DOPPLER RADAR

Objectives:

Weather radar routinely provides valuable information on storm size, shape, intensity, and direction of movement. Additionally, it employs the Doppler effect to monitor details of atmospheric circulation within storms. For instance, air motions that indicate possible tornado development can be detected. This information provides advance warning to the public of severe weather and saves lives.

After completing this investigation, you should be able to:

• Describe aspects of the actual wind that are detected by Doppler radar.
• Determine the speed of the wind toward or away from the radar site.
• Construct the wind pattern as detected by Doppler radar.

Materials: Red and green pens or pencils.

Introduction:

Radar detection of motion is based on the Doppler effect, the change in frequency (or phase) of a sound or electromagnetic wave reaching a receiver when the receiver and source are moving relative to one another. A frequency shift occurs when a radar signal is reflected from a moving target such as a cluster of raindrops. If the raindrops are moving toward the radar, the reflected signals returned to the radar have a higher frequency than if the target were stationary. On the other hand, if the raindrops are moving away from the radar, the returned signal’s frequency is lowered. The magnitude of the frequency shift is a measure of the parts of raindrops’ motions that are directly toward or directly away from the radar.

Doppler weather radar is especially useful for detection of severe weather conditions. One of the most devastating and potentially deadly of severe weather phenomena is the tornado. A tornado is a rapidly rotating column of air in contact with the ground. Tornadoes are almost always associated with thunderstorms.

Before tornadoes develop their intense, ground-level circulation, a broader-scale horizontal rotation is often evident within the parent thunderstorm. This internal thunderstorm rotation is called a mesocyclone. As the air entering the thunderstorm begins to swirl in the mesocyclone, raindrops are carried along and reflect radar energy back toward the radar antenna.

Figure 1a is a schematic view of a radar beam detecting a mesocyclone, depicted as a rotating cylinder embedded within a severe thunderstorm. As viewed from above in the Northern Hemisphere, the mesocyclone’s rotation is typically counterclockwise. Figure 1b is a view from above with the size of the mesocyclone exaggerated (not to scale). The dashed lines represent the radar beam in positions 1 through 5 as it sweeps through the mesocyclone. The arrows drawn around the mesocyclone column in Figure 1b represent the actual wind at dots located at the tails of the arrows. Each arrow shows the instantaneous direction of air movement at that point, and the length of the arrow represents the speed of that wind.

Figure 1. Schematic view of (a) radar beam detecting a mesocyclone with tornado and (b) sensing radial velocities.

1. In this example, the actual winds circulating around the mesocyclone all have the same speed, as shown by arrows whose lengths are [(the same)(different)].

2. From one location to another around the mesocyclone, the wind directions are [(the same)(different)].

3. Doppler radar detects only those motions or components of motion that are directly toward or away from the radar. At two locations within the rotating wind pattern no air motion is detected by the Doppler radar beam. At those points, the actual wind blows along paths perpendicular to the radar beam (dashed line). Hence, the Doppler radar senses no Doppler wind speed at these locations. These two locations are sensed by the radar when its beam is in the [(1)(2)(3)(4)(5)] position. Draw a small circle around each of the dots associated with those two arrows to denote this 0-Doppler wind speed.

4. Two arrows on the column circle are along the direction of the radar beam, one directly toward the radar and one directly away. These two locations are sensed when the radar beam is at the [(1 and 3)(4 and 2)(3 and 5)(5 and 1)] positions. Because these arrows are oriented directly away from or directly toward the radar site along the beam direction, the radar will sense the full wind speed, away or toward, respectively.

Where the wind arrow is oriented directly toward the radar, use a green pencil to solidly color the wind arrow. Where the wind arrow is oriented directly away from the radar, use a red pencil to solidly color the wind arrow. [The NWS color convention uses “cool” colors such as greens and blues for motions toward the radar and “warm” colors such as reds and oranges for those away from the radar.]

5. Disregarding the arrows identified in questions 3 and 4, at the other four arrow locations shown around the mesocyclone, wind arrows are neither directly toward or away, nor perpendicular to the radar beam direction. Where the radar beam direction and the actual wind arrow make an angle other than 0 or 90 degrees, Doppler radar senses only the component of the total motion that is directly toward or away from the radar. For the two arrows that are directed partly toward the radar, use the green pencil to draw approximately half-length green arrows, from the location dots, that are aimed directly toward the radar along the dashed beam direction. These two locations are at the [(2)(3)(4)(5)] radar beam position.

6. For the two arrows that are directed partly away from the radar, use the red pencil to make similar half-length red arrows, drawn from the location dots, which are aimed directly away from the radar. These two locations are at the [(1)(2)(3)(4)] radar beam position.

Finally, with the green pencil, shade across the nearly semi-circular area of the mesocyclone where the arrows are green. Shade the lightest from near the 0-Doppler wind speed position, becoming darker where the green arrow is longest. With the red pencil, shade across the portion of the mesocyclone where the arrows are red. Graduate the shading from lightest near the 0-Doppler wind speed position, becoming darker where the red arrow is longest.

Observe the colored arrows of your mesocyclone depiction. These are the Doppler winds as detected by the radar utilizing the Doppler effect. Respond to Items 7 -11 as either T for true, or F for false, based on the colored arrow pattern of the radar display associated with the mesocyclone.

7. [(T)(F)] The green arrows are directed toward the radar.

8. [(T)(F)] The red arrows are directed away from the radar.

9. [(T)(F)] Along the radar beam, when in position 3, the Doppler wind speed is zero.

10. [(T)(F)] The green shaded area depicts air motions toward the radar.

11. [(T)(F)] The red shaded area depicts air motions away from the radar.

The color scheme you have drawn on Figure 1b could represent the severe weather “signature” of a mesocyclone on a Doppler weather radar display. The signature has regions of green and red appearing on opposite sides of a radial line from the radar’s location along which no wind speed is detected, called the 0-Doppler wind speed boundary by radar meteorologists. Meteorologists have identified other identifiable Doppler radar patterns associated with fronts, gust fronts and outflow boundaries from thunderstorms, wind shear, and other forms of severe weather.

A midlatitude cyclonic system with its cold front passed across the central and eastern U.S. on October 21st of 2013. The advancing cold front produced lifting of relatively warm, humid air creating precipitation along the front during its advance. Here we look at the primary technology tool used to monitor precipitation associated with weather systems.

12. Figure 2 is the surface weather map for 00Z 22 OCT 2013. At map time, the cold front of this storm system was located [(to the west of)(at)(to the east of)] Buffalo in western New York State.

Figure 2. Surface weather map for 00Z 22 OCT 2013.

13. Buffalo’s station model reported the temperature as 59 °F, dewpoint 44 °F, overcast skies and wind from the [(north-northwest)(south-southwest)(east-southeast)(north-northeast)] at about 5 knots.

14. Precipitation echoes as shown by radar shadings on this Figure 2 at 00Z [(did)(did not)] occur to the west and north of Buffalo along the front.

Figure 3 is the Buffalo National Weather Service (BUF) Doppler radar displays of the Base Reflectivity on the left and the Base Velocity on the right at 0212Z on 22 OCT 2013. These displays were acquired a short time following the Figure 2 map time.

Figure 3. Buffalo, NY (BUF) Doppler radar displays of Base Reflectivity (left) and Base Velocity (right) at 0212Z, respectively, on 22 OCT 2013.

The location of the radar site is denoted by a black dot in the center of the reflectivity and velocity displays. The reflectivity (left map) is related to the intensity of the radar return signal according to the scale along the lower right margin of the reflectivity display.

15. The yellow and orange shadings generally northwest of Buffalo in the reflectivity view were returns from moderate to heavy rates of precipitation embedded in the area of generally lighter precipitation denoted by green returns. (The reflectivity values in the scale to the lower right of the map correlate to rainfall intensities.) The precipitation echoes shown by the shadings in the reflectivity display [(did)(did not)] generally cover a large area to the west and north of the radar site. The reflected energy from these raindrops provides information on both the number and size of the drops which determines precipitation intensity.

The reflected energy from rain, snow and other particles in the atmosphere can also be used to determine the particle motions. The Doppler Base (Radial) Velocity display (right map) is interpreted using the scale to its lower right. Negative numbers/green hues indicate winds with radial components toward the radar site. Positive numbers/red hues denote radial components of the wind away from the radar. The radial velocity color scale further shows magnitudes of the radial velocity in knots. The purple ring surrounding the red and green shades represents where the radar cannot distinguish whether the motions are uniquely toward or away from the radar at the greater distances.

16. The green and red radial velocity shadings inside and beyond the purple ring [(do)(do not)] generally cover most of the same areas shaded in the reflectivity return display depicted to the left.

In the Base Velocity view (right in Figure 3), the boundary between light reddish-gray and light greenish-gray shadings oriented generally northwest/southeast indicates 0 “Doppler wind speed”.

17. In the radial Velocity display on the right, the area of radial wind components flowing toward the radar site (bright green shading) is located generally to the [(west)(east)] of the radar site.

18. In the radial Velocity display, the area of greatest radial wind components flowing away from the radar site (bright red shading) is located generally to the [(northwest)(northeast)(southeast)(southwest)] of the radar site.

19. Draw a straight line from the radar site along the 0-Doppler wind speed boundary into Canada. This “zero-speed” situation occurs when the radar beam direction is [(perpendicular)(parallel)] to the actual wind direction (or there are calm conditions) and there is no wind motion component directly towards or away from the radar.

20. Draw a short (about 1 cm) arrow perpendicular to your 0-Doppler wind speed boundary line about midway between Buffalo and Toronto. Place an arrowhead on the red end to indicate the Doppler-detected wind direction at the station. The direction of your arrow, signifying the wind direction in the lower layers of the atmosphere sensed by the radar signal, is generally from the [(northeast)(northwest)(southwest)(southeast)].

21. The surface wind direction from the Buffalo station model in Figure 2 (item 13), is [(generally consistent with)(in the opposite direction from)] the arrow you drew on the radial Velocity display of Figure 3.

As the radar beam pulse travels outward from the radar site, the beam curves downward, but with less curvature than the underlying Earth’s surface. Therefore, the beam is sampling air at increasing altitudes as distance away from the radar site increases. Shading patterns therefore give information on wind speeds and directions at higher and higher altitudes. The essentially straight line of the 0 “Doppler wind speed” boundary across the area indicates the winds were relatively consistent in direction with altitude.

22. The 00Z 22 OCT 2013 Buffalo, NY Stüve diagram (not shown) provides a plot of wind directions from the sounding. A radiosonde tracked for winds is called a rawinsonde. The Buffalo wind profile showed southwest winds at all levels from the surface to 100 mb. These rawinsonde measured winds [(were)(were not)] generally consistent in direction with the Doppler radar display of wind directions.

Doppler velocity depictions may be quite complex, especially during storm episodes, and require interpretation by trained radar meteorologists. While many TV stations claim they are providing “Doppler” radar information, the views they present are generally of reflectivity. The real Doppler velocity images would prove confusing to most viewers. Additionally, light “clear air” reflectivities from dust, insects and temperature gradients that provide wind information in non-precipitation cases would not be shown on local TV shows as they can be misleading.

You might practice calling up NWS sites and viewing both reflectivity and velocity displays when precipitation is occurring in your area. More information on Doppler radar and its imagery can be found at http://www.srh.noaa.gov/srh/jetstream/doppler/doppler_intro.htm ().

We will examine additional radar views of air motions in a later investigation dealing with tornadoes. Such radar views can be found from the course website’s Radar section with the NWS Radar Page link. Additional discussion of radar imagery interpretation can be found at: http://ww2010.atmos.uiuc.edu/(Gh)/guides/rs/rad/home.rxml ().

Suggestions for further activities: The radar images in this investigation were from sites located via “NWS Radar Page” link from the course website. Another source of radar imagery is http://www.intellicast.com/ (). Also, a discussion of Doppler technology, including storm relative velocities, can be found at: http://www.crh.noaa.gov/lmk/soo/88d/index.php () with additional images at: http://www.crh.noaa.gov/lmk/soo/88dimg/index.php ().