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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
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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.
11/14/16 Aviation Weather Services AC 00-45H 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 the 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.
11/14/16 Aviation Weather Services AC 00-45H 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: 97/16/40 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
(12) 60
(11) 55
(10) 50 ЛОАЯ 300108000
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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11/14/16 Aviation Weather Services AC 00-45H 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
11/14/16 Aviation Weather Services AC 00-45H 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 Hydrometeorological 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 http://www.wdtb.noaa.gov/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 super-cooled (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 Satellite.
3.4.1 Description.
Satellite is perhaps the single most important source of weather data worldwide, particularly over data-sparse regions, such as countries without organized weather data collection and the oceans.
The National Oceanic and Atmospheric Administration’s (NOAA) Geostationary Operational Environmental Satellite (GOES) imagery can be found on the AWC’s Web site, as well as all NWS WFO Web sites. Additional satellite imagery for Alaska can be found on the NWS Alaska Aviation Weather Unit (AAWU) Web site.
3.4.2 Imagery Types.
Three types of satellite imagery are commonly used: visible, infrared (IR), and water vapor. Visible imagery is only available during daylight hours. IR and water vapor imagery are available day or night.
3.4.2.1 Visible Imagery.
Visible imagery (see Figure 3-27, Visible Satellite Image—U.S. Example) displays reflected sunlight from the Earth’s surface, clouds, and particulate matter in the atmosphere. Visible satellite images, which look like black and white photographs, are derived from the satellite signals. Clouds usually appear white, while land and water surfaces appear in shades of gray or black.
11/14/16 Aviation Weather Services AC 00-45H The visible channel senses reflected solar radiation. Clouds, the Earth's atmosphere, and the Earth's surface all absorb and reflect incoming solar radiation. Since visible imagery is produced by reflected sunlight (radiation), it is only available during daylight.
Figure 3-27. Visible Satellite Image U.S. Example 12 15 18 21 24 27 30 33 36 42 45 48 51 54 57 60 63 66 69 02 05 28 81 84 87 90 03
3.4.2.1.1 Visible Image Data Legend.
The data legend (see Figure 3-28, Visible Satellite Data Legend) on a visible image displays albedo, or reflectance, expressed as a percentage. For example, an albedo of 72 means 72 percent of the sunlight that struck a feature was reflected back to space.
Figure 3-28. Visible Satellite Data Legend 6 9 12 15 18 21 27 30 33 36 39 42 45 48 51 54 57 60 63 66 69 72 75 28 81 84 87 00 93 The gray shades (values) represent albedo, or reflectance, expressed as a percentage.
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3.4.2.2 Infrared (IR) Imagery.
IR images (see Figure 3-29, Infrared (Color) Satellite Image—U.S.
Example, and Figure 3-30, Unenhanced Infrared (Black and White) Satellite Image—U.S. Example) display temperatures of the Earth’s surface, clouds, and particulate matter. Generally speaking, the warmer an object, the more IR energy it emits. The satellite sensor measures this energy and calibrates it to temperature using a very simple physical relationship.
Clouds that are very high in the atmosphere are generally quite cold
(e.g., -50 ºC), whereas clouds very near the Earth’s surface are generally quite warm (e.g., +5 ºC). Likewise, land may be even warmer than the lower clouds
(e.g., +20 ºC). Those colder clouds emit much less infrared energy than the warmer clouds and the land emits more than those warm clouds.
The data measured by satellite is calibrated and colorized according to the temperature. If the temperature of the atmosphere decreases with height (which is typical), cloud-top temperature can be used to roughly determine which clouds are high-level and which are low-level.
When clouds are present, the temperature displayed on the IR images is that of the tops of clouds. When clouds are not present, the temperature is that of the ground or the ocean. A major advantage of the IR channel is that it can sense energy at night, so this imagery is available 24 hours per day.
Figure 3-29. Infrared (Color) Satellite Image—U.S. Example
The scale is in degrees Celsius. Blue/purple colors indicate colder temperatures, while orange/red colors indicate warmer temperatures.
11/14/16 Aviation Weather Services AC 00-45H 3-60 Figure 3-30. Unenhanced Infrared (Black and White) Satellite Image—U.S. Example
The scale is in degrees Celsius. Lighter gray shades indicate colder temperatures, while darker gray shades indicate warmer temperatures.
3.4.2.2.1 Infrared Image Data Legends.
The data legend (see Figure 3-31, Infrared (Color) Satellite Image Data Legend, and Figure 3-32, Unenhanced Infrared (Black and White) Satellite Image Data Legend) on an IR image is calibrated to temperature expressed in degrees Celsius. The legend may vary based on the satellite image provider.
Figure 3-31. Infrared (Color) Satellite Image Data Legend
The colors (values) represent temperature in degrees Celsius.
Figure 3-32. Unenhanced Infrared (Black and White) Satellite Image Data Legend
The gray shades (values) represent temperature in degrees Celsius.
3.4.2.3 Water Vapor Imagery.
The water vapor imagery (see Figure 3-33, Water Vapor Satellite Image— U.S. Example) displays the quantity of water vapor generally located in the middle and upper troposphere within the layer between 700 mb (approximately 10,000 ft MSL) and 200 mb (approximately flight level (FL)
390). The actual numbers displayed on the water vapor images correspond to temperature in degrees Celsius. No direct relationship exists between these values and the temperatures of clouds, unlike IR imagery. Water vapor