Geoscience Reference
In-Depth Information
3. Windshsear hazard factor based on TDWR
In aviation meteorology, windshear refers to a sustained change of wind speed and/or wind
direction that causes the aircraft to deviate from the intended flight path. Low-level
windshear (below 1600 feet) could be hazardous to the arriving/departing aircraft. Hong
Kong is situated in a subtropical coastal area and it is common to have intense convective
weather in the spring and summer. To alert low-level windshear associated with microburst
and gust front, a TDWR is operated by the Hong Kong Observatory (HKO) in the vicinity of
HKIA (Figure 1). It is a C-band radar with 0.5-degree half-power beam width scanning over
the airport and determines convergence/divergence features along the runway orientation
from the Doppler velocities. Windshear alerts are generated when the velocity change is 15
knots or more.
Another index that quantifies the windshear threat is the F-factor (Proctor et al., 2000). It is
based on the fundamentals of flight mechanics and the understanding of windshear
phenomena. The F-factor could also be calculated from the Quick Access Recorder (QAR)
data recorded on the commercial jets (Haverdings, 2000). In this study, an attempt is made
to calculate F-factor for some typical microburst events at HKIA based on the TDWR
measurements and the results are compared with the F-factor determined from the QAR
data.
F-factor is calculated from TDWR's radial velocity data in two steps. First of all,
convergence/divergence features are identified from the TDWR data. Then F-factor is
determined from each convergence/divergence feature by assuming a wind field model of
microburst. The two steps are briefly described below.
To compute convergence/divergence features, the method described in Merritt (1987) is
adopted. The TDWR microburst detection algorithm identifies microburst by searching for
significant velocity difference along a radial in a search window of 4 range gates (4 x 150
metres per gate = 600 metres in length, and one degree in azimuth). If the windshear along
a search window is divergent (i.e. radial wind generally increases with increasing distance
from the radar), the search window is taken to be a divergence shear segment. Likewise,
convergence shear segment is also identified.
Two divergence/convergence segments are associated as a divergence/convergence shear
features if their minimum overlap in range is 0.5 km or if their maximum angular spacing is
2 degrees azimuth. A divergence/convergence region contains at least 4 shear segments
with a minimum length of 0.95 km and a minimum area of 1 km
2
. Moreover, the maximum
velocity difference among the shear segments inside a divergence region should be at least 5
m/s. As such, the shear within a divergence region is at least 5 m/s per 600 m, i.e. 0.008
m/s/m.
F-factor is related to the total aircraft energy and its rate of change, and is defined to be:
W
w
x
F
(1)
gV
a
where
W
is the component of atmospheric wind directed horizontally along the flight path
(direction
x
) and
W
its rate of change,
g
the acceleration due to gravity,
w
the updraft of
x
