1 RADAR METEOROLOGY P.S.Biju [email protected]Courtesy: Presentations of Dr.D.Pradhan, Scientist-G, DDGM(UI),New Delhi, Shri.S.B.Thampi,DDGM,Chennai & Dr.Y.K.Reddi, Scientist-F,MCHyderabad Chapter 1: Introduction RADAR is an acronym for Radio Detection and Ranging. Similar principle is Light Detection and Ranging (LIDAR) used in ceilometers. So many other similar principles are there with Detection and Ranging (DAR) having the same equation for range measurement. Radar principle is explained in the following figure: The similar principle LIDAR is illustrated below:
RADAR is an acronym for Radio Detection and Ranging. Similar principle is
Light Detection and Ranging (LIDAR) used in ceilometers. So many other
similar principles are there with Detection and Ranging (DAR) having the same
equation for range measurement.
Radar principle is explained in the following figure:
The similar principle LIDAR is illustrated below:
2
Range of Radar
Radar is an electronic device which is capable of transmitting an
electromagnetic signal, receiving back an echo from a target, and determining
various things about the target from the characteristics of the received signal.
Range is the distance of the target given by the values of c and t , which
is explained as h = ct/2 .
Milestones of weather radar
• 1842 : Doppler effect
• 1888: Electromagnetic waves discovered by Hertz
• 1922 : Detection of ships by radio waves by Marconi
• 1947: The first weather radar in Washington D.C.
• 1990: Introduction of Doppler weather radar
• 2000: Doppler weather radars in India
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Electromagnetic wave
A wave propagation containing mutually perpendicular electric and magnetic
fields perpendicular to the direction of propagation.
Light wave is an example of electromagnetic wave
Polarisation of radar signal
The direction of propagation of electric field in an electromagnetic wave is
known as polarisation. Hence an electromagnetic wave used in radar is either
horizontally or vertically polarised.
S-band Doppler weather radar of IMD is horizontally polarised and C-band is
dual polarised (both horizontal and vertical).
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Wavelength (λλλλ)and frequency(νννν)
Length of one wave is known as wavelength
Time taken to travel one complete wavelength is known as period (T) . Number
of wavelengths travelled in one second is known as frequency(ν) .
Hence T = 1/ν and Velocity C=λ/T = νλ
Electromagnetic spectrum
The arrangement of electromagnetic wave according the order of wavelength
is known as electromagnetic spectrum.
Radar signal uses wavelength in the microwave region ( 1mm to 1 m) in the
following bands.
5
IMD utilised S (10 cm), C ( 5 cm) and X ( 3 cm) bands in DWR, Polarised
radar and Multimet radar respectively.
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X-band become obsolete in IMD ?
Attenuation of radar signal while passing through a medium is
inversely proportional to wavelength as per the following equation:
X-band radars are not suitable for the tracking of clouds, cyclones etc
due its smaller wavelength gives more attenuation while passing
through it. Hence Cyclone detection radars and Doppler weather
radars at coastal stations uses S –band only.
Doppler effect
Doppler effect observed in sound was described by Christian Andreas Doppler
that the sound waves from a source coming closer to a standing person have a
higher frequency while the sound waves from a source going away from a
standing person have a lower frequency.
The approach of Doppler in sound waves proved to be valid for light waves
also. Light waves from a source coming closer to an observer have a higher
frequency (lower wavelength-Blue shift) while the light waves from a source
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going away from an observer have a lower frequency (Larger wavelength-Red
shift).
Doppler effect in Radar
In Doppler weather radar (DWR) this principle is adopted by considering
radar as observer and the moving target as the source of light ( In fact the
original source is also radar, but the scattered light is reflected is from the target.
Hence for the radar (observer) the source is the target)
Doppler shift in frequency (∆ν) is given by the basic equation;
Where V is the velocity of target.
Hence Doppler weather radar will give both range and velocity of the target.
Distinguish between conventional radar and DWR Conventional radar
1. Gives only position of a target
2. Analog technology and mostly black and white pictures
3. No provision for unattended operation
Doppler weather radar
1. DWR gives both position and velocity of a target
2. Automatic control and Mostly unattended operation
3. User friendly colour images
4. Large number of products for various applications like aviation,
hydrology, weather forecasting etc
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Block diagram of a Radar
Transmitter: This part requires high power for the transmission of
electromagnetic signal upto 500 Km range. The basic component is a Radio
Frequency generator (RF Generator). The generated RF frequency signal is
amplified to high power electromagnetic signals by the one of the following
transmitters:
1. Magnetron
2. Klystron
3. Solid state transmitters
Magnetron has Lighter weight, Easy to carry and 200 MW or more power.
But its frequencies are not purer, which is essential for Doppler weather radar.
Conventional radars used magnetron as the transmitter
Klystron has Heavier weight, Purer frequencies, Wave forms can be controlled
and generate power of more than 200 MW. Doppler weather radar uses
Klystron as the transmitter.
Solid state transmitters have power only up to 50 W, but desirable power can
be achieved by making an array of a large numbers of transmitters. But seldom
used for meteorological purposes.
Modulator: Modulator is the ON/OFF switch of the Radar Transmitter. When
and which duration it should transmit will be decided by the modulator. It also
decides the correct wave form of the transmitted signal.
Master clock and PRF: Master clock controls the entire radar system. It
determines how often the radar will transmit signal into space. The rate at
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which the radar transmits is called Pulse Repetition Frequency (PRF).
Usually its value is between 200Hz to 3000 Hz. The duration of transmitter
signal names either pulse duration or pulse length. Typical value of Pulse
duration is 0.1 to 10 µs. DWR of IMD uses two Pulse widths 1µs and 2 µs.
Antenna: Antenna is a device for radiating and receiving of EM waves. It
can be isotropic or non isotropic. An antenna that sends the radiation equally in
all directions is called isotropic antenna. It is similar to the light of a candle
except the bottom portion. Radar antennas are more like flash lights
Main parameters in the selection of an antenna are:
� Wave length
� Diameter of reflector ( small as a foot to 30 ft)
� Gain
Gain is the ratio of power received at a point in space on the centre of the beam
axis to the power received at the same point from an isotropic antenna.
As shown above, gain has no unit. But logarithm of gain multiplied by 10 has a
unit called deciBell. Typical gain is 20 dB to 45 dB.
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Ideal antenna would direct all of the radar energy into a single direction and
this is practically impossible. Practically radar signal have a bright spot called
the main lobe and also having energy off to the side of the main lobe called side
lobes. Radar signal also have energy behind the antenna called back lobes.
Relation between gain and beam width: Beam width is the angular distance
across the antenna beam at the point where the power is reduced to one half of
the peak power which exists along the centre axis of the antenna beam pattern
k2 depends on the kind and shape of the antenna and for circular reflector
k2=1
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deci Bell (dB) unit
For example the output power of Klystron is about 800kW. It can be expressed
in dB as:
In dBm as
Half power in dB
10 log (1/2) = - 3 dB,
i.e. the power reduced to half means power is reduced by 3 dB
For example power reduced from 8W to 4W
10 log (8)=9 dB and 10 log (4) = 6 dB.
Clearly the reduction is 3 dB
10 log (1/4)=-6 dB i.e. the power reduced to one-fourth means power reduced
by 6 dB
For example power reduced from 8W to 2W
10 log (8)=9 dB and 10 log (2) = 3 dB. Clearly the reduction is 6
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Wave guide
Regular wires and coaxial cables cause so much loss of signals that they are not
useful at radar frequencies
Wave guide is a conductor connecting transmitter/receiver and antenna .Wave
guide is usually a hollow, rectangular, metal conductor whose interior
dimensions depend upon of the wavelength of the signal being carried.
T/R Switch or Duplexer
Most of the radars transmit power from 1000 W to more than 1 MW. At the
same time it is capable of receiving powers as small as 10-10
W or less. If
transmitter sent power in to the receiver it would burn up quickly. An automatic
switch known as T/R switch or Duplexer is added in the radar system to protect
the receiver from the high power of the transmitter.
When the transmitter is turned on , the duplexer acts to direct the strong pulse
of energy to the antenna and as soon as the transmitter stops sending a signal,
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the duplexer switches to connect antenna with the receiver and transmitter will
be disconnected from the antenna.
Receiver
Receivers detect and amplify the very weak signal received by the antenna. Most of the radars used super heterodyne type receivers where the high
frequency received signal is mixed with a reference signal and converts it into a
much lower frequency (typically 30 to 60 Hz).,which can be easily processed.
Co-axial cables can be used to connect receivers with displays since frequency
and distance are less.
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Display
The earliest and easiest display is to put the radar data in to an oscilloscope
where horizontal axis is time and vertical axis is signal strength. Time base can
be changed in to distance and vertical scale can be changed in to power. This is
known as A-scope display.
But this display will not give the direction of target. The most universal
displays for radar are Plan position Indicator (PPI) and Range Height Indicator
(RHI). This different type of display products are obtained due to difference in
the scanning modes.
When scanning in PPI mode, the radar holds its elevation angle constant but
varies its azimuth angle. The returns can then be mapped on a horizontal plane.
If the radar rotates through 360 degrees, the scan is called a "surveillance scan".
If the radar rotates through less than 360 degrees, the scan is called a "sector
scan".
When scanning in RHI mode, the radar holds its azimuth angle constant but
varies its elevation angle. The returns can then be mapped on a vertical plane.
The elevation angle normally is rotated from near the horizon to near the zenith
(the point in the sky directly overhead).
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PPI displays the radar data in a map-like format with the radar at the centre.
Distance is given by adding range marks (called range rings) around the radar. Most of the radar put the north at the top, east to the right, south at the bottom
and west to the left.
RHI display gives distance in the horizontal axis and height above the radar in
the vertical axis.
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Chapter 2 : Reflectivity
Radar Equation
Radar transmits energy into space through antenna. Consider a target at a
distance r from an isotropic antenna. Now we can imagine this target as a point
on the surface of a spherical region of energy with centre as radar.
Area of the spherical energy field = 4π r2
Power density of the sphere,
where Pt is the total power
For an antenna in use (non isotropic antenna) ,gain (g) factor should be added.
If Aσ is the area of the target (Target aperture) ,then the power received at the
target can be represented as:
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If there is no loss of energy the same power (Pσ) will be reflected back from
the target towards the antenna
Power density of signal reflected from the target is
Let Ae (Antenna aperture) be the effective area of the receiving antenna , then
Power received by the antenna is
Substituting the value of value of Pσ , the received power Pr will be obtained as:
Effective area (Ae) of the antenna is related to gain (g) and wavelength(λ) as
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Now the received power(Pr)of the antenna is
Actually the received power is the balance obtained after the scattering of radar
beam with the target. Hence Aσ is nothing but the backscattering cross
sectional area (σ)of the target.
Received power(Pr) is
Backscattering cross sectional area (σ) depends on the size, shape, and kind of
matter of the target as well as the wavelength of the radar . But most of the
hydrometeors are approximately spheres
When sphere is large compared to the wavelength (D/λ) >10, σ is equal to the
geometric area:
When sphere is small compared to the wavelength (D/λ) <0.1, then the sphere
is in the Rayleigh region where σ is proportional to sixth power of the diameter.
Many meteorological targets are in the Rayleigh region. Then the equation for σ
is:
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Intermediate region also important called Mie or resonant region, which is
important to detect the presence of hail
Practically there may be many rain drops or cloud particles within the radar
beam at the same time. Then the total backscattering cross-sectional area is the
sum of all of the individual backscattering cross-sectional area in a sample
volume
per unit sample volume
Sample volume of radar beam is given by (considering all energy confined
within half power beam width)
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θ, φ are horizontal and vertical beam widths, r is the distance of sample volume
from radar and h is pulse length:
Here it is assume that the smallest radial distance a pulse can occupy half of
pulse width as per illustration( remember that pulse will be travelling out to a
target, scatter off it and propagate back to the radar. So the radar pulse volume
will be
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Total backscattering cross section area more then written as
Where the volume given by
Real radar antennas do not have all the power confined within half power beam
widths. So correction factor also may be applied
i.e. 2 ln 2, ( natural logarithm)
Total backscattering cross section area will get the equation
Substitute the value of σ in the equation for received power
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Now the received power will be changed as
For most of the meteorological radars with wavelength 3 cm and larger, almost
all rain drops can be considered small compared to the wavelength, so the
Rayleigh approximation applies.
Substitute the value in Received power
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Define the reflectivity factor z as
Then Received power will be changed in to
One more factor we have to add in the equation which is the attenuation factor
(ι)This is the loss of power in travelling through a medium ( atmosphere, cloud,
rain, snow, hail etc ) and its value lies between 0 and 1
This is known as radar equation
Now Define radar constant C1 . The radar equation becomes
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|K|2 is the magnitude of complex index of refraction m= n+ik ( n is the index of
refraction and k is the absorption coefficient). |K|2 depends on material,
temperature and wavelength. |K|2 for water is 0.93 and for ice is 0.197. These
two values differ by 7 dB.
Define another constant C2
A very simplified equation will be obtained as:
Reflectivity
The factor z is also called reflectivity
The parameter Di is the diameter of ith droplet in the unit volume. The unit of
diameter of droplet is mm and unit of volume is m3 . So the unit of z is
mm6 / m
3
Reflectivity may range from 0.001 mm6
/ m3 (fog, weak clouds,etc) to as much
as 50,000,000 mm6
/ m3
(Very heavy hail ). Hence it is very convenient to
express it in logarithmic scale
Z varies from - 30 dBZ for fog to + 75 dBZ for heavy hail
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Chapter 3: Doppler velocity
Doppler shift
Doppler shift is the frequency shift due to the relative motion between the
object and the observer. Here the observer is Radar and the object is the moving
target
Number of waves in the distance of 2r is :
Distance in radian is :
This is the phase shift produced in distance 2r
The term Phase shift ism illustrated below :
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If φ0 is the initial phase of the transmitted signal from the radar, then phase of
the returned signal will be
The change of phase with time from one pulse to the next is given by
Velocity of the object is
Angular velocity is
f is the frequency shift
then
and
This is known as Doppler shift
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Doppler dilemma
As per Nyquist theorem, then the maximum frequency shift can be detected is
related to Pulse repetition frequency (PRF) as ;
Then the maximum velocity which can be measured accurately by a doppler
weather radar is given by :
This is known as Maximum unambiguous velocity
It says that if we want to be able to detect high velocities, we must use long
wavelengths and large PRF’s or both.
The maximum phase shift a radar can detect is ππππ radians, since a phase shift of
2π radians is in effect zero phase shift. It is equivalent to λ/2 in wavelength and
T/2 in time. Then the maximum range which can be measured by a Doppler
weather radar is :
Then
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So
This is known as Maximum unambiguous range. Now we are having
Maximum unambiguous Range increases with the of PRF. But the increase of
PRF may decrease the maximum unambiguous velocity. Both the equations can
be then combined as
This is called Doppler dilemma
If we want to have a large Vmax, we must have a small rmax and vive versa. But
S-band radars are more useful than X band radar in solving the Doppler
dilemma. C is intermediate of these two radars
But increasing the wavelength is a real solution, but it may increase the size of
the radar and is practically impossible
Identification of Multi-trip echoes
Velocity /Range ambiguities are also known as Velocity/Range folding or
aliasing. An echo (r) beyond the range rmax will be displayed at a range of
and
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( r- rmax ). These are known as second-trip or multi-trip echoes. These wrong
echoes are identified by
1. If the radar shows a nearby storm in a particular direction, but is
nothing outside, it is probably a multi-trip echo
2. Second-trip echo can be recognised with its reflectivity. The echoes at
smaller distance should have an expected reflectivity,since it decreases
with distance ( Typically less than 10-12 dB)
3. A narrow wedge –like echo points towards the radar may be a multi-
trip echo
4. If a convective type echo (8-15 km height) appears on the radar display
with less than the normal height may be a second-trip echo.
5. Second trip echoes may not show in the velocity product
6. Change of PRF and scan again to see the difference
7. Phase coding helps to discriminate first and second trip echoes
Velocity folding
In velocity folding, if the velocity of the target V is greater than Vmax ,
then it will be displayed in the range – Vmax to + Vmax .
For example if an object is moving away from radar with a velocity +30
m/sec greater than the Vmax =25 m/sec, then it will be displayed in the range -25
m/s to +25 m/sec. Range is 50 m/sec and hence it will display as -20 m/sec ( 30-
50)
if the storm is moving away and part of it is moving away faster than
,then strong receding velocities would surround a region with apparently strong
approaching velocities.
The velocity folded can be unfolded to its true velocity using staggered
PRF or dual PRF technology. Here we are using two PRF in the ratios 2:3, 3:4,
and 4: 5.
2: 3 ratio increases the limit of velocity measurements by two times and 3: 4
by three times and 4:5 by four times.
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Example 2:3 gives 32 m/sec ( if PRF =1200 Hz & 800 Hz)
3: 4 gives 48 m/sec ( if PRF =1200 Hz & 900 Hz)
4: 5 gives 64 m/sec ( if PRF = 1200 Hz & 960 Hz)
Internal variability and spectrum width
When there are many targets within the sample volume (rain storm etc),then
each individual target would produce a frequency shift related to its radial
velocity. Then according the quality of processor a Doppler radar may produce
the mean velocity.
Spectrum width is a measure of the width of spectrum of frequencies
measured from different moving targets within the volume of measurement. The
term generally used to indicate this variance is the standard deviation of velocity
( σ )
Vi is the velocity of an individual target and Vave is the mean velocity and N is
the number of velocities in the sample
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Chapter 4: Echoes
Minimum detectable signal (MDS): Minimum Detectable Signal (MDS) is a specific value of minimum receivable
power (Pr (min)).
The minimum detectable signal is defined as the useful echo power at the
antenna, which gives at the output of the IF amplifier (just before detection), a
signal which lies 3 dB above the mean noise level.
The MDS is generally expressed in dBm; typical values are around -114
dBm.
Standard targets:
A target of known characteristics, usually a sphere. Spheres can be tied to
balloons and released and tracked by a radar. Considering it as a point target at a
known distance from the radar ,the gain of the antenna can be calculated by
considering the back scattering cross sectional area as;
Another standard target of known characteristics is Flat-plate reflector, where
the back scattering cross sectional area is;
Clear air return
Birds, insects and particulates are the clear air echoes
Birds can be detected by radar by considering it as a point target and as a
water body. But they are very small targets and hence they can be detected
within a few miles.
Insects are more in warmer months. The reflectivity factor from insects play
an important role in the detection of gust fronts from thunderstorm because lot
of insects are picked up and swept along with gust fronts, which is a hazard for
aviation services
Microbursts also can be detected from the return from insects
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Rain
An empirical relation for the relationship between reflectivity (z) and rain
rate(R) is
Z is measured in mm6 /m
3 and R is measured in mm/h,
A and b are empirical constants
The most commonly used Z-R relationship is given by Marshal and Palmer
Bright band
Reflectivity of snow and ice is less than that of water ( about 7 dB less).
When the snow is falling with slow terminal velocity, its outer surface will
melt and a film of water forms on the outside of the snow flake. It will be
reflected as a giant water droplet and hence it will give high reflectivity in radar
known as bright band . After melting level, speed will increase and the size will
reduce rapidly and hence the reflectivity also may reduce.
Bright band occur primarily during stratiform or stable situations. But the
decaying stage of thunderstorm also bright band will occur
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Anomalous propagation
Refractive index of the medium is
Where c is the velocity of light in air and u is the velocity of light in the medium
The value of n for air 1.0003 and for vacuum is 1.0000. This says that the
important part is in the fourth decimal places . Hence a more convenient term is
defined known as refractivity (N).
The relation connecting N with atmospheric temperature, pressure and vapour
pressure is
A ray of light bend away from the normal when it travels from a denser medium
to the rarer. Density is proportional to refractivity. So a radar beam bend along
with the curvature of earth since N is different at different points due to the
change in temperature and pressure.
While travelling through non uniform atmosphere the radar beam bend more or
less relative to earth
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Then the effective radius of earth is
Consider the case of radar ray bending exactly the same as the earth
Then
Hence
and
Radius of earth R = 6374 km, then the refractive index gradient δn/δH needed
for a ray to follow the earth’s surface is -1.57 × 10-4
km-1
or in N units ,this is
-157 N unit/km
But for straight radar rays, standard refraction condition may apply where
δN/δH is -39 N unit/km.
35
Height of radar beam under this condition is
The downwards bending of radar rays stronger than the normal is known as
super refraction. It occurs when temperature increases with height ( inversion).
Then the radar will detect ground targets to much longer distance than normal
conditions.
The condition of extended range of detection of ground targets is called
Anomalous propagation.
If the refractions of the radiation is strong enough, the radar wave trapped in a
layer of the atmosphere. It is called ducting.
Ducting occurs when N ≤ -157 N unit/km
Chapter 5: Dual Polarisation
Dual polarization use both vertical and horizontal polarization in radar
illustrated as shown below:
Differential reflectivity
The basic parameter is the differential reflectivity
Heavy rain ( > 30 mm/hr
Then
Light rain ( < 5 mm/hr)
ice particle, Hail
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Chapter 5: Dual Polarisation
Dual polarization use both vertical and horizontal polarization in radar
illustrated as shown below:
Differential reflectivity
The basic parameter is the differential reflectivity
Heavy rain ( > 30 mm/hr)
Dual polarization use both vertical and horizontal polarization in radar and
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Differential phase
Differential phase measures the difference in phase between horizontally
polarised returns and vertically polarised returns
Its value depends on the intensity of the precipitation and orientation and the
type of hydrometeor
Specific differential phase
This is an important term in rain measurements similar to Marshal-palmer
relation
Advantages of R-KDP relation over R-Z relation
1. Less affected by attenuation
2. Independent of radar calibrations
3. Less influenced by difference in drop size distributions
4. Less affected by the presence of hail, anomalous propagation, birds and
insects
5. Good estimator for liquid water in rain hail mixture
6. Together with ZH can detect small hail
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Chapter 6: Scanning strategy
Scanning is the motion of the radar antenna during data collection
In Horizontal scanning, used to generate PPI displays, the antenna is
continuously rotated in azimuth around the horizon or is rotated back and forth
in a sector (sector scanning). At the completion of each 360 or sector scan, the
elevation angle of the scan typically is increased;
In Vertical scanning, used to generate RHI displays, is accomplished by
holding the azimuth constant while continuously varying the elevation angle of
the antenna; at the completion of each vertical scan, the azimuth typically is
incremented and the vertical scan proceeds in the opposite direction.
Scanning strategy satisfies the following needs:
1. No important weather events should be missed
2. Range and velocity ambiguities do not occur
3. Clutters are minimal
4. High data resolution
5. Minimal noise
6. Shortest lived phenomena like thunderstorm, tornado etc should not
be missed
Volume Coverage Pattern
Volume scans are typically performed by conducting a series of horizontal and
vertical scans to develop three-dimensional views of the reflectivity field and
the radial velocity field
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A Volume Coverage Pattern is a series of 360 degree sweeps of the antenna
at selected elevation angles completed in a specified period of time.
Modes of Volume scanning
In this Clear air mode , scans are made at five different elevations starting at
0.5° and incrementing by 1° for elevation angles 0.5°, 1.5°, 2.5°, 3.5° and 4.5
using long pulse and complete in 10 minutes. At each elevation angle, the radar
makes two full azimuthal rotations. One rotation is to collect reflectivity data
and the other is to collect Doppler data.
Because snow has a low reflectivity, this mode will sometimes be used to detect
light snowfall.
In this Precipitation mode, scans are made at fourteen different elevations
starting at 0.5° and increasing up to 19.5 typically separated by 1° (Higher
elevation it can be higher) using short pulse and complete in 5 minutes. Two
full rotations are made at each elevation.
A second precipitation mode strategy, is used to observe more distant storms; it
uses a short pulse and sweeps 9 elevation angles in 6 minutes.
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Scan schedule for bad weather
1. Long range single elevation scan ( lowest elevation) up to 500 km
range for general observation
2. A medium range ( upto 250 km range) multiple elevation scan, called
volume scan for detailed probing of atmosphere
3. RHI scan is done only in manual mode as and when required
4. A 10 minutes temporal spaced scan strategy for the period of bad
weather or expected bad weather
5. A 3 hour temporal spaced scan strategy for fair weather in winter
DWR data
Doppler weather radar produces three kinds of data
1. Raw data
2. Product data
3. Image data
Raw data is the base data that is measured/ reported by the Radar Signal
Processor after correcting for following errors:
1. Range Normalisation
2. Clutter Filtering
3. Earth Curvature
4. Range folding, if any
5. Velocity folding, if any
6. Speckle removal etc
The Raw data mainly consists of three parameters Reflectivity (Z), Velocity(V)
and Spectrum width (σ)
In Gematronik Radar (Chennai, Machalipatnam, Visakhapatnam and
Kolkata) , the Unix work station running Rainbow software captures the scan
data from Radar processor, construct raw data files (Gematronik specific),
archives them, process raw data and generates product data. The raw data sets
are available in separate files for Z,V and σ.In Metstar Radar , RCP8 server
generates one Raw product file for each scan ( Volume or azimuth) which
contains all the base parameters Z,V and σ. The soft ware used in Mestar radar
is IRIS (Interactive Radar Information System)
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Chapter 7: Doppler weather radar products
Main products are
� Plan position Indicator – Reflectivity (PPZ)
� Plan position Indicator – Velocity (PPV)
� Plan position Indicator – Surface Rainfall Intensity(SRI)
� Plan position Indicator –
24 hours Precipitation Accumulation (PAC)
� Maximum Reflectivity (Max Z)
� Vertical Wind Profile (VVP2)
PPI products
Image presented on a conical surface of a constant elevation . The displayed
range is the slant range and this is different for different elevations.
In PPI (Z) Eye of the cyclone and two spiral bands is shown as High
reflecivity region. Highest reflectivity (about 50dBZ) area corresponds to the
heaviest rain fall
In PPI (V) a couplet (2Ds) of two maximum radial velocities of opposite
direction. The maximum radial velocity in the couplet corresponds to
observation when radar beam is parallel to wind direction in the rotating wind
field in the eye-wall region
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RHI Products
Range Height Indicator is generated from Z or V products with the range on the
X-axis and the height on the Y-axis. A Cartesian grid is displayed as an overlay
to facilitate reading height of clouds. This grid is seen bending along the X-axis
to due to correction for earth curvature.
MAX-Z product
The MAX-Z takes a polar volume raw data set, converts it to a Cartesian
volume, generates three partial images and combines them to the displayed
image:
(1) A top view of the highest measured values in Z-direction. (each vertical
column) (2) A north-south view of the highest measured values in Y-direction
(each horizontal line) (3) An east-west view of the highest measured values in
X-direction (each horizontal column)
This single product provides distribution of parameters measured by DWR
in three dimensional spaces.
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CAPPI product
The Constant Altitude Plan Position Indicator (CAPPI) product takes a volume
data set of the selected data type as input and the CAPPI algorithm generates an
image of the selected data type in a user-definable height (layer) above ground.
No echo is observed near the radar location in cone of silence
PSEUDO-CAPPI Product
This product is generated in the same way as for the standard CAPPI product.
Additionally, the possible "no data" areas of the standard CAPPI close to the
Radar site and at lager ranges are filled with data of the corresponding
elevation: at short ranges the data are taken from the highest elevation until this
beam crosses the defined height, and for large ranges, where the lowest beam is
higher than the defined height, the data accumulation follows the lowest beam.
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Main wind products
Volume velocity Processing :(VVP_2) displays the horizontal wind velocity
and the wind direction in a vertical column above the radar site.It provides
vertical wind profile over DWR station
Uniform Wind Technique: It provides horizontal wind vectors at different
user defined grid points
Hydrological products
Surface Rainfall Intensity (SRI): The SRI generates an image of the rainfall
intensity in a user selectable surface layer with constant height above ground.
The estimated values of converted to SRI by using marshall-palmer
relationship
Precipitation Accumulation (PAC): The PAC product is a second level
product. It takes SRI products of the same type as input and accumulates the
rainfall rates in a user-definable time period. Every time a new SRI product is
generated, the PAC again.
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Vertical Integrated Liquid (VIL): Vertical Integrated Liquid (VIL) product is
to give an instantaneous estimate of the liquid water content residing in a user-
defined layer in the atmosphere.
C and D are constants
Z is the reflectivity in mm6 /m
3 and M is the Liquid water content
(g/m3 )
Aviation products-Shear products
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Warning Products
Hail warning Product (HHW): Hall warning product, Red colour represents
areas of probable hail and yellow areas of very probable hail.
Thunderstorm prediction
Thunderstorms typically initiate along boundary layer convergence lines that
are visible on Doppler radars. monitoring of these boundary layer-convergence
lines can be used to successfully prepare very short period forecasts of
thunderstorm initiation. Detection of strong echo (50 dBZ) at elevated heights
(8km) indicates a possible severe storm, especially a large hail producer.
Hook shaped echo may be an indication of a supercell thunderstorm associated
with tornado
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Cyclone tracking and Prediction
1. Fixing the current position and estimating intensity
2. Locating the centre of the circular region of cloud or rainfall
encompassing the eye using animation of previous images
3. Estimating the horizontal velocity using radial velocity couplet
