The Earth Story

Radar return

A couple years ago, the United States congress funded a major upgrade of the nation’s weather radar systems. These new systems are able to recognize smaller objects and they can actually tell their shapes– differentiating between water droplets and flying debris can tell if a tornado is on the ground, for example.

One cool result of these radar upgrades is that every summer, the new systems catch something like this cloud seen on the border of Texas and Oklahoma. If you were in this area when this radar shot was taken…it wouldn’t have been raining.

The radar returns for water droplets show that they’re pretty symmetric, almost spherical, and about the same size, so different polarizations of radar directions bounce off them the same way. Objects with odd shapes and sizes return different radar signatures in different polarizations. These objects were wide, flat, and varied in size a lot.

This is actually a swarm of grasshoppers and beetles flying at an elevation of over 500 meters. You’re looking at the radar returns from insects.

For more discussion of how this works, check out this video: http://tmblr.co/Zyv2Js1qHeois

-JBB

Image credit: US NWS http://wapo.st/1JhbWFz

Radar Revolution If you watched the weather reports, or heck let’s just admit it…if you were tuned into the Weather Channel during last week’s severe storms in the southern U.S., you probably heard weather forecasters say several times that they knew there were tornadoes on the ground based on debris in the air. This type of report ought to be recognized for what it is; the coming out party for a brand new type of weather radar known as dual-polarization radar. Polarization of light is a property you might have heard of; many sunglasses offer polarized lenses. Polarization is the concept of how a light wave vibrates in space. As light moves, the waves can vibrate in any direction; up and down, side to side, and all angles in-between. But, when light passes through a filter or bounces off many objects, the angle of vibration changes and light comes off only in a single direction; a single polarization. Sunglasses contain filters that only allow light with a single polarization direction through; all of the other directions get filtered out. Sunlight reflecting off surfaces often becomes polarized; so when polarized light hits the polarized filter in sunglasses, glare gets filtered out. Radar waves work the same way; they can vibrate up and down or side to side. Traditional weather radar sent out waves that were polarized only in the x direction – they only vibrated from side to side. Dual-pol radar includes the up and down direction, the y direction, as you see in the image at the left. Traditional radar waves head out from the source and bounce off of material in the atmosphere. By measuring the waves that bounce back, a radar receiver can tell how much stuff, like water droplets, is in the air. But, that radar can’t tell objects apart; it can’t tell a log flying in the air from water droplets very easily, it’s all just flying objects. By measuring radar returns in 2 directions, weather forecasters can actually tell what shapes they are seeing. Water droplets have certain shapes in the air and so they give different radar responses in the 2 directions. Flying debris, on the other hand, is random; the objects are being tossed around by the wind, so the 2 radar directions give identical returns. You can see this reflected in the 2 radar images; the traditional radar shows a storm that could have a tornado, but the dual-pol setup has a specific signal returned directly from the tornado striking Tennessee last week. In other words, this new radar system can specifically tell when a tornado is on the ground and picking up objects. This is a huge advance for tornado forecasting and safety. Last week, the National Weather Service issued several “Tornado Emergency” statements. I’m used to tornado warnings; saying that there may be a tornado on the ground, but a Tornado Emergency is the upgrade; that is the weather service saying they have a confirmed tornado. With this technology, the weather service can tell when tornadoes are on the ground even in rural areas or at night when tornado spotters can’t see them. The Weather Service can now give meaning to the Emergency status; they can specifically say “we see the radar signature of debris”. They can literally tell you when it’s coming. On top of that, it can pick out other hazards like hail, giving warnings for those types of storms as well. These new dual-pol radars have only recently been installed. In 2011-2012 they were in the test-phase, but in 2012 Congress began appropriating funds for the Weather Service to upgrade its radar systems nationwide. It took a cost of about $50 million to upgrade the whole country, but as of 2013 almost all areas now have this capability. Now that tornado season is underway, we should give credit to the Weather Service for developing this system and to the US Government for actually funding its deployment. People will get warnings in a way they never before could thanks to this system. This new radar will save lives. -JBB Image credits: http://www.roc.noaa.gov/wsr88d/dualpol/default.aspx http://earthsky.org/earth/nws-upgrades-to-dual-polarization-radar-for-better-look-at-precipitation-tornadoes Read more: http://usatoday30.usatoday.com/weather/news/story/2012-01-17/national-weather-service-new-radar-system/52746738/1 http://www.wdtb.noaa.gov/courses/dualpol/outreach/ http://www.wdtb.noaa.gov/courses/dualpol/documents/FAQsOnDualPolRadar.pdf http://www.cimms.ou.edu/~schuur/radar.html https://www.nssl.noaa.gov/tools/radar/dualpol/ http://blogs.agu.org/wildwildscience/2014/04/30/nws-weather-radar-upgrade-proves-wildly-successful-in-one-night/