Data Assimilation ofWeather Radar and LIDAR for Convection Forecasting and Windshear Alerting in Aviation Applications - Data Assimilation for Atmospheric, Oceanic and Hydrologic Applications - page 535

Fig. 22.13 Wind vectors and streamlines based on the retrieved velocity at 14:29 UTC overlaid
on the LOS velocity ( color shades ) measured by the LIDAR. Positive values indicate LOS velocity
away from LIDAR. White contours show the mountainous topography near HKIA at 100 m
intervals
at locally the highest rate in the flow. Practically, they are identified using finite-
domain finite-time Lyapunov exponents (FDFTLE) method. Technical details of the
method can be found in Tang et al. ( 2011 ).
Figure 22.13 shows the 3DVAR retrieved wind overlaid on the line of sight (LOS)
velocity at 14:29 UTC 19 April 2008. In the airport region, sometimes a long
and distinct ridge of updraft is persistent as an organizing structure. We show the
evolution of this updraft between 14:36 UTC and 14:41 UTC, 19 April 2008, in
Fig. 22.13 , at 150 second intervals. Discussion and illustrations on the results of
3DVAR retrieved wind can be referred to Tang et al. ( 2011 ). This ridge of updraft
originates downwind of Lin Fa Shan, a mountain on Lantau Island to the south of
HKIA. The ridge could correspond to the merging of gap flows on the two flanks of
the mountain peak, leading to convergence and updraft when they meet. Unlike other
coherent structures which either stay in the vicinity of mountain topography or move
with the background flow and quickly dissipate, this ridge is larger in scale and stay
longer in time. More importantly, this ridge is transversal to the runway corridor,
where many flights passed through. In Fig. 22.14 a-c, the LOS velocity is shown
and its magnitude is represented by the color shades. Not much of the velocity
structures can be directly inferred from these plots, though locations of strong wind
convergence could be associated with gradient of LOS velocity. The FDFTLE plots
in Fig. 22.14 e, f, g, however, reveal the the locations of LCS corresponding to the
updraft structure. In addition, we plot the Hovmoller diagram of the LOS velocity