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
Radar Meteorology
M. D. Eastin
Radar Components
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 Meteorology
Sidelobes
PULS
E
ic
Electr
Field
ANTENNA
Half-power beamwidth
TRANSMITTER
Duplexer
switch
Klystron
Amplifier
Pulse
modulator
Frequency
Mixer
Frequency
Mixer
STALO
Microwave
Oscillator
Amplifier
DISPLAY
COHO
Microwave
Oscillator
Phase
Detector
RECEIVER
M. D. Eastin
Radar Components
A 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
Sidelobes
PULS
E
ic
Electr
Field
ANTENNA
Half-power beamwidth
TRANSMITTER
Duplexer
switch
Klystron
Amplifier
Pulse Repetition Frequency (PRF)
• Sets the timing between each pulse
• Fixed (operational radars)
• User-controlled (research radars)
Frequency
Mixer
Frequency
Mixer
STALO
Microwave
Oscillator
Amplifier
Radar Meteorology
Pulse
modulator
DISPLAY
COHO
Microwave
Oscillator
Phase
Detector
RECEIVER
M. D. Eastin
Radar Components
A 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
Radar Meteorology
Sidelobes
PULS
E
ic
Electr
Field
ANTENNA
Half-power beamwidth
TRANSMITTER
Duplexer
switch
Klystron
Amplifier
Pulse
modulator
Frequency
Mixer
Frequency
Mixer
STALO
Microwave
Oscillator
Amplifier
DISPLAY
COHO
Microwave
Oscillator
Phase
Detector
RECEIVER
M. D. Eastin
Radar Components
A 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
Sidelobes
PULS
E
ic
Electr
Field
ANTENNA
Half-power beamwidth
TRANSMITTER
Duplexer
switch
Klystron
Amplifier
Half-power Beam Width
• Function of radar design and range
• Radius of a conical cross-section
(i.e. a circle) at a given range
Frequency
Mixer
Frequency
Mixer
STALO
Microwave
Oscillator
Amplifier
Radar Meteorology
Pulse
modulator
DISPLAY
COHO
Microwave
Oscillator
Phase
Detector
RECEIVER
M. D. Eastin
Radar Components
A 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
Sidelobes
Doppler winds:
E
ANTENNA
Half-power beamwidth
TRANSMITTER
Duplexer
switch
Reflectivity:
• Amplitude difference between echo
and known amplification
PULS
ic
Electr
Field
Klystron
Amplifier
Pulse
modulator
Frequency
Mixer
Frequency
Mixer
STALO
Microwave
Oscillator
Amplifier
DISPLAY
COHO
Microwave
Oscillator
Phase
Detector
RECEIVER
• Related to frequency difference between
transmitted wave and echo (later…)
Radar Meteorology
M. D. Eastin
Signal Characteristics
Transmitted Signal:
Quantity
Symbol Units
Units
Typical Value
Comments
Frequency
ft
hertz
MHz, GHz
3000 MHz
c = f tλ
Wavelength
λ
meter
cm
10 cm
c = f tλ
Pulse Duration
τ
second
μs
1 μs
Pulse Length
h
meter
m
300 m
Pulse Repetition
Frequency
F
s-1
s-1
400 s-1
Pulse Repetition
Period
Tr
second
ms
2.5 ms
Time 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
1J
Integral of the average power
over one complete pulse
Average Power
Pav
watt
kW
400 W
Power averaged over one
complete pulse repetition period
Pav = WF
Radar Meteorology
Length of pulse as it travels
through the atmosphere
h = cτ
M. D. Eastin
Signal Characteristics
Transmitted 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)
Radar Meteorology
M. D. Eastin
Signal Characteristics
Transmitted 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
)
Radar Meteorology
M. D. Eastin
Signal Characteristics
Transmitted Signal: How to Express Power
Ratio of two powers:
 P1 
db  10 log 
 P2 
where
(decibels)
P1 = Observed power
P2 = Reference power (constant)
 P 
dbm  10 log 1 
 1 mw 
Radar Meteorology
M. D. Eastin
Signal Characteristics
Received Signal (Radar Echoes):
Quantity
Symbol Units
Units
Typical Value
Comments
Frequency
fr
hertz
MHz, GHz
~3000 MHz
Differs from the transmitted
frequency by the Doppler shift
(usually less than a few kHZ)
Wavelength
λr
meter
cm
~10 cm
c = f rλ r
Pulse Repetition
Frequency
F
s-1
s-1
400 s-1
Same as transmitted PRF
Pulse Repetition
Period
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
Radar Meteorology
M. D. Eastin
Signal Characteristics
Received 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 power
Larger values denote a greater “total” cross-section by the target(s)
Minimum Detectable Signal (MDS) → weakest return power that can
discriminated from the ever
present background noise
Time of Arrival:
Used to determine target’s range (r) from the radar following:
r
Radar Meteorology
ct
2
M. D. Eastin
Display - NEXRAD
Radar Meteorology
M. D. Eastin
Display - NEXRAD
Plan-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
Radar Meteorology
M. D. Eastin
Display - NEXRAD
Volume Scanning Strategies:
Precipitation mode scan geometry
Severe weather scan geometry
Saves time…fewer elevations
Radar Meteorology
M. D. Eastin
Display - NEXRAD
Volume Scanning Strategies:
Clear air mode scanning geometry
Fewer elevations, slower antenna
rotation achieves greater sensitivity
for clear air turbulence, clouds,
Insects, drizzle, or light snowfall.
Radar Meteorology
M. D. Eastin
Display - NEXRAD
Radar 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…)
Reflectivity Factor (dBZ)
65
Radar Meteorology
55
45
35
25
15
5
M. D. Eastin
Display - NEXRAD
Precipitation Mode:
• Used once liquid precipitation
is observed
Radar Meteorology
M. D. Eastin
Display - NEXRAD
Clear-Air Mode:
• Used for snow and detecting
the onset of deep convection
Radar Meteorology
M. D. Eastin
Display - NEXRAD
Base Reflectivity:
• Echo intensity at the lowest PPI scan level (0.5°) measured in dBZ
Radar Meteorology
M. D. Eastin
Display - NEXRAD
Composite Reflectivity:
• Maximum echo intensity at any PPI scan level measured in dBZ
Radar Meteorology
M. D. Eastin
Display - NEXRAD
Storm Total Precipitation:
• Time integral of base reflectivity after NWS selected start time (measured in inches)
• Primary tool to predict flash flooding
Radar Meteorology
M. D. Eastin
Display - NEXRAD
Vertically 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)
Radar Meteorology
M. D. Eastin
Display - NEXRAD
Radial Velocity:
• Observed velocity component along the radar beam direction (measured in knots)
Radar Meteorology
M. D. Eastin
Display - NEXRAD
Storm-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
Radar Meteorology
M. D. Eastin
Display - NEXRAD
Combined Radar Reflectivity and Radial Velocity:
• Used to detect most severe weather
Radar Meteorology
M. D. Eastin
NEXRAD Data
Available 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 resolution
Raw volumetric data
Level-III: Derived products most used by forecasters
Base reflectivity
Composite reflectivity
Base radial velocity
Base storm-relative radial velocity
Vertically-integrated liquid (VIL)
Echo tops (ET - maximum height of 10 dBZ echo)
Storm total precipitation
and many more….
Radar Meteorology
M. D. Eastin
Display – Other Types
Range-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
Radar Meteorology
M. D. Eastin
Display – Other Types
Examples 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)
Radar Meteorology
M. D. Eastin
Display – Other Types
Examples of RHI Scans:
• Vertical cross-section
through a squall line
reconstructed via
RHI slices through
a PPI volume
Radar Meteorology
M. D. Eastin
Display – Other Types
Time-Height Scanning Strategy:
• Radar is pointed vertically as storm passes over
Radar Meteorology
M. D. Eastin
Display – Other Types
Horizontal 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…)
Radar Meteorology
M. D. Eastin
Display – Other Types
Vertical 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
Radar Meteorology
M. D. Eastin
Display – Other Types
Radar 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)
Radar Meteorology
M. D. Eastin
Display – Phenomena
Beam Blockage:
• Caused by tall buildings, trees, water towers, and cell towers near radar…
Blocked
Beam
Radar Meteorology
M. D. Eastin
Display – Phenomena
Non-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 departing
At 1114 UTC
Radar Meteorology
M. D. Eastin
Display – Phenomena
Non-Meteorological Targets:
• Ground clutter, aircraft, etc…
Ground clutter and
diverted aircraft
Columbia shuttle
Break-up
Radar Meteorology
M. D. Eastin
Display – Phenomena
Bright 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
Stratiform area
Convection
Altitude (km)
BB
Distance (km)
Reflectivity factor (dBZ)
Radar Meteorology
M. D. Eastin
Display – Phenomena
Convective Storms: PPI Displays
Radar Meteorology
M. D. Eastin
Fundamentals of Radar and Display
Summary:
• Radar Components
• Signal Characteristics
• Display – NEXRAD
• Display – Other Types
• Display – Phenomena
Radar Meteorology
M. D. Eastin