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:
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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 ( λλλ)and frequency( ννν) 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.
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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
6 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
8 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 1 µs 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,
12 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 = 4 π r 2 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 A σ 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 (P σ) will be reflected back from the target towards the antenna Power density of signal reflected from the target is
Let A e (Antenna aperture) be the effective area of the receiving antenna , then Power received by the antenna is
Substituting the value of value of P σ , the received power P r will be obtained as:
Effective area (A e) of the antenna is related to gain (g) and wavelength( λ) as
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Now the received power(P r)of the antenna is
Actually the received power is the balance obtained after the scattering of radar beam with the target. Hence A σ is nothing but the backscattering cross sectional area ( σ)of the target.
Received power(P r) 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 C 1 . 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 C 2 A very simplified equation will be obtained as:
Reflectivity The factor z is also called reflectivity
th The parameter D i is the diameter of i droplet in the unit volume. The unit of diameter of droplet is mm and unit of volume is m 3 . So the unit of z is 6 3 mm / m Reflectivity may range from 0.001 mm 6 / m 3 (fog, weak clouds,etc) to as much as 50,000,000 mm 6 / m 3 (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 πππ 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
and
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 V max , we must have a small r max 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 r max will be displayed at a range of
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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 V max , then it will be displayed in the range – Vmax to + V max . For example if an object is moving away from radar with a velocity +30 m/sec greater than the V max =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 V ave 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 mm 6 /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.
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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
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Chapter 5: Dual Polarisation Dual polarization use both vertical and horizontal polarization in radar and 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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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 Z H 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 mm 6 /m 3 and M is the Liquid water content (g/m 3 )
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
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