Measuring the intensity of rain as it is falling is
indispensable to anticipating rapidly rising waters
and reacting to the associated flood risk. To the
classical rain gauges installed throughout the area,
new radar technologies were added in the 1990s to
detect rain and measure the accumulation of
precipitations in real time. Today, Météo France has
a national network of 24 weather radars available
within approximately 100 km. However, all regions in
France are not covered by this mesh, in particular
the mountain zones where the relief masks downstream
rain zones by creating an obstacle to wave
displacement. Within the FRAMEA1
project, a new radar technology developed by
Novimet ² is being tested at the
Aix-en-Provence Cemagref. The experiments conducted
in the Maures massive have proven to be highly
promising.
1 Flood forecasting using Radar in Alpine and
Mediterranean Areas (FRAMEA)
2 A young company split off from the CNRS
More compact and less expensive
radars
The radars used at the beginning
to monitor planes flying overhead were extended to
the detection and quantification of precipitations.
Large-scale radars, 6–8 m in antenna diameter, are
used today in weather stations located in the
plains. In mountain zones, the number of radars must
be multiplied in relation to the relief, which
requires smaller and less expensive models. The new
Hydrix radar responds to these demands. However, by
reducing the diameter of the parabolic antenna to
1.5 m, the wave frequency must be boosted, which
increases the attenuation of waves during their
displacement.
To compensate this signal attenuation effect, a
profiling algorithm (ZPHI) is used. Finally, the
radar operates in double polarization, which
provides information on the size of the rain drops
and estimates precipitations without resetting
ground network observations. Today, in a doctoral
dissertation supervised by both Cemagref and the
firm Novimet, this new radar technology is being
tested in the Var department, a mountainous region
that experiences very intense flash floods.
Results that are coherent with ground readings
The Hydrix is installed near Réal Collobrier,
Cemagre’s instrumented research catchment, located
in the Maures massif. The total rainfall in autumn
2006 calculated by the radar was compared to the
rain gauge readings on the ground and to the
accumulation estimated by one of the nearby radars
belonging to the Météo France network. Within a 60-
to 80-km radius, the data supplied by the radar were
in coherence with the quantities of rain collected
on the ground. In addition, the algorithmic signal
processing retransmitted rain gauge data in real
time that were as good quality as the data sent by
the classical radar managed by Météo France. Today,
the research is continuing so as to integrate the
rain gauge data supplied by the radar into existing
rainfall-runoff models. By converting rainfall into
runoff, these mathematical tools can calculate the
runoff of rivers at the outlet of a catchment. These
rainfall and runoff data then feed the flood warning
systems, such as the Aiga system developed by
Cemagref and Météo France in 2005. By completing the
existing radar network, the Hydrix technology will
contribute to the extension of the flood warning
system over the entire area, including mountainous
zones.
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