Using the Global Precipitation Measurement (GPM) Core Observatory satellite, launched in 2014, NASA and Japan Aerospace Exploration Agency researchers have taken the first 3D images of raindrops and snowflakes. The GPM snapshots will help research meteorologist Joe Munchak determine precipitation rates and support the improvement of weather models.

NASA Tech Briefs: What can raindrop size tell us?

Joe Munchak: It tells us a lot about how the raindrop formed, and the type of storm that produced it. Very large raindrops have to form from melting snowflakes or hailstones. There’s simply no other way to grow them from just a collection of water; they would break up hydrodynamically.

NTB: Why is a three-dimensional image so valuable?

Munchak: One of the capabilities we have with the Global Precipitation Measurement (GPM) radar is to actually probe into storms and get very fine vertical details. The vertical component really matters because we can see, at the top of the storm cloud, how the particles start to form and how they grow. Are they collecting other particles by collisions? Are they shrinking due to evaporation? Where do they melt and become rain? Or [are the particles] falling on the ground as snow? That’s the detail that we can get from the 3D picture.

NTB: How can that information be used to improve forecast models?

Munchak: By knowing the size of the raindrops, we actually get a more accurate measurement of the amount of rain that’s falling; that’s very important to know. This is the first time we’ve had the technology to observe this on a global scale. With more accurate numbers, we can now determine whether or not a particular storm is likely to produce a flash flood. As we collect this data on the micro-physical properties of the storm – as well as environmental parameters such as the humidity and temperature, and where they vary regionally – we start to build a global picture.

NTB: How are these images taken? What technology makes this possible?

Munchak: The GPM satellite has a couple ways of measuring precipitation. The Dual-frequency Precipitation Radar (DPR) was provided and built by the Japanese Space Agency. The two frequencies determine the raindrop size. The way that the radar energy reflects off of the raindrops is very dependent on both the wavelength of the radar signal and the size of the raindrop. When you have two different wavelengths for the same raindrop size, you’ll get two different reflectivities. From that, you can back out the two pieces of relevant information: the average size of the raindrops and the overall number of raindrops.

NTB: What’s next for the GPM mission?

Munchak: This is the first time we’ve had this capability in space. We need to spend some time in the next few years to understand all the potential sources of error that come from these spaceborne measurements. Once we get a handle on those, I think we’ll have global maps of raindrop size, and we’ll start to understand the variable processes that cause those to change globally.

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