CarterLake.org NEXRAD radar - How it works
CarterLake.org's NEXRAD radar is live NEXRAD Level III data from the National Weather Service. Such data is provided by the NOAA WSR-88D radar dome in Valley, Nebraska. There are 153 such stations located around the United States.
The Valley office with WSR-88D dome. Photo credit: NOAA
NEXRAD (Next Generation Radar) obtains weather information (precipitation and wind) based upon returned energy. The radar emits a burst of energy. If the energy strikes an object (rain drop, bug, bird, etc), the energy is scattered in all directions. A small fraction of that scattered energy is directed back toward the radar.
This reflected signal is then received by the radar during its listening period. Computers analyze the strength of the returned pulse, time it took to travel to the object and back, and phase shift of the pulse. This process of emitting a signal, listening for any returned signal, then emitting the next signal, takes place very fast, up to around 1300 times each second.
The WSR-88D Doppler radar is operated in one of two modes -- clear air mode or precipitation mode. In clear air mode, images are updated every 10 minutes. In precipitation mode, images are updated every four to six minutes.
Clear Air mode is used when there is no rain within the range of the radar. In this mode, the radar is in its most sensitive operation state. This mode has the slowest antenna rotation rate which permits the radar to sample a given volume of the atmosphere longer. This increased sampling increases the radar's sensitivity and ability to detect smaller objects in the atmosphere than in precipitation mode.
When precipitation is occurring, the radar does not need to be as sensitive as in clear air mode as rain provides plenty of returning signals. At the same time, meteorologists want to see higher in the atmosphere when precipitation is occurring to analyze the vertical structure of the storms. This is when the meteorologists switch the radar to precipitation mode.
Radar image and dBZ
Approximate rainfall rates
Currently, CarterLake.org only provides the long range base reflectivity image from the radar. This image shows precepitation out to 286 miles. How the radar builds an image of the sky is by taking snapshots in slices. These slices are called "tilts" as the radar changes its angle to the horizon. The base tilt is taken at the lowest elevation (.5 degrees)
"dBZ" stands for decibels of Z, Z being the reflectivity factor. Reflectivities in the range between 5 dBZ and 75 dBZ are detected when the radar is in precipitation mode. Reflectivities in the range between -28 dBZ and +28 dBZ are detected when the radar is in clear air mode.
Rainfall rate estimates 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.
Ground Clutter is the most common false echo and is usually seen in every radar image. When atmospheric conditions are such where there are low-level inversions (air temperature increasing with height instead of the typical decreasing with height) ground clutter can be very pronounced.
Ground clutter is the easiest false echo to recognize since it does not move in any organized fashion, it has no kind of structure to it that is similar to real precipitation, and it is usually close to the radar. The ground clutter in clear air mode is often more prevalent than precipitation mode. This is because the radar is in its most sensitive operation thereby "seeing" smaller objects such as dirt, dust and bugs.
Explanation of radar artifacts
Temperature - The temperature readings on our radar map are provided by the National Weather Service's Automated Weather Observing System (AWOS). These sensor suites are located at area airports and provide temperature updates approximately every hour.
The software which powers our images on this site is GRLevel3. GRLevel3 is a Windows-based viewer for NEXRAD Level III data from the NWS Radar Product Central Collection Dissemination Service.
The basic engine for the software is a true Geographic Information System, which allows the importation of virtually limitless user add-on files. Beyond that, the software has intuitive pan, tilt, and (up to street level) zoom capability - rivaling the radar software we see on television.
The software has the ability to display probable storm hail, storm direction and speed, and cloud rotation. It also can display thunderstorm, flash flood, and tornado warnings.
With built-in FTP capabilities, GRLevel3 can upload images directly to the CarterLake.org webserver. We then use PHP to post-process the image and display it as users see it.