Technology to better predict rain from storms

Posted on June 11, 2017  /  0 Comments

The government predicted rainfall more than 150 mm on the 25th of May. Over 500 mm of rain fell. Technically, they were not wrong (550 mm is within the range of “more than 150 mm”), but obviously, forecasts like this might as well not be made. [an error was corrected in the above para]

But it is wrong to condemn the Met Department which operates even without Doppler radar, though they have been talking about it since 2012. But as discussed below, Doppler radar is old and can only tell about large rain drops. The more appropriate technology is Lidar.

A satellite taking elevation measurements over Bangladesh. NASA

A satellite taking LIDAR (Light Detection and Ranging) elevation measurements over Bangladesh. NASA

Worth reading this article in the Economist, to get a sense of the science that is being applied to weather prediction these days. The Indians have become experts in satellite launches. We should work with them to improve our common weather hazard detection and monitoring capabilities.

The eight-satellite swarm, which was launched in December, listens for radio signals that come from GPS satellites directly above it in space, and for the same signals when they have been reflected from the ocean’s surface beneath the hurricane being studied. Differences between the reflected signal and the original are a consequence of the state of that surface, and CYGNSS can use them to infer wind conditions there.

Satellite measurements like this are useful, but it also helps to get as close as possible to the hidden bottom kilometre of a storm. NOAA is doing this with drones called Coyotes, built by Raytheon, an aerospace company. Coyotes are released from tubes in the bellies of NOAA’s research planes, then piloted remotely in order to gather data from the region in a storm that is just above the ocean’s surface. The data the drones collect complement those from dropsondes, which are sensors that are pushed out of the same tubes and plunge down through a storm like bombs, transmitting as they go.

The research planes have also started using a device called a Doppler wind lidar to measure a hurricane’s moisture content more accurately. Radar, a standard instrument on these planes, works at radio frequencies, which means it is reflected only from large drops of water. Lidar’s use of light, which is also reflected by small—even microscopic—drops, paints a more accurate picture of the way moisture is distributed within a storm.

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