METR 2413
4 February 2004
Radar
Observations
Radar Basics
RADAR - acronym for RAdio Detection And
Ranging; a radio device or system for locating an
object by means of ultrahigh-frequency radio waves
reflected from the object and received, observed, and
analyzed by the receiving part of the device in such a
way that characteristics (as distance and direction) of
the object may be determined.
Each Radar system consists of an antenna and a
receiver.
Radar History
The WSR-57 and WSR-74 NWS Weather Surveillance Radar
units were replaced by WSR-88D units.
The WSR-88D (Weather Surveillance Radar - 1988 Doppler)
is a NEXRAD unit.
NEXRAD (NEXt-Generation Weather RADar) is a network of
high-resolution WSR-88D Doppler radars operated by the
NWS.
Other Radar systems exist including the Doppler on Wheels
(DOWS), the ELDORA, Polarmetric radar, etc.
Radar History
The Norman
Doppler radar
at NSSL,
with an older
WSR-57
radar (right)
Radar Fundamentals
The WSR-88D radar transmits a stream or "beam" of energy
in discrete pulses which propagate away from the radar
antenna at approximately the speed of light (~3 × 108 ms-1).
The volume of each pulse of energy will determine how many
targets are illuminated. This directly determines how much
energy (power) is returned to the radar. The shape of the radar
antenna, the wavelength (l) of the energy transmitted, and the
length of time the radar transmits determine the shape and
volume of each radar pulse.
Radar Fundamentals
The power density determines how much energy targets
intercept and reflect or "backscatter" toward the radar antenna.
Two pulses of energy have been transmitted by the radar in the
following figure. The pulse on the right was transmitted first.
Due to its greater range from the radar, it has a larger volume
and lower power density than the second transmitted pulse on
the left.
If two radars transmitted the same amount of power but had
different beamwidths, then the one with the narrower beam
would have greater sensitivity due to its greater power density.
This would result in the detection of smaller targets at greater
ranges.
Radar Fundamentals
Radar Fundamentals
As pulse volumes within the radar beam encounter targets,
energy will be scattered in all directions. A very small portion
of the intercepted energy will be backscattered toward the
radar. The degree or amount of backscatter is determined by
target
*
*
*
*
size (radar cross section)
shape (round, oblate, flat, etc.)
state (liquid, frozen, mixed, dry, wet)
concentration (number of particles per unit volume).
Radar Fundamentals
We are concerned with two types of scattering, Rayleigh and nonRayleigh (there are several types such as Mie scattering). Rayleigh
scattering occurs with targets whose diameter (D) is much smaller
(D < l/16) than the wavelength of the transmitted radio waves.
The WSR-88D wavelength is approximately 10.7 cm, so Rayleigh
scattering occurs with targets whose diameters are less than or
equal to about 7 mm or ~0.4 inch. Raindrops seldom exceed 7 mm
so all liquid drops are Rayleigh scatterers.
Nearly all hailstones are non-Rayleigh scatterers due to their larger
diameters. However, since the vast majority of targets sampled by
the WSR-88D are raindrop size or smaller, the Rayleigh
assumption is used in all computations of radar reflectivity.
Radar Fundamentals
The Probert-Jones (P-J) radar reflectivity equation will help to quantify the
physical aspects of pulsed E-M energy and the associated limitations of target
(e.g., precipitation) detection. The P-J equation is described below as
where:
Pr = power returned to the radar from a target (watts)
Pt = peak transmitted power (watts)
G = antenna gain,
q = angular beamwidth
H = pulse length,
p = 3.14159
K = physical constant (target character)
L = signal loss factors associated with attenuation and receiver detection
Z = target reflectivity,
l = transmitted energy wavelength
R = target range
Radar Fundamentals
For the WSR-88D, the only variables that are not fixed are returned power (Pr),
reflectivity (Z), attenuation factor (La), and range (R). The fixed variables are
combined to create a new term which we will refer to as the radar constant, Cr.
By combining the fixed variables into a radar constant, the previous simplifies
into
where Cr is the radar constant. Solving for Z, the above equation becomes
By knowing the power returned which the radar can easily measure, the above
equation indirectly estimates target reflectivity.
Radar Fundamentals
Range-normalized values of reflectivity, Z, can range over many
orders of magnitude. To compress this large range of values for
operational use, Z is displayed in decibels of Z, that is, dBZ.
Converting Z to dBZ is simply done by using
For example, if Z = 4000 mm6m-3, then
dBZ = 10(log10 4000)  10 x 3.6 = 36 dBZ.
Radar Fundamentals
Due to the WSR-88D’s increased sensitivity, reflectivities as low as 32 dBZ can be detected in clear air mode near the RDA.
How can there be such a thing as a negative dBZ?
If 0 < Z < 1, log10Z < 0 and thus dBZ < 0.
Very low dBZ values indicate the presence of extremely small sized
particles (e.g., dust, haze, smoke).
The WSR-88D can also detect reflectivity values as high as 95 dBZ.
As an example, a one cubic meter volume containing just one 38.3
mm (~1.50 inch) diameter water-coated hailstone would yield a
reflectivity value of approximately 95 dBZ. However, giant hail
frequently occurs with reflectivities less than 70 dBZ. This is a good
indication that such large targets do not meet the Rayleigh
approximation
Radar Fundamentals
If PRT is the time from the beginning of one pulse to the beginning
of the next pulse, and t is the time actually spent transmitting, then
PRT-t = t is the listening period. For example, if the WSR-88D is
operating for 1.57 µsec and using a PRT of 1000 µsec (0.001 s or 1
millisecond), then the listening period is t = PRT- t = 1000 - 1.57
µsec = 998.43 µsec (or 0.99843 millisecond). As a result, for each
hour the radar is active at this PRT, only about 5.7 seconds is spent
transmitting. This means that 99.843% of the time the WSR-88D is
listening for signal returns.
In long pulse, the radar transmits 17.1 seconds every hour,
spending 99.525% of its time listening.
Radar Fundamentals
SMART-R (http://www.nssl.noaa.gov/smartradars/)
is a collaborative radar meteorology research program.
Two mobile 5-cm Doppler radars are used to study convective and
mesoscale atmospheric processes to help improve forecasts of
significant weather events such as flash floods, hurricanes and
tornadoes.
WSR-88D Radar Products
Base Reflectivity is one of the basic quantities that a
Doppler radar (like NEXRAD) measures. Base
Reflectivity basically corresponds to the amount of
radiation that is scattered or reflected back to the radar by
whatever targets are located in the radar beam at a given
location (units are in dBZ). These targets can be
hydrometeors (snow, rain drops, hail, cloud drops or ice
particles) or other targets (dust, smoke, birds, airplanes,
insects). The colors on the Base Reflectivity product
correspond to the intensity of the radiation that was
received by the radar antenna from a given location.
WSR-88D Radar Products
WSR-88D Radar Products
Like Base Reflectivity, Base Velocity is a base
product measured by the radar. Base Velocity is
the average radial velocity of the targets in the
radar beam at a given location. Radial velocity is
the component of the target's motion that is along
the direction of the radar beam. Positive values
(warm colors) denote out-bound velocities that
are directed away from the radar. Negative
values (cool colors) are in-bound velocities that
are directed towards the radar.
WSR-88D Radar Products
WSR-88D Radar Products
Composite Reflectivity is the maximum base reflectivity
value that occurs in a given vertical column in the radar
umbrella. NEXRAD scans in several pre-defined "volume
coverage patterns (VCPs), where the radar makes a 360degree horizontal sweep with the radar antenna tilted at a
given angle above the horizontal, then changes the
elevation angle, and completes another 360-degree
sweep, and so on. Composite reflectivity gives a plan
view of the most intense portions of thunderstorms, and
can be compared with Base Reflectivity to help
determine the 3-D structure of a thunderstorm.
WSR-88D Radar Products
WSR-88D Radar Products
The Rainfall Accumulation products attempt to estimate
the amount of rainfall that has fallen in a given area under
the radar's umbrella. NEXRAD does this by making
certain assumptions about the number and kind of
raindrops it detects. There are certain limitations involved
with radar estimation of rainfall, which is a subject of
current meteorological research, and there are plans to
improve the way that NEXRAD produces its rainfall
estimates. A given rainfall product should generally be
compared with a product from another radar or with rain
gage reports, if they're available.
WSR-88D Radar Products
WSR-88D Radar Products
WSR-88D Radar Products
Storm-Relative Radial Velocity is Base
Velocity with the average motion of all storm
centroids subtracted out. Storm-Relative
Radial Velocity can be useful in finding
mesocyclones or other circulation patterns.
WSR-88D Radar Products
WSR-88D Radar Products
Vertically Integrated Liquid, or VIL, is a
calculation that converts a column of
reflectivity into its liquid water equivalent.
However, it turns out that VIL is seasonally
and geographically correlated to hail size.
WSR-88D Radar Products
WSR-88D Radar Products
The VAD Wind Profile is a time series of
estimate of the horizontal wind at specific
heights above the radar. It is useful in
diagnosing the locations and structure of
fronts, the movement of moisture from the
Gulf of Mexico, and other meteorological
phenomena.
WSR-88D Radar Products