Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
4.1.3 Echo Intensities
Clear Air Mode Precipitation Mode Figure 4-1. WSR-88D Weather Radar Echo Intensity Legend
The colors on radar images represent different echo reflectivities (intensities) measured in dBZ (decibels of Z). The dBZ values increase based on the strength of the return signal from targets in the atmosphere. Each reflectivity image includes a color scale that represents a correlation between reflectivity value and color on the radar image. Figure 4-1 depicts these correlations for both Clear Air and Precipitation Mode. For Clear Air Mode the scale ranges from -28 to +28 dBZ, for Precipitation Mode the scale ranges from 5 to 75 dBZ. The color on each scale remains the same in both operational modes, only the dBZ values change. The scales also include ND correlated to black which indicates no data was measured.
Reflectivity is correlated to intensity of precipitation. For example, in Precipitation Mode, when the dBZ 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 purposes. Table 4-1 defines this correlation.
Table 4-1. 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 AC 00-45F, CHG 2 Section 4: Radar and Satellite Imagery Page 4-2
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
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 be used to determine the intensity of snowfall. However, snowfall rates generally increase with increasing reflectivity.
4.1.4 Products The NWS produces numerous radar products of interest to the aviation community. The next section will discuss Base Reflectivity and Composite Reflectivity both available through National Weather Service (NWS) Doppler Radar Images web site at: http://radar.weather.gov/ridge/
4.1.4.1 Base Reflectivity Base Reflectivity is a display of both the location and intensity of reflectivity data. Base Reflectivity images encompass several different elevation angles (tilts) of the antenna. The Base Reflectivity image currently available on the ADDS website begins at the lowest tilt angle
(0.5°), more specifically 0.5° above the horizon.
Both a short range (Figure 4-2) and long range (Figure 4-3) image is available from the 0.5º Base Reflectivity product. The maximum range of the short range Base Reflectivity product is 124 NM from the radar location. This view will not display echoes farther than 124 NM from the radar site, although precipitation may be occurring at these greater distances. Other options to view precipitation beyond 124 NM from the radar site include selecting the long-range view which increases coverage out to 248 NM, selecting adjacent radars, or viewing a radar mosaic.
Figure 4-2. WSR-88D Weather Radar Short Range (124 NM) Base Reflectivity Product Example
AC 00-45F, CHG 2 Section 4: Radar and Satellite Imagery Page 4-3
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009) DBZ Radar Image from National Weather Service: KPAH 19:03 UTC 04/29/2005 + Kankakee + Fort Wayne 75 Lamoni Peoria + Logansport + Lima -70 + Kirksville + Champaign.
+ Quinci Springfield talindianapolis -65 + Terre Haute -60 + Bloomington Columbia 55 50 • Evansville Louis ville 45 + Springfield Swing Green romar Blu 35 * pray con -30 lyersbun shville + Crossville 25 + Jackson + Lawrenceburg 20 + Memphis + Chattanboga HOttle Rock Horence « Hunts vine 15 + Pine Blurg!™ • Oxfort Tupela + Rome 10 + Gadsden + Columbus + Birgingham.
Greenville + El Dorado + luscaloosa J IND Radar Image from National Weather Service: KPAH 19:03 UTC 04/29/2005 Figure 4-3. WSR-88D Weather Radar Long Range (248 NM) Base Reflectivity Product Example
4.1.4.1.1 Base Reflectivity Use The Base Reflectivity product can be used to determine the location of precipitation and nonprecipitation echoes, the intensity of liquid precipitation, and the general movement of precipitation when animating the image.
If the echo is precipitation, the product can be used to determine if it is convective or stratiform in nature. Stratiform precipitation (Figure 4-4) has the following characteristics:
• Widespread in areal coverage,
• Weak reflectivity gradients,
• Precipitation intensities are generally light or moderate (39 dBZ or less),
• Occasionally, precipitation intensities can be stronger
• Echo patterns change slowly when animating the image.
AC 00-45F, CHG 2 Section 4: Radar and Satellite Imagery Page 4-4
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
Figure 4-4. WSR-88D Weather Radar Stratiform Precipitation on the 0.5ºBase Reflectivity Product Example
Hazards associated with stratiform precipitation include possible widespread icing above the freezing level, low ceilings and reduced visibilities.
Convective precipitation (Figure 4-5) can be described using the following characteristics:
Figure 4-5. WSR-88D Weather Radar Convective Precipitation on the 0.5ºBase Reflectivity Product Example
y Echoes tend to form as lines or cells, y Reflectivity gradients are strong, AC 00-45F, CHG 2 Section 4: Radar and Satellite Imagery Page 4-5
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009) y Precipitation intensities generally vary from moderate to extreme, − Occasionally precipitation intensities can be light y Echo patterns change rapidly when animating the image
Numerous hazards are associated with convective precipitation. They include: turbulence, lowlevel wind shear, strong and gusty surface winds, icing above the freezing level, hail, lightning, tornadoes and localized IFR conditions with heavy precipitation.
4.1.4.1.2 Strengths of Base Reflectivity The strengths of the Base Reflectivity product include:
• The location of precipitation and non-precipitation echoes is depicted, and
• The intensity and movement of precipitation is relatively easy and straight forward to determine.
4.1.4.1.3 Limitations of Base Reflectivity Limitations associated with the Base 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.
4.1.4.1.3.1 Radar Beam Overshooting Radar beam overshooting may occur because the radar beam (typically the 0.5 degree slice) 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 Base Reflectivity radar beam is at an altitude of approximately 18,000 feet; at 248 NM the beam height is approximately 54,000 feet. Any precipitation with tops below these altitudes and distances will not be displayed on the image. Therefore, it is quite possible that precipitation may be occurring where none appears on the radar image.
4.1.4.1.3.2 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 beam blockage area will remain clear of echoes even as precipitation and other targets pass through.
AC 00-45F, CHG 2 Section 4: Radar and Satellite Imagery Page 4-6
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
Figure 4-6. WSR-88D Weather Radar Beam Blockage on Base Reflectivity Product Example
4.1.4.1.3.3 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.
Figure 4-8. WSR-88D Weather Radar Ground Clutter Example AC 00-45F, CHG 2 Section 4: Radar and Satellite Imagery Page 4-7
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
4.1.4.1.3.4 Ghost A Ghost (Figure 4-9) 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 superrefraction during the warm season.
The ghost develops as an area of low reflectivity echoes (typically less than 15dBZ) near the radar site and quickly expands. When animating the imagery, the ghost echo shows little movement.
Figure 4-9. WSR-88D Weather Radar Ghost Example
4.1.4.1.3.5 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 donutshaped echo with low reflectivity values (Figure 4-10). When animated, the echo expands 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 are departing from caves.
AC 00-45F, CHG 2 Section 4: Radar and Satellite Imagery Page 4-8
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
Figure 4-10. WSR-88D Weather Radar Angle Example This angel was caused by bats departing Selman Bat Cave at Alabaster Caverns State Park, Oklahoma around sunset
4.1.4.1.3.6 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.
AC 00-45F, CHG 2 Section 4: Radar and Satellite Imagery Page 4-9
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
BASE REFLECTIWITY
KILX - 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/"CP: Д / 11 ELEV ANGLE: 0.50 ° MAX: 73 dBZ RANGE 248 NM Legend: (Category) dBZ
(16) 75
(14) 70
(13) 85
(12) 60
(11) 55 ПОЛА @C01002 8888990 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 AC 00-45F, CHG 2 Section 4: Radar and Satellite Imagery Page 4-10
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009) The maximum range of the long range Composite Reflectivity product (Figure 4-13) is 248 NM from the radar. The "blocky" appearance of this product is due to its lower spatial resolution as it has one-fourth the resolution of the Base Reflectivity product.
DBZ Radar image trom National Weather Service: КРАН 19:03 UTC 04/29/2005 + Kankakee * Fort Wayne 75 Lamoni Peoria + Logansport + Lima + Kirksville Champaign + Quincy Springfield atindianapolis L+ Tèrre Haute + Bloomington Columbia vandalia, Clinton T7+ Louisville 50 evansville tut 45 40 * Springfield wing Green 35 eat Branson 30
• Crossville 25 + Lawrenceburg Memphis 1+ Chattanooga Little Rock -Horence - Huntsvme + Pine Blurg + Oxford Tupelo + Rome + Gadsden + El Dorado Da Greenville Columbus: + Tuscaloosa J + Bimpingham INDI Radar Image from National Weather Service: КРАН 19:03 UTC 04/29/2005 Figure 4-13. WSR-88D Weather Radar Long Range (248 NM) Composite Reflectivity Product Example For a higher resolution (1.1 x 1.1 NM grid) Composite Reflectivity image, users must select the short range view (Figure 4-14). The image is less "blocky" as compared to the long range image. However, the maximum range is reduced to 124 NM from the radar location.
AC 00-45F, CHG 2 Section 4: Radar and Satellite Imagery Page 4-11
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
Figure 4-14. 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 4-15 includes the 0.5º Base Reflectivity and Composite Reflectivity images for the same location and period of time.
In Figure 4-15, 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.
AC 00-45F, CHG 2 Section 4: Radar and Satellite Imagery Page 4-12
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
Base Reflectivity Composite Reflectivity Figure 4-15. 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 lowtopped 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; at 248 NM the beam height is approximately 54,000 feet. Any precipitation with tops below these altitudes and distances 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.
AC 00-45F, CHG 2 Section 4: Radar and Satellite Imagery Page 4-13