Radar Pulses and Backscattered Energy

Radio Detection and Ranging (RADAR) systems emit a pulse, whose volume expands with distance. The current radar system is the Weather Surveillance Radar 1988-Doppler (WSR-88D) or Next Generation Radar (NEXRAD).

As pulse volumes within the WSR-88D radar beam encounter targets, energy will be scattered in all directions. A very small portion of the intercepted energy will be backscattered (termed "reflectivity") toward the radar and referred to as an "echo." The degree or amount of backscatter is determined by target

  • size (radar cross section)
  • shape (round, oblate, flat, etc.)
  • state (liquid, frozen, mixed, dry, wet)
  • concentration (number of particles per unit volume)

Meteorologists are concerned with two types of scattering, Rayleigh and non-Rayleigh. Rayleigh scattering occurs with targets whose diameter (D) is much smaller than the wavelength of the transmitted energy. The WSR-88D's wavelength is approximately 10.7 cm, so Rayleigh scattering occurs with targets whose diameters are less than or equal to about 7 mm or ~0.4 inch. Raindrops seldom exceed 7 mm so all liquid drops are Rayleigh scatters.

Nearly all hailstones are non-Rayleigh scatterers due to their larger diameters. However, since the vast majority of targets sampled by the WSR-88D are raindrop size or smaller, the Rayleigh assumption is used in all computations of radar reflectivity.

Base Reflectivity (from http://www.srh.noaa.gov/jetstream/remote/baserefl.htm#rainrate)

Typical base reflectivity radar beam cross section.Taken from the lowest (1⁄2°) elevation scan, base reflectivity is excellent for surveying the region around the radar to look for precipitation. However, remember the radar beam increases in elevation as distance increases from the radar. This is due, in part, to the elevation angle itself but is more because the earth's surface curves away from the beam.

This can lead to underestimating the strength and intensity of distant storms. For this reason, it is wise to always check the radar images from different locations to help provide the overall picture of the weather in any particular area.  

Also, radar pulses travel in "line of sight" arcs. This leads to detection problems in the mountainous west. We have particular problems with this in our area of California.

Base reflectivity image from the Doppler radar in Frederick, OKThis image (right) is a sample base reflectivity image from the Doppler radar in Frederick, OK. The radar is located in the center of the image. 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.

These dBZ values equate to approximate rainfall rates indicated in the table below.

 dBZ   Rain Rate 
65 16+
60 8.00
55 4.00
52 2.50
47 1.25
41 0.50
36 0.25
30 0.10
20 Trace
< 20 No rain

These are hourly rainfall rates only and are not the actual amounts of rain a location receives. The total amount of rain received varies with intensity changes in a storm as well as the storm's motion over the ground.

Also, thunderstorms can contain hail which is often a good reflector of energy. Typically, a hailstone is coated with a thin layer of water as it travels through the thunderstorm cloud. This thin layer of water on the hailstone will cause a storm's reflectivity to be greater, leading to a higher dBZ and an over estimate the amount of rain received. Value of 20 dBZ is typically the point at which light rain begins. The values of 60 to 65 dBZ is about the level where ¾" hail can occur.