11/14/16 Aviation Weather Services AC 00-45H 3-41 “reflectivity,” is measured in terms of decibels (dBZ). A decibel is a unit that describes the change of power emitted versus the power received. Since the power emitted is constant, the power received is related to the intensity of the precipitation target. Each reflectivity image includes a color scale that describes the relationship among reflectivity value, color on the radar image, and precipitation intensity. Figure 3-8, WSR-88D (NEXRAD) Weather Radar Echo Intensity Legend, depicts the correlations for most NWS radar images. The scale ranges from -30 dBZ to greater than 75 dBZ. The scale also includes ND correlated to black, which indicates no data was measured. The colors and decibel scale can vary depending on the service provider and Web site.
Reflectivity is correlated to intensity of precipitation. For example, in Precipitation Mode, when the decibel value reaches 15, light precipitation is present. The higher the indicated reflectivity value, the higher the rainfall rate. The interpretation of reflectivity values is the same for both Clear Air and Precipitation Modes.
Reflectivity is also correlated with intensity terminology (phraseology) for air traffic control (ATC) purposes. Table 3-7, WSR-88D Weather Radar Precipitation Intensity Terminology, defines this correlation.
Table 3-7. WSR-88D Weather Radar Precipitation Intensity Terminology Reflectivity (dBZ) Ranges Weather Radar Echo Intensity Terminology <30 dBZ Light 30-40 dBZ Moderate >40-50 dBZ Heavy 50+ dBZ Extreme Note: En route ATC radar’s weather and radar processor (WARP) does not display light precipitation.
Values below 15 dBZ are typically associated with clouds. However, they may also be caused by atmospheric particulate matter such as dust, insects, pollen, or other phenomena. The scale cannot reliably be used to determine the intensity of snowfall.
However, snowfall rates generally increase with increasing reflectivity.
3.3.5 Radar Products.
The NWS produces many radar products that serve a variety of users. Some of these products are of interest to the aviation community. This paragraph will discuss radar mosaics, composite reflectivity, base reflectivity, and the radar coded message products.
3.3.5.1 Radar Mosaic.
A radar mosaic consists of multiple single-site radar images combined to produce a radar image on a regional or national scale. Regional and national mosaics can be found on the Web sites of NWS, AWC, all NWS WFOs, as well as commercial aviation weather providers.
11/14/16 Aviation Weather Services AC 00-45H 3-42 Radar mosaics can be assembled from either composite reflectivity (see paragraph 3.3.5.2) or base reflectivity (see paragraph 3.3.5.3), depending on the Web site or data provider. At this time, NWS national (see Figure 3-9, NWS National Radar Mosaic Example, Which Utilizes NEXRAD Base Reflectivity) and regional (see Figure 3-10, NWS Regional Radar Mosaic Sector Example, Which Utilizes NEXRAD Base Reflectivity), and Alaska (see Figure 3-11, Alaska Radar Mosiac Example) radar mosaic sectors are assembled using only base reflectivity data (0.5º radar beam angle with a 124 NM range) and are set up to display all echoes (precipitation and non-precipitation). Most commercial aviation weather providers use composite reflectivity for their mosaics and configure the display to eliminate most non-precipitation echoes. NEXRAD radar data data-linked to aircraft cockpit displays via FAA Flight Information Service-Broadcast (FIS-B) use the composite reflectivity data for their radar mosaics.
Figure 3-9. NWS National Radar Mosaic Example, Which Utilizes NEXRAD Base Reflectivity
11/14/16 Aviation Weather Services AC 00-45H 3-43 Figure 3-10. NWS Regional Radar Mosaic Sector Example, Which Utilizes NEXRAD Base Reflectivity
Figure 3-11. Alaska Radar Mosaic Example
Many areas of Alaska do not have radar coverage. These areas are shaded gray.
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3.3.5.2 Composite Reflectivity.
Because the highest precipitation intensity can be at any altitude, the composite reflectivity product (see Figure 3-12, WSR-88D Weather Radar Composite Reflectivity, Single-Site Product Example) is needed. Composite reflectivity is the maximum echo intensity (reflectivity) detected within a column of the atmosphere above a location. During its tilt sequence, the radar scans through all of the elevation slices to determine the highest decibel value in the vertical column (see Figure 3-13, Creation of a Composite Reflectivity, Single-Site Product), then displays that value on the product. When compared with base reflectivity, the composite reflectivity can reveal important storm structure features and intensity trends of storms (see Figure 3-14, Weather Radar 0.5º Base Reflectivity (left) versus Composite Reflectivity (right) Comparison).
NEXRAD radar displays on airplane avionics use the composite reflectivity data for their radar mosaics.
Figure 3-12. WSR-88D Weather Radar Composite Reflectivity, Single-Site Product Example
11/14/16 Aviation Weather Services AC 00-45H 3-45 Figure 3-13. Creation of a Composite Reflectivity, Single-Site Product
The composite reflectivity product displays the highest reflectivity of all elevation scans.
Figure 3-14. Weather Radar 0.5º Base Reflectivity (left) versus Composite Reflectivity (right) Comparison
This composite reflectivity shows that in many locations the highest precipitation intensity occurs at an altitude higher than precipitation detected at the elevation of the base elevation angle.
3.3.5.3 Base Reflectivity.
Base reflectivity product is a display of both the location and intensity of reflectivity data from the lowest elevation angle, or 0.5º above the horizon.
11/14/16 Aviation Weather Services AC 00-45H 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.
Precipitation 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 base 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
11/14/16 Aviation Weather Services AC 00-45H Figure 3-16. WSR-88D Weather Radar Long-Range (248 NM) Base Reflectivity, Single-Site Product Example DBZ Radar Image from National Weather Service: KPAH 19:03 UTC 04/29/2005 + Kankakee —a+ Fort Wayne 75 Lamoni + Reoria + Logansport + Lima + Kirksville + Champaign.
• Quinc Springfield te Indianapolis 65 + Terre Haute -60 + Bloomington Columbia 55 Cinton 50 Evansville, t Louisville 45 + Springfield 40 Dwing Green -35 • Branson Poplar Blutta 30 ayersbun lashville + Crossville 25 Jack + Lawrenceburg -20 OMemphis + Chattanboga Little Rock Forense * Huntsvine 15 + Pine Blufg!" * + Oxford Tupelo + Rome -10 + Gadsden Columbus + Bingingtiam +* Greenville + El Dorado + Tuscaloosa J -IND 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 http://www.aviationweather.gov (see Figure 3-17, Radar Coded Message (RCM) Display), as well as other weather service provider's Web sites.
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 3-47
11/14/16 Aviation Weather Services AC 00-45H 3-48 maximum echo top for each radar’s area of coverage. The other tops shown on the display are derived from the satellite images.
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 distinguished 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 Stratiform Precipitation.
Stratiform precipitation (see Figure 3-19, WSR-88D Weather Radar Stratiform Precipitation on the 0.5º Base Reflectivity Product Example) has the following 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.
11/14/16 Aviation Weather Services AC 00-45H 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 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
11/14/16 Aviation Weather Services AC 00-45H 3-51 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 Example) 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.
11/14/16 Aviation Weather Services AC 00-45H 3-52 Figure 3-21. WSR-88D Weather Radar Beam Blockage on Base Reflectivity Product Example
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