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
3-53
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).
3-55
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.
3-58
3.4.2 Data Access.
3.4.2.1 Radar Data Access.
Weather radar data at NCEI are stored on a tape archive system and are accessed by placing orders through the NCEI website. There are several ordering systems available, optimized for different situations, including small orders, multisite and multiday orders, and large bulk orders. Currently, TDWR-SPG data are only available from the “Select by Site” and “Select by File” ordering systems. Small orders are typically completed in less than 15 minutes.
There are occasional gaps and missing data for each site in the archive. These gaps are caused by a number of reasons, including scheduled maintenance at the radar sites, unplanned downtime due to severe weather, communications problems, or archival problems. The data access web pages include lists or visualizations of file availability.
3.4.3 Products.
3.4.3.1 TDWR-SPG Products.
This complete list of all available data products (called Level-III products) includes descriptions and possible uses. There are 26 TDWR-SPG Level-III products routinely available from NCEI, including precipitation estimates, storm relative velocity, and echo tops.
3.4.3.2 Comparison of the TDWR to the WSR-88D.
The range resolution of the TDWR is finer than what is available in the WSR-88D or any other FAA radar that has weather channel capability. The TDWR utilizes a range gate resolution of 150 m for Doppler data. It has a resolution of 150 m for reflectivity data within 135 kilometers (km) and 300 m from beyond 135 km to 460 km. By contrast, the WSR-88D employed by the NWS, FAA, and DOD has a maximum range gate resolution of 250 m for Doppler and 1 km for surveillance data.
The angular (azimuth) resolution of the TDWR is nearly twice what is available in the WSR-88D. Each radial in the TDWR has a beam width of
0.55 degrees. The average beam width for the WSR-88D is 0.95 degrees. The following table shows a comparison of technical specifications between the TDWR and the WSR-88D.
3-59 Table 3-8. Technical Specifications: WSR-88D vs. TDWR WSR-88D TDWR Wavelength 10 cm 5 cm Volume Scan Time 4 minutes in Volume Coverage Pattern (VCP) 12 1 min 0.2 degree, Hazard (HAZ) Bean Width 1.25 degrees 0.5 degrees Range Gate 0.13 NM in velocity
0.54 NM in reflectivity
0.067 NM Max Unambiguous Velocity Up to 62 kts 20–30 kts Max Doppler Range 230 km 90 km The National Oceanic and Atmospheric Administration’s (NOAA) Radar Operations Center (ROC) has detailed information (http://www.roc.noaa.gov/).
3.4.4 How Does the Radar Collect Data?
The FAA designed the TDWR to look for low-altitude phenomena, such as wind shifts over the runways, windshear along the immediate approach and departure corridors, and downbursts. These radars are located close to major airports with the scanning strategy optimized to sample the atmosphere over its associated airport. The TDWR employs a scanning strategy called monitor mode, which is similar to the WSR-88D clear-air mode.
The TDWR remains in monitor mode until one of the two following conditions are recognized:
1. A region of 20 dBZ echoes located within 24.3 NM from the associated airport with a nominal areal extent of 1.3 NM and an altitude of at least
1.3 NM AGL, or
2. The radar detects windshear or a microburst.
This is a departure from the WSR-88D operations that can switch from clear-air mode to precipitation mode if the areal coverage of precipitation exceeds an adaptable parameter anywhere on the scope. The TDWR ties its decision on the area of influence directly to its associated airport.
3.4.5 Volume Coverage Pattern (VCP) 90.
VCP 90 is the TDWR monitor mode (clear air) and consists of 17 scans in about 6 minutes. The first cut is always a low Pulse Repetition Frequency (PRF), long-range scan (276 km). All remaining scans are short range (90 km). There is a small variation in the elevation angles of the lower scans, but all scans above 5 degrees are the same in all systems. Cuts two and three employ a split cut strategy. Cut two contains no dealiased Doppler data. Cut three (at the same elevation as cut two) is the first cut to contain dealiased Doppler velocity data. The first three cuts are used to initialize wind field models and velocity unfolding algorithms.
3-60
3.4.6 Volume Coverage Pattern (VCP) 80.
VCP 80 is the TDWR hazardous mode (precipitation mode). The first cut is always a low PRF, long-range scan (276 km). Cuts two and three are the short-range, split cut scans used to collect information for unfolding and clutter migration algorithms. The first cut with dealiased Doppler data is cut three. There are two sub-volumes or aloft scans contained in each full volume, as denoted by the use of red and orange. Every fourth scan, the TDWR provides a short-range elevation scan at the same elevation as cuts two and three.
3.4.7 Where Can I Get Details on TDWR-SPG?
1. NOAA’s ROC maintains papers, presentations, and detailed information on the TDWR-SPG system (https://www.roc.noaa.gov/spg/default.aspx).
2. Details on a subset of the TDWR-SPG products are available in the Federal Meteorological Handbook No. 11 (FMH-11), Doppler Radar Meteorological Observations (WSR-88D). The handbooks are availabl e for download from the Office of the Federal Coordinator for Meteorology (OFCM) (http://www.ofcm.gov/).
3. NCEI provides descriptions and possible uses of radar products generated from the TDWR network operated by the FAA. All data are available digitally at NCEI for free (visit http://www.ncdc.noaa.gov/nexradinv/) and paper copies are available for purchase. Certification of the paper data is al so available at https://www.ncdc.noaa.gov/customer-support/certifica tion-data.
3.4.8 TDWR Level-II (Base) Data.
The TDWR Level-II Base Data is not available from NCEI.
3.4.9 TDWR Level-III Products.
There are 26 TDWR Level-III products routinely available from NCEI. Most Level-III products are available as digital images, color hard copy, grayscale hard copy, or acetate overlay copies. Each copy will include state, county, and city background maps.
The Certification of Data page (https://www.ncdc.noaa.gov/customersupport/certification-data) has further information on hard copy radar products, pricing, and certification information. A detailed list of Lev el-III product codes is also available at http://www1.ncdc.noaa.gov/pub/data/radar/RadarProductsDetailedTable.pdf.
Brief descriptions and possible uses of these products are included below.
3.4.9.1 General Products.
General products include Base Reflectivity and Base Velocity, as well as graphical products derived from algorithms, including Spectrum Width, Vertically Integrated Liquid, and the Velocity Azimuth Display (VAD) Wind Profile.
3.4.9.1.1 Digital Base Reflectivity (TR0, TR1, TR2/181).
A display of echo intensity measured in decibels. These products are used to detect precipitation, evaluate storm structure, locate boundaries, and
3-61 determine hail potential. The three lowest elevation angles are available, with a maximum range of 48 NM.
3.4.9.1.2 Digital Base Velocity (TV0, TV1, TV2/182).
A measure of the radial component of the wind either toward the radar (negative values) or away from the radar (positive values). Cool colors (green) represent negative values and warm colors (red) represent positive values.
These products are used to estimate wind speed and direction, locate boundaries, locate severe weather signatures, and identify suspected areas of turbulence. The three lowest elevation angles are available, with a maximum range of 48 NM.
3.4.9.1.3 Long Range Digital Base Reflectivity (TZL/186).
Digital Reflectivity data are measured at an elevation angle of 0.6 degrees with a maximum range of 225 NM.
3.4.9.1.4 Composite Reflectivity (NCR/37).
A display of maximum reflectivity for the total volume within the range of the radar. This product is used to reveal the highest reflectivity in all echoes, examine storm structure features, and determine intensity of storms.
3.4.9.1.5 Vertically Integrated Liquid (NVL/57).
The water content of a 2.2 x 2.2 NM column of air, which is color-coded and plotted on a 124-NM map. This product is used as an effective hail indicator, to locate most significant storms, and to identify areas of heavy rainfall.
3.4.9.1.6 Echo Tops (NET/41).
An image of the echo top heights color-coded in user-defined increments. This product is used for a quick estimation of the most intense Convection and higher echo tops, as an aid in identification of storm structure features, and for pilot briefing purposes.
3.4.9.1.7 VAD Wind Profile (NVW/48).
A graphic display of windbarbs plotted on a height staff in 500-ft or 1,000-ft increments. The current (far right) and up to 10 previous plots may be displayed simultaneously. This product is an excellent tool for meteorologists in weather forecasting, severe weather, and aviation.
3.4.9.2 Precipitation Products.
Precipitation products are estimated ground accumulated rainfall. Estimates are based on a reflectivity and rainfall rate relationship called Z-R (https://www.weather.gov/tae/research-zrpaper).
3.4.9.2.1 One-Hour Precipitation (N1P/78).
A display of estimated 1 hour precipitation accumulation on a 1.1-NM x 1-degree grid using the Precipitation Processing System (PPS) algorithm. This product is used to assess rainfall intensities for flash flood warnings, urban flood statements, and special weather statements.