When you turn on the TV to watch the local weather report, most likely you will see the local on-air meteorologist display and explain radar imagery. You have probably noticed that on days when there is no precipitation falling nearby, the radar does not show a whole lot. However when there is precipitation nearby, and especially when severe weather is in the area, suddenly the radar is alive with a whole lot of colors and movement.
The radar is a complex tool, and it is vitally important to National Weather Service operations as it is the tool meteorologists use to inform the public of any hazardous weather that is in their path. In other words, they use the Doppler radar to issue the severe thunderstorm and tornado warnings you see scroll across the bottom of your screen occasionally.
How Doppler Radar Works
The radar transmits pulses of microwave radiation. Part of the energy of each pulse bounces off raindrops, insects, snowflakes, etc. back to the radar.
The image above (courtesy of the National Weather Service) illustrates how the radar emits a pulse of microwave radiation, and how part of the energy from this pulse bounces off a rain drop (or snow flake), back to the radar. Meteorologists analyze the strength of the returned pulse and the time it took to travel to the rain drop and back.
The most visible and recognizable component to the Doppler radar images you see on TV is the base reflectivity imagery. These types of images paint the picture of what is occurring nearby. No matter what the precipitation type is, radar images rely on a lot of different colors to indicate the intensity of the precipitation. The colors represent the strength of returned energy to the radar expressed in values of decibels (dBZ). The color scale is located at the lower right of each image. As dBZ values increase so does the intensity of the rainfall.
Base Reflectivity image showing precipitation in Oklahoma. Photo courtesy of the National Weather Service.
Despite the vast improvements to Doppler radar over the years, it simply cannot read everything. The reason for this is relatively straightforward. The radar is projected about 0.5° off the ground. The radar scan “sees” the lower parts of storms when they’re close to the radar and higher parts of storms when they’re further away from the radar (due to Earth’s curvature). The radar then tilts upward and does another rotation for a higher elevation scan. This process repeats several times, depending on which scanning mode it’s in.
At increasing distance, the radar is viewing higher and higher in storms and the beam may overshoot the most intense parts. Photo courtesy of the National Weather Service.
In other words, when meteorologists are monitoring storms quite a distance away, the radar beam will miss all the “important” information contained in the lower levels of the storm, such as rotation which could be indicative of a developing tornado (although not all rotation within storms is indicative of a developing tornado. I will discuss rotation in storms next.) These limitations of the radar may make you wonder how meteorologists can accurately warn on a storm located quite a distance away. There is a work around to this issue. National Weather Service meteorologists have the ability to select an adjacent radar, typically located around 200 miles away, which may offer a more advantageous position to sample storms. Also trained storm spotters (members of the public who attend special classes each year to learn how to discern various weather phenomena) often act as the “eyes and the ears” of the National Weather Service and relay reports of such weather phenomena.
What separated the Doppler radar from previous generation National Weather Service radars is its ability to detect motion. Although Base Reflectivity is important to diagnose precipitation intensity, this is only half the picture. Meteorologists rely on velocity imagery to indicate whether there is rotation within a storm, detect strong winds from storms, detecting the speed of frontal boundaries, and even the general motion of winds near the radar.
Base velocity, like Base Reflectivity, provides a picture of the basic wind field from the lowest (0.5°) elevation scan. But to see the wind, there needs to be radar “returns” before the radar can determine the velocity. Remember, the radar beam elevation increases with increasing distance from the radar. Therefore, the reported value will be for increasing heights above the earth’s surface.
Also, know WHERE the radar is located in the image. The radial velocity colors only has the proper meaning if you know how it is blowing relative to the location of the radar. Outbound winds (red colors) on one radar might be inbound winds (green colors) at an adjacent radar. If the radar cannot determine (called range folding) inbound or outbound then it paints the wind in purple.
Velocity image from Charleston, WV. The location of the radar is in the middle, near where the small white circle is located. The greens indicate inbound winds and the reds indicate outbound winds. In other words, winds are out of the east.
Putting it together
Now that you have a good basic understanding of the Doppler radar, I want to show you how National Weather Service meteorologists use these two radar depictions to warn on a tornado. Even to an untrained eye, it is clear that something is going on in the images below. You see a lot of the reds and purples in the left panel, indicating intense rainfall. I’m sure your eye is drawn to the area of green and reds converging together. This is what is called an inbound/outbound couplet and this is a very strong indicator of strong rotation within a storm. National Weather Service meteorologists spend a lot of time learning how to operate and interpret radar images and this is about as clear as it gets of indicating a tornado on the ground. Sure enough, these images were taken from the Tuscaloosa, AL tornado back in 2011.
A side-by-side view of the Tuscaloosa-Birmingham tornado that occurred on April 27, 2011. The left panel is the Base Reflectivity panel, and the right panel indicates the winds associated with the storm. The tight couplet of inbound and outbound winds indicate the location of the tornado. Photo courtesy of the National Weather Service.
Doppler radar is a complex and multi faceted tool meteorologists at the National Weather Service rely on to do their job, which is to save the life and property of the public. This post just serves as a brief introduction to this tool. For another perspective on how the Doppler radar works, you can check out another meteorologist’s blog here.