Radar Products
Base Reflectivity
NEXRAD radars work by bouncing radio waves off particles in the air. Those particles could be raindrops, hail, snow, or even dust and insects. The amount of energy that bounces off of those particles and returns to the radar is called "reflectivity" and is represented by the variable "Z". Reflectivity covers a wide range of signal strength, from very weak to very strong, so it is measured on a decibel (logarithmic) scale in units of dBZ, or decibels of Z. The higher the dBZ value, the larger the number and/or size of the particles the radar beam is seeing.
The dBZ values increase as the strength of the signal returned to the radar increases. The scale of dBZ values is related to the intensity of rainfall. It is important to remember, however, that the radar shows only areas of returned energy and not necessarily precipitation. So the presence of a return, especially a very weak return below 20 dBZ, doesn't always mean that it's raining.
The colored squares along the bottom of the map correspond to precipitation types and intensities. When you move your finger across the squares, RadarScope will display a value for each color. NEXRAD radars can't distinguish between different types of precipitation with absolute certainty. However, reflectivity values can be somewhat roughly associated with different precipitation types:
- 10 dBZ (green) - Very light rain or light snow
- 20 dBZ (green) - Light rain or moderate to heavy snow
- 30 dBZ (yellow) - Moderate rain or sleet showers
- 40 dBZ (orange) - Moderate to heavy rain or sleet showers
- 50 dBZ (red) - Heavy thunderstorms
- 60 dBZ (pink) - Intense to severe thunderstorms with hail
If you're seeing values less than 5 dBZ, the radar is operating in clear air mode, which means it's likely not raining in your area. Such low values usually indicate dust or insects in the air rather than precipitation.
Like in the movie "Pirates of the Caribbean," these reflectivity values are more like guidelines than rules. This is a rough guide only. The atmosphere is a complex system, so you can't always associate particular values with precise conditions or events. As a general rule, the higher the dBZ value, the heavier the concentration of objects at that location in the atmosphere.
You can learn more about base reflectivity products on this NWS web page: http://www.srh.noaa.gov/jetstream/doppler/baserefl.htm
Composite Reflectivity
Composite reflectivity combines data from all elevation scans, or tilts, to create a single product. The resulting image shows the highest reflectivity value from the vertical cross section at that location. Composite reflectivity can reveal important features in a storm's structure that might not be seen in the base reflectivity product.
Because it combines data from all the tilts, the composite reflectivity product is one of the last to be produced during a volume scan. As with all NEXRAD products, it's important to remember that the data displayed in the image depict conditions that have already happened rather than what is happening right now.
Learn more about composite reflectivity from this National Weather Service web page: http://www.srh.noaa.gov/jetstream/doppler/comprefl.htm.
Base Reflectivity 248 nmi
The 248 nautical mile base reflectivity product offers twice the range of the standard base reflectivity product, but only half the pixel resolution. At the extended range of this product, the radar beam is at a very high altitude, so only fairly intense storms will be detected at those distances.
Base Velocity
Base velocity indicates storm motion toward or away from the radar, measured in knots. One knot is equal to one nautical mile per hour, or about 1.15 miles per hour. The velocity products in RadarScope use the Doppler effect to determine how fast the particles in the air are moving relative to the radar itself. Negative values (green in RadarScope) indicate motion toward the radar, while positive values (red in RadarScope) indicate motion away from the radar. They can be difficult to interpret without training and experience, but Doppler velocity products can be used to detect the overall movement of a storm as well as relative motion within the storm itself, such as rotation.
Note that the radar can only detect the component of the velocity vector along the radar beam, so this isn't a full picture of the wind field. But it gives you a fairly good idea which way a storm is heading. The attached velocity image below is from a tornado that touched down near Oklahoma City on February 10, 2009. The rotation in the storm is evident from the bright green and red values near each other between Union City and Mustang. Note that the radar beam isn't looking near the ground; it's much higher up in the air. So while it's seeing a large area of rotation at higher levels of the storm, it can't see the actual tornado that is near the ground.
Since the beam is sent out at an angle to the ground, it is looking higher up in the atmosphere as it gets farther from the radar. So the data you see in a radar image are often thousands of feet above the ground. At that height, wind speeds are often higher than they are on the ground. Doppler velocity products are valuable tools for meteorologists to use to determine motion in storm systems. But if you're interested in surface level winds, your best bet is to look at data from weather stations on the ground. There are several iPhone apps and other sources on the web which provide such information.
You can learn more about base velocity products on this National Weather Service page: http://www.srh.noaa.gov/jetstream/doppler/basevel.htm
Storm Relative Velocity
Storm relative velocity is simply base velocity with the average storm motion subtracted out. When storms are moving quickly, this makes it easier to spot green/red velocity couplets that are indicative of rotation and which might be masked out by the motion of the storm. As with base velocity, green is motion towards the radar and red indicates motion away. Below is the storm relative velocity image from the same time as the base velocity image shown above. You can see how factoring out the overall motion of the storm makes the area of rotation stand out more clearly.
It's also worth noting that the above rotation images are ideal cases. We aren't always lucky enough to get such prominent radar signatures from tornadoes. The radar isn't looking at ground level, so it can't actually see the tornado itself. It's seeing rotation higher up in the storm covering an area that is several miles wide. The height and width of the radar beam increases with its distance from the radar. So the farther away a storm is from the radar, the higher up the radar is seeing and the wider the beam, making it is less likely to detect the rotation associated with a tornado.
You can learn more about storm relative velocity on this National Weather Service page: http://www.srh.noaa.gov/jetstream/doppler/srm.htm
Estimated Rainfall
The rainfall products are estimates of how much rain has fallen at a particular location. The National Weather Service has computers that analyze the reflectivity values returned by the radar and estimate how much rain has fallen. It is not, of course, perfectly accurate but it usually gives you a good idea of the relative amount of rainfall at various locations within the radar's coverage area. The One Hour Surface Rainfall product provides an estimate of how much rain has reached the ground in the past hour. The Storm Total Surface Rainfall product does the same thing for an arbitrary period of time specified by the radar operator, usually corresponding to the beginning of a rainfall event. Since this product is based on the relationship of reflectivity (Z) to rainfall rate (R), it is important to note that it is not an indicator of snowfall accumulation.
You can learn more about precipitation estimates on these National Weather Service pages:
http://www.srh.noaa.gov/jetstream/doppler/ohp.htm
http://www.srh.noaa.gov/jetstream/doppler/stp.htm
Vertically Integrated Liquid
The vertically integrated liquid (VIL) product estimates the amount of water in a column of air. High values for VIL can indicate heavy rainfall or the presence of hail. When VIL values fall rapidly, it may indicate a downburst. VIL is subject to radar limitations and seasonal dependencies, so it's a tricky product to interpret.
Learn more about vertically integrated liquid from this Wikipedia web page: http://en.wikipedia.org/wiki/Vertically_integrated_liquid.
Echo Tops
The echo tops product shows the maximum height of precipitation echoes detected by the radar between 5,000 and 70,000 feet that exceed 18 dBZ. Higher echoes are often associated with stronger areas of a storm. This product is useful for identifying strong updrafts, and a sudden drop can indicate the onset of a downdraft. Some storms are too close to the radar for the beam to see the top, so echo tops is often underestimated for strong storms near the radar.
Clear Air Mode
When there's no precipitation in the area, it's common for the radar to be operating in what is called "clear air mode." In this mode, the radar is scanning more slowly so that it can be more sensitive and pick up much weaker returns. This allows it to see more details and detect finer particles in the atmosphere, including things like dust and insects. While the reflectivity color scale ranges from 5 to 75 dBZ in precipitation mode, it ranges from -28 to 28 dBZ in clear air mode.
This more sensitive mode of operation allows meteorologists to see what's going on in the atmosphere even though no rain is falling. Clear air mode gives meteorologists the ability to see things like cold fronts and subtle airmass boundaries. When conditions are right, these boundaries can become the focal point for storm initiation, so being able to see them is extremely important. Clear air mode is also useful for detecting very light drizzle and light snow. Sometimes these phenomena do not generate a strong enough return signal to be detected in precipitation mode, but are clearly visible in the more sensitive clear air mode. For this reason, the NWS will sometimes leave a radar in clear air mode when it's snowing.
RadarScope supports a couple of display options for clear air mode. By default, RadarScope shifts the color values in clear air mode to more closely match those used for precipitation mode. But if you enable "Expert Mode" in the RadarScope preferences, it shifts the color scale to that normally used by NWS meteorologists. This reveals more of the detail seen by the radar, but it means that what you often see is a big plume of dust, insects, and other clutter surrounding the radar. The following two images provide an example of this using the same clear air mode base reflectivity product. The first image uses RadarScope's default color scale. The second image uses RadarScope's expert mode color scale.
When precipitation begins within the coverage area of a particular radar, the NWS usually switches to precipitation mode. This mode looks more like what you'd expect when looking at radar images on various web sites.
You can learn more about clear air mode on this National Weather Service page: http://www.srh.noaa.gov/radar/radinfo/radinfo.html#clear.
Tilts
The radar beam is sent into the air at varying angles, or tilts, from the horizon. The lowest angle (tilt 1) is about 0.5 degrees for most radars. The highest angle (tilt 4) is between 3 and 4 degrees from horizontal. Higher tilts allow you to see higher levels of the storm structure. With any tilt, the farther the beam gets from the radar the higher it is looking in the air. Because of the steeper angle, that effect is more pronounced in the higher tilts. The curvature of the Earth also comes into play, so even if there were no tilt to the radar beam whatsoever, it is looking higher above the ground the further it gets away from the radar. Meteorologists use the higher tilts to get an idea of the vertical structure of a storm. But because of the steeper angle, those products can be a little more difficult to interpret.
For most purposes, the casual user will want to stick with tilt 1, which is closest to the ground. But keep in mind, even the lowest tilt can be sampling at thousands of feet above the ground depending on the distance from the radar. There can still be a lot of weather happening in that lowest few thousand feet beneath the beam, even for the lowest tilt.
You can learn more about radar tilts on this National Weather Service page: http://www.srh.noaa.gov/jetstream/doppler/vcp_max.htm.
Resolution
RadarScope renders NEXRAD Level 3 data that it receives from the National Weather Service at its true resolution. Level 3 data translates to about 1 kilometer per radar pixel radially as you move away from the radar, and about a 1 degree angle as the radar rotates. Like a flashlight beam, the radar beam widens as it gets farther from the radar itself and the width of the pixels increases as a result. So while the data pixels are still 1 kilometer radially, they become significantly wider and are thus lower resolution as you move away from the radar. RadarScope displays images at the true resolution of this data, so what you see is the best that NEXRAD Level 3 data can provide.
Below is a sample image from the Oklahoma City radar. In this case, the radar is located just east of the "Moore" label. You'll notice that the radar pixels become wider as you move away from the radar.
Update Interval
Collecting data is not an instantaneous process for NEXRAD radars. It takes a certain amount of time to rotate the antenna and collect data for all the different tilts. Collectively these tilts make up what is called a volume scan. Depending on whether the radar is operating in clear air mode or precipitation mode, each volume scan takes a different amount of time. When operating in precipitation mode, a volume scan takes 5-6 minutes. In clear air mode, since the antenna is rotating more slowly, a volume scan takes about 10 minutes.
As a radar collects a volume scan, it first collects a 360 degree sample at an elevation angle of 0.5 degrees (tilt 1), then a scan for tilt 2, and so on, increasing elevation angle with each revolution. Once all the tilts for a given volume scan have been collected, it will recycle back down to tilt 1 and do it all over again.
This is why NEXRAD radar images update on a 5-10 minute interval. RadarScope is tuned to the NEXRAD volume scan strategy. Checking for updates more often is an unnecessary waste of battery power because new data will not exist until the volume scan is complete.
More Information
As you can see, there's a lot of useful information in radar images, but interpreting them can be a tricky prospect. It takes a good understanding of how the radar works as well as how the atmosphere behaves to make sound judgements during severe weather events. The NEXRAD network offers high density coverage of the U.S., but it still can't see everything. RadarScope is one of many tools you can use to stay informed. But it should always be used in conjunction with official information from the National Weather Service, local emergency management officials, and your local news media.
The National Weather Service has some good information on its web site about NEXRAD radar products. Here are a couple of good pages that provide starting points for learning more about NEXRAD radar: