Geoscience Reference
In-Depth Information
SWIRLS updates and outputs nowcast products at 6-minute intervals. For severe
thunderstorms, the major results are visualized as an image product called the Severe
Weather Map on its client workstation in the forecasting office, as well as a web page named
SPIDASS (
S
WIRLS
P
anel for
I
ntegrated
D
isplay of
A
lerts on
S
evere
S
torms) dedicated for
severe weather alerts (Fig. 11).
Fig. 11. (a) shows an example of the Severe Weather Map. Textual alerts with quantitative
details were printed at the bottom. (b) , the main panel of SPIDASS web page provides a
compact view of all alerts arranged in rows and colour-coded for different severity levels.
The Hong Kong Observatory has also developed separate multi-sensor thunderstorm
nowcasting systems for the aviation community and the public utilities services (Li, 2009). A
lightning nowcasting system, named the Airport Thunderstorm and Lightning Alerting
System (ATLAS), covers the Hong Kong International Airport (HKIA). It combines rapidly
updated CG lightning strike information, radar reflectivity and TREC wind information to
nowcast lightning strikes using a modified Semi-Lagrangian advection scheme. Depending
on the predicted distance from HKIA, ATLAS will automatically generate RED (1km) or
AMBER (5 km) alerts.
ATLAS is equipped with two ensemble algorithms, to take into account the possible rapid
development nature of lightning (transient and sporadic). The Weighted Ensemble (WE)
algorithm sums all available 12-minute CG forecasts with decreasing weight with time. If
the sum exceeds an optimized threshold, alerts are created. WE has proved to be effective
for alerting persistent and wide-spread thunderstorms. The Time Lagged Ensemble (TLE)
algorithm sums the 1-minute forecasts valid at the same time from the twelve 1-minute
forecasts provided in the past 12 minutes with decreasing weight over time. TLE is proved
to be more skilful in predicting rapidly developing, small or wide-spread thunderstorms
than WE. Figure 12 shows a snapshot of the ATLAS product.