Corporate Profile

1
METR 2413 4 February 2004
Radar Observations
2
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.
3
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.
4
Radar History
The Norman Doppler radar at NSSL, with an older
WSR-57 radar (right)
5
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.
6
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.
7
Radar Fundamentals
8
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).
9
Radar Fundamentals
We are concerned with two types of scattering,
Rayleigh and non-Rayleigh (there are several
types such as Mie scattering). Rayleigh
scattering occurs with targets whose diameter (D)
is much smaller (D lt 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.
10
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  
11
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.
12
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.
13
Radar Fundamentals
Due to the WSR-88Ds 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 lt Z lt 1,
log10Z lt 0 and thus dBZ lt 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
14
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.
15
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.
16
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.
17
WSR-88D Radar Products
18
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.
19
WSR-88D Radar Products
20
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
360-degree 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.
21
WSR-88D Radar Products
22
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.
23
WSR-88D Radar Products
24
WSR-88D Radar Products
25
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.
26
WSR-88D Radar Products
27
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.
28
WSR-88D Radar Products
29
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.
30
WSR-88D Radar Products