Geoscience Reference
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3.
J
D
controls the retrieved wind field to follow weak anelastic mass constraint:
@
N
2
X
J
D
D
1
2
u
@x
C
@
N
v
@y
C
@
N
w
D
(22.6)
@
z
i;j;k
where
N
is the average atmospheric density; and
4.
J
S
controls the smoothness of the retrieved wind field:
2
4
X
i;j;k
3
u
s
r
2
u
2
C
X
i;j;k
v
s
r
2
v
2
C
X
i;j;k
w
s
r
2
w
2
J
S
D
1
2
5
(22.7)
where r
2
is the Laplacian operator.
The weights of the various quantities are taken following the consideration of the
order-of-magnitude of the respective terms and experiments with the actual radar
data:
1
D
2
D
1;
u
b
D
v
b
D
w
b
D
1
10
4
;
D
D
1
10
5
;
u
s
D
v
s
D
w
s
D
100
22.3.2
Illustration of Radar Wind Retrieval Data
in a Significant Convection Event on 28 July 2010
The case of intense convective weather occurred in the afternoon of 28 July 2010
over Hong Kong where more than 100 mm of rainfall were recorded over part of the
territory between 0600 UTC and 1000 UTC (1400-1800 HKT where HKT D UTCC
8
h). About 4,000 numbers of cloud-to-ground lightning strokes were registered
over Hong Kong, nearly half of them occurred in the region of the Hong Kong
International Airport (HKIA) and Lantau Island (Fig.
22.3
). On the side of aviation
weather service, the Airport Thunderstorm Lightning Alerting System (ATLAS)
operated by the Observatory at HKIA issued red alert for two and a half hours, i.e.
cloud-to-ground lightning occurred or was expected to occur within 1 km from the
boundary of HKIA, so that the ground personnel had to operate their operation and
should look for shelters. This is the longest period for the issuance of red alert since
the operation of ATLAS in March 2008. During the period when red alert was in
force, there were 290 times of cloud-to-ground lightning strokes within 1 km of the
boundary of HKIA. The thunderstorms also brought about significant windshear and
microburst at the airport area, leading to significant disruption of air traffic at HKIA.
Synoptically, there was a trough of low pressure over the surface starting from
27 July 2010, bringing showers and thunderstorms to south China coast. Active
southwesterly jets on 850 and 700 hPa levels were observed over the southern
China that favoured transport of warm and moist airstream towards the Pearl River
Delta region. In the middle troposphere (500 hPa level), a trough was found over
southeastern coast of China. Upward motion could be analyzed over south China
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