Aviation Weather Services, Advisory Circular 00-45G, Change 1 (July 2010)
BASE REFLECTIVITY
KTLX - OKLAHOMA CITY, OK ANOMALOUS 05/26/2000 12:15:59 GMT PROPAGATION 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
(12) 60
(11) 55
(10) 50
(9) 45
(7) 35
(6) 30
(5) 25
(4) 20 ПОАА (2) 10
(1) 5 Figure 4-11. WSR-88D Weather Radar Anomalous Propagation (AP) Example
4.1.4.2 Composite Reflectivity Composite reflectivity is the maximum echo intensity (reflectivity) detected within a column of the atmosphere above a location. The radar scans through all of the elevation slices to determine the highest dBZ value in the vertical column (Figure 4-12) 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.
THE VALUE DISPLAYED ON THE COMPOSITE REFLECTIVITY PRODUCT IS:
Figure 4-12. Creation of a Composite Reflectivity product Section 4: Radar and Satellite Imagery Page 4-10
Aviation Weather Services, Advisory Circular 00-45G, Change 1 (July 2010)
Figure 4-13. WSR-88D Weather Radar Short Range (124 NM) Composite Reflectivity Product Example
4.1.4.2.1 Composite Reflectivity Use The primary use of the Composite Reflectivity product, which offers the highest reflectivity value in a vertical column, is to determine the vertical structure of the precipitation. The image must be compared with the Base Reflectivity image to determine the vertical structure of the precipitation. Figure 4-14 includes the 0.5º Base Reflectivity and Composite Reflectivity images for the same location and period of time.
In Figure 4-14, within location A, the intensity of the echoes is higher on the Composite Reflectivity image. Also, within area B, many more echoes present on the Composite Reflectivity. Since the Composite Reflectivity product displays the highest reflectivity of all elevation scans, it is detecting these higher reflectivities at some higher altitude/elevation than the Base Reflectivity product, which is sampling closer to the ground. This often occurs when precipitation and especially thunderstorms are developing.
Section 4: Radar and Satellite Imagery Page 4-11
Aviation Weather Services, Advisory Circular 00-45G, Change 1 (July 2010) Base Reflectivity Composite Reflectivity
Figure 4-14. WSR-88D Weather Radar 0.5º Base Reflectivity versus Composite Reflectivity Comparison
4.1.4.2.2 Strengths of Composite Reflectivity The primary strength of the Composite Reflectivity product is its three-dimensional view of reflectivity. The method used to determine this three-dimensional view is described in section
4.1.4.2.1.
4.1.4.2.3 Limitations of Composite Reflectivity Limitations associated with the Composite Reflectivity product include:
The radar beam may overshoot targets, and The image may be contaminated by:
o Beam blockage o Ground clutter o Anomalous Propagation (AP) and o Ghosts.
4.1.4.2.3.1 Radar Beam Overshooting Radar beam overshooting may occur because the lowest base reflectivity tilt (0.5) 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 radar beam is at an altitude of approximately 18,000 feet above the radar. Any precipitation with tops below this altitude and distance will not be displayed on the image. Therefore, it is quite possible that precipitation may be occurring where none appears on the radar image.
4.1.4.2.3.2 Beam Blockage Beam blockage (Figure 4-6) occurs when the radar beam is blocked by terrain and is particularly predominant in mountainous terrain. This impacts both the Composite Reflectivity and Base Reflectivity images.
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 Section 4: Radar and Satellite Imagery Page 4-12
Aviation Weather Services, Advisory Circular 00-45G, Change 1 (July 2010) beam blockage area will remain clear of echoes even as precipitation and other targets pass through.
4.1.4.2.3.3 Ground Clutter Ground clutter (Figure 4-8) is radar echoes returns from trees, buildings, or other objects on the ground. It appears as a roughly circular region of high reflectivities 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.
4.1.4.2.3.4 Ghost A Ghost (Figure 4-9) is a diffused echo in apparently clear air that is 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 superrefraction 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.
4.1.4.2.3.5 Angels Angels are echoes caused by a physical phenomenon not discernible by the eye at the radar site. They are usually causes by bats, birds or insects. Angels typically appear as a donutshaped echo with low reflectivity values (Figure 4-10). When animating, the echo expends and becomes more diffuse with time.
Angels typically only appear only 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 take flight from caves.
4.1.4.2.3.6 Anomalous Propagation (AP) Anomalous propagation (AP) (Figure 4-11) is an en 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 to two is determined by animating images. Thunderstorms move with a smooth, continuous motion while AP appears to “bloom up” and dissipate randomly.
4.1.5 Radar Mosaics 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 at the National Weather Service (NWS) Doppler Radar Images web site: http://radar.weather.gov/ridge/ The mosaics are located toward the bottom of the page.
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Aviation Weather Services, Advisory Circular 00-45G, Change 1 (July 2010)
4.1.5.1 0.5º Mosaics – Contiguous U.S. and Hawaii
The NWS produces a set of regional and national mosaics (Table 4-2) in the contiguous U.S.
using the 124 NM 0.5º Base Reflectivity product (Figure 4-15).
Table 4-2. NWS Radar Mosaic Products Pacific Northwest Pacific Southwest Upper Mississippi Valley Southern Mississippi Valley Northeast Southeast Southern Rockies Northern Rockies Southern Plains Great Lakes Low Resolution National High Resolution National
Figure 4-15. Great Lakes Regional Radar Mosaic Example
The most recent image from single site radars is used to create the product. Single site data older than 15 minutes from the current time of the product are excluded from the image.
Therefore, data on the mosaics will be no greater than 15 minutes old. Where radar coverage overlaps, the highest dBZ value will be plotted on the image.
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Aviation Weather Services, Advisory Circular 00-45G, Change 1 (July 2010)
4.1.5.2 0.5º Mosaics – Alaska The Alaskan mosaic (Figure 4-16) differs from the contiguous product in only one way: it is created using the 248 NM 0.5º Base Reflectivity single site product.
Figure 4-16. Alaskan Regional Radar Mosaic Example
The long range Base Reflectivity product is used because the radar sites are located at greater distances from each other. Even with the use of the long range product, many areas of Alaskan do not have radar coverage. These areas are shaded gray on Figure 4-16.
Section 4: Radar and Satellite Imagery Page 4-15
Aviation Weather Services, Advisory Circular 00-45G, Change 1 (July 2010)
4.2 Satellite
4.2.1 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.
GOES satellite imagery can be found on the NWS Aviation Digital Data Service (ADDS) website at: http://adds.aviationweather.noaa.gov/satellite/. Additional satellite imagery for Alaska can be found on the Alaska Aviation Weather Unit (AAWU) website at:
http://aawu.arh.noaa.gov/sat.php
4.2.2 Imagery Types Three types of satellite imagery are commonly used: Visible, infrared (IR), and water vapor.
Visible imagery is available only during daylight hours. IR and water vapor imagery are available day or night.
4.2.2.1 Visible Imagery Visible imagery (Figures 4-17 and 4-18) displays reflected sunlight from the Earth’s surface, clouds, and particulate matter in the atmosphere. These images are simply black and white pictures of the Earth from space. During daylight hours, visible imagery is the most widely used image type because it has the highest resolution and approximates what is seen with the human eye.
Gray shades displayed on visible imagery can be correlated with particular features.
Assuming a high sun angle, thick clouds and snow will appear white, thin clouds will appear translucent gray, land appears gray, and deep bodies of water such as lakes and oceans will appear black.
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Aviation Weather Services, Advisory Circular 00-45G, Change 1 (July 2010)
Figure 4-17. Visible Satellite Image – U.S. Example Section 4: Radar and Satellite Imagery Page 4-17
Aviation Weather Services, Advisory Circular 00-45G, Change 1 (July 2010) 701 UTC Tue 03 May 2005 Visible Satellite http://adds.aviationweather.go 9 12 15 18 21 24 27 30 33 36 39 45 48 51 57 60 63 66 69 72 25 28 81 84 87 00 03 Figure 4-18. Visible Satellite Image - Regional-Scale Example
4.2.2.1.1 Visible Image Data Legend The data legend (Figure 4-19) on a visible image displays albedo, or reflectance, expressed as a percentage. For example, an albedo of 72 means 72 percent of the sunlight which struck a feature was reflected back to space.
6 9 12 15 18 21 24 27 30 33 36 39 42 45 48 51 54 57 60 63 66 69 72 05 78 81 84 87 00 03 Figure 4-19. Visible Satellite Data Legend The gray shades (values) represent albedo or reflectance expressed as a percentage.
4.2.2.2 Infrared (IR) Imagery Infrared (IR) images (Figure 4-20 through 4-23) display temperatures of the Earth's surface, clouds, and particulate matter. Generally speaking, the warmer an object, the more infrared energy it emits. The satellite sensor measures this energy and calibrates it to temperature using a very simple physical relationship.
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Aviation Weather Services, Advisory Circular 00-45G, Change 1 (July 2010) Clouds that are very high in the atmosphere are generally quite cold (perhaps -50°C) whereas clouds very near the earth's surface are generally quite warm (perhaps +5°C). Likewise, land may be even warmer than the lower clouds (perhaps +20°C). 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 infrared images is that of the tops of clouds. When clouds are not present, the temperature is that of the ground or the ocean.
Figure 4-20. 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.
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Aviation Weather Services, Advisory Circular 00-45G, Change 1 (July 2010) 101 UTC Tue 03 May 2005 -68 -64 -60 -56 552 48 40 540 536 532 528 521 520 516 512 8 01 0 0 8 12 16 20 24 28 Figure 4-21. Infrared (Color) Satellite Image - Regional-Scale Example The scale is in degrees Celsius. Blue/purple colors indicate colder temperatures, while orange/red colors indicate warmer temperatures.
150 58 51 70 0 52 90 6 50 51 020 20 23 0 Figure 4-22. 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.
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Aviation Weather Services, Advisory Circular 00-45G, Change 1 (July 2010)
Figure 4-23. Unenhanced Infrared (Black and White) Satellite Image – Regional- Scale Example The scale is in degrees Celsius. Lighter gray shades indicate colder temperatures, while darker gray shades indicate warmer temperatures.
4.2.2.2.1 Infrared Image Data Legends The data legend (Figure 4-24 and Figure 4-25) on an infrared image is calibrated to temperature expressed in degrees Celsius. The legend may vary based on the satellite image provider.
Figure 4-24. Infrared (Color) Satellite Image Data Legend The colors (values) represent temperature in degrees Celsius.
Figure 4-25. Unenhanced Infrared (Black and White) Satellite Image Data Legend The gray shades (values) represent temperature in degrees Celsius.
4.2.3.3 Water Vapor Imagery The water vapor imagery (Figure 4-26 and Figure 4-27) displays the quantity of water vapor generally located in the middle and upper troposphere within the layer between 700 millibars (FL100) to 200 millibars (FL390). 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 imagery does not really "see" clouds but "sees" high-level water vapor instead.
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