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in the USA was the potential to detect mesocyclones and tornadoes. The investment in this
technology paid off as demonstrated by the graph in Fig. 15. Trend of improvement is seen
on all three performance indicators with the steepest rise in the years the Doppler radar
network (NEXRAD) was being installed. This is logical: as the new tool was spreading
across the country more forecasters were beginning to use it. Improvement continues few
years past the completion of the network likely because it took time to train all forecasters
and gain experience with the Doppler radar. The data indicates a plateau from about 2002
until present suggesting maturity of the technology with little room left for significant
advancements. Further progress might come from combining radar data with short term
numerical weather prediction models and/or introduction of rapidly scanning agile beam
phase array radars (Zrnic et al., 2007 and Weber et al., 2007).
Fig. 15. Probability of detection, false alarms and lead time in tornado warnings issued by
the National Weather Service as function of year. (Figure courtesy Don Burgess).
Doppler velocities are potent indicators of diverging (converging) flows such as observed in
strong outflows from collapsing storms. These “microbursts” have been implicated in
several aircraft accidents motivating deployment of terminal Doppler weather radars
(TDWR) at forty seven airports in the USA (Mahapatra, 1999, sec 7.4). Vertical profiles of
reflectivity and Doppler velocity in Fig. 16 indicate a pulsing microburst; the intense
reflectivity core (red below 5 kft) near ground is the first precipitation shaft and the
elongated portion above is the following shaft. On the velocity display the yellow arrows
indicate direction of motion. Clear divergence near ground and at the top of the storm (in
the anvil) is visible and so is the convergence over the deep mid storm layer (5 to 14 kft). The
horizontal change in wind speed near ground of ~ 20 kts at this stage is not strong to pose
treat to aviation (35 kts is considered significant for light aircraft).
An atmospheric undular bore (Fig. 17) was observed with the WSR-88D near Oklahoma
City. This phenomena is a propagating step disturbance in air properties (temperature,
pressure, velocity) followed by oscillation. Spaced by about 10 km the waves propagate in a
surface-based stable layer. The layer came from storm outflow and the bore might have been
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