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Clouds and Precipitation 13&14

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CHAPTER 14

PRECIPITATION

  • Precipitation types
  • From the book:
  • Drizzle
  • Rain
  • Snow
  • Snow grains
  • Ice crystals
  • Ice pellets
  • Hail
  • Snow pellets
  • Variations of the list from the book
  • Freezing drizzle
  • Freezing rain
  • Graupel
  • Sleet
  • Ice needles
  • Snow grains

What’s the dif?

  • Rain is larger than drizzle
  • .02 inch or bigger is rain and is widely separated
  • Ice pellets often referred to as sleet are frozen rain droplets
  • Hail is frozen ice balls, graupel is frozen ice balls covered with a layer of water
  • ¼ inch or bigger
  • Snow pellets are actually small hail white in color
  • ¼ inch or smaller
  • Snow grains basically frozen drizzle

The growth process

  • Two growth processes
  • Collision-coalescence
  • Ice crystal process also known as the Bergeron process

Collision/coalescence process

  • Sometimes referred to the warm rain process
  • Warm is relative here as the temp needs to be above -15°C
  • As the droplet falls, air retards the falling drop
  • When the air resistance reaches an amount equal to the pull of gravity it has reached its terminal velocity
  • Larger droplets overtake smaller droplets collide and coalescence is achieved
  • Collision does not always result in coalescence
  • Sometimes they just bounce

Droplet size

  • Just to give you an idea of how big a droplet we are talking about here
  • Normal human hair is about 60 to 80 microns in diameter
  • A micron is a millionth of a meter

Collision/coalescence process

  • It takes about 4,000 feet of cloud to produce droplets big enough to fall
  • In cumulus clouds the largest droplets will be first on the scene because they fall the fastest and have had the most collisions
  • Droplets tend to break apart if larger than 5,000 microns so they seldom get any larger
  • The most important factors in droplet growth are
  • liquid water content (the most important)
  • The range of droplet sizes
  • The cloud thickness
  • The updrafts
  • The electric charge of the droplets and of the electrical field within the cloud

Ice crystal or Bergeron process

  • Extremely important in the middle and high latitudes (like where we live)
  • In this process the temps are well below freezing
  • However water may still exist in temps down to -40°c
  • This is referred to as supercooled water (more in the icing chapter)
  • So we’ve got supercooled water alongside ice crystals
  • Since water escapes easier in a liquid state than frozen, the ice crystals take water molecules from the droplet
  • Saturation vapor pressure just above the droplet is greater than the saturation vapor pressure above the crystal
  • This causes the molecules to diffuse to the crystal
  • This process will continue as long as there is supercooled water available

Ice crystal or Bergeron process

  • Other processes in play here are:
  • Accretion or riming (rime ice)
  • Where ice crystals collide with supercooled droplets and freeze together on contact creating graupel
  • If the graupel splinters it may provide nuclei for other supercooled droplet to freeze and a rapid chain reaction occurs
  • Like in the fall streak cloud example
  • Aggregation
  • This happens when the ice crystals collide and stick to one another

Precipitation types

  • Snow when the temp is freezing all the way to the surface

Precipitation types

  • Ice pellets or sleet occur when snow passes through a warm layer causing partial melting then refreezing

Precipitation types

  • Freezing rain requires a relatively deep layer of above freezing temps on top of a freezing layer
  • The lower freezing layer must be shallow enough to supercool the droplets without freezing them
  • If the lower layer is too thick the result is ice pellets (frozen rain droplets)

Precipitation types

  • Rain occurs when there is a deep layer of above freezing air based at the surface

Chapter 15
Weather Radar

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.
There are 155 Doppler radar stations in the U.S.

Antenna
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.

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.

Backscattered Energy
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

Power Output
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
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.

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.

Attenuation
The amount of precipitation attenuation is related to the wavelength of the radar

Attenuation Lear 35

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)

Resolution
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.

Resolution
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.

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.

Wave Propagation
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.

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

Superrefraction
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.

Ducting
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.

Limitations
Beam Overshoot
Curvature of the earth contributes
Use a mosaic or a closer site

Limitations
Beam Undershoot
Happens at elevations above the base scan close to the station

Limitations
Beam Blockage
May be minimized by looking at the mosaics

Limitations
Ground Clutter
Trees, buildings and stuff
Computer is supposed to eliminate most of this

Limitations
Ghost
Caused by
insects
superrefraction
certain settings used on the radar equipment when the real echo is farther out

Base Reflectivity Limitations
Angels
Bats, Birds, Insects
Usually only happens in clear air mode

Limitations
Anomalous Propagation
One of the biggest enemies of radar
Changes in air density can bend the radar
Computer enhancement and use of multiple sites to defeat AP

Base Reflectivity Limitations
Wind farms
If within 10 NM they cause
Beam blockage
False echoes
High reflectivity values

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.

Intensity of Dragon flies

Bright Band
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.