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Author Topic: How does radar detect rain?  (Read 691 times)

Offline chris

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How does radar detect rain?
« on: 16/06/2016 22:00:39 »
We see weather presenters on the TV talking about "the radar images show rain..." - but how do they "see" rain? What signal is received / detected to discriminate rain from clouds or other sources of moisture? How does it work?


 

Offline Colin2B

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Re: How does radar detect rain?
« Reply #1 on: 16/06/2016 23:44:43 »
Most radars will detect rain but most try to filter it out. Generally selection is based on the intensity of return echo, hard targets eg boats or aircraft giving a stronger return than softer eg rain. Shorter wavelengths 1-10cm  are better at picking up rain (better backscatter at rain particle sizes) but the shorter wavelengths attenuate faster hence have less range so it's a trade off. Dedicated cloud radar is often even shorter at 1-10mm because of the much smaller particle size.
Rain detecting radar will often use Doppler technique to detect movement. Another technique to increase sensitivity makes use of the fact that falling raindrops flatten as they fall due to air resistance, hence are wider than tall, so 2 radar pulses are transmitted, one horizontal polarisation the other vertical to discriminate rain vs other returns.
 
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Offline evan_au

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Re: How does radar detect rain?
« Reply #2 on: 17/06/2016 11:50:07 »
Waves of all kinds experience partial reflection when they enter a region of different impedance.

Radar uses electromagnetic waves, and water has a different impedance than air, so there will be a reflection when the radar strikes the front of the raindrop, and another when it the radar entering the raindrop strikes the rear of the raindrop.

If the rain is intense, the radar pulses can bounce around amongst the raindops, muddying the image beyond the region of intense rainfall.
 

Offline evan_au

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Re: How does radar detect rain?
« Reply #3 on: 18/06/2016 05:09:10 »
NASA's Global Precipitation Measurement satellite uses radar at 14GHz for rain, and 35GHz which is better for measuring snow.
It also has thermal imaging to measure the heat released in thunderstorms.

You can see some of the sophisticated ground-based equipment and airborne sensors they are using to calibrate the space-based measurements here (including a device with cameras that photograph the shape of every raindrop falling through its detector):
http://spectrum.ieee.org/aerospace/satellites/why-did-the-mule-climb-the-mountain-because-nasa-said-so
 

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Re: How does radar detect rain?
« Reply #3 on: 18/06/2016 05:09:10 »

 

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