- 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.
- There are 155 Doppler radar stations in the U.S.
- The antenna 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.
- 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.
- The amount of energy returned directly back to the radar after striking a target is called backscattered energy
- Targets include precip, birds, dust, insects buildings, air mass boundaries, terrain
- Reflectivity is a measurement of backscattered energy
- An Echo is what shows up on the display
- This display is usually enhanced using computer software
- The WSR-88D has a peak power output of 750 kilowatts.
- Wavelength of 10cm
- 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.
- Wavelength of 3cm
How Radar Works
- Basic radar emits a radio wave and measures the reflection
- The energy is scattered in all directions
- Using ranging calculations it is possible to detect the distance from the radar antenna
- Older radar transmitted and received energy simultaneously
How Doppler Radar Works
- Doppler uses a pulse method, listening for returns between pulses
- By keeping track of the phase shift of the returning signal, the radar can compute speed to and from the antenna
- Positive shift indicates motion toward the antenna while negative shift indicates motion away
- Incredibly the pulse lasts .00000157 sec with a .00099843 sec listening period
- This amounts to the radar transmitting 7 sec/hour the remaining 59 min and 53 sec listening
- Attenuation is any process which reduces energy within the radar beam.
- This reduces the amount of backscattered energy.
- Precipitation attenuation is the decrease of the intensity of energy within the radar beam due to absorption or scattering of the energy from precipitation particles.
- 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
- 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)
- Beam resolution is the ability of the radar to identify targets separately at the same range, but different azimuths
- 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.
- 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.
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.
- 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
- 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.
- 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.
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.
Intensity of Liquid Precipitation
- The most significant factor in determining the reflectivity of liquid particles is the size of the precipitation particle.
- Larger particles have greater reflectivity than smaller particles.
- For example, a particle with a 1/4-inch diameter backscatters the same amount of energy as 64 particles that each have a 1/8-inch diameter.
- Intensity of Liquid Precipitation
- Radar images/intensity scales are associated with reflectivities that are measured in decibels of Z (dBZ).
- The dBZ values increase based on the strength of the return signal from targets in the atmosphere.
Intensity of Liquid Precipitation
- Typically, liquid precipitation-sized particle reflectivities are associated with values that are 15 dBZ or greater.
- Values less than 15 dBZ are typically associated with liquid cloud-sized particles.
- However, these lower values can also be associated with dust, pollen, insects, or other small particles in the atmosphere.
Intensity of Snow
- A radar image cannot reliably be used to determine the intensity of snowfall.
- However, in general, snowfall rates generally increase with increasing reflectivity.
- Bright band is a distinct feature observed by radar that denotes the freezing (melting) level
- The term originates from a band of enhanced reflectivity that can result when a radar antenna scans through precipitation.
- The freezing level in a cloud contains ice particles that are coated with liquid water. These particles reflect significantly more energy (appearing to the radar as large raindrops) than the portions of the cloud above and below the freezing layer.
- Latitude of 23.5º N to 23.5º S
- Intertropical convergence zone – where the northeast and southeast trades converge (near the equator)
- Lots of moisture to great heights
- As a result severe thunderstorms are possible if air is unstable
SUBTROPICAL HIGH PRESSURE BELTS
- Semi-permanent highs occur over the water
- generally good weather
- Lows form over the land because of temp. differential
- land is generally warmer
- Commonly an inversion is set up under the subtropical high
- The inversion is strongest when the east side of the high is over the west edge of a continent
- Dry weather – California
- The inversion is weakest and highest when the west side of the high is over the east edge of the continent
- Wet and thunderstormy in Florida
TRADE WIND BELTS
- Northeasterly in the Northern hemisphere
- Southeasterly in the Southern hemisphere
- Flying wx is generally good over the ocean (uniform temp)
- Where they blow from land to sea generally dry terrain
- Where they blow from sea to land generally wet terrain
MONSOON, THE MOTHER OF ALL SEA BREEZES
- India gets average of 400 inches of rain a year from monsoons
- June to October is the monsoon season
- In summer, wind blows inland (wet air)
- In winter, wind blows seaward (dry air)
- This happens when the air mass has been modified to conditions similar to the present mass it replaces
- All that is left is a front line or difference in wind direction
- This may happen to polar front in the U.S.
- May influence storms in the Atlantic or Gulf of Mexico
- AKA Tropical Upper Tropospheric Trough (TUTT)
- Spreads extensive clouds to t he east of the trough line
- Lots of rain 460” of rain on Mt Waialeale Hawaii
- Forms on the southeast side of the subtropical high
- Travel from east to west
- Preceded by good wx
- Followed by cloudiness in a North South line
- Any low that is born in the tropics
- Tropical depression up to 34 kts
- Tropical storm 35 – 64 kts
- Hurricane or Typhoon above 65 kts
- Super typhoon winds 130 kts or greater
- Low level convergence, high level divergence
- Sets up a chimney effect
- Large quantities of water release lots of latent hear furthering the updrafts
- Rise in temp. lowers surface pressure which increases the low level convergence
- Generally move in a west/northwest path
- Saffir-Simpson Hurricane Scale