), have
been removed from the calculation EDR presented in Figure 9(a). Thus the EDR should not
be biased by strong shear of mean radial wind. Thunderstorm outflow may be another
reason for the severe turbulence in this region. Because there is no strong horizontal shear
of the Doppler velocity field in the region 270
o
and 25 km, we conclude that the large EDRs
presented in in that region of Figure 9(a) are unrealistic. After a threshold SNR> 20 dB is
applied, it can be seen that these large EDR values are removed (Figure 9(b)).
Using the Hong Kong TDWR observations in 2006 and 2008, many EDR maps were
produced and examined. Here wind shear contribution has been removed from spectrum
width measurements. Here the mean wind shears in horizontal and vertical directions are
calculated by using mean radial velocity field smoothed by a 9 points median filter along the
radar beam in the Eq.(8). Figure 10 shows two typical EDR maps during light rain at 21:32
UTC on 27 April 2006 (Figure 10(a)) and during a thunderstorm at 13:17 UTC on 13 June
2008 (Figure 10(b)). For most of the scanned area, EDR is low and turbulence is classified as
insignificant or light (green and light blue). Small pockets of moderate and severe
turbulence (yellow and red) are scattered in the scanned area. Near the Lantau Island,
moderate and severe levels of turbulence are frequently observed in the cases we studied.
The blockage of the Island on the ambient flow may be a reason for the occurrence of the
turbulent airflow. Based on the numerical simulations, Clark et al. (1997) and Chan (2009)
found that mechanical effect of a mountainous island is a source of the generation of the
turbulence.
Clear air cases have been investigated as well, but we found that SNR of the Hong Kong
TDWR is too low to provide reliable and meaningful EDR maps.
After the EDR maps were generated, EDR profiles along the flight paths can be compared with
aircraft measured EDR. A total of 14 cases are selected to make the comparison. The aircraft
EDRs are estimated based on the vertical wind measured by aircraft (Cornman et al., 2004).
Radar derived EDR profile is constructed by selecting the EDR in a resolution volume V
6
closest to the flight path and at an elevation angle of 0.6
o
. There are still differences in the
measurement heights between the aircraft and the radar beam for these two EDR datasets.
Only a part of the flight path is covered by the radar beam. For example, aircraft
approaching runway 25RA is in the radar beam only at the distance between 0.5 and 1.5 nm
from the end of runway. From this point of view, EDRs estimated by aircraft and the radar
would be compared within this distance interval. It should also be mentioned that radar
estimated EDR is based on the spectrum width of the Doppler velocity, i.e. velocity in the
radial direction along a radar beam. On the other hand, the aircraft estimated EDR is based
on the vertical wind. As such, the two EDR datasets are derived from different components
of the wind. Put aside errors in measurement, in order to have agreement turbulence must
be isotropic.
