“Our old 5 cm radar and the 3 cm Omera will be destroyed! Are there people interested? » The tweet was posted on June 8 by Jean-François Berthoumieu, director of the ACMG (Climate Association of Moyenne Garonne). These are devices installed near the Agen airfield, where the former premises of the association were located, which has since migrated to the Technopole; the Smad (Joint Union for the departmental aerodrome) requested their destruction. A page turns ; climate experts have adopted more advanced techniques, but these radars were very useful in their time.
“With this radar, we could see pockets of hail forming at altitude”
“The radar sends a wave in one direction, and we measure its echo, like when you shout near a cliff,” explains Jean-François Berthoumieu. The wavelength of the ACMG Omera in the 1970s (the smallest) was 3 cm, and I used it until the 1980s. It allowed us to see thunderstorms up to at 80 km, but with very low precision. In Canada, then in the Gers, in 1980, I worked with a 5 cm wavelength radar, capable of going up to 300 km, with very good representation at 100 km. We can see with details of 500 m side; observe structures inside thunderstorms, where rain or hail forms, and watch them fall to the ground. »
Jean-François Berthoumieu monitors the progression of possible storms and rain on Météo-France sites.
JML
“With a radar of this type, we could see pockets of hail forming at altitude, and warn the ground that it was going to fall, in order to recover it and study how it was formed. From there, we confirmed that less than 8% of storms produced hail, and discovered that silver iodide could not be used to combat it, because it would have taken effect in four minutes! »
In 1992-93, a succession of hailstorms hit Lot-et-Garonnais farmers, who called for help. “A Canadian colleague mentioned potassium chloride, the one the Americans used to drown hostile submarines in mist, and we tested hygroscopic salts to accelerate the rain processes, which reduces the formation of ice. But here, we needed a 5 cm radar, which we bought second-hand. »
Interested Romanians
With this radar and software called Titan, the ACMG could then guide the pilots who were going to seed the clouds, this time with calcium chloride. “There were good results, which were however random,” regrets François Berthoumieu. We then abandoned the radar to prevent hail, and I tried to keep it for the risk of flooding. But as it cannot see beyond a distance of 10 km, it would have had to be moved to monitor Agen, and we had no funding. The equipment gradually aged, the most useful parts were sold to South Africa and Romania. Romanians are also interested in the current parable. »
Depending on where this sensor is placed, we can say that we have half an hour or an hour before the flood wave hits us.”
The ACMG then closely followed the method of anti-hail nets, “the ideal solution for those who can afford them. » It took fifteen years for this solution to be adopted in Lot-et-Garonne, on apple and kiwi orchards. On the other hand, the vine is always vulnerable, like the ente plums. Michelin is currently developing technology that deploys in the event of rain and hail, while allowing the sun to pass through. A technology that resembles an accordion structure.
In this case, the radar could still be used to alert users to implement this protection, “but we now have a network of automatic stations, for example to measure rain, which are more robust in terms of maintenance, which each cost between 1 500 and 2,500 euros, and we even have sensors that sink into waterways to measure their height, and alert us effectively every twelve minutes to prevent the risk of flooding. »
Follow Météociel
François Berthoumieu and his colleagues also use, on Météociel, radar and satellite images from Météo-France. “I saw on my screen the storms of Villefranche-du-Queyran forming, which fell on the spot like a bucket of water, and our stations indicated the moment when the water levels overflowed. We thus see that on the Ourbise, in Villeton, the alert threshold was exceeded at 12:05 a.m. on June 9. »
This set can constitute an alert system to warn municipalities or river unions. “Depending on where this sensor is placed, we can say that we have half an hour or an hour before the flood wave hits us. The idea is to put these sensors, costing less than 1,000 euros, everywhere in streams. »
A system that would allow time to evacuate an area, houses or close systems that divert water, “but we also have to think about the places where we build, because we can’t stop the water… » This device was presented to local elected officials and river technicians, who must now find funding for these sensors.
