However, a value of 60 to 65 dBZ does not mean that severe weather is occurring at that location. When all returns from all elevation scans are compiled an image is created which takes the highest dBZ value from all elevations, called Composite Reflectivity. It is a picture of the strongest returns from all elevations. When compared with Base Reflectivity, the Composite Reflectivity can reveal important storm structure features and intensity trends of storms.
This is important because often during the development of strong to severe thunderstorms, rain-free areas or areas with light rain develop as a result of strong updrafts. Yet, because it requires all elevation scans to be completed, unlike the Base Reflectivity being the first image created, Composite Reflectivity is the last image created in each volume scan.
Therein lies an important point when viewing composite reflectivity images; always check the time of the image. Often, the base reflectivity image and composite reflectivity image will not have the same time with the base reflectivity image being the most recent. In the loop right below it will change to the base reflectivity image from the same time as the composite view. The first thing you will notice about the composite image is there is much more "green" color near the radar, located at the center.
When higher elevation scan information is included in the composite reflectivity, it appears to indicate more widespread rain. However, the base reflectivity image does not show that rain so it is probably not reaching the ground but evaporating as it falls from very high in the atmosphere. Evidence of very strong updrafts leading to the possibility of severe weather can be seen when comparing the two images.
The radar's computers will calculate the shift and determine whether the precipitation is moving toward or away from the radar, and how fast, then apply a corresponding color to those directions and speeds. Red is typically a target moving away from the radar, while green is applied to targets moving toward the radar. The intensity of these colors determines its estimated speed. Learn more about Velocity here. In the image above, you can see the velocity data that is associated with a strong storm depicted in the reflectivity data.
This is a great example of what a tornado looks like in the velocity display. Click on the image for better detail. The radar is located to the southeast, or to the bottom right of the computer screen. Note the bright red, or strong outbound velocities right next to the bright green, or inbound velocities.
This indicates a strongly rotating column of air. When coupled with a reflectivity pattern that exhibits a hook signature, as in this case, there is often a tornado occurring or about to occur. If there is a "target" out there and it reflects radar energy back to the radar, the radar will display it as if it was precipitation. The radar does have some logic built in to help it discriminate between precipitation and non-precipitation targets.
But, sometimes we see curious things on our radar display. Here are a few:. Bird Roost Rings. These are most common in the fall around bodies of water that typically have temperatures warmer than the surrounding land at night. It is also the time birds are gathering for the seasonal migration.
Just before sunrise, there is often a coordinated lift off and dispersion of the birds out into the surrounding fields for feeding during the day. Click on the image to the left for a quick animation of the bird rings.
Wind Farm Interference. If close enough within a few kilometers they can partially block a significant percentage of the beam and attenuate data down range of the wind farm. They can also reflect energy back to the radar and appear as clutter AP on the radar image and contaminate the base reflectivity data.
The reflectivity data is used by radar algorithms to estimate rainfall and to detect certain storm characteristics. Learn more here. Sun Interference. Twice a day, at sunrise and sunset, the radar experiences interference from the electromagnetic energy emitted by the sun. There is a point at sunrise and sunset where the radar dish points directly at the sun and is hit with this energy. This is then displayed as a spike of returned energy on our display. It is brief, typically only occurring during one volume scan.
Notice in the image to the left that sunset is slightly south of due west. The date is March 11, In less than 2 weeks, we will be at the Spring Equinox. The sun will set due west of the radar. I want to quickly summarize some of these common radar reflectivity products.
Radar Mosaics : While the National Weather Service maintains a network of individual radars covering most of the United States , you've learned that the range of each radar is only miles.
That's not very helpful for tracking very large areas of precipitation, so meteorologists often create "radar mosaics," which "stitch together" the reflectivity from the individual radars into a single image covering a larger region or even the entire country , as shown in the example on the right.
Note that rain, mixed precipitation, and snow each has its own color key in this larger version of the radar mosaic above on the right. While the exact methods for creating such images vary, they all start with radar reflectivity, and often incorporate surface temperature and other observations to give a "best guess" of precipitation type.
Radars now have some additional capabilities to help discern precipitation type, too, which we'll cover in the next section. Still, keep in mind that such "precipitation-type" radar images aren't perfect they don't always show the correct precipitation type. Composite Reflectivity: The radar reflectivity derived from a single radar scan is called "base reflectivity.
That's right, radars scan at more than just the 0. NEXRAD units are capable of tilting upward and regularly scanning at angles of elevation as large as For example, if a powerful thunderstorm erupts fairly close to the radar, a scan at the shallowest angle of 0.
Such a single, shallow scan falls way short of painting a proper picture of the storm's potential. As a routine counter-measure, the radar tilts upward at increasingly large angles of elevation, scanning the entire thunderstorm like a diagnostic, full-body MRI. So, a radar image created from composite reflectivity will likely display a higher dBZ level more intense colors than a radar image of base reflectivity. For example, on August 19, , Tropical Storm Fay was moving very, very slowly over the Florida peninsula.
At the time, heavy thunderstorms were pounding the east-central coast of Florida. Now shift your attention to the image of composite reflectivity on the right above. Notice the reddish colors near Melbourne, which indicate higher dBZ values compared to the corresponding radar echoes on the image of base reflectivity.
Composite reflectivity may not be representative of current precipitation rates at the ground, but it can show the potential if the precipitation causing the highest reflectivity often well up into the cloud can fall to the surface. Now that you know how to interpret radar reflectivity, we're going to look at some additional capabilities of NEXRAD, which allow weather forecasters to infer wind velocities and and better detect severe weather like hail and tornadoes.
Read on. Skip to main content. Ultimately, the intensity of the return echo and therefore, reflectivity depends on three main factors inside a volume of air probed by the radar "beam": the size of the targets the number of targets the composition of the targets raindrops, snowflakes, ice pellets, etc. An image of radar reflectivity from Z on June 1, The line of high reflectivity values denotes large numbers of large rain drops characteristic of thunderstorms. Indeed, rainfall rates when this "squall line" passed through exceeded 3.
The radar reflectivity of nimbostratus clouds on the left relatively far away from the radar can be too low because the radar beam probes their upper layers, where precipitation rates are relatively light or the radar beam misses the cloud altogether.
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