19-9 Supercooled water freezes on impact with an aircraft. Clear icing can occur at any altitude above the freezing level, but at high levels, icing may be rime or mixed rime and clear. The abundance of supercooled water makes clear icing very rapid between 0 °C and -15 °C, and encounters can be frequent in a cluster of cells. Thunderstorm icing can be extremely hazardous.
19.6.6 Hail. Hail is precipitation in the form of balls or other irregular lumps of ice produced by thunderstorms. Thunderstorms that are characterized by strong updrafts, large supercooled liquid water contents, large cloud-drop sizes, and great vertical height are favorable to hail formation.
An individual unit of hail is called a hailstone. Hailstones can range in size from a pea
(0.25 inch diameter) to larger than a softball (4.5 inch diameter). Hailstones that are 3/4 inch in diameter and larger can cause significant damage to aircraft and make it difficult to control. A hailstone was collected at Vivian, South Dakota, on July 23, 2010, that measured 8 inches in diameter, 18.62 inches in circumference, and weighed 1.93 pounds (see Figure 19-10).
Figure 19-10. Vivian, South Dakota Record Hailstone
Hail is most frequently found in the interior of continents within the mid-latitudes and generally confined to higher elevations within the tropics. In the United States, hail is most common across the Great Plains region east of the Rocky Mountains. Hail is more common aloft and at higher elevations because the stones begin to melt when they fall below the freezing level, and the smaller stones may melt into raindrops before they reach the surface.
19-10 When viewed from the air, it is evident that hail falls in paths known as hail swaths. They can range in size from a few acres to an area 10 miles wide and 100 miles long. Piles of hail in hail swaths have been deep enough to require a snow plow to remove them, and occasionally hail drifts have been reported.
19.6.7 Rapid Altimeter Changes. Pressure usually falls rapidly with the approach of a thunderstorm, then rises sharply with gust frontal passage and arrival of heavy rain showers in the cold downdraft, falling back to normal as the storm moves away. This cycle of pressure change may occur in as little as 15 minutes.
19.6.8 Static Electricity. Static electricity, a steady, high level of noise in radio receivers, is caused by intense corona discharges from sharp metallic points and edges of flying aircraft. It is encountered often in the vicinity of thunderstorms. When an aircraft flies through clouds, precipitation, or a concentration of solid particles (e.g., ice, sand, or dust), it accumulates a charge of static electricity. The electricity discharges onto a nearby surface or into the air, causing a noisy disturbance at lower frequencies.
The corona discharge is weakly luminous and may be seen at night. Although it has a rather eerie appearance, it is harmless. It was named “St. Elmo’s Fire” by Mediterranean sailors, who saw the brushy discharge at the top of ship masts.
19.6.9 Tornado. A tornado is a violently rotating column of air in contact with the ground, either pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud. When tornadoes do occur without any visible funnel cloud, debris at the surface is usually the indication of the existence of an intense circulation in contact with the ground. A waterspout is any tornado occurring over a body of water.
Tornadoes can occur almost anywhere in the world, but are most common in the central and eastern United States during spring afternoons and evenings. They typically last only a few minutes and travel a few miles, but can persist for more than 90 minutes and track more than 100 miles in extreme cases.
On a local scale, the tornado is the most intense of all atmospheric circulations. Its vortex is typically a few hundred yards in diameter, but can range in width from less than 10 yards to over 2 miles. Wind speeds are typically estimated on the basis of wind damage using the Enhanced Fujita Scale (Enhanced F Scale) (see Table 19-1).
19-11 Table 19-1. Enhanced Fujita Scale (Enhanced F Scale) for Tornado Damage EF-Rating Class 3 Second Wind Gust Description Relative Frequency mph km/h EF-0 Weak 65-85 105-137 Gale 53.5% EF-1 Weak 86-110 138-177 Weak 31.6% EF-2 Strong 111-135 178-217 Strong 10.7% EF-3 Strong 136-165 218-266 Severe 3.4% EF-4 Violent 166-200 267-322 Devastating 0.7% EF-5 Violent >200 >322 Incredible <0.1% Note: The Enhanced F Scale is a set of wind estimates (not measurements) based on damage. The 3 second gust is not the same wind as in METAR/SPECI surface observations.
Note: Confirmed tornadoes with no reported damage (i.e., those that remain in open fields) are always rated EF-0.
Over 80 percent of all U.S. tornadoes are produced by supercell thunderstorms. Multiple tornado occurrences associated with a particular large-scale weather system is termed a tornado outbreak. On rare occasions, one supercell can produce multiple tornadoes over many hours.
An aircraft entering a tornado is almost certain to suffer structural damage. The tornado vortex has been documented to extend to the cloud top in violent tornadoes.
20-1 CHAPTER 20. WEATHER RADAR
20.1 Principles of Weather Radar. The most effective tool to detect precipitation is radar.
Radar, which stands for Radio Detection and Ranging, has been utilized to detect precipitation since the 1940s. Radar enhancements have enabled more precision in detecting and displaying precipitation.
The radar used by the National Weather Service (NWS) is called the Weather Surveillance Radar-1988 Doppler (WSR-88D). The prototype radar was built in 1988.
It is essential to understand some principles of weather radar. This will allow you to correctly interpret WSR-88D images. This section will also include a comparison between some WSR-88D principles and aircraft radar principles. These comparisons will help explain the strengths and limitations of the WSR-88D and aircraft radar.
20.1.1 Antenna. The antenna (see Figure 20-1) alternately emits and receives radio waves into the atmosphere. Pulses of energy from the radio waves may strike a target. If it does, part of that energy will return to the antenna.
Figure 20-1. Radar Antenna
The shape of an antenna determines the shape of a beam. The WSR-88D has a parabolic-shaped antenna. This focuses the radio waves into a narrow, coned-shaped beam. The antenna can be tilted to scan many altitudes of the atmosphere.
20.1.2 Backscattered Energy. The amount of energy returned directly back to the radar after striking a target is called backscattered energy (see Figure 20-2).
20-2 Figure 20-2. Backscattered Energy
Targets may include precipitation, clouds, dust, birds, insects, buildings, air mass boundaries, terrain features, etc. Reflectivity is a measurement of the amount of backscattered energy. An echo is the appearance, on a radar display, of the backscattered energy (i.e., reflectivity).
20.1.3 Power Output. The WSR-88D has a peak power output of 750 kilowatts. This allows for better detection of low reflectivity (small) targets in the atmosphere, such as clouds, dust, insects, etc.
Most aircraft radar have a peak power output of less than 50 kilowatts. Therefore, smaller targets are difficult to detect with aircraft radar.
20.1.4 Wavelengths. The wavelength is the distance between two crests or two troughs within the radio wave emitted from the radar (see Figure 20-3). The WSR-88D has a wavelength of 10 centimeters. Most aircraft radars have a wavelength of 3 centimeters. Although shorter wavelengths are better at detecting smaller targets, they are significantly more attenuated than longer wavelengths.
20-3 Figure 20-3. Wavelengths
20.1.5 Attenuation. Attenuation is any process which reduces energy within the radar beam.
This reduces the amount of backscattered energy.
20.1.5.1 Precipitation Attenuation. Precipitation attenuation (see Figure 20-4) is the decrease of the intensity of energy within the radar beam due to absorption or scattering of the energy from precipitation particles.
Figure 20-4. Precipitation Attenuation
Precipitation close to the radar absorbs and scatters energy within the radar beam. Therefore, very little, if any, energy will reach targets beyond the initial area of precipitation. Because of precipitation attenuation, distant targets
(i.e., precipitation) may not be displayed on a radar image.
The amount of precipitation attenuation is related to the wavelength of the radar (see Figure 20-5).
20-4 Figure 20-5. Precipitation Attenuation Versus Wavelength
As the wavelength of the radar decreases, the amount of precipitation attenuation increases.
The WSR-88D’s 10-centimeter wavelength is not significantly attenuated by precipitation. However, aircraft radars, which typically have 3-centimeter wavelengths, have a significant precipitation attenuation problem. As a result, aircraft weather radar typically only shows the leading edge of extreme intensity echoes.
20.1.5.2 Range Attenuation. Range attenuation is the decrease of the intensity of energy within the radar beam as the beam gets farther away from the antenna. If not compensated for, a target that is farther away from the radar will appear less intense than an identical target closer to the radar.
Range attenuation is automatically compensated for by the WSR-88D.
However, most airborne radars only compensate for range attenuation out to a distance of 50 to 75 nautical miles (NM). Targets beyond these ranges will appear less intense than they actually are.
20.1.6 Resolution. Resolution is the ability of the radar to show targets separately.
20.1.6.1 Beam Resolution. Beam resolution is the ability of the radar to identify targets separately at the same range, but different azimuths (see Figure 20-6).
20-5 Figure 20-6. Beam Resolution
Two targets must be separated by at least one beam width (diameter) in order to be displayed as two separate echoes on a radar image.
The WSR-88D has a beam width of 0.95°. Therefore, at a range of 60 NM, targets separated by at least 1 NM will be displayed separately. At a range of 120 NM, targets separated by at least 2 NM will be displayed separately.
Aircraft radar have beam widths that vary between 3 and 10°. Assuming an average beam width of 5° at a range of 60 NM, targets separated by at least
5.5 NM will be displayed separately. At a range of 120 NM, targets separated by at least 10 NM will be displayed separately.
The beam resolution is better for the WSR-88D than aircraft radar (see Figure 20-7).
20-6 Figure 20-7. Beam Resolution Comparison Between WSR-88D and Aircraft Weather Radar
In the example above, the targets (thunderstorms) are at the same range in azimuths for both the aircraft and WSR-88D radar. At 10 NM, the beam width is small enough for both the WSR-88D and aircraft radar to display the thunderstorms separately. At 60 NM, the WSR-88D beam width is still small enough to display both thunderstorms separately. However, the aircraft radar beam width is larger, which results in the two thunderstorms being displayed as one echo.
Note that the beam becomes wider at greater distances from the radar.
Therefore, the beam resolution decreases with increasing range from the radar.
As a result, lines of precipitation may appear to break up as they move closer to the radar. In reality, the breaks in the precipitation were most likely always there.
20-7
20.1.7 Wave Propagation. Radar beams do not travel in a straight line. The beam is bent due to differences in atmospheric density. These density differences, caused by variations in temperature, moisture, and pressure, occur in both the vertical and horizontal directions, and affect the speed and direction of the radar beam.
In a denser atmosphere, the beam travels slower. Conversely, in the less dense atmosphere, the beam travels faster. Changes in density can occur over very small distances, so it is common for the beam to be in areas of different densities at the same time as it gets larger. The beam will bend in the direction of the slower portion of the wave.
20.1.7.1 Normal (Standard) Refraction. Under normal (i.e., standard) conditions, the atmosphere’s density gradually decreases with increasing height. As a result, the upper portion of a radar beam travels faster than the lower portion of the beam. This causes the beam to bend downward (see Figure 20-8).
Figure 20-8. Normal Refraction
The radar beam curvature is less than the curvature of the Earth. Therefore, the height of the radar beam above the Earth’s surface increases with an increasing range.
20.1.7.2 Subrefraction. Atmospheric conditions are never normal or standard.
Sometimes, the density of the atmosphere decreases with height at a more than normal rate. When this occurs, the radar beam bends less than normal.
This phenomenon is known as subrefraction (see Figure 20-9).
20-8 Figure 20-9. Subrefraction
Subrefraction may cause the radar beam to overshoot objects that would normally be detected. For example, distant thunderstorms may not be detected with subrefraction. Subrefraction may also cause radar to underestimate the true strength of a thunderstorm. Thunderstorms may appear weaker on radar because subrefraction causes the radar beam to strike the thunderstorm near the top of the cumulonimbus cloud, where the precipitation particles tend to be smaller.
20.1.7.3 Superrefraction. Conversely, sometimes the density of the atmosphere decreases with height at a less than normal rate, or even increases with height.
When this occurs, the radar beam will bend more than normal. This phenomenon is called superrefraction (see Figure 20-10).
Figure 20-10. Superrefraction
20-9 Superrefraction causes the radar beam to travel closer to the Earth’s surface than what would occur in a normal atmosphere. This can lead to overestimating the strength of a thunderstorm, as the beam would detect the stronger core of the storm, where precipitation-sized particles are larger.
If the atmospheric condition that causes superrefraction bends the beam equal to, or greater than, the Earth’s curvature then a condition called ducting, or trapping, occurs (see Figure 20-11).
Figure 20-11. Ducting
When ducting occurs, the radar beam will hit the surface of the Earth, causing some of the beam’s energy to backscatter. This often leads to false echoes, also known as anomalous propagation (AP), to appear in the radar display.
20.1.8 Intensity of Precipitation. The intensity of precipitation is determined from the amount of energy backscattered by precipitation, also known as reflectivity. Reflectivity is determined by:
• The size of precipitation particles;
• The precipitation state (liquid or solid);
• The concentration of precipitation (particles per volume); and
• The shape of the precipitation.
20.1.8.1 Intensity of Liquid Precipitation. The most significant factor in determining the reflectivity of liquid particles is the size of the precipitation particle (see Figure 20-12).