fundamentals of radar and display

Radar Meteorology M. D. Eastin

Fundamentals of Radar and Display



Outline

• Radar Components• Signal Characteristics• Display – NEXRAD• Display – Other Types• Display – Phenomena

PULSEElectric

FieldSidelobes

DuplexerKlystronAmplifier

Pulsemodulator

STALOMicrowaveOscillator

FrequencyMixer

COHOMicrowaveOscillator

Amplifier

PhaseDetector

DISPLAY

switch

Half-power beamwidth

TRANSMITTER

RECEIVER

ANTENNA

FrequencyMixer

A Typical Pulse Radar System: Four Basic Components

• Transmitter• Antenna• Receiver• Display

• System is designed to transmit microwave pulses in shorts bursts from the antenna, and then activate the receiver to “listen” for any returns associated with that pulse

• Returns are then amplified and displayed as radar reflectivity

Duplexer: Switch which allows the same antenna to transmit pulses and receive returns


Radar Components

PULSEElectric

FieldSidelobes


Pulsemodulator


FrequencyMixer


Amplifier

PhaseDetector

DISPLAY

switch


TRANSMITTER

RECEIVER

ANTENNA

FrequencyMixer


Radar ComponentsA Typical Pulse Radar System: Transmitter

• A microwave tube (“Klystron”) produces pulses of power at a desired frequency (or wavelength – 10 cm – S-band)

• A pulse modulator controls the timing of each pulse. Typical pulse durations are ~1 μs with each pulse separated by a few milliseconds to allow time for unique returns at large ranges

Pulse Repetition Frequency (PRF)

• Sets the timing between each pulse• Fixed (operational radars)• User-controlled (research radars)

PULSEElectric

FieldSidelobes


Pulsemodulator


FrequencyMixer


Amplifier

PhaseDetector

DISPLAY

switch


TRANSMITTER

RECEIVER

ANTENNA

FrequencyMixer


Radar ComponentsA Typical Pulse Radar System: Antenna

• Output from the antenna is a pulse modulated microwave-frequency sine wave.

• Waves travel along a microwave transmission line (or “waveguide”) through the duplexer to the antenna

• The antenna concentrates waves into the desired shape – often a narrow cone (or “beam”) for most meteorological radars

• Transmitted beams travel through the environment until they strike an object (meteorological or not!)

• A very small portion of the beam is reflected back toward the antenna

PULSEElectric

FieldSidelobes


Pulsemodulator


FrequencyMixer


Amplifier

PhaseDetector

DISPLAY

switch


TRANSMITTER

RECEIVER

ANTENNA

FrequencyMixer


Radar ComponentsA Typical Pulse Radar System: Antenna

Sidelobes:

• No radar antenna is perfectly built!• Small construction flaws allow for a portion of the transmitted signal to escape through “holes” as the beam is being formed• Can also strike environmental targets and have power reflected back

Half-power Beam Width

• Function of radar design and range• Radius of a conical cross-section (i.e. a circle) at a given range

PULSEElectric

FieldSidelobes


Pulsemodulator


FrequencyMixer


Amplifier

PhaseDetector

DISPLAY

switch


TRANSMITTER

RECEIVER

ANTENNA

FrequencyMixer


Radar ComponentsA Typical Pulse Radar System: Receiver

• The echo power is very small compared the transmitted power

• Echoes are first converted to an “intermediate frequency” by mixing the unique return echo frequency with the constant transmitted frequency

• Intermediate wave are then amplified by a known amount before being sent to the Doppler phase detector and display unit

Reflectivity:

• Amplitude difference between echo and known amplification

Doppler winds:

• Related to frequency difference between transmitted wave and echo (later…)


Signal CharacteristicsTransmitted Signal:

Quantity Symbol Units Units Typical Value Comments

Frequency ft hertz MHz, GHz 3000 MHz c = ftλ

Wavelength λ meter cm 10 cm c = ftλ

Pulse Duration τ second μs 1 μs

Pulse Length h meter m 300 mLength of pulse as it travels

through the atmosphereh = cτ

Pulse Repetition Frequency

F s-1 s-1 400 s-1

Pulse RepetitionPeriod

Tr second ms 2.5 msTime between pulses

Tr = 1 / F

Peak Power Pt watt kW, MW 1 MW 1 MW = +90 dBm(reference is 1 milliwatt)

Pulse Energy W joule J 1 J Integral of the average power over one complete pulse

Average Power Pav watt kW 400 WPower averaged over one

complete pulse repetition periodPav = WF


Signal CharacteristicsTransmitted Signal: Considerations

Wavelength: Choice is a function of the target to be studied and budget

Larger wavelengths → Precipitation detection Require large antennas ($)

Pulse Duration: Choice a function of sensitivity and range resolution

Longer durations → Better sensitivity (i.e. less error in a given dBZ) Poorer range resolution (i.e. no detailed structure)

PRF: Choice dictates the maximum range at which a target can be detected ( after a pulse has been transmitted, the radar must wait long enough ) ( to allow echoes from the most distant detectable targets to return ) ( “second trip echoes” → Returns observed after the next pulse )

Larger frequencies → Greater range→ Multiple echoes of same target (better sensitivity)→ Less motion by radar between consecutive pulses (better angular resolution of target)


Signal CharacteristicsTransmitted Signal: Considerations

Peak Power: The power of the return echo from a target increases with the transmitted power of the pulse → large peak powers are desired

Pulse Energy: Radar sensitivity increases with pulse energy → large magnitudes desired

Average Power: Directly related to peak power and pulse energy → large values desired

( Quantity most often calibrated for modern radars )( Most radar achieve accuracies of < 0.1 dBz )


Signal CharacteristicsTransmitted Signal: How to Express Power

Ratio of two powers:

(decibels)

where P1 = Observed power P2 = Reference power (constant)

2

1log10P

Pdb

mw

Pdbm

1log10 1


Signal CharacteristicsReceived Signal (Radar Echoes):

Quantity Symbol Units Units Typical Value Comments

Frequency fr hertz MHz, GHz ~3000 MHzDiffers from the transmitted

frequency by the Doppler shift (usually less than a few kHZ)

Wavelength λr meter cm ~10 cm c = frλr

Pulse Repetition Frequency

F s-1 s-1 400 s-1 Same as transmitted PRF

Pulse RepetitionPeriod

Tr second ms 2.5 ms Same as for transmitted pulse

Received Power Pr watt mW, nW 10-6 mW 10-6 mW = -60 dBm

Time of Arrival Δt second ms 1 ms Measured from the time of the transmitted pulse


Signal CharacteristicsReceived Signal (Radar Echoes): Considerations

Frequency: Difference between the transmitted and received frequencies is the“Doppler shift” → Proportional to the radial velocity of the target

→ More on this later…

Received Power: Many orders of magnitude smaller than the transmitted powerLarger values denote a greater “total” cross-section by the target(s)

Minimum Detectable Signal (MDS) → weakest return power that candiscriminated from the ever present background noise

Time of Arrival: Used to determine target’s range (r) from the radar following:

2

tcr


Display - NEXRAD


Display - NEXRADPlan-Position Indicator (PPI) Scanning Strategy: Single Elevation Angle

Data collected on a cone are projected onto a plane

Echoes close to the radar are at a low

elevation

Echoes far from the radar are at a high

elevation


Display - NEXRADVolume Scanning Strategies:

Precipitation mode scan geometry Severe weather scan geometry

Saves time…fewer elevations


Display - NEXRAD

Clear air mode scanning geometry

Fewer elevations, slower antennarotation achieves greater sensitivityfor clear air turbulence, clouds,Insects, drizzle, or light snowfall.

Volume Scanning Strategies:


Display - NEXRADRadar Reflectivity:

• A measure of the power scattered back to the radar from objects in the path of a radar beam• Proportional to the sum of the sixth power of the diameter of all the particles illuminated by a pulse provided the particles are smaller than the radar wavelength (more on this later…)

65 55 45 35 25 15 5

Reflectivity Factor (dBZ)


Display - NEXRADPrecipitation Mode:

• Used once liquid precipitation is observed


Display - NEXRADClear-Air Mode:

• Used for snow and detecting the onset of deep convection


Display - NEXRADBase Reflectivity:

• Echo intensity at the lowest PPI scan level (0.5°) measured in dBZ


Display - NEXRADComposite Reflectivity:

• Maximum echo intensity at any PPI scan level measured in dBZ


Display - NEXRADStorm Total Precipitation:

• Time integral of base reflectivity after NWS selected start time (measured in inches)• Primary tool to predict flash flooding


Display - NEXRADVertically Integrated Liquid (VIL):

• Integral of reflectivity (or water mass) through a column (measured in kg /2)• Used to estimate the presence of hail and hail size (large VIL = large hail)


Display - NEXRADRadial Velocity:

• Observed velocity component along the radar beam direction (measured in knots)


Display - NEXRADStorm-relative Radial Velocity:

• Velocity component with the component of the storm motion along the radar beam removed• Best display for detecting mesocyclones, tornado vortex signatures, or microbursts


Display - NEXRADCombined Radar Reflectivity and Radial Velocity:

• Used to detect most severe weather


NEXRAD DataAvailable Data:

• All NEXRAD data since 1992 has been archived and is publically available through the NCDC at: http://hurricane.ncdc.noaa.gov/pls/plhas/has.dsselect

Level-II: Radar reflectivity and radial velocity at original sampling resolutionRaw volumetric data

Level-III: Derived products most used by forecasters

Base reflectivityComposite reflectivityBase radial velocityBase storm-relative radial velocityVertically-integrated liquid (VIL)Echo tops (ET - maximum height of 10 dBZ echo)Storm total precipitationand many more….


Display – Other TypesRange-Height Indicator (RHI) Scanning Strategy:

• Radar is scanned in elevation at a fixed azimuth• Volume scans are accomplished by rotating slowly in azimuth while scanning rapidly in elevation


Display – Other TypesExamples of RHI Scans:

• Sequence of RHI scans showing development of shallow cumulus along the south Florida coast

Note ground clutter and echo from tall buildings

(echo from radar side lobe)


Display – Other TypesExamples of RHI Scans:

• Vertical cross-section through a squall line reconstructed via RHI slices through a PPI volume


Display – Other TypesTime-Height Scanning Strategy:

• Radar is pointed vertically as storm passes over


Display – Other TypesHorizontal cross-sections:

• Radar data is interpolated from cylindrical to Cartesian coordinates and displayed in Cartesian space

• Often done when constructing analyses from multiple Doppler radars (more on this later…)


Display – Other TypesVertical cross-sections:

• Radar data is interpolated from cylindrical to Cartesian coordinates and displayed in Cartesian space

• Cartesian grid can be sliced similar to RHI scan

• If constructed from multiple Doppler radars, the vertical wind component can be estimated and displayed


Display – Other TypesRadar Composites:

• Composite reflectivity from multiple PPI scans are projected onto a single display to show regional or national precipitation distributions• The rain-snow distinction determined by surface observations (not the radar)


Display – PhenomenaBeam Blockage:

• Caused by tall buildings, trees, water towers, and cell towers near radar…

BlockedBeam


Display – PhenomenaNon-Meteorological Targets:

• Insects and bats often rest during the day and travel at night → take-off at sunset• Birds rest at night and travel during the day → take-off at sunrise

Birds departingAt 1114 UTC


Display – PhenomenaNon-Meteorological Targets:

• Ground clutter, aircraft, etc…

Columbia shuttleBreak-up

Ground clutter and diverted aircraft


Display – PhenomenaBright Band:

• Enhancement of radar reflectivity at the melting level as large aggregate snowflakes develop a thin film of water on their surface before they collapse to a smaller drop

D istan c e (km )

R ef le ct iv ity fa cto r (d B Z )

S tr a t i fo rm a rea C on v ec t io n

B BAltitude (km)


Display – PhenomenaConvective Storms: PPI Displays



Summary:

• Radar Components• Signal Characteristics• Display – NEXRAD• Display – Other Types• Display – Phenomena

fundamentals of radar and display

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