#teamradar

INSAR

Radar is an extremely useful tool for monitoring changes in the Earth’s surface. Interferometric synthetic aperture radar, or INSAR, can accurately measure tiny changes in the Earth’s surface.

Radar waves pass through Earth’s atmosphere and will bounce off land surfaces. Radar is a form of light - electromagnetic radiation - so like light it is a type of wave with a frequency/wavelength. When radar bounces from the surface to a spacecraft, it arrives back at a specific point in each wave. It's difficult to turn small changes in wavelength from pixel to pixel into any useful information since they’re a function of many properties of the surface, but if 2 images taken close together in time are subtracted from each other, tiny changes from one frame to another stand out. Those tiny changes in the wavelength of the returning energy are reflected in changes in the colors on an INSAR interferogram like this one. As the wavelength of the returning energy changes, it either builds constructively on the waves in the previous pass or interferes destructively.

INSAR is a tool that can show changes in the Earth’s surface from one scene to the next – changes like deformation in a volcano or motion from an earthquake. If 2 closely timed radar images are available, earthquake motions can be understood and volcanic eruption warnings can be given, but doing so requires a satellite built to do the measurement.

Previous satellites could do these measurements, but they weren’t built to do rapid overpasses. A few years ago, the European Space Agency launched its Sentinel-1A satellite (http://on.fb.me/1B1f6Jj), the first in its next-generation series of Earth-observing spacecraft. That satellite carries radar specifically designed for INSAR – it will be able to image every point on Earth’s surface every 12 days.

This INSAR image shows ground deformation in an earthquake in Oaxaca , Mexico last year. The ground moved by a maximum  of about 40 centimeters in this earthquake, so each of the contours of color represents about 3 centimeters of ground motion. The points marked show estimates of the earthquake epicenter based on seismic measurements made by the USGS and Mexican seismic networks; clearly based on this image, the nearby seismic instruments from the Mexican seismic network produced a more precise estimate of the epicenter location.

-JBB

Why do storms break up and go around (name your town here)?

If you’ve ever watched a line of storms moving into your area on the radar, you may have asked this question yourself. A line of storms may appear solid and menacing at a distance, and then as the storms get closer, gaps appear in the line, and your house may get missed by the rain. After the storms pass, the gaps disappear, and you see a solid line of storms again. The storms are not actually breaking up and reforming around your town. What you are seeing is a result of the changing size of the radar beam as it moves away from the radar. At close range, the beam is narrower and can see finer details, such as gaps in the storms. At a distance, the radar beam is much wider and cannot see the gaps between storms, so a line of storms at a distance appears to be solid, when it is really not.

Think of the radar beam as a beam of light from a flashlight. If you hold a flashlight close to a wall in a dark room, you will see a small beam of light. As you back away from the wall, the beam of light gets larger. The energy coming from the radar works in much the same way. Near the radar, the beam of energy is much narrower, but as the beam moves out away from the radar, it grows in size, much like a flashlight beam.

The spreading radar beam affects the resolution of the image. Small features can be seen close to the location of the radar, but they are often obscured at a greater distance. So, at a distance from the radar, a line of storms may appear solid, because the beam is too wide to resolve the gaps between storms. As the storms get closer to the radar, the narrower beam can “see” the gaps.

Also, the life cycle of a typical thunderstorm is only around 30 minutes to 1 hour. Storms are changing and evolving all the time. A storm at a distance that is an hour away will change considerably by the time it reaches your location. It may weaken and die out before it ever reaches you, or it may change the direction it is moving and miss your location completely.

Here’s just one more thing to keep in mind. Due to the curvature of the earth, the radar beam gets higher in the sky as it moves away from the radar. The radar beam is sampling the upper levels of storms at a distance, and looking at the lower levels of storms close to the radar. This can also affect what you are seeing on the radar screen.

-Amy

For more on how radars work, visit the JetStream mini-course on radars:

http://oceanservice.noaa.gov/education/yos/resource/JetStream/doppler/doppler_intro.htm