SO 254 Spring 2017 LCDR Matt Burich Radar is the most commonly used active sensor in meteorology RADAR RA dio D etection A nd R anging Active refers to the fact that a radar alternately switches between transmitting ID: 582920
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Slide1
Intro to Radar
SO 254 – Spring 2017
LCDR Matt BurichSlide2
Radar is the most commonly used
active
sensor in meteorology
RADAR: RAdio Detection And Ranging
“Active” refers to the fact that a radar alternately switches between transmitting
and
receiving pulses of microwave energy via a common antenna
After transmitting a short but powerful pulse, the “switch” (or duplexer) places the system into receive mode where the antenna collects the “echo” of the pulse off of targets in its path
The antenna focuses the microwave energy into a narrow beam so that transmitter power is concentrated in a specific direction.
Echoes collected by the antenna are processed in the receiver where a few basic parameters of the returned signal are measured.Slide3
Measured parameters:
RADAR:
RA
dio Detection And Ranging
Where
is the speed of light and
is the elapsed time between the pulse transmission and received echo
1)
Range
to the targets via:
Strength of the returned echo from which the equivalent
reflectivity
of the targets is derived
The Doppler shift in frequency (phase) of the echo vs. the transmitted pulse indicating the component of
target velocity in the line of sight of the radar ()
The polarization of the echo from
which information on target shape
may be derived Slide4
Radar Reflectivity (Z)
The radar beam simultaneously illuminates many meteorological targets at once within
a
resolution volume The resolution volume is a spherical rectangle, the dimensions of which are defined by the performance parameters of the radar (its pulse length, beam width, and beam height)Reflectivity is derived from the amount of power returned in a radar echo as collected by the antenna and processed in the receiver
In general, larger number concentrations of precipitation particles (hydrometeors) and larger sized hydrometeors will produce larger values of reflectivitySlide5
Radar Reflectivity (Z)
The radar beam simultaneously illuminates many meteorological targets at once within
a
resolution volume The resolution volume is a spherical rectangle, the dimensions of which are defined by the performance parameters of the radar (its pulse length, beam width, and beam height)Reflectivity is derived from the amount of power returned in a radar echo as collected by the antenna and processed in the receiver
In general, larger number concentrations of precipitation particles (hydrometeors) and larger sized hydrometeors will produce larger values of reflectivitySlide6
Radar Reflectivity (Z)
Reflectivity is measured logarithmically in decibel units (
dBZ
)-50 to 0 dBZ = marginally detectable precipitation0-10 dBZ = drizzle, very light rain or snow10-30 dBZ = moderate rain / heavier snow
30-60
dBZ
= moderate to heavy rain / melting snow60-70 dBZ = hail
Some characteristic values:Slide7
Radar Reflectivity (Z)
Reflectivity is measured logarithmically in decibel units (
dBZ
)-50 to 0 dBZ = marginally detectable precipitation0-10 dBZ = drizzle, very light rain or snow10-30 dBZ = moderate rain / heavier snow
30-60
dBZ
= moderate to heavy rain / melting snow60-70 dBZ = hail
Some characteristic values:Slide8
Radial Velocity
Radial velocity is the component of a target’s velocity in the
line of sight
of the radar (motion either directly toward or directly away)
Target moving 100
kts
100% of target motion is directly toward or away from the radar (along a radial)Slide9
Radial Velocity
Radial velocity is the component of the target’s velocity in the
line of sight
of the radar (motion either directly toward or directly away)
Target moving 100
kts
In the image, the wind is from the south at 20
kts
everywhere (black arrows) – the component of the wind in the
line of sight
of the radar (radial velocity) is indicated by blue arrows
On the radar display, velocities are color-coded to indicate motion toward (
green
) or away (
red
) from the radar site
-20
20
14
14
-14
-14
0
0
Target
not
moving directly toward the radarSlide10
Radial Velocity
Radial velocity has proven to be invaluable at locating areas of small-scale rotation in thunderstorms and can be displayed both from a ground-relative sense (i.e., velocity relative to the radar) and a “storm-relative” sense (by subtracting storm motion from ground-relative velocity)
Areas where strong velocity toward the radar and strong velocity away from the radar are located immediately adjacent to one another are suggestive of such areas which may produce tornadoes if the spin is sufficiently concentrated
The image to the right shows the rotation associated with an EF-4 tornado that struck Tuscaloosa, Alabama in April of 2011
radarSlide11
Radial Velocity
Doppler radar determines radial velocity by comparing the frequency difference (really, phase difference) between the transmitted and received pulse and computing a Doppler “beat frequency”
which is proportional to velocity in the line of sight
Targets moving toward the radar will cause the frequency of the echo to increase over the transmitted pulse while targets moving away will cause the frequency of the echo to decreaseSlide12
Echo polarization
Up until recently, most weather radars transmitted pulses with a horizontal polarization only since larger raindrops tend to be “oblate” (flat) due to aerodynamic drag (therefore, a stronger echo was returned)
In recent years, our primary National Weather Service radars have been upgraded to have a “dual polarization” capability – that is, they transmit both horizontally polarized and vertically polarized pulses
This capability allows for the determination of target shape
(among other things) and has shown skill at distinguishing heavy rain (large horizontal return and smaller vertical return) from spherical hail (nearly equal horizontal and vertical returns)Slide13
National Weather Service Radar
The National Weather Service maintains a network of dual-polarization Doppler weather radars designated WSR-88D (Weather Surveillance Radar 1988 Doppler)
The word “surveillance” refers to the radar’s primary scan mode which is a 360 degree sweep (a surveillance scan) as opposed to a sector scan which would be something less than 360 degrees or an elevation scan which would hold a constant azimuth and vary elevation angleSlide14
National Weather Service Radar
Despite the fact that the WSR-88D performs a surveillance scan, it
does
vary elevation angle between each 360 degree sweep to interrogate a three-dimensional volume of spaceEssentially, after completing a 360 degree scan at one elevation angle, the radar will adjust its elevation angle (“tilt”) slightly and complete another 360 degree scan, and so onDepending on the volume coverage pattern (VCP) desired (and there are several), an entire volume scan
usually takes between 4 and 6 minutes after which point the radar returns to the lowest elevation angle (called the
base
scan…usually 0.5 degrees above the horizon) and begins the process againSlide15
The image below shows the elevation angles scanned (a total of 14) for VCP 21…A VCP may also be referred to as a radar’s “scan strategy”
Base scan (0.5 degrees)
The base scan is what is typically shown in most applications and
should represent best the precipitation intensity near the ground. However…Note that the base elevation angle scan becomes “higher” off the ground as distance from the radar increases…this must be kept in mind when examining echoes at the far extent of a radar’s rangeSlide16
Composite Reflectivity
Composite reflectivity displays the highest value of echo return from a particular azimuth and range from
any
elevation angleEssentially, it gives a somewhat more three-dimensional look at precipitation and will readily reveal areas where precipitation is falling aloft, but evaporating before reaching the ground (compare between base and composite)Base scan
CompositeSlide17
Finally, for today, we note that radar is particularly effective at locating the “melting layer” where frozen hydrometeors (e.g., snowflakes) are melting into rain. This layer appears as a ring of high reflectivity called a
bright band
on radar as the frozen hydrometeors become “wet” as they’re melting and appear to the radar to be very large rain droplets