Unraveling Radar: History, Function, and Applications

1. Lecture 8 (10/28)METR 1111 Radars

2. Radar & its History • RADAR is an acronym • Stands for RAdio Detecting And Ranging • In 1930’s, radar used to monitor shipping traffic • In WWII, used to detect aircraft • Precipitation would cause interference • After WWII, radar used for weather too

3. How Does it Work? • Radar creates a very strong (~1 megawatt) pulse of electromagnetic energy (or radiation) • Antenna (or dish) focuses beam and it travels through the atmosphere • Anything the beam hits is called a target • Target scatters radiation back (some straight back at the antenna) • This reflected radiation is measured • You can figure out distance with speed beam travels at and the time the process takes.

4. Properties of Targets • Amount of energy reflected back is called reflectivity. • Reflectivity depends on the number of targets in an area and their size • Problem: a few large drops can reflect the same radiation as a bunch of small drops • That’s why radar is not an accurate tool for measuring rainfall

5. Radar Images • Reflectivity images show the location and amount of reflectivity on a display. • This is what you see on TV • Velocity Images - product of Doppler radar • Shows component of the wind towards or away from the radar • This is useful for detecting rotation in storms (tornadoes), & damaging winds

6. Reflectivity Image • Typical reflectivity image on next slide • Color scale tells you what the image means • Can tell if returns are light, moderate, heavy, or severe • Earth curves away from beam so you see below clouds close to the radar site and above clouds far away • Farther away, you get less resolution since beam spreads out

7. Typical Reflectivity Image

8. Doppler Radar • Doppler radar can measure winds towards and away from the radar. But how? • Because of the same reason why a train whistle sounds louder as it approaches you than as it moves away • Doppler radar measures the change in frequency of targets (actually measures change in phase which is related to freq.)

9. Typical Velocity Image

10. Velocity Images • Color scale is again important. • Winds toward the radar are usually negative & winds away from radar usually positive • There are limitations in the maximum wind speeds you can detect. • In event of very strong winds, can get velocity or range folding (where it goes off one extreme end of the scale and onto the other)

11. NEXRAD WSR-88D • This is what the National Weather Service calls their radars. • NEXRAD stands for: NEXt generation RADar • WSR-88D stands for Weather Survillance Radar 1988 Doppler • For short, you can use one or the other to describe the current NWS radar network

12. Phased Array Radar • This is the next step in Radar development • Originally developed for use by the Navy • Currently being tested at the National Severe Storms Laboratory

13. How a Phased Array Works Unlike conventional radar, phased arrays • Use multiple beams • Use multiple frequencies • Controlled electronically • Advantages • Up to 6 times faster scanning atmosphere (3-D view) • Can track multiple targets (several storms at once) • Potentially increase tornado warning lead times from 12-22 minutes

14. Good web sites to visit For more info on radars: • University of Illinois site or Intellicast site For current radar imagery: • OK-FIRST Data -- Great data for the southern plains. Use username = "synoptic", and password = "scale" to access the data. You will need to download the software first. • See radar links here http://weather.ou.edu/wx/ • A good pay site is www.weathertap.com ($5.95/month). For a little more detail about Phased Arrays • http://www.noaanews.noaa.gov/stories/s850.htm

15. For Next Time: • Read Chapter 10 of the Weather Book (Sky Watching) • We’re going to go over some basics again with cloud types next week • After this, we’re in the home stretch • Numerical Models • Winter Storms (may switch with Severe Wx) • Severe Weather • Job Opportunities