Data Assimilation ofWeather Radar and LIDAR for Convection Forecasting and Windshear Alerting in Aviation Applications - Data Assimilation for Atmospheric, Oceanic and Hydrologic Applications - page 540

second-order Adam-Bashforth scheme that are derived analogously as for the first
order scheme.
As an example of the application of the 4DVAR method, the analysis of a
gustnado event is presented here. On 6 September 2004, a trough of low pressure
associated with Typhoon Songda affected the coast of southern China. Troughing
flow could be analyzed on the surface up to 850 hPa level (not shown). In the
upper troposphere (e.g. 200 hPa level), divergence could be analyzed in the region
in association with a broad anticyclone (not shown). The atmosphere was humid
and unstable as revealed in the radiosonde data on that day. The K index was
35 at 0000 UTC (0800 HKT) and rose to 41 at 1200 UTC (2000 HKT). Under
the unstable atmosphere, intense convective development was triggered over inland
areas of southern China due to solar heating during the day and the thunderstorms
so produced moved south towards the coast in the evening.
Starting from about 08 UTC (16 HKT) on that day, the radarscope of Hong Kong
showed that there were isolated thunderstorms along the south China coast. Among
them, one storm appeared to the west of HKIA at about 0930 UTC (1730 HKT) and
brought westerly winds to the western part of the airport. The westerly was basically
rain-free and appeared to be the gust front associated with the thunderstorm. It
spread eastwards and converged with the background east to southeasterly winds
over the eastern part of HKIA. The tornado developed over the convergence zone of
the two airstreams.
The LIDAR's PPI scan image at the climax of the tornado (about 09:54 UTC,
6 September 2004) is shown in Fig. 22.15 . The tornado under study is encircled
in red. It could be found in the southern side of HKIA at which the cargo apron is
located. 4DVAR analysis is carried out with a horizontal resolution comparable with
the radial resolution of the LIDAR, viz. 100 m.
Figures 22.16 and 22.17 show the 4DVAR-analyzed horizontal wind vectors (as
arrows) and perturbation horizontal velocities (u - , in colour

contours) at a height of 200 m above sea level. The anticyclonic flow associated
with the tornado is captured successfully by the 4DVAR-retrieved wind field. The
perturbation horizontal velocities also show a couple of features:

< u > andv- < v >

(a) From the plot of u - , the perturbed easterly wind at the southern part of

the tornado is generally stronger than the perturbed westerly wind at the
northern part of the system. The former reaches about 8 m/s. Similarly, from
the plot of v -

< u >

, the perturbed northerly wind at the eastern part of the
tornado is generally stronger than the perturbed southerly wind at the western
part of the system. The former reaches about 10 m/s;
(b) A series of small-scale cyclones and anticyclones (each with a size of several
hundred metres) is analyzed at the convergence zone between the westerly flow
associated with the gust front and the background east to southeasterly flow at
the eastern part of HKIA; and
(c) Even with the mean wind subtracted, the perturbed v-component still shows the
southerly jet emerging from a gap of Lantau Island at the lower right corner of
the analysis domain (Fig. 22.17 ).

< v >