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Appendix A
Criteria for Evaluation of a Weather Radar System
The criteria for comparison of the Next Generation Weather Radar (NEXRAD) and pre-
NEXRAD systems fall into three broad areas:
1.
2.
3.
technical aspects including detection capabilities;
warning performance; and
products and services offered.
RADAR TECHNICAL ASPECTS
This section provides a system description and radar performance specifications and discusses
the key radar technical aspects that were considered in developing criteria for addressing the quality
of radar coverage. Key radar technical aspects include coverage at specific altitudes, spatial and
temporal resolution, sensitivity, Doppler coverage, and radar availability.
Other technical aspects of radar operation (e.g., clutter suppression, antenna beam patterns,
algorithms, and data archiving) are also discussed in this section. However, these did not enter directly
into the evaluation of the detectability of weather phenomena for the "quality of service" assessment.
The discussion of radar technical aspects concludes with a description of NEXRAD technical improve-
ments.
System Description
The pre-NEXRAD NWS radar network consisted of 128 WSR-57, WSR-74S, and WSR-74C
radars. The WSR-57 and WSR-74S radars, both LO-cm-wavelength systems, were configured as
manually operated surveillance radars; the 68 WSR-74C, 5-cm radars functioned as "local warning"
radars. The NEXRAD network, when complete, will consist of 138 lO-cm WSR-~8Ds in the
contiguous United States, including 22 Department of Defense (DoD) radars. Beyond the contiguous
United States, seven NEXRADs are planner! in Alaska, four in Hawaii, three in the Caribbean, and
several other systems in locations around He world.
65
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66
Appendix A
Radar Performance Specifications
Table A-! gives the technical characteristics of the NEXRAD and comparable values for the
pre-NEXRAD radars. NEXRAD is a Doppler radar system that routinely generates products involving
both reflectivity and velocity information. The reflectivity estimates of the NEXRAD have significantly
higher data quality than the pre-NEXRAD systems due to higher spatial resolution of the narrow
antenna beam, increased sensitivity to weak echoes, new Doppler clutter-suppression processing, and
new calibration procedures. Radar coverage with limited Doppler capability was available in the pre-
NEXRAD NWS network only at Montgomery, Alabama; Marseilles, Illinois; and Huntsville, Alabama.
Therefore, for weather phenomena that can only be detected using Doppler features (e.g.,
mesocyclones or microbursts), limitations in the NEXRAD network Doppler coverage are not a factor
for most of the country under the "no degradation of service" criterion. Where a combination of
Doppler and reflectivity features is used for phenomena detection (e.g., supercelIs), limitations in the
NEXRAD Doppler coverage could be a factor in those geographic areas where NEXRAD coverage
at long range must be compared to pre-NEXRAD coverage at short range.
TABLE A-1 Radar Characteristics-Pre-NEXRAD and NEXRAD
NEXRAD
WSR-57
WSR-74S
WSR-74Ca
P. = Transmitter peak output power (MOO)0.750 0.5000.5000.250
G = Antenna gain (dB)45.5 38.538.540
= Wavelength (cm)10.53 10.7110.715.33
t2 = Propagation loss (dB) @˘ 0.50 ~ 50km0.7 0.70.70.8
~ 230 km2.5 2.52.52.8
al = Beam width (de")0.95 2.02.01.5
~ = Pulse width Cases)1.57 4.004.003.00
Bn = Noise band width (MHz)0.79 0.751.500.80
TSYS = System noise temp (K)600 170010002150
Z.,,,,, = Min detectable dBZ ~ 50 km (single pulse) -7.6 -0.2 0.3 -1.7
Doppler Feature yes no no no
Clutter Suppression yes no no no
a The Huntsville, Alabama, WSR-74C has enhanced capabilities G = 43 dB, 8, = 1.0 decree ~ = ~ O
psec, Z.,,,,, <-5 dBZ, Doppler and clutter suppression.
_ ~, . _,
The NEXRAD has three major components, as shown in Figure Am. These components are the
radar data acquisition (RDA) unit, the radar product generator (RPG), and the principal user processor
(PUP) (Crum and Alberty, 1993~. The RDA includes the transmitter, antenna, receiver, signal proces-
sor, and Level 2 base data (wide band) communications interface. The RPG includes the computing
power that generates the image products and the narrow band communications interfaces. The PUP is
the processor workstation with which the forecaster requests and receives NEXRAD products. "Associ-
ated" PUPs are permanently connected with the RPG and routinely receive specified sets of products.
"Non-associated" PUPS are intermittently connected to any NEXRAD via a dial-up, low-speed
communication line and receive only requested products.
OCR for page 67
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68
Appendix A
The Doppler capability of the WSR-88D allows measuring storm kinematics (i.e., velocity
information) to generate more-accurate and timely weather advisories and warnings while suppressing
interfering ground echoes; thereby improving short-range and low-altitude coverage. Another important
design improvement is the availability of digital information in a variety of displays from the
NEXRAD.
Spatial Coverage
In planning the layout of the NEXRAD network, consideration was given to the distribution of
population within the contiguous United States. In meeting the needs of the sponsoring agencies,
further siting criteria included proximity to existing or planned Weather Forecast Offices and to
high-use commercial airports and designated high-value military facilities.
On a smooth spherical Earth, the coverage provided by a particular radar would be limited by
the curvature of the Earth and the refraction of the microwaves in the atmosphere. Because of those
factors, the maximum range of potential coverage for a given altitude is represented by a circle with
the radar located at the center. Due to blockage of the radar beam by intervening terrain features,
however, the area actually covered at a given altitude is not circular in some cases (particularly in the
West). When the coverage zones of all operational radars are plotted and superimposed, the resulting
"coverage map" provides one of the aids for evaluating the adequacy of radar coverage. Maps shown
later in this appendix illustrate this coverage, or radar field of view. Radar resolution, sensitivity, and
the forecaster's expertise must be added to the field-of-view in determining overall quality of service.
Generally the ability to detect weather phenomena decreases with increasing radar range because
the low-altitude coverage, the radar resolution, and radar sensitivity all deteriorate with increasing
range. In addition, there is a "cone of silence" above the radar, as illustrated in Figure A-2. This
deficiency may be alleviated by observations from neighboring NEXRADs over most of the United
States east of the Rocky Mountains.
At close ranges (up to, perhaps, 40 km) the interfering effects of ground clutter are at a
maximum." The NEXRAD clutter suppression feature mitigates this limitation to a considerable extent.
At long ranges both the Earth's curvature and the increasing resolution cell size reduce the radar's
ability to detect weather phenomena. These two features are illustrated in Figure A-3. For each weather
phenomenon shown, no combination of resolution and sensitivity can compensate for the fact that the
radar beam is simply too high above the grounc! for effective detection. Weather targets Hat protrude
into the lower edge of Me beam may be detected because of the NEXRAD's sensitivity. The restrictive
effect of the Earths curvature is a function of the height of the weather phenomenon and the degree
to which signals received from high levels can be interpreted effectively in terms of weaker at lower
heights. At long ranges, the width of the beam may prohibit resolution of fine details of Me echo
structure needed to identify particular weather phenomena. Representative ranges for detecting various
weaker phenomena are discussed in Chapter 2.
The coverage at various altitudes for both the pre-NEXRAD and NEXRAD networks has been
mapped. Figure A-4a shows Be NEXRAD altitude coverage, that is, where the NEXRAD coverage
~ All radars receive signals scattered from the ground. These signals are called "ground clutters because the
returns interfere with the signals from weather targets and often obscure the weather signal entirely. This problem
is particularly severe at close range (approximately out to 40 km) but is also strongly influenced by terrain,
especially in hilly or mountainous regions. Cities with tall buildings also produce strong clutter signals that can
obscure weather above them.
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MELEES
O DEGREES
11 ~ I ~ 11 COVERAGE IN CONE OF SILENCE
\ ~ ~ ~ ~ FROM NEIGHBORING RADAR
345 km (186 nml) AWAY
(REPRESENTATIVE SEPARATION
DISTANCE WEST OF ROCKIES)
HORIZONTAL SCALE, 1 INCH = 30 km
VERTICAL SCALE, 1 INCH = 7.5 km
SILENCE ~ ~7~
_ 40,000 fit__
___ ____
SITE ~
O DEGREES
I COVERAGE IN CONE OF SILENCE
FROM NEIGHBORING RADAR
230 km (124 nmi) AWAY
(REPRESENTATIVE SEPARATION
DISTANCE EAST OF ROCKIES)
~1
VERTICAL EXAGGERATION, 4 TIMES
Figure A-2 Cone of silence for the NEXRAD. The cone of silence has a radius of 34 km at 40,000 fit and a radius of 8.5 km at 10,000 ft.
Elevations higher than 19.5 degrees are not scanned using the existing volume coverage patterns; therefore, radar echoes are not received
from within this cone directly above each radar. Overlapping coverage from adjacent radars is needed to cover this region and is shown for
two radars operating with a 1-degree beam width centered at 0.5 degrees elevation. Courtesy of SRI International.
. ~ 0 OOO fit
ye
us
to
HORIZONTAL SCALE, 1 INCH = 50 km
VERTICAL SCALE, 1 INCH = 2 km
VERTICAL EXAGGERATION, 25 TIMES
1 1 1 1
0 40 80
- .
.
I. | to
\ / O r,EGREES
i, ---
- . _
I
·
· !5
° ~ I ~
by.
in
1 1 1 1 1 1 1 1 1
120 160 200 240
RANGE (km)
~ ~E
us
/~>, ,~` TORNADO E
~S~w, j
Figure A-3 Diagram illustrating the effect of range and earth curvature (with standard atmospheric refraction) on NEXRAD cross-beam
resolution and coverage of low-level weather phenomena. Courtesy of SRI International.
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70
Appendix A
at 0.0 degrees flower half-power edge of a Degree beam that is centered at 0.5 degrees) extends down
to or below 4,000 It (red), between 4,000 ft and 6,000 It (yellow), between 6,000 It and 10,000 ft
(green), and between 10,000 ft and 40,000 ft (blue) above site level (ASL). All altitudes in the figure
are given ASL. Figure A-4b shows a map indicating differences between the two networks in
minimum-altitude coverage over the contiguous United States and bordering areas red indicating that
the NEXRAD coverage reaches to lower altitudes than the pre-NEXRAD coverage, blue the converse,
and green indicating regions of similar coverage. The blue areas indicate regions of potential degrada-
tion of service with the NEXRAD network. The pie chart in Figure A-4b shows that the NEXRAD
coverage is similar to, or better than, the earlier network over most of the contiguous United States.
However, over about 7 percent of the area, the earlier network coverage extends to lower altitudes.
Altitude coverage, alone, is not equivalent to service, however, as discussed elsewhere in the report
and in this appendix.
Resolution
Spatial resolution is important from a meteorological perspective since better radar resolution
(i.e., a smaller radar-resolution cell) allows detection of smaller-scale features, such as tornado
circulations, mesocyclone circulations, and hail or rain shafts, at greater ranges. Spatial resolution is
defined as the maximum dimension of the volume "illuminated" by a pulse. At close ranges the
resolution is governed by the pulse length, and at long ranges it is governed by the antenna beam
width. The narrower beam of the NEXRAD antenna gives it an advantage over earlier National
Weather Service (NWS) radars in resolution capability at long range. Figure A-3 depicts the NEXRAD
beam-width dimension as a function of range.
Radar systems often average information over several adjacent resolution cells along the range
direction to improve detectability and to measure the returned signal strength more precisely, but this
is done at the expense of range resolution. In the NEXRAD reflectivity mode, four such ceils of 250-m
length are averaged together to yield an effective range resolution of ~ km. Comparable range
resolution was obtained from the pre-NEXRAD radars using longer transmitted pulses. Table A-2 gives
.
.
TABLE A-2 Approximate Spatial Resolution Dimensions
Radar Range Resolution
Cross-Beam Resolution At:
6 km 60 km 180 km
NEXRAD 1.0 km (reflectivity) 100 m 1.0 km 3.0 km
0.3 km (velocity)
WSR-74C 0.5 km 150 m 1.5 km 4.5 km
WSR-74S 0.6 km 200 m 2.0 km 6.0 km
WSR-57 0.6 km 200 m 2.0 km 6.0 km
OCR for page 71
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OCR for page 72
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Radar System Evaluation Criteria
73
the approximate spatial resolution of the base data for each of the radars at ranges of 6 km, 60 km,
and, IS0 km. Note that cross-beam resolution at 60 km is ~ km for the NEXRAD and about 2 km for
the pre-NEXRAD radars.
Figure A-Sa shows the resolution of the NEXRAD network for the 10,000-ft ASL coverage.
The colors on the figure indicate resolution dimensions of less than ~ km, between ~ and 2 km, and
between 2 and 4 km. Figure A-5b compares the resolution capabilities of the two networks for
10, 000-ft coverage over the contiguous United States and bordering areas. The red areas indicate where
the NEXRAD network provides better resolution; the blue areas indicate where the old network had
better resolution capabilities; and green areas indicate where the resolutions were essentially the same.
The blue areas do not imply a degradation of service but indicate regions that are candidates to
examine for potential degradation of service. As indicated by the cumulative distributions in Figure
A-6, the NEXRAD network will cover about 86 percent of the contiguous United States and bordering
areas with resolution better than 4 km. (The percentage would be reduced slightly without the DoD
radars.) The pre-NEXRAD network provided 4-km resolution over only 46 percent of the area.
Temporal resolution is also important in observing rapidly evolving weather features. The
NEXRADs operate on predetermined scan patterns, and the temporal resolution of the volume data is
5 to 6 min (i.e., the scans are repeated every 5 to 6 min). Access to this continuously evolving
volumetric data allows the forecaster to analyze time sequences that yield a better understanding of
evolving weather. Furthermore, the PUP processing of volume-scan data allows analysis of vertical
cross-sections of the radar echo to determine the vertical structure of the storms. The temporal
resolution of the pre-NEXRAD network was considerably more variable and more difficult to assess
because the radars were manually operated for any volume-scan functions. When used interactively,
pre-NEXRAD radars could detect changes in severe storm phenomena in less than ~ min. However,
on the basis of the evidence available to the pane! from NEXRAD operators experienced win the old
radars, He pane! concludes that the type and amount of information from NEXRAD's set scan strategy,
especially use of the four-panel storm and velocity structure display, is as effective as a vertical scan
and has contributed to the improved probability of detection for severe storms.
Sensitivity
The NEXRAD is about 10 times more sensitive than any of the previous radars. The sensitivity
of a radar as a function of range can be determined using conventional weather radar equations (Batten,
1973; Doviak and Zrnic, 1993~. Figure A-7 shows the sensitivity versus range for all four radar types
of concern.
The sensitivity curves of Figure A-7 show the single-puIse radar sensitivity, defined as the
reflectivity factor that would yield unity signal-to-noise ratio (SNR= I) of the received echo. The
processing gain of the digital video integrating processor (DV1P) in the pre-NEXRAD systems and the
signal processor of the NEXRAD reduce the variance of the reflectivity by averaging adjacent range
gates. However, when data thresholds are used, this process frequently precludes realizing any gain
in detectability. Since the pre-NEXRAD systems were not operated to observe low-reflectivity echoes,
the DV1P typically suppressed any echoes below iS dBZ, yielding acceptable operational performance.
The NEXRAD more effectively processes the received signals and observes the low-reflectivity echoes
in generating some products.
The network sensitivity at a given altitude for any contiguous United States location is the best
sensitivity value of all network radars that provide coverage to that altitude at that location. Figure A-
Sa shows He sensitivity of the NEXRAD network on the 10,000-ft coverage map. Figure Ahab com-
pares He sensitivities of the NEXRAD and pre-NEXRAD networks at the 10,000-ft level. In the
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74
Appendix A
figure, red denotes locations where the NEXRAD network has better sensitivity; blue denotes locations
where Me pre-NEXRAD network has beKer sensitivity; and green indicates equivalent sensitivity.
White indicates no coverage at that altitude by either system. As before, the blue regions indicate a
potential degradation in quality of service. However, one must also consider the sensitivity question
in the context of the ability to observe specific weather phenomena as discussed in Chapter 2.
The cumulative radar sensitivity distributions in Figure A-9 indicate that the NEXRAD network
will cover more than 80 percent of the contiguous United States and bordering areas at the 10,000-ft
level with sensitivity sufficient to detect all precipitation echoes of consequence (5 dBZ or greater) as
well as many echoes of nonprecipitation origin. The pre-NEXRAD network has this capability over
only 30 percent of the area. For most purposes radar sensitivity does not limit the operational perfor-
mance of the NEXRAD, but its greater sensitivity allows detection of a greater percentage of weak
weather phenomena than the pre-NEXRAD radars could observe.
Doppler Coverage
The Doppler coverage (i.e., the region over which Doppler radial velocities are provided) is
important in determining network capability to detect a number of weather phenomena, as discussed
in Chapter 2. The Doppler coverage may differ from the coverage shown earlier due to two factors.
These factors are as follows:
If minimum signal-to-noise ratio thresholds are imposed, a stronger echo may be
required for Doppler velocity than for reflectivity measurement. This can reduce the
effective range of Doppler coverage.
The Doppler velocities are measured with transmitted waveforms having ambiguities at
multiples of approximately Il5 On for the NEXRAD.i Thus, although the maximum
stated Doppler range is approximately 230 km, the transmitted signals are such that
Doppler data can sometimes be provided for only half of that range. Even ~en, if there
are comparable weather echoes in the first two ambiguity intervals, for example, at x
and (x + ~15) km, no Doppler information may be available in either interval. Regions
where Doppler velocity information is not available due to SNR or range-ambiguity
considerations are portrayed on the PUP in a purple color, thus giving rise to the
NEXRAD "purple-haze" display.
The impact of the above considerations is that NEXRAD Doppler coverage is not always
available beyond the ~15 km range and that it is sometimes impaired even at lesser ranges.
Availability Of Radars
For successful support of NWS operations, it is essential that the radars be available for use on
a nearly full-time basis. System availability and reliability affect the accuracy and timeliness of
forecasts and warnings of many weather phenomena. The NEXRAD radars have been designed for
continuous, unattended operation with the expectation of an increased availability of He radars. Not
~ The NEXRAD program office is modifying the radar wave forms and software to extend the unambiguous
range to approximately 165 km.
OCR for page 75
- ~
-r
l
Figure A-Sa NEXRAD resolution coverage at 10,000 ft above site level. Red depicts the area over which the beam-
resolution cell is less than 1 km; yellow, between 1 km and 2 km; and green, between 2 km and 4 km. Courtesy of
SRI International.
-
~ I
Figure A-Sb Areas of the contiguous United States and border regions having better resolution coverage at 10,000 ft
above site level with NEXRAD (red), with equal resolution by pre-NEXRAD and NEXRAD networks (green), and
with better resolution by the pre-NEXRAD network (blue). Courtesy of SRI International.
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100
90
80
70
_ 60
50-
a)
40
30
20
10
O - ,
Y
~1
/ 1
/ · _
/ .~
/ ., I it 1
0 2 ~
NEXRAD Network
Pre-NEXRAD
Network
__
4 6 8 10 12
Resolution Cell (km)
Figure A-6 Cumulative resolution distribution functions. The figure indicates the percentage of desired coverage area (contiguous United
States plus bordering areas) at 10,000 fit above site level having resolution cell size smaller than that shown on the abscissa for pre-NEXRAD
and NEXRAD networks. Courtesy of SRI International.
20
10-,
I
m 0
·-, -10
._
ED -20 -
cn
-30
-40
,,~.~-
.. - ~
NEXRAD
-- WSR-57
......... WSR-74S
- -- ~ WSR-74C
1 1
1 1 0 1 00 1 000
Range (km)
Figure A-7 Radar sensitivity curves showing reflectivity factor (dBZ) versus range for the single-pulse radar return power that yields equal
signal and noise power, SNR = 1. (The sensitivity curves for He WSR-57 and the WSR-74S are nearly superimposed.) The processing gain
to reduce the variance of base data reflectivity estimates is not shown in these curves. The NEXRAD value is based on the short pulse mode;
however, the NEXRAD long pulse increases sensitivity by 5 dB. Courtesy of SRI International.
77
OCR for page 78
78
Appendix A
only must the RDA, RPG, and PUP subsystems be available, but also the communication links among
the three must be equally available for controlling the radar, requesting products, and receiving the
base data and products. Important factors that impact system availability and reliability are
availability of spare parts;
responsiveness of parts distribution system;
availability of qualified maintenance staff;
maintenance education and training programs;
availability of qualified radar meteorologists;
radar operations education and training programs;
reliable and effective communications to serve operations and maintenance activities;
and
adequate funding to support operations and maintenance activities.
Table A-3 gives availability statistics for the first 56 installed NEXRADs over a recent 27-month
period, along with roughly comparable (and the best obtainable) statistics for the 3 pre-NEXRAD
systems over the preceding 50 monks. These initial availability data show that He NEXRAD systems
are performing at the expected 96 percent availability level required for commissioning. The
pre-NEXRAD systems indicate higher availability, but downtime for preventive maintenance, logistics
delays, and system modifications was not included in Be computation (such downtime data were never
logged). This factor makes it difficult to provide a comparative assessment of network availability.
TABLE A-3 Weather Radar Availability Statistics
Radar
Availability (I)
NEXRAD
WSR-57
WSR-74S
WSR-74C
96.3 (does not include DoD radars)
98.5a
98 e2a
99. la
Not considered reliable data or representative because outage times
were not as rigorously defined nor adhered to in the pre-NEXRAD
systems.
The availability of the DoD radars in the contiguous United States is important in considering
NEXRAD network coverage and quality of services. Statistics on DoD radar availability were not
available, but the pane! received some reports of slow response in DoD maintenance of a DoD
NEXRAD. The budgetary pressures on DoD, including possible teas-e closings, raise additional
concerns about availability of those NEXRADs. The possible impact of limited DoD NEXRAD avail-
ability has been addressed in this study by considering both the full NEXRAD network and a degraded
network that includes only NWS NEXRADs when assessing the ability to detect certain weather
phenomena.
OCR for page 79
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82
Appendix A
NEXRAD Technical Improvements
There are a number of technical improvements in the NEXRAD system. Improvements have
been made in terms of the transmitter and receiver, antenna, Doppler signal processing, and the digital
information system. These technical improvements are discussed in the following sections.
Transmitter arm Receiver
The NEXRAD transmitter is a phase-stable, high-power, klystron-based transmitter, in contrast
to the lower-power magnetron systems of the pre-NEXRAD radars. The phase stability allows digital
processing to improve the processing and display quality and to reject strong interfering clutter signals.
The NEXRAD receiver uses modern low-noise amplification and wide-dynamic-range amplifiers to
yield higher SNR signals from low-reflectivity weather targets even at distant ranges (Heiss et al.,
1990).
Antenna
An important aspect of radar coverage is defined by the radar antenna beam pattern. The
majority of the transmitted energy is radiated into a conical main beam about ~ degree wide. This beam
width governs the resolution capability of the radar, except at very short ranges. However, a significant
fraction of the energy is spread into the antenna "sidelobes" at angles outside the main beam. Typical
azimuthal cuts of the antenna patterns for the NEXRAD and WSR-57 radars are shown in Figure A-IO.
The early radars, particularly the WSR-57s, had antenna sidelobe levels so high that ground
clutter often made short-range and low-altitude coverage virtually impossible unless the radar operator
manually raised the antenna elevation enough to reduce the clutter. The WSR-57 antenna has sidelobes
only about IS dB down from the main lobe response (Wilk et al., 1965~. Short-range and low-altitude
coverage of He NEXRAD is improved by virtue of significantly better antenna sidelobe response and
active clutter filtering in the radar processor. The NEXRAD has measured sidelobes down 29 dB from
the main lobe response, using linear polarization with the radome in place (Sirmans, 1993~. The much-
improved sidelobe suppression provides an additional 20 dB (two way) of clutter suppression.
Doppler Signal Processing
Some weather radars, namely, the NEXRAD and three pre-NEXRAD systems with limited
Doppler capability, measure the velocity of the weather target toward or away from the radar (i.e., the
"radial velocity") in addition to measuring the reflectivity of the target. Knowledge of the radial-
velocity characteristics of a weather target provides important assistance in recognizing and quantifying
certain significant weather phenomena, such as tornadic storms, wind shear, or high surface winds with
precipitation. This knowledge is also useful in general weather forecasting. Additionally, the Doppler
feature provides vertical wind profiles and enables the radar to reject returns from stationary objects
(i.e., "ground clutter") based on the radial-velocity information of the radar echo (Federal
Meteorological Handbook, 1990~.
The NEXRAD Doppler signal processor uses modern digital signal processing to compute
accurate estimates of radar reflectivity and velocity while simultaneously suppressing interference and
artifacts in He data, such as ground clutter. The WSR-57s and WSR-74s had no clutter-cancellation
capability. The NEXRAD electronic clutter-cancelIation feature reduces clutter by 30 dB in reflectivity
mode. In Doppler mode, filters reduce clutter by 50 dB. Thus, including the 20 dB of additional clutter
suppression through the reduction of antenna siclelobes, the NEXRAD gives about 50 to 70 dB of
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Radar System Evaluation Cnteria
83
WSR Antenna Pattems
NEXRAD f
(WSR-88D) /
1. ~ /
-1 0-
-
m
`:L -20-
a)
o
-30
-40 L
Gain = 45.5 dB
\ Beamwidth = 0.95 den
WSR-57 ,'
a .,
, ~
/
I
I .
I 1
1 1
1 1
· I I
I 1
I
,
1
~-5
ti`~1 :11
-
]'''.V v W 11
-4 -3 -2 -1 0
Angle off beam center (deg)
'` Gain = 38.S dB
~ Beamwidth = 2.0 deg
1
1 1
1 : ,,-%
1 1
1 1 1
1 1 1
1 1
1 1
1 1
1 1
1
1 1
1 1 1
it,,
1 2 3 4 5 ~
`-1 8 dB
1
1 1
1 1
1 1
· I
\ I
I
t
I
I
Figure A-10 Antenna radiation patterns (relative scale) showing main beam and first few sidelobes. The NEXRAD has a half-power beam
width of 0.95 degrees while that of Me WSR-57 is 2.0 degrees; the sidelobes of We NEXRAD are typically 10 dB lower Ran those of the
WSR-57. The on-axis gain of the NEXRAD antenna is about 7 dB higher than Rat of We WSR-57. Based on data from Wilk et al., 1965
and Sirmans, 1993.
clutter suppression as compared with the WSR-57. This allows better coverage of the closest ranges,
while suppressing isolated clutter, including much anomalously propagated ground clutter.
Digital Information System
The digital information system encompasses features that provide tools or capabilities for
forecasters (Crum et al., 1993~. They include the following:
signal processing that provides digital or analog radar data from which weather products
can be displayed or derived;
algorithms that translate radar data into weather information- for example, radar
reflectivity translated into rainfall rate and cumulative rainfall over some period of time,
or reflectivity and velocity data combined in a hail detection and had! severity product;
products that present this information in graphical image or alphanumeric format
(Klazura and Imy, 1993~;
displays, which provide presentations of radar data and derived products; and
· data archiving (recording), which allows post-facto analysis to improve aIgori~ms and
products.
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84
Appendix A
One technological advance incorporated into the NEXRAD systems is the provision of color
displays. These displays enable the meteorologist to see informative images quantifying the weather
conditions with the spatial and temporal resolution necessary to comprehend the mesoscale situation
and the effects on local conditions. Moreover, the ability to make and view time-lapse imagery allows
more accurate, targeted, and timely forecasts to be issued.
The NEXRAD automatically generates a specific list of products and distributes them to the
associated PUPs without operator intervention. With these communications capabilities, the RPG can
serve multiple users (multiple PUPs), meeting the specific needs of each. In addition, meteorologists
can request additional products, which are produced by the RPG as processing time permits.
The Level 2 base data are the three quantities computed for each range bin of each radial of
information: (~) the radar reflectivity factor (~-km resolution); (2) the velocity (at 250-m resolution);
and (3) the spectrum width (at 250-m resolution). These base data are archived on ~ mm digital data
tapes for later perusal and analysis by station meteorologists or the broader meteorological community
in investigation of particularly interesting events. This can aid in developing new products, permit
analysis of system performance, and serve as a climatological data base. New product development
may require both full-range resolution reflectivity data (i.e., 250-meter spacing) and enhanced
processing to improve the base data quality. The Level 3 product data are a subset of the various
products. The data are routinely stored on disk for a relatively short period of time for later retrieval
and analysis and are also archived for permanent record.
The NEXRAD network allows any given user to access data from radars in adjacent service
areas. Furthermore, the four private NEXRAD information dissemination service data providers create
national mosaics of NEXRAD products (Beer, 1991~. These mosaics, because of their increased
quantitative products, are superior to those provided through the pre-NEXRAD network.
WARNING PERFORMANCE
The issuance of weather advisories and warnings is supported by automated functions in the
radar, however, it depends primarily on the expertise of well-trained forecasters. Evaluation of a
system's warning performance is likewise partly objective and quantifiable and partly subjective and
empirical. The pane! has accounted for both the detection performance described above and the
forecaster warning experience; taken together, they define the overall system performance. It is clear
that the detection capability of the NEXRAD directly affects the warning performance. As such, both
detection capabilities and warning performance represent the core of the most significant conclusions
in this report and were used to develop assessment criteria for evaluating the potential degradation of
service. The detection capability is discussed in detail in Chapter 2, "Radar Network Configuration and
Detection Capabilities," and in Chapter 3, "Comparison of Weather Services: Pre-NEXRAD and
NEXRAD. "
PRODUCTS AND SERVICES OFFERED
Advisories and warnings of significant weather phenomena are relayed to the public and other
users through a wide variety of specific products and services. These products and services are tailored
to particular user communities (i.e., aviation, marine, agricultural, general public, etc.) and are
disseminated through specific communications media (i.e., NOAA Weather Wire Service and Weather
Radio, Federal Aviation Administration radio broadcasts, military meteorological offices, local radio
and television stations, and private-service vendors). The warnings and advisories are also channeled
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Radar System Evaluation Cntena
85
to the "Family of Services" medium, which provides external user access to near real-time weather and
hydrologic data and information through a dial-up service.
The NEXRAD network must be considered in the context of the modernized weather service,
whereas the pre-NEXRAD radar network must be considered in the context of the former structure.
The technical features of the NEXRAD allow the development and provision of new products and
services that were, heretofore, unavailable. Many of the new NEXRAD products are available to users
from the NEXRAD information dissemination service providers. The number and breadth of the
automated products and forecasting aids have increased considerably, thereby adding to the complement
of former products and services. Improvements in these radar products and services bring the overall
NWS modernization another step closer to its goal of providing the nation with more timely and
accurate warning of hazardous weather and flooding.
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Representative terms from entire chapter:
contiguous united
