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3.2.1.12.1 Windshear.
Ten kts or more fluctuations in wind speed (+/-10KTS), within 2,000 ft of the surface, require an Urgent (UUA) pilot report. When LLWS is entered in a pilot report, LLWS is entered as the first remark in the remarks (/RM) group.
Example:
/RM LLWS +/-15 KT SFC-008 DURC RY22 JFK Remarks, LLWS, air speed fluctuations of plus or minus 15 kts, surface to 800 ft during climb, Runway 22, John F.
Kennedy International Airport, NY.
3.2.1.12.2 FUNNEL CLOUD, TORNADO, and WATERSPOUT.
Funnel cloud, tornado, and waterspout are entered with the direction of movement, when reported.
Example:
/RM TORNADO W MOV E Remarks, tornado west moving east
3.2.1.12.3 Thunderstorm.
Thunderstorm coverage is coded as isolated (ISOL), few (FEW), scattered (SCT), numerous (NMRS) followed by description as line (LN), broken line (BKN LN), solid line (SLD LN), when reported. This is followed with TS, the location and movement, and the type of lightning, when reported.
Example:
/RM NMRS TS S MOV E Remarks, numerous thunderstorms south moving east
3.2.1.12.4 Lightning.
Lightning frequency is coded as occasional (OCNL) or frequent (FRQ), followed by type as lightning in cloud (LTGIC), lightning cloud to cloud (LTGCC), lightning cloud to ground (LTGCG), lightning cloud to air (LTGCA), or combinations, when reported.
Example:
/RM OCNL LTGICCG Remarks, occasional lighting in cloud, cloud to ground
3.2.1.12.5 Electrical Discharge.
For an electrical discharge, DISCHARGE is coded followed by the altitude.
3-35 Example:
/RM DISCHARGE 120 Remarks, discharge, 12,000 ft MSL
3.2.1.12.6 Clouds.
Remarks are used when clouds can be seen but were not encountered and reported in the sky condition group (/SK).
Examples:
/RM CB E MOV N Remarks, cumulonimbus east moving north /RM OVC BLO Remarks, overcast below
3.2.1.12.7 Plain Language.
If specific phraseology is not adequate, plain language is used to describe the phenomena or local geographic locations. Remarks that do not fit in other groups like during climb (DURC), during descent (DURD), reach cruising altitude (RCA), top of climb (TOP or TOC), are included.
Examples:
/RM DONNER SUMMIT PASS
3.2.1.12.8 Volcanic Eruptions.
Volcanic ash alone is an Urgent PIREP. A report of volcanic activity includes as much information as possible including the name of the mountain, ash cloud and movement, height of the top and bottom of the ash, etc., is included.
Example:
/UUA/OV ANC240075/TM 2110/FL370/TP DC10/WX VA/RM VOLCANIC ERUPTION 2008Z MT AUGUSTINE ASH 40S MOV SSE Urgent Pilot Weather Report, 240 degrees at 75 NM from Anchorage International Airport, AK, 2110 UTC, flight level 370, a DC10 reported volcanic ash, remarks, volcanic eruption occurred at 2008 UTC Mount Augustine, ash 40 NM south moving south-southeast.
3.2.1.12.9 SKYSPOTTER.
The SKYSPOTTER program is a result of a recommendation from the Safer Skies FAA/Industry Joint Safety Analysis and Implementation Teams. The term SKYSPOTTER indicates a pilot has received specialized training in observing and reporting in-flight weather phenomenon or PIREPs.
When a PIREP is received from a pilot identifying themselves as a SKYSPOTTER aircraft, the additional comment “/AWC” is added at the end of the remarks section of the PIREP.
3-36 An AWC-WEB/xxxx in the remarks indicates the pilot report was submitted by an airline dispatcher or Center Weather Service Unit (CWSU) meteorologist directly to the Aviation Weather Center (AWC). The xxxx represents the airline abbreviation or Air Route Traffic Control Center (ARTCC) of the CWSU that submitted the PIREP.
Example:
PIREP TEXT/RM REMARKS/AWC
PIREP TEXT/RM REMARKS/AWC-WEB/KZFW
3.2.2 Air-Reports (AIREP).
AIREPs are messages from an aircraft to a ground station. AIREPs are normally comprised of the aircraft’s position, time, flight level (FL), estimated time of arrival (ETA) over its next reporting point, destination ETA, fuel remaining, and meteorological information. It is beyond the scope of this AC to describe the details of all the elements in the AIREP, but this paragraph will focus on the meteorological information.
The AWC’s website provides AIREPs over the CONUS and portions of the Atlantic and Pacific Oceans.
3.2.2.1 AIREP Types and Content.
There are two types of AIREPs, routine or position report (ARP) and special (ARS). AIREPs can be reported by the pilot but the majority of routine AIREPs are automated and downlinked from the aircraft to a service provider,
e.g., a flight planning company, for processing and forwarding to an airline and the NWS.
The majority of AIREPs report wind and temperature at selected intervals along the flight route, derived from onboard sensors and probes. Some aircraft are equipped with sensors and probes to measure humidity/water vapor, turbulence, and icing data.
The format for the AIREP is governed by ICAO. The AWC’s website includes AIREPs on their PIREP Web page that is formatted for Web display with some weather elements decoded.
3.2.2.2 AIREP Examples.
The following examples are from https://www.aviationweather.gov. The actual airline’s call sign was replaced with a fictitiou s call sign and the Special AIREP was created from the routine report.
Routine AIREP Example:
ARP XXX836 2443N 15516W 2229 F350 M43 315/128 TB LGT
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.