|WEATHER RADAR FAQ SECOND SET
METEOROLOGIST JEFF HABY
21. How is the reflectivity gradient important to severe weather?
The reflectivity gradient is defined as how much the value of radar reflectivity changes over distance. If
the reflectivity changes significantly over a small distance then that would be a strong reflectivity
gradient. If the reflectivity changes only slightly over a significant distance then that would be a weak
reflectivity gradient. Determining what is a strong gradient, weak gradient or a gradient that is in-between
takes practice. With a few examples we will show how each look on radar.
First we will look at a commonly used scale for reflectivity values. The scale is shown below:
The green colors represent light reflectivity (light rain aloft), the yellow colors are more of a moderate
reflectivity (moderate rain aloft) and the red colors represent heavy reflectivity (heavy rain and possible
An example of a strong reflectivity gradient is a red color next to no reflectivity while an example of weak
reflectivity would be a gradual transition from green to yellow colors. Below is an example of a weak, moderate
and strong reflectivity gradient.
WEAK REFLECTIVITY GRADIENT
MODERATE REFLECTIVITY GRADIENT
STRONG REFLECTIVITY GRADIENT
The reflectivity gradient is important because it can give clues to if
severe convective wind gusts are occurring.
Severe convective wind gusts are more likely to occur when there is a strong reflectivity gradient. The leading
edge of severe thunderstorms often have an abrupt transition zone of
heavy precipitation and wind. In this
region the outflow from the storm is progressing into the environmental air ahead of the storm. Strong convective
winds will force the reflectivity into an abrupt zone where the reflectivity changes rapidly
over a small distance. Look for severe convective wind gusts when red reflectivity is next to very little
or no reflectivity. This is especially true if the storms are in a line segment that is bowing.
22. What are echo tops and their importance?
An echo top is the radar indicated top of an area of precipitation. Once the precipitation intensity drops
below a threshold value as the radar beam samples higher elevations of a storm or precipitation region
then the echo top is located. The cloud top will often extend above the echo top since clouds are more
difficult to detect by radar.
Echo tops can be used to assess the intensity of a storm. The rule of thumb is that the higher the echo tops
are in a storm then the stronger the
updraft is that produced that storm. A stronger updraft makes
convective wind gusts and
large hail more likely. When there are several storms on radar, the ones with the higher echo
tops may be the most likely ones to produce the most significant
severe weather (convective wind gusts and hail).
23. What is bright banding and how does it occur?
Bright banding occurs due to the higher reflectivities associated with snow that is melting as it
is falling aloft. Ice is a better absorber of radar radiation compared to liquid water. Because of this,
snow will show a lower reflectivity on radar when it has the same moisture content as a rain event. When
the snow is melting however, a film of water forms on the outside of the snowflake. Since snowflakes
can be fairly large, when there is a film of water on the snowflake it has the same reflectivity as a
a giant raindrop or small wet hail.
A radar beam will generally sample a higher elevation as it moves away from the radar site. Because the
melting of the snowflakes occurs within a specific elevation range aloft, there will be a higher
reflectivity as the radar beam moves through this layer. This can produce a circular or arcing band
of higher reflectivity around the radar site on the reflectivity display.
Below are some bright banding examples (more examples will be added over time):
Dallas / Ft. Worth Area Bright Band
24. VIL and updraft strength?
VIL (Vertically Integrated Liquid) is a summation of reflectivity through a vertical column of the troposphere.
VIL is most accurate at the medium ranges from the radar site since the radar is able to sample most of
the vertical column of the storm. If the storm is too close to the radar then part of the storm will be
in the cone of silence and if the storm is too far from the radar then the bottom portion of the storm under
the lowest tilt angle will not be sampled.
The higher an
updraft penetrates through the troposphere then it is more likely significant
moisture has been
funneled and suspended in that vertical column. If it is not apparent on reflectivity which storms have the
strongest updrafts then the VIL can be used to determine which storms are most likely to have the
strongest updrafts. Higher VIL values occur with suspended
heavy rain and precipitation extending through
a deep vertical depth of the troposphere.
25. How does snow look on radar?
Snow often has the following features on radar:
a. A fairly low reflectivity for
dry snow. Wet snow can have a much more significant reflectivity. Determine
the likely water content of the snow along with examining the radar images.
b. The gradient between colors tends to be gradual. Often there are varying shades of the lower
c. It tends to have a grainy or fuzzy appearance. The edges of the precipitation areas may not
have a well defined edge in light snow situations.
Below are some examples of snow on radar (more examples will be added over time):
SNOW ON RADAR
26. What creates a bow echo?
Bow echoes, when they occur, usually occur with a grouping of multicell storms that are
arranged into a
squall line. The upper tropospheric winds steer storms. These winds
help determine the speed and direction that the storms move. The upper tropospheric winds
will not always be constant along a squall line. In the regions these winds are
stronger that portion of the squall line will surge forward. Also, in regions these
winds are drier that portion of the squall line will surge forward because
evaporative cooling creates negative buoyancy that will further accelerate a downdraft toward the surface.
Since the downdraft from a squall line approaches the earth's surface at an angle, the faster the
downdraft winds the faster the storms may migrate forward. Below is an example of a bow echo:
The next example is that of a bow echo with a line-end vortex on the north side of the squall line:
27. What is an inflow notch?
There are two types of inflow notches that will be discussed which are the low level inflow
notch into the
updraft of a storm and the mid-level rear inflow notch into the back side of a storm. The
characteristic that both types of inflow notches have is that there tends to be reduced
reflectivity in the region they occur since the air is either too dry or is moving
too quickly to allow precipitation to develop or fall through this air.
First will be shown a couple of examples of inflow notches into a
supercell. Within the inflow
region is the updraft. There tends to be lower reflectivities in the updraft region and higher
reflectivities in the downdraft region. Within an inflow notch is the low level wind into the
updraft of the storm. A classic supercell will take on a hook like feature and an HP supercell
will take on a kidney bean feature. An example of each is shown below:
A rear inflow notch enters the backside of a storm in the middle levels of the troposphere. These
inflow notches are particularly conducive to
severe weather if they ingest high momentum and
dry air into the storm. If the air is dry it will cool through evaporative cooling. This will
increase the negative buoyancy of the air and it will accelerate toward the earth's surface. This
negative buoyancy acceleration along with the air's initial high momentum can produce severe
convective wind gusts at the leading edge of the storm or storm complex. An example of a storm
complex with a rear inflow notch is shown below:
28. Anvil blowoff on radar
Much of the light precipitation detected on radar does not reach the ground. This is especially true
if there is a dry layer of air between the surface and from where the precipitation is falling. This is
even more especially true if the precipitation falls through a deep dry layer and the precipitation begins
the fall from high aloft. Rain falling from high aloft is very common in the
downwind portion of a
virga are common on the downwind side of a storm. When the radar beam gets high enough it
will detect these thick clouds and virga aloft. This may mislead the radar operator into thinking
precipitation is reaching the ground in those locations when it is not.
Strong winds will shear the top of a thunderstorm. This moves thick cloud, precipitation and virga
downwind from the storm. If this shows on radar as a green color it is likely not reaching the ground.
Keep the following in mind:
1. Anvil blowoff will be especially evident at long ranges from the radar since the radar beam increases
in elevation away from the radar site.
2. Anvil blowoff will generally show as light reflectivity (usually color coded green) and this
reflectivity generally does not result in precipitation reaching the ground.
3. Strong updrafts in a strong
shear environment (strong upper level wind) will often have
the anvil blowoff showing on radar.
4. Anvil blowoff tends to show up best on composite reflectivity since it is using multiple tilt angles and
showing reflectivity from all the different angles.
Here are some example of anvil blowoff from storm(s) on radar:
29. Recognizing flooding potential on radar
Radar is an important nowcasting tool for recognizing flooding potential.
when too much rain falls over a given time period for the ground surface to support. The flooding potential
will be greater when storms move over previously saturated land, snowmelt combines with rainfall or
rain falls over land that has a low permeability. Any of the following seen on radar
can produce flash flooding especially if the land is already saturated:
1. Training thunderstorms- thunderstorms developing and moving over the same areas that previously
2. Very intense slow moving thunderstorms- a single slow moving thunderstorm can produce several inches of rainfall per
3. Consistent rain- rain
(especially heavy rain) falling over an extended period of time.
30. Calculation of radar shear
There are many different types of shear. When it comes to the use of the term shear on
a Doppler radar product what is usually being referred to is the addition and inbound
and outbound winds that are adjacent to each other. For example, if a radar meteorologist
says there are 80 knots of shear it means the winds going toward the radar added to the
winds going away from the radar on two adjacent radar range gates is 80 knots.
Another term that is used in radar meteorology is the rotational velocity. This is found by adding the inbound and
outbound winds and dividing by 2. If the inbound velocity is 30 knots and the outbound velocity is 52 knots, then
the rotational velocity will be 41 knots.
31. Inversions and radar ground clutter
The temperature profile of the troposphere makes a strong contribution to how radar emitted radiation
will refract in the troposphere.
Superrefraction is the beam bending more toward the earth's surface than
in normal tropospheric conditions and subrefraction is the beam bending less toward the earth's surface than
in normal tropospheric conditions.
An inversion is a situation in which the temperature increases with height. Thus it is a situation where there
is colder air under warmer air. An inversion layer is a layer of stability since cold air under warm air is
a stable situation. A common type of inversion is the radiational cooling inversion in which overnight the
earth's air near the surface cools by ground surface longwave radiation emission. The optimum conditions
for a radiation inversion is a dry, clear and long night. Inversions at and near the earth's surface
can also occur due to shallow cold front passages and
evaporative cooling in the
boundary layer. An
inversion promotes superrefraction.
Ground clutter is returns to the radar from radar emitted energy scattering off of objects on and near the
earth's surface. Ground clutter is most evident when low tilt angles are used since the radar energy travels
close to the earth's surface especially at close ranges to the radar. Since a superrefraction situation causes
the radar beam to travel closer to the earth's surface, superrefraction will promote an
increase in ground clutter.
Thus, the combination of a low tilt angle and an inversion at and near the earth's surface promotes an
abundance of ground clutter. Below is an example radar images using the lowest tilt angle (0.5 degrees)
taken in the morning when a radiation inversion was in place.
32. Rmax and Vmax as it relates to the Pulse Repetition Frequency
The Pulse Repetition Frequency (PRF) is the number of radiation pulses emitted by radar in 1 second. For example, if
the radar emits 400 pulses in one second then the PRF is 400 pulses/second. Think of pulses like the pulses
of a strobe light. A strobe light alternates between light and dark and there is light a certain number of
times within a
given period of time. Radar is similar except the number of pulses is much more per second than a strobe light and
radar emits microwave type wavelength radiation. Another difference is that the radar spends less than 1% of the time
emitting radiation and over 99% of the time sensing for returned radiation. Radar can sample the troposphere
very fast because the speed of light is fast (about 300,000,000 meters per second).
Rmax stands for the maximum range the radar can detect. If the radar emitted a pulse of energy and waited as long
as needed for returning radiation then the radar could detect to any range. However, since
the speed of light is so fast compared to the distances we need to measure
returns in the troposphere the radar is not required to wait more
than a tiny fraction of a second for return energy to come back. Thus, the radar can be set to emit and listen
for 100s of pulses per second and we can still measure ranges that cover a broad area. However, the faster the
PRF becomes the smaller of a range that can be detected. If the PRF is set too fast then there is not as much time
to sample the troposphere in one pulse before the next pulse is sent out. Energy returned from one pulse after
another pulse has been sent will be range folded.
Suppose the PRF is 500 pulses per second. The formula for Rmax is C / (2 * PRF). C stands for the speed of light.
With a PRF of 500 pulses/s, the Rmax is = 300,000,000 m/s / (1,000 s^-1) = 300,000 m which is equal to 300 km. Thus
the radar can sample up to 300 km during each pulse. If a return is beyond 300 km then it will be range folded
and will show up at a distance closer to the radar than the return really is because the radar thinks it
is getting returns from a second pulse it already sent out.
Suppose we increase the PRF to 1,200 pulses per second. Rmax then becomes 300,000,000 m/s / (2,400 s^-1) =
125 km. From these two examples you can see that as the PRF increases, then the Rmax becomes a smaller range. This
makes sense because the faster pulses are emitted the less time there is for the pulse to travel and come back
to the radar before the next pulse is emitted. If we want to detect echoes beyond 125 km we will need to
decrease the PRF from 1,200 to a smaller number.
When a reflectivity image is put into motion we can see where the precipitation areas are moving toward and
how fast they are moving. However, we can not see the motions within the precipitation areas very well. To help
with that problem Doppler radar has come along.
Vmax stands for the maximum velocity the radar can detect. Precipitation particles that are sensed are either
moving closer to the radar over time, further from the radar over time or stay the same distance from the radar
over time. It is this motion we want to detect because from it the motions inside rain clouds can be sensed.
The motion of precipitation particles or other particles is determined by the phase shift that occurs from
radar radiation striking the particle. Suppose you throw a ball at a wall that is moving at you and then
throw another ball at the same velocity at a wall that is moving away from you. The ball you threw at the wall
that is moving toward you will rebound faster off the wall back toward you. Thus, the velocity of
the ball changes relative to you
depending on if the wall is moving further or closer to you even if you throw it at the same velocity toward the
wall both times. This principle does not work with light because the
speed of light is a constant. However, the frequency (number of light waves passing a point over time) and
wavelength (distance from beginning to end of each wave) does
change. It is from the phase shift of light that is used by radar to determine whether an object is moving toward or
away from the radar and the magnitude of that motion.
Suppose the PRF is 500 pulses per second using a 0.1 meter wavelength radar. The formula for Vmax = (PRF * wavelength) /
4. With a PRF of 500 pulses/s, the Vmax is = (500 s^-1 * 0.1 m) / 4 = 12.5 m/s. Thus the radar can only sample
motions that are equal to or less than 12.5 m/s. If the actual velocity of an object is 17.5 m/s that echo will
be velocity folded and will have a value of (17.5 - 12.5 = 5 m/s). If we want to detect higher velocities without
them being folded the PRF needs to be increased.
Suppose we increase the PRF to 1,200 pulses per second. Vmax becomes (1,200 s^-1 * 0.1 m) / 4 = 30 m/s. From these
two examples you can see that as the PRF increases, then the Vmax becomes higher. Think of a strobe light once again and
the strobe light shining on a bouncing ball. If the strobe light flickers more quickly (higher PRF) and we watch
it in slow motion then we can predict where the ball will be each time the light shines on it again. However, if the
pulses are longer (PRF decreased) to the point where the light shines again slower than the time it takes the ball
to make one bounce we will not know whether the ball is rising or falling (it has folded and we
can no longer be sure where it will be when the light shines on the ball on the next pulse). The phase shift of
light is smaller the higher the PRF is. As the PRF decreases the phase shift becomes more. Once the phase shift
becomes too much then the velocity will be folded.
33. Recognizing veering and backing wind on radial velocity
Please read the following Haby Hint:
On radial velocity, red and yellow colors represent motion away from the radar and greens and blues
represent motion toward the radar.
As the radar beam moves away from the radar site it will usually increase in altitude as range from the
radar increases. Thus, locations near the radar site will be sampled close to the surface while locations
at the outer range of the radar will be sampled at a much higher elevation.
Radial velocity colors will only be shown where there is hydrometeors or other particles in the troposphere
for the radar beam to scatter off. It is precipitation and thunderstorm areas where the best returns will occur.
The images below are idealized since they show colors across the entire radar sampling area. This is reality
would generally only occur if precipitation was occurring across the entire radar sampling region.
Look at the
image below depicting an idealized
veering wind. The winds near the radar are from the east. We know this
because of the yellow colors immediately to the west of the radar (motion away) and the blue colors
immediately to the east of the radar (motion toward). The white curving line on the image is the zero radial motion.
Within the area in white motion is neither toward or away from the radar but is rather motion that is remaining
equidistant from the radar site at that point in space. A key point to remember is that the winds will
flow perpendicular to this white zero radial velocity line. At the middle ranges from the radar site the
winds are from the southeast and south. We know this is the wind direction here because winds cross
the white radial velocity perpendicularly and motion is from the blue and green colors toward the
yellow and red colors. At the outer ranges of the radar the wind is from the southwest. The outer ranges of
the radar will be the highest in elevation sampled.
Thus, going from the surface to aloft the winds
shift from easterly to southeasterly to southerly to southwesterly. This is a veering wind since the wind
is turning clockwise with height. A veering wind is associated with
warm air advection since low level winds
from a southerly direction will generally transport in warmer air. Remember the initials CVW, where these
letters stand for Clockwise, Veering, Warm Air Advection.
A veering pattern on radial velocity will have an S-shaped pattern. See the veering wind image below and
notice the S-shaped signature made by the white zero radial velocity radial. A student posted a message
for the way to remember that an S-shaped pattern is associated with warm air advection is to remember
that Superman has a warm heart. Superman has an S on the shirt.
The second image below shows a backing wind. The wind is from the east at the surface, then gradually shifts
to the northeast and then to the north at the outer range. A
backing wind will shift counterclockwise with
height. Remember the initials CCBC, where these letters stand for Counter-Clockwise, Backing, Cold Air Advection.
A backing wind pattern will have a backward-S shaped radial velocity pattern.
VEERING WIND WITH HEIGHT
BACKING WIND WITH HEIGHT
34. Wavelength and frequency of light and perspective of observation
In a vacuum light travels at the maximum velocity which is 299,792,458 m/s. Light will travel slower
than this speed and will decelerate when it travels through gases (such as the earth's atmosphere),
liquids (such as the earth's ocean) and land. The denser the object and the longer light has to
travel through the object, the more light will slow down and absorb into the object. If
the substance is dense enough such as land or very deep waters it will absorb the energy completely.
Light has a couple of important properties which are frequency and wavelength. Light travels as
both a particle and a wave. Waves have a frequency (number of waves passing a fixed point through time) and
a wavelength (length of one complete wave). The frequency is measured in Hertz (waves passing per second) and
the wavelength is measured in meters or fractions of a meter.
While the maximum speed that light travels is not a function of relativity, the motion of an object compared to
the motion of surrounding objects will cause the wavelength and frequency to be different from different perspectives.
Suppose an object emits radiation with a wavelength of 7 micrometers and this object is moving quickly
away from an observer. The observer will discover the wavelength as being greater than 7 micrometers when
it reaches the observer. This is called a red shift since the wavelength is observed as being longer than
what the emitter radiates. The light from many galaxies is red shifted and this is used as evidence that
the universe is expanding since many galaxies are moving away from each other.
Imagine a very long rope with waves traveling through the rope. Suppose you are stationary and
notice 100 waves passing you per minutes. Now suppose you
are set in motion in the same direction the waves are moving through the rope. From
this new observational perspective you will notice less than 100 waves passing you per minute. The frequency
has decreased. This is a red shift. When the frequency of light decreases the wavelength increases.
When an emitter of radiation is getting closer to the observer over time the observer will notice a higher
frequency and shorter wavelength than what the emitter is radiating. This is known as a blue shift.
Suppose you are stationary and notice 100 waves within a very long rope passing you per minutes. Now suppose you
are set in motion in the opposite direction the waves are moving through the rope. From
this new observational perspective you will notice more than 100 waves passing you per minute. The frequency
has increased. When the frequency of light increases the wavelength decreases.
See this link for information on universe expansion and evidence for it:
35. Increasing the power returned to the radar
The power returned to the radar is much less than the power transmitted by the radar. Imagine the
sun as a transmitting radar and the planets are the hydrometeors. Some of the sun's energy goes from
the sun and is reflected off the planets back toward the sun. This energy that makes it back to the
sun is only the slightest of a tiny fraction of the original energy the sun emitted.
The return that makes it back to the radar is a function of several variables. The radar equation
will show us these variables.
The radar equation (power received back to radar) is =
(Pi^3*Pt*g^2*O*o*h*K^2*l*z) / 1024*Ln(2)*wavelength^2*r^2
Where Pt is power transmitted, g is gain, 0 and o are beam widths, h is pulse length, K is refraction term,
l is attenuation term, z is radar reflectivity factor, wavelength is the wavelength used by the radar and
r is the radius from the radar to the precipitation echoes.
Most of the variables in the radar equation are constants for any given radar, thus the equation
Pr (power returned to radar) = (c2*z)/r^2
c2 is the single value of all the constants put together. This constant will be different for
different radars and we will go over how power received can be increased by using a different radar
later in this essay.
First we will look at the power received as a function of the radar reflectivity factor and the radius from
the radar to the precipitation echoes. Notice in the equation Pr = (c2*z)/r^2, that z is in the numerator while
r is in the denominator. Since z is in numerator, if z increases while r remains constant then the
power returned to the radar must increase. This makes sense because z increases by increasing the size or
number of hydrometeors. The return to the radar will be stronger for larger and more numerous drops for any
given radius to the hydrometeors. Since r is in the denominator, when r increases and z is constant then the
power returned must decrease. This makes sense because an object further from the radar will receive less
radar radiation to scatter off of it than an object closer to the radar. Think of the planet examples again. Mercury
and Venus get much more solar energy scattering off of them than Pluto does since Mercury and Venus are
closer to the sun. Thus, both the size/number of hydrometeors and the distance (radius) to those
hydrometeors determines how much radiation is scattered back to the radar.
We mentioned earlier that some of the constants in the radar equation are different for different radars.
These terms include the power transmitted, gain, beam widths, pulse length and wavelength. If a term
is in the numerator of the radar equation and that term is increased, then the power returned to the
radar should increased. An example exception to this is when increasing one term in the numerator causes another
term in the numerator to decrease more than the original term was increased. Let's go through intuitively
how a different radar will cause the returned energy to the radar to increase.
Power transmitted: If the radar emits more energy than it is intuitive there will be more energy to
scatter back toward the radar if hydrometeors are present. Thus, more powerful radars are going to
receive more backscattered radiation. For example, if our sun in the solar system increased it's power
transmission then the earth would receive more solar radiation and more solar radiation would be
scattered off of the earth.
Gain and beam widths: These terms are intimately links because changing one can change the others. The more
confined a beam is the more energy that will be within that beam. If there is more energy within a beam then
there will be more scattering of radiation off of the hydrometeors that beam intersects. Increasing the
gain will increase the power returned. Increasing the beam widths however will decrease the power
returned because the increase in beam width is more than offset by the decrease in gain caused by
the beam being more spread out.
Pulse length: Pulse length is a function of how long the radar emits radiation within a beam. For example,
a flashlight that is turned on for 30 seconds will emit more total radiation than a flashlight turned on
for 15 seconds. Increasing the pulse length will increase the returned energy to the radar.
Wavelength: Wavelength is in the denominator of the radar equation. Thus, when wavelength increases then
the power returned decreases. Thus, radars that emit shorter wavelength radiation will get a more powerful
return. Shorter wavelength radiation has more energy than longer wavelength radiation.
The Pi term and 1024*Ln(2) term are simply numbers thus they are always constant.
The last two terms we need to discuss are the attenuation (l) term and the complex index of refraction term (K^2).
Attenuation is power loss due to radar radiation absorbing into the atmosphere or less radiation being able
to scatter back toward the radar do to the presence of hydrometeors. For any given radar,
this term varies depending on the weather conditions thus
this term is often ignored and set to a constant of 1 since multiplying the radar equation by 1 yields the same
result. Attenuation does have a function of the wavelength of radar used. Shorter wavelength radars will
attenuate more than longer wavelength radars.
The complex index of refraction is a function of the material state of the hydrometeor. Generally less energy
will be scattered off of ice than liquid water. With the same mass, there will be less returned radiation from
dry snow than from rain.
36. Range folding and detecting range folded echoes
A range folded echo is one that is detected beyond the maximum unambiguous range. These echoes
will show up at a distance from the radar equal to R - Rmax, where R is the distance from the radar
to the actual echo returns and Rmax is the maximum unambiguous range. For example if a storm is
300 miles from the radar and the maximum unambiguous range is 270 miles, the storm will be shown
on radar at a range of only 30 miles. This is because the reflectivity echoes from this storm arrive
after the radar has sent out another pulse. The radar assumes it gets reflectivity only from the
pulse it has most recently sent.
The following information covers how a radar operator can distinguish between real reflectivity and range
1. Look outside to visually verify the precipitation
2. Range folded echoes are often long and thin. The range gates are skinnier closer to the radar. Thus, precipitation
that is far from the radar will be compacted into skinnier bands when it is brought closer to the radar.
3. Range folded echoes generally have anomalous low cloud tops. This is because the radar beam generally increases in
altitude further away from the radar. Thus when a storm top far from the radar is brought closer to the
radar the height of that echo will decrease.
4. Range folded echoes generally do not have a high reflectivity. Since storms at the outer edge of the radar
are sampled at a very high altitude, the reflectivity from this precipitation will generally be low.
Thus, range folded echoes often show in the low reflectivity colors such as green near the radar.
5. Range folded echoes will change location when the Pulse Repetition Frequency (PRF) is changed. As
the PRF is decreased, the range folded echoes will eventually go away.
6. Check other nearby radars to see if the reflectivity in question shows up on those radars also.
7. Use multiple tilt angles. Range folded echoes if they show on a low tilt angle may
not show on a higher tilt angle.
37. Echo height errors due to superrefraction and subrefraction
A standard radar will assume normal refraction takes place. Radar determines an echo height by
calculating how much the beam changes in elevation with distance from the radar and how the
earth's surface curves under the radar beam.
Errors in the echo height can occur from the beam not refracting normally and land surface elevation changes
at the earth's surface. The land surface elevation change errors can be removed if the radar is given
topographic data of the earth's surface. The refraction errors can be reduced from soundings inputted
into the radar so that the radar determines whether refraction will be more than normal, normal or
less than normal.
If the radar assumes normal refraction, significant echo height errors can occur when
subrefraction take place. Suppose there is a storm that is 100 kilometers from the radar site and the echo
top of the storm in the actual troposphere is 40,000 feet. Suppose superrefraction is taking place and the
radar assumes normal refraction. The radar will not indicate the actual echo top of 40,000 feet since the
beam is not refracting as the radar assumes it is. The radar under superrefraction conditions will
indicate an echo top greater than 40,000 feet in this example. Thus superrefraction overestimates
the echo top height. Using this same line of logic, subrefraction underestimates the echo top height thus
it will indicate a echo top of less than 40,000 feet in this example.
38. Radar reflectivity pitfalls
Below is a list and explanation of radar reflectivity pitfalls:
1. Earth's curvature- The Earth's curvature causes more of a storm to be unsampled the further the storm
is from the radar site. This makes it more difficult to detect accurate VIL values and mesocyclonic
circulations at long ranges from the radar.
2. Topography- Elevated terrain can increase
ground clutter and
anomalous propagation. Valley regions
are not sampled if the radar is on the other side of elevated terrain.
3. Unusual temperature gradients- Strong
inversions and other strong temperature lapse rates will
refract the radar beam atypically. This will result in echo height errors, can increase ground clutter
in the case of inversions, and can causes sampling errors of storms.
4. Ground clutter- Overestimates precipitation intensity for echoes near the radar site. Ground clutter
will be reduced by using a higher tilt angle. Ground clutter also tends to be less when the lower
troposphere is unstable.
5. Beam spreading- The resolution of range gates decreases as range from the radar increases. Precipitation
areas will look bigger and pixilated at the longer ranges.
6. Attenuation- Radar beam is less powerful as it moves into the longer ranges from radar as the radar
beam moves through precipitation areas that scatter away the beam progressively as it moves away
from the radar. This causes an underestimation of echo intensity at the long radar ranges.
7. Unsampled regions- The cone of silence (cone created immediately above radar bounded by rotating
highest tilt angle used 360 degrees) is not sampled. The regions below lowest tilt angle is also
8. Location of precipitation- Position of precipitation aloft may not be position precipitation
strikes the Earth's surface.
9. Virga- Often much of the light precipitation that shows on radar
evaporates before reaching the ground.
39. Severe storm tracking techniques
The most dangerous portion of a storm is the mesocyclone. If a
large hail occur it will
generally be near this portion of the storm. Thus, it is a good idea to use this portion of the
storm as the central position of the storm when plotting the storm's movement.
It is a good idea to remind that storms and
severe thunderstorms often produce tornadoes even when
no tornado warning is out yet. While radar can be used to determine the circulations associated with
a tornado, the radar can not tell if the circulation is connected to the ground.
Storm spotters are very
helpful in determining whether the circulation is in contact with the ground.
When plotting the movement of a storm focus on the cities in the path of the storm since the storm
is likely already impacting those cities. Radar data is often several minutes old.
When plotting the movement of a storm it will not always move in a straight line. Development within
the storm and shear can cause the storm to take a curving and wobbling path. Adjust the anticipated
path of the storm on each radar update.
Be aware of new storms that develop and do not become overly fixated only on storms that have a warning
out on them. Severe storms can develop in a matter of minutes.
Geographic features, roads and landmarks make it easier for viewers to understand where a storm is
Be careful about zooming in too close for too long on a storm when running the radar. Keep a close watch
on all the viewing area. Also keep radar display simple enough so that viewers can understand what is
40. What is a pulse storm?
A pulse storm is a thunderstorm that produces strong to
severe weather in a short period of time. The
environmental conditions conducive to pulse storms are strong
CAPE and weak
wind shear. The strong
CAPE contributes to a strong
thunderstorm updraft. Strong and severe thunderstorms often have
strong updrafts associated with them. The weak wind shear is what causes the duration of the storm
to be small. Contributing to weak shear are weak upper tropospheric winds and weak winds within the
troposphere in general. Since the shear is weak, the downdraft will fall into the vicinity of the updraft and
cut off the inflow into the updraft. The downdraft will also reduce the momentum within the