3-46 The Base Reflectivity product is one elevation scan, whereas Composite Reflectivity looks at all elevation scans. Base Reflectivity products are available several minutes sooner than Composite Reflectivity products.
Prec ipitation at any location may be heavier than depicted on the Base Reflectivity image because it is occurring above the lowest elevation angle.
Both a short-range (see Figure 3-15, WSR-88D Weather Radar Short-Range (124 NM) Base Reflectivity, Single Site Product Example) and long-range (see Figure 3-16, WSR-88D Weather Radar Long-Range (248 NM) Base Reflectivity, Single Site Product Example) image are available from the
0.5º Bas e Reflectivity product. The maximum range of the short-range, single-site radar Base Reflectivity product is 124 NM from the radar location.
Long-range, single-site, Base Reflectivity product’s range is 248 NM from the radar location.
When using a single-site radar, i.e., not using a radar mosaic, echoes farther than 124 NM (short-range) or 248 NM (long-range) from the radar site will not be displayed, even if precipitation may be occurring at these greater distances.
Figure 3-15. WSR-88D Weather Radar Short-Range (124 NM) Base Reflectivity, Single-Site Product Example
Figure 3-16. WSR-88D Weather Radar Long-Range (248 NM) Base Reflectivity, Single-Site Product Example DBZ Radar Image from National Weather Service: PAH 19:03 UTC 04/29/2005 + Kankakee Lf+ Fort Wayne 75 Lamoni • Heona + Logansport + Lima + Kirksville Champaign + Quincy Springfield Landian apolis + Terré Haute + Bloomington Columbia Cinton + Louisville evansvilles + Springfield wing Green Poplar Bluff, + Branson ashville Dyersburt + Crossville + Lawrenceburg OMemphis 2+ Chattanboga + Little Rock Forence « Huntsvie V + Pine Blufg *+ Oxford Tupelo + Rome Gadsden + El Dorado * Greenville Columbus + luscaloosa J + Bimingham Radar Image from National Weather Service: КРАН 19:03 UTC 04/29/2005
3.3.5.4 Radar Coded Message (RCM).
With the deployment of the WSR-88D (NEXRAD) radar network in the 1990s, the manual radar observations, known by the acronyms ROB, SD, and RAREP, was replaced by the RCM. The RCM is the encoded and transmitted report of radar features observed by a WSR-88D radar. The RCM is generated automatically and has no human input or oversight.
The actual RCM is highly detailed, complicated, and not intended for use by the pilot or operator in raw format. However, the RCM is used to generate certain radar charts or displays, such as "The Current RCM Radar Plot," available on https://www.aviationweather.gov (see Figure 3-17, Radar Coded Message (RCM) Display), as well as other weather service provider's websites.
The RCM display is updated every 30 minutes. The display is an image representation of the NEXRAD radar Composite Reflectivity overlaid with cloud and echo tops and centroid movement. The RCM display includes the maximum echo top for each radar's area of coverage. The other tops shown on the display are derived from the satellite images.
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3-48 The echo tops shown on the display can be erroneous in some cases. The algorithm computes echo tops (the altitude of echoes greater than or equal to 18 dBZ) within the range of the radar. This can lead to spurious values when precipitation is far from the site. Values greater than 50,000 ft (500 ft on the plot) can be disregarded, especially if their locations do not correspond to any precipitation.
Figure 3-17. Radar Coded Message (RCM) Display
The display is an image representation of the NEXRAD radar Composite Reflectivity overlaid with cloud and echo tops and centroid movement.
3.3.6 Precipitation.
3.3.6.1 Convective Precipitation.
Convective precipitation (see Figure 3-18, WSR-88D Weather Radar Convective Precipitation on the 0.5º Base Reflectivity Product Example) is disting uished by the following radar characteristics:
• Echoes tend to form as lines or cells,
• Reflectivity gradients are strong,
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• Precipitation intensities generally vary from moderate to extreme,
• Occasionally precipitation intensities can be light, and
• Echo patterns change rapidly when animating the image.
Numerous hazards are associated with Convective precipitation. They include:
turbulence, LLWS, strong and gusty surface winds, icing above the freezing level, hail, lightning, tornadoes, and localized instrument flight rules (IFR) conditions below the cloud base due to heavy precipitation.
Figure 3-18. WSR-88D Weather Radar Convective Precipitation on the 0.5º Base Reflectivity Product Example
3.3.6.2 Stra tiform Precipitation.
Stratiform precipitation (see Figure 3-19, WSR-88D Weather Radar Stratiform Precipitation on the 0.5º Base Reflectivity Product Example) has the fo llowing radar characteristics:
• Widespread in aerial coverage;
• Weak reflectivity gradients;
• Precipitation intensities are generally light or moderate (39 dBZ or less);
• Occasionally, precipitation intensities can be stronger; and
• Echo patterns change slowly when animating the image.
3-50 Figure 3-19. WSR-88D Weather Radar Stratiform Precipitation on the 0.5º Base Reflectivity Product Example
Hazards associated with stratiform precipitation include possible widespread icing above the freezing level, low ceilings, and reduced visibilities.
3.3.7 Limitations.
Limitations associated with Composite Reflectivity and Base Reflectivity include:
• The radar beam may overshoot or undershoot targets, and
• The image may be contaminated by:
o Beam blockage, o Ground clutter, o Anomalous Propagation (AP), o Ghosts, o Angels, and/or o Other non-meteorological phenomena.
3.3.7.1 Radar Beam Overshooting and Undershooting.
Radar beam overshooting may occur because the lowest radar beam (typically the 0.5 degree elevation) can be higher than the top of precipitation. This will most likely occur with stratiform precipitation and low-topped convection. For
3-51 example, at a distance of 124 NM from the radar, the 0.5º radar beam is at an altitude of approximately 18,000 ft; at 248 NM the beam height is approximately 54,000 ft. Any precipitation with tops below these altitudes and distances will not be displayed on a single-site radar image. Therefore, it is quite possible that precipitation may be occurring where none appears on the single-site radar image.
Radar overshooting occurs more often in the mountainous western United States where some radars are located on a mountain top (e.g., the WSR-88D near Cedar City, UT).
Radar undershooting occurs when precipitation occurs above Base Reflectivity beam, usually with high-cloud based precipitation near the radar site. This often occurs in the western United States during the summer months.
Undershooting may occur at and near the radar site even in Composite Reflectivity products when the precipitation is above the highest elevation angle. This region above the radar is known as the “cone of silence” (see Figure 3-20, Cone of Silence).
Figure 3-20. Cone of Silence
3.3.7.2 Beam Blockage.
Beam blockage (see Figure 3-21, WSR-88D Weather Radar Beam Blockage on Base Reflectivity Product E xample) occurs when the radar beam is blocked by terrain and is particularly predominant in mountainous terrain.
Beam blockage is most easily seen on the 0.5º Base Reflectivity images where it appears as a pie-shaped area (or areas) perpetually void of echoes.
When animating the imagery, the beam blockage area will remain clear of echoes even as precipitation and other targets pass through. In many cases, the beam blockage effect seen on a single-site radar can be minimized by viewing mosaic images.
Figure 3-21. WSR-88D Weather Radar Beam Blockage on Base Reflectivity Product Example shone Beam Blockage + A† Soda Springs Malad City + Prestor LOÿ Brigham City Ogden
3.3.7.3 Ground Clutter.
Ground clutter (see Figure 3-22, WSR-88D Weather Radar Ground Clutter Example) is radar echoes' returns from trees, buildings, or other objects on the ground. It appears as a roughly circular region of high reflectivity at ranges close to the radar. Ground clutter appears stationary when animating images and can mask precipitation located near the radar. Most ground clutter is automatically removed from WSR-88D imagery, so typically it is does not interfere with image interpretation.
Figure 3-22. WSR-88D Weather Radar Ground Clutter Example Radar Image from National Weather Service: KCLE 16:46 UTC 05/11/2005 + Alma + Saginaw rt Huron + London + Findlay Akron Youngstown + Mansfield Ground Clutter Pittsburgh + Columbus - Cambridge - wheeling Zanes ville + Moundsville® Dayton + New Lexington * Morgan town -Fairmont
• 16:46 UTC 05/11/2005 3-52
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3.3.7.4 Ghost.
A ghost (see Figure 3-23, WSR-88D Weather Radar Ghost Example) is a diffused echo in apparently clear air caused by a “cloud” of point targets, such as insects or by refraction returns of the radar beam in truly clear air.
The latter case commonly develops at sunset due to super-refraction during the warm season. The ghost develops as an area of low reflectivity echoes (typically less than 15 dBZ) near the radar site and quickly expands. When animating the imagery, the ghost echo shows little movement.
Figure 3-23. WSR-88D Weather Radar Ghost Example
3.3.7.5 Angels .
Angels are echoes caused by a physical phenomenon not discernible by the eye at the radar site. They are usually caused by bats, birds, or insects. Angels typically appear as a donut-shaped echo with low reflectivity values (see Figure 3-24, WSR-88D Weather Radar Angel Example). When animated, the echo expands and becomes more diffuse with time.
Angels typically only appear when the radar is in Clear Air Mode because of their weak reflectivity. Echoes caused by birds are typically detected in the morning, when they take flight for the day. Echoes caused by bats are typically detected in the evening, when they are departing from caves.
3-54 Figure 3-24. WSR-88D Weather Radar Angel Example
This angel was caused by bats departing Selman Bat Cave at Alabaster Caverns State Park, OK around sunset.
3.3.7.6 Anomalous Propagation (AP).
AP (see Figure 3-25, WSR-88D Weather Radar Anomalous Propagation (AP) Example) is an extended pattern of ground echoes caused by superrefraction of th e radar beam. Superrefraction causes the radar beam to bend downward and strike the ground. It differs from ground clutter because it can occur anywhere within the radar’s range, not just at ranges close to the radar.
AP typically appears as speckled or blotchy, high-reflectivity echoes. When animating images, AP tends to “bloom up” and dissipate, and has no continuity of motion. AP can sometimes be misinterpreted as thunderstorms;
differentiating between the two is determined by animating the images.
Thunderstorms move with a smooth, continuous motion while AP appears to “bloom up” and dissipate randomly.
Figure 3-25. WSR-88D Weather Radar Anomalous Propagation (AP) Example
BASE REFLECTIVITY
KTLX - OKLAHOMA CITY, OK ANOMALOUS 05/26/2000 12:15:59 GMT PROPOGATION LAT: 35/19/58 N LON: 97116140 W ELEV: 1277.0 FT MODE/VCP: A / 11 ELEV ANGLE: 0.50 ° MAX: 73 dBZ RANGE 248 NM Legend: (Category) dBZ
(15) 75
(14) 70
(13) 65
(11) 55
(9) 45
(8) 40 ЛОЛА 3000003
3.3.7.7 Other Non-Meteorological Phenomena.
3.3.7.7.1 Wind Farms.
Wind farms can affect the return signal of the radar beam. Depending on the proximity of the wind farm to the radar site (generally within 10 NM), wind farm turbines can result in beam blockage, false echoes, or high reflectivity values (see Figure 3-26, Wind Farms Have Been Known to Make Benign Echoes Appear as Small Storms. This Example Is from the Buffalo Radar).
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3-56 Figure 3-26. Wind Farms Have Been Known to Make Benign Echoes Appear as Small Storms. This Example Is from the Buffalo Radar
3.3.8 Use.
Radar mosaic products based on Composite Reflectivity radar data provide a more complete depiction of precipitation than mosaics based on Base Reflectivity data.
3.3.9 Future Products.
The NWS is working on two new types of radar displays that are not available for description in time for publication.
3.3.9.1 Product Update Rate.
Today, composite radar images are updated after volume scans are complete (about every 5 minutes). In the future, composite radar products will be updated with every tilt sweep.
3.3.9.2 Mosaicked Composite Reflectivity.
The NWS will add mosaics of Composite Reflectivity. This is especially important for aviation as the heaviest precipitation at any altitude in the
3-57 vertical will be depicted in mosaics. Today, only the base elevation angle
(i.e., closest to the ground) contributes to the mosaic.
3.3.9.3 Hydr ometeorological classification.
The implementation of dual polarization radar across the NEXRAD system means more information about detected targets can be observed. For more information on what dual polarization is, refer to Dual-Polarization Radar Training for NWS Partners provided by the NWS Warning Decision Training Division at https://training.weather.gov/wdtd/courses/dualpol/Outreach/.
Dual polarization allows the NWS to make a best guess at the precipitation type detected from a pre-defined list. The precipitation types of interest to aviation include:
• Graupel (also known as snow pellets). Graupel is ice crystals encased in a layer of ice. When graupel hits the ground, you can sometimes see it bounce. Graupel is formed when small cores of ice crystals fall through a region of supercooled (below freezing) liquid water droplets. These conditions are also favorable for airframe icing, thus detection of graupel implies potential icing conditions above the freezing level.
• Hail (or hail mixed with rain). Research is continuing to identify hail size from dual-polarization radar information.
3.4 Terminal Doppler Weather Radar (TDWR).
The TDWR network is a Doppler weather radar system operated by the FAA, which is used primarily for the detection of hazardous windshear conditions, precipitation, and winds aloft on and near major airports situated in climates with great exposure to thunderstorms in the United States. The National Centers for Environmental Information (NCEI) archives the products produced by the NWS Supplemental Product Generator (SPG) (called Level III) which are in the same data format as NEXRAD Level III. NCEI does not archive the base data (called Level II).
3.4.1 History.
TDWR was developed in the early 1990s at Lincoln Laboratory, which is part of the Massachusetts Institute of Technology (MIT). The TDWR system was funded by the FAA to assist air traffic controllers by providing real-time windshear detection and high-resolution precipitation data.
In 2004, the NWS established the policy with the FAA to allow the operational use of TDWR data and began to develop the SPG. Initial deployment of the SPG began in 2005 and finished in 2008.
As of 2016, there were 47 operational TDWR-SPG radar systems in major metropolitan locations across the United States and Puerto Rico.