3-37 ARP Routine report XXX836 Aircraft call sign 2423N 15516W Location in latitude and longitude, 24 degrees 23 minutes north, 155 degrees 16 minutes west F350 Flight level or altitude, FL350 M43 Temperature in Celsius, minus 43 ºC 315/128 Wind direction (true) and speed, 315 degrees (true) and 128 kts TB LGT Light turbulence Special AIREP Example:
ARS XXX836 2443N 15516W 2229 F350 M43 315/128 TB SEV ARS Special AIREP Same as the routine example except:
TB SEV Severe turbulence
3.3 Radar Observations.
3.3.1 Description.
Weather radar observations and their resultant images are graphical displays of precipitation and non-precipitation targets detected by weather radars. Weather Surveillance Radar–1988 Doppler (WSR-88D), also known as next generation weather radar (NEXRAD), displays these targets on a variety of products, which can be found on the websites of all NWS Weather Forecast Offices (WFO), the AWC, Storm Predication Center (SPC), and various flight planning and weather service providers.
3.3.2 Issuance.
WSR-88D radars are continuously generating radar observations. Each radar observation, called a volume scan, consists of 5 to 14 separate elevation “tilts,” and takes between 4 and 11 minutes to generate, depending on the radar’s mode of operation. Once one observation is complete, the next one begins. Radar observation times are not standard, nor are they synchronized with other radars. The valid time of the observation is the time assigned to the product, which is the end of the last radar scan.
3.3.2.1 WSR-88D Radar (NEXRAD) Network.
The WSR-88D radar network consists of 160 radars operated by the NWS, FAA, and Department of Defense (DOD). Figure 3-6, Location of WSR-88D Weather Radar in the CONUS and Their Respective Coverage at 4,000 Feet AGL, 6,000 Feet A GL, and 10,000 Feet AGL, and Figure 3-7, Location of
WSR-88D Weather Radar Outside of the CONUS, show the location of the radars.
Figure 3-6. Location of WSR-88D Weather Radar in the CONUS and Their Respective Coverage at 4,000 Feet AGL, 6,000 Feet AGL, and 10,000 Feet AGL VCP12 Coverage 4,000 ft above ground level* 6,000 ft above ground level* 10,000 ft above ground level* * Bottom of beam height (assuming Standard Atmospheric Refraction).
Terrain blockage indicated where 50% or more of beam blocked.
125 250 500 750 Miles 3-38
3-39 Figure 3-7. Location of WSR-88D Weather Radar Outside of the CONUS
3.3.2.2 Coverage.
Figu re 3-6 depicts the radar coverage over the CONUS at 4,000 ft AGL, 6,000 ft AGL, and 10,000 ft AGL (i.e., above the height of the radar). Several WSR-88D radars are located in mountainous areas, such as the western United States. For example, the radar in southern Utah (near Cedar City) is on top of a 10,000-foot mountain. This means that the coverage begins at 10,000 ft AGL in that area. Any precipitation from low-topped clouds would not be detected by this radar due to overshooting of the radar beam (see paragraph 3.3.7.1).
3.3.3 Modes of Operation.
The WSR-88D employs scanning strategies in which the antenna automatically raises to higher and higher preset angles, or elevation scans, as it rotates. These elevation scans comprise a volume coverage pattern (VCP). Once the radar sweeps through all elevation slices, a volume scan is complete. The WSR-88D radar can use several VCPs.
There are two main classes of VCPs, which are commonly referred to as Clear Air and Precipitation Modes.
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3.3.3.1 Clear Air Mode.
In Clear Air Mode, the radar is in its most sensitive operation. This mode has the slowest antenna rotation rate, which permits the radar to sample the atmosphere longer. This slower sampling increases the radar’s sensitivity and ability to detect smaller objects in the atmosphere. The term “clear air” does not imply “no precipitation” mode. Even in Clear Air Mode, the WSR-88D can detect light, stratiform precipitation (e.g., snow) due to the increased sensitivity.
Many of the radar returns in Clear Air Mode are airborne dust and particulate matter. The WSR-88D images are updated approximately every 10 minutes when operating in this mode.
3.3.3.2 Precipitation Mode.
Precipitation targets typically provide stronger return signals to the radar than non-precipitation targets. Therefore, the WSR-88D is operated in Precipitation Mode when precipitation is present, although some non-precipitation echoes can still be detected in this operating mode.
The faster rotation of the WSR-88D in Precipitation Mode allows images to update at a faster rate approximately every 4 to 6 minutes.
3.3.4 Echo Intensities.
Figure 3-8. WSR-88D (NEXRAD) Weather Radar Echo Intensity Legend
The colors on radar images represent the reflective power of the precipitation target. In general, the amount of radar power received is proportional to the intensity of the precipitation. This reflective power, commonly referred to by meteorologists as
3-41 “ref lectivity,” 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 NW S 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 website.
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 websites of NWS, AWC, all NWS WFOs, as well as commercial aviation weather providers.
3-42 Rad ar mosaics can be assembled from either Composite Reflectivity (see paragraph 3.3.5.2) o r Base Reflectivity (see paragraph 3.3.5.3), depending on the website or data provider. At this time, NWS national (see Figur e 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 Al aska (see Figure 3-11, Alaska Radar Mosiac Example) radar mosaic sectors are assembled using only B ase 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
3-43 Figure 3-10. NWS Regional Radar Mosaic Sector Example, Which Utilizes NEXRAD Base Reflectivity
F igure 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 C omposite 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 w ith 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 (rig ht) 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
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.
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.
Prec ipitation 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 Bas e 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
Figure 3-16. WSR-88D Weather Radar Long-Range (248 NM) Base Reflectivity, Single-Site Product Example DBZ Radar Image from National Weather Service: PAH 19:03 UTC 04/29/2005 + Kankakee Lf+ Fort Wayne 75 Lamoni • Heona + Logansport + Lima + Kirksville Champaign + Quincy Springfield Landian apolis + Terré Haute + Bloomington Columbia Cinton + Louisville evansvilles + Springfield wing Green Poplar Bluff, + Branson ashville Dyersburt + Crossville + Lawrenceburg OMemphis 2+ Chattanboga + Little Rock Forence « Huntsvie V + Pine Blufg *+ Oxford Tupelo + Rome Gadsden + El Dorado * Greenville Columbus + luscaloosa J + Bimingham 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 https://www.aviationweather.gov (see Figure 3-17, Radar Coded Message (RCM) Display), as well as other weather service provider's websites.
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 maximum echo top for each radar's area of coverage. The other tops shown on the display are derived from the satellite images.
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3-48 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 disting uished by the following radar characteristics:
• Echoes tend to form as lines or cells,
• Reflectivity gradients are strong,