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2
Radar Network Configuration and Detection Capabilities
NETVVORK COVERAGE AT 10,000 FRET ABOVE SITE LEVEL
The pre-NEXRAD NWS radar network consisted of 128 WSR-57, WSR-74S, and WSR-74C
radars concentrated primarily in the central and eastern United States. The WSR-57 and WSR-74S
radars, both 10-cm wavelength systems, were configured as manually operates} surveillance radars. The
68 WSR-74C radars functioned as "local warning" radars and often were operated only during periods
of significant weather. in recent years, three of the pre-NEXRAD radars had some Doppler capability;
two of these have been replaced by NEXRADs at nearby sites.
The pre-NEXRAD radars routinely scanned only at low elevation angles in the surveillance
mode and were operated manually for other scans, such as vertical scans in the range-height indicator
mode. Figure 2-la indicates the composite coverage provided by the pre-NEXRAD network at 10,000
ft above site level over the contiguous United States and bordering greasy Here "coverage" means that
the lower edge of the radar beam. as determined from the radar horizon with standard atmospheric
~. ~,
_ ~
refraction and with due consideration of any blockage by intervening terrain, would intercept any
weather phenomenon reaching at least that altitude (see Appendix A, pages 68-70~. The altitude of
10,000 ft is used here only because the charge to the panel asked for an examination of any reduction
in radar coverage at 10,000 ft. This request stems from the definition in P.L
102-567, Sec. 702~4)
that "degradation of service' means any decrease in or failure to maintain the quality and type of
weather services provided by the National Weather Service to the public in a service area, including
... a reduction in existing weather radar coverage at an elevation of 10,000 feet." From a scientific
point of view, there is no compelling reason to specify this or any other specific height, however,
because the requisite altitude coverage for detecting various weather phenomena differs for each
phenomenon. Thus, coverage at the 10,000-ft level gives no guarantee that every phenomenon of
concern can be detected.
The NEXRAD network, when complete, will consist of 138 10-cm wavelength NEXRADs in
the contiguous United States, including 22 operated by the DoD. All of these NEXRADs are computer-
controlled radars that are designed to operate continuously in a three-dimensional, volume-scan mode.
Figure 2-Ib indicates the composite coverage at the 10, 000-it level that will be provided by He
~ Above site level (ASL) altitudes are used since the siting documentation and data base are given in terms of
ASL. Certain radars in the West (e.g., Tucson) are located on mountains high above the surrounding ground level
(AGL). Therefore, ASL and AGL altitudes may differ by several hundred or even thousands of feet.
8
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Representative terms from entire chapter:
contiguous united
it li0 '.
PORTL4H] ~ :~ISS~U~ MASTS
ma- He. ·: : :...:... I:. /~ PLUM ~ O FM=6 WE
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COLUM8IA~ --~ V ~ ~~ ~ ACHARI£S~ ~ mv
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UT1LE TUPELO IHUH~UE O OAUY~A a1ARLEST
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-~: ~i ~ -anew rat ~ _ . ..
_ v ~: :~::.:: : ~;::: -: ; :.- : :' 1 ~) ~ v`UCON ~lr
IlIDLAND O S EN U E O O5H r o o AN ' ~OOLUU8US
O A L E O o L FW ~J/Aa
Network Configuration and Detection
1 1
NEXRAD network in the contiguous United States. Coverage will be nearly complete over the central
and eastern United States. in addition, the previous gaps in coverage over the western United States
will be reduced substantially. Figure 2-Ib also shows (striped blue) where the coverage by the
NEXRADs at 10,000 feet is reduced from that provided by the pre-NEXRAD radars. The 10,000-foot
requirement is defined in P.~. 102-567, Sec. 702~4~.
Over land, the areas where coverage is reduced include small portions of North Dakota,
Montana, and Wyoming. As noted in Chapter ~ in the section on study process, to make a meaningful
assessment of adequacy of coverage, radar coverage also must be considered in terms of the resolution
and sensitivity of the radars with respect to specific weather phenomena. These aspects of radar
coverage are examined and discussed later in this chapter and in Appendix A.
The NEXRAD network will be capable of detecting tall storms anywhere in the contiguous
United States and in most of the 230-km bordering area. Specifically, any storm reaching a height of
at least 26,000 ft above site level will extend through at least the lower half of the beam of some
NEXRAD in the network (scanning at the lowest elevation angle). However, for shallower phenomena,
NEXRAD will not provide detection coverage at the desired altitudes over the entire contiguous United
States: therefore coverage for locations far from NEXRAD sites. but close to ~re-NEXRAD radars.
7 ~ ~ ~
~ . ~ · ~ ~ ~ __. . · · . ~ ~ C ~ me · ~ _` . . · me ~ ~
needs to be examined more closely. the section entitled comparisons or Detection Coverage by ~re-
NEXRAD and NEXRAD Networks," later in this chapter, discusses the panel's review of this
coverage. It is important to note that the old network did not provide full coverage for shallow weather
phenomena.
CALCULATED VERSUS OBSERVED DETECTION RANGES
Critical to assessing the possibility that any degradation of service may exist in a particular
geographic area is the need to quantify the range to which significant weather phenomena can be
detected by the old and new radars. In some situations the radar detects weather events directly,
leading the forecaster to issue warnings or advisories. These events include hurricanes, strong surface
winds, heavy rain, heavy snow, and wind-shift lines. In other situations, features detected by He radar
are only indirectly related to the actual weather event. This is the case for tornadoes, hail, and precipi-
tation amount. Large tornadoes at relatively close radar range are an exception because the tornado
circulation can be seen by a radar in Doppler mode (appearing as a "tornadic vortex signature," or
TVS), even though the circulation may not be fully resolved.2
Research has shown (Burgess and Lemon, 1990) that rotating thunderstorms, which are
associated with mesocyclones or misocyclones, often produce tornadoes or other severe weather like
hail and strong surface winds. However, not all rotating thunderstorms produce severe weather, and
not all severe weather is associated with rotating thunderstorms. Thus, more than radar information
goes into the issuance of a tornado or severe-storm warning. The forecaster integrates atmospheric
stability and melting level information, wind profiles, numerical model outputs, and other
environmental characteristics to determine the potential for significant weather. Spotter reports, features
~ For a NEXRAD operating with a 1-degree beamwidth and scanning at 0.5 degrees elevation, the lower edge
of the detection cone crosses the 10,000-ft elevation dictated by P.L. 102-567 when the beam is 230 km from the
radar. (Terrain or other obstructions to the beam will reduce the range of coverage. Technology considerations
limit the usable NEXRAD Doppler range to 230 km.)
2 Drawings and descriptions of weather phenomena are provided in Appendix D.
12
Assessment of NEXRAD Coverage
observed in satellite data, and other weather observations supplement the radar data as aids for the
forecaster to make a decision to issue a warning.
The pane} used two methods to determine the maximum range at which significant weather
phenomena can be detected by the old and new radars. The first method was to describe the
phenomenon in terms of its physical dimensions and radar reflectivity factor and use the radar
characteristics to calculate the maximum range at which the phenomenon could be detected. The second
method was to survey the observational experience of research and operational meteorologists. The
results of these two methods are presented as Tables 2-l and 2-2. To ensure credibility of the results,
the pane! distributed the tables twice to a large cross-section of the weather research and operational
communities for Heir comments. A total of 35 people (listed in Appendix E) provided comments. After
the revisions this process produced, there was no significant disagreement among the pane! members
or He 35 respondents as to the data shown in the tables.
The pre-NEXRAD network contained three radars win various levels of Doppler capability.
These radars were located in Huntsville and Montgomery, Alabama, and in Marseilles, Illinois. The
radars in Montgomery and in Marseilles were replaced by nearby NEXRADs; the radar in Huntsville
is still in use. The NWS currently plans to decommission the Huntsville radar and provide detection-
coverage service for that area from NEXRADs at Birmingham, Alabama; Nashville, Tennessee; and
other adjacent NEXRADs. The capabilities of the Huntsville WSR-74 Doppler radar are included in
Table 2-l to allow a comparison of the existing radar with the planned NEXRAD coverage.
Table 2-} describes the characteristics (top, limiting horizontal extent, and reflectivity) that the
panel used to calculate the maximum ranges at which various weather phenomena can be detected. The
typical spread of the values for each characteristic is listed, along with an estimated median value.
These values take into account regional and seasonal differences, but the limits do not include extreme
cases. In the case of reflectivity, the values refer to average values within the phenomena. The spread
of values is based on the scientific literature (see references) as well as on the panel's experience and
that of those who responded to the mailings.
Calculations of the range for each radar and each phenomenon shown in the table were based
on the panel's assumption that a phenomenon can be detected if (~) the center of the beam at a 0.5
degree elevation angle is at or below the top of the phenomenon; (2) the half-power beam width for
supercells, mini-supercells, and misocyclones is less than one-half of the horizontal diameter of the
rotation, while for all other phenomena the half-power beam width is less than the smallest azimuthal
dimension; and (3) the minimum detectable signal at a particular range is less than the radar reflectivity
factor of the phenomenon. The minimum detectable signal for each radar is found in Appendix A,
Table A-~.)
Table 2-2 provides a comparison of calculated and experienced maximum ranges for detection
of various radar-observed phenomena by pre-NEXRAD radars and NEXRAD. The table is based on
two sources of information: the theoretical calculations given in Table 2-1 and forecaster/researcher
experience.
In Tables 2-1 and 2-2, the WSR-57 and WSR-74S radars are grouped together because their
beam widths and minimum detectable signals are very similar. The most range-limiting of the three
characteristics of the phenomena is given in Table 2-2 as the calculated maximum range. The altitude
of the top of the phenomenon is often the limiting characteristic. For the calculations of maximum
range, the altitude of the top of the phenomenon is assumed to be radar-independent. In two cases,
convergence lines and lake-effect snow, the minimum detectable signal is also a significant factor. The
horizontal dimension is the limiting characteristic for mesocyclone and misocyclone; the vertical
dimension is He limiting characteristic for the bright band.
As the tables indicate, the Doppler feature of the NEXRAD provides a new capability for
directly measuring several weather phenomena and winds within the phenomena. These phenomena
Network Configuration and Detection
13
include tornadoes, mesocyclones, misocyclones, microbursts, macrobursts, convergence lines, and
winds within hurricanes, precipitation, and the clear-air boundary layer. The confidence in detecting
supercelis, mini-supercelis, and macrobursts using Doppler radar is much higher than with non-Doppler
radars, since the detection is direct and reliable. Supercelis are rotating thunderstorms whose
circulation can be detected directly in the Doppler velocities, but which can only occasionally be
inferred indirectly by a hook-shaped radar reflectivity feature with non-Doppler radars. Macrobursts
are strong surface winds associated with some thunderstorms, which (using the old radars) occasionally
can be inferred from a bow-shaped radar reflectivity signature. Fujita (1985) has shown that the most
`dangerous phase of downburst storms occurs when radar returns are developing a distinctive bow-echo
appearance; at that time wind damage has already occurred in many cases. The NEXRAD can provide
earlier precursor information, and the Doppler capability can detect the wincis directly.
In addition to listing the calculated maximum range values, Table 2-2 also lists We observed
maximum range for each phenomenon and each radar in the "Experience" column. These values reflect
the actual experience of forecasters and researchers who use the radars to observe specific weather
phenomena.) The spread of maximum range values shown within the brackets under the "Experience"
column represents differences caused by seasonal, geographic, and meteorological factors. The number
in front of the bracket is a subjective estimate of a median maximum range value that would be
applicable within the contiguous United States.
The calculated values admittedly contain many assumptions, but they are, nevertheless, very
useful for providing an independent consistency check of the experience values. Comparison of the
calculated and experience values in Table 2-2 shows general agreement. However, there are some
apparent disagreements, including the following:
.
.
The maximum-range values for detecting mesocyclones and misocyclones with the
NEXRAD, as shown in the "Experience" column in Table 2-2, are lower Pan the calcu-
lated values. The pane] believes that this difference is due to the assumed detection
criteria; that is, only two samples are needed over the diameter of the weather
phenomena circulation. While theoretically this low number of samples would allow He
feature to be detected, its magnitude would be greatly underestimated in the actual
observation (Carbone et al., 1985~.
The calculated maximum ranges for detecting stratiform snow and lake-effect snow,
particularly for the wicler-beamed pre-NEXRAD radars, are greater than experience
indicates. The pane! believes that the calculations do not fully account for partially filled
beam effects, which are particularly prevalent for these features because of the relatively
sharp decrease in reflectivity near the top of the phenomenon (see cloud/snow illustra-
tion in Appendix A, Figure Add. As a result, in these situations, the measured
reflectivity could even be below detectable thresholds. Also, when the contoured dis-
plays of the phenomena were viewed on the old radars, the display thresholding tended
to remove much of the snow return.
~ A list of forecasters and researchers who contributed to the experience values is provided in Appendix E.
This list included responses from experts at 11 NWS operational sites, many with experience in operating NEXRAD
and pre-NEXRAD radars. In many instances, the participants conferred with their colleagues at the site listed in
developing their estimates. Their experience values were in good agreement with those from other operational sites
and from technical experts from research and development locations. These values were in general agreement with
the panel's calculated values and reflected relatively conservative interpretation of detection coverage for each
significant weather phenomenon.
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16
Assessment of NEXRAD Coverage
TABLE 2-2 Companson of Calculated and ~Expenenced" Maximum Range Values for Detecting
Various Radar-Observed Phenomena for the WSR-57, WSR-74S, WSR-74C and NEXRAD.
Calculated values are the most range-lim~t~ng value for each phenomenon In Table 2-1. The
~Expenence~ columns reflect the actual expenence of trained forecasters and researchers who have
used the equipment. The typical spread of values is due to seasonal, geographic, and meteorological
vanations. The number In front of the brackets is the estimated median, deemed here as the
"expenenced~ median maximum range. NC = No Capability.
Phenomena Calculated
Median Maximum Range (km)
WSR-57 -74C NEXRAD WSR-57
-74S -74S
Inexperienced
Median Maximum Range (Icm)
and [typical maximum range valuesl
-74C NEXRAD
Hurricane
eye wall 386 386 386350[250-450]375[275-450]400[300-460]
winds NC NC 230aNCNC230&
Supercell
mesocyclone NC NC 230.NCNC180[150-230]
hook echo 120 126 12675[20-120]85[25-140]100[40-160]
Mini-Supercell
miso- or NC NC 120NCNC100[70-150]
mesocyclone
hook echo 60 77 7745[15-55]55[20-80]70[30-110]
Misocyclone NC NC 60NCNC35[10-70]
Tornadic Vortex NC NC *bNCNC45[10-130]C
Signature
Microburst NC NC 45NCNC35[20-50]
Macroburst
reflectivity
signature 165 165 165150[100-200]150[100-225]150[100-250]
velocity
signature NC NC 77NCNC70[50-100]
Precipitation Detectiond
convective
rain 327 327 327300[150-450]325[175-450]350[200-460]
stratiform
rain 255 255 255225[125-300]235[140-325]250[150-350]
stratiform
snow 160 210 226120[60-150]140[70-160]180[120-240]
lake-effect
snow 130 130 14560[30-90]80[40-140]120[80-160]
Network Configuration and Detection
17
. . .
Phenomena Calculated
Median Maximum Range (km)
WSR-57 -74C NEXRAD WSR-57
-74S -74S
"Experience.
Median Maximum Range (Icm)
and [typical maximum range valuesl
-74C NEXRAD
Winds
boundary
layer NC NC 103 NC
(warm season)
high surface
winds with
preeip. NC NC
profiles within
cloud and
precip.
NC NC *.
Convergence Lines
(warm season) 77 77 77
Melting level
(bright band) 9 14
NC
103 NC
NC
NC
NC
80[20-120]
70[20-120]
*e
limitedf limitedf 80[40-120]
18 20[10-30] 35[20-40] 45[25-70]
NOTES
a. Doppler velocity is available only to 230 km and sometimes only to half of that range.
b. A theoretical value is not possible (see footnote letter g of Table 2-1).
e. The estimated median value is skewed to the low end of the maximum range distribution. This is because there are many
more relatively weak tornadoes with small diameters. The likelihood of detecting a tornadic vortex signature increases as the
diameter and rotational speed increases. Fortunately, the more-intense tornadoes (F3-F5) will typically have tornadie vortex
signatures observable to greater ranges. The estimated median range for these eases is near 100 km.
d. Precipitation measurement is discussed separately at the end of the section entitled "Calculated Versus Observed Detection
Ranges. ~
e. Range is not an issue; this is a profile for the immediate vicinity of the radar.
f. The limited observation of thin lines with the WSR-57, WSR-74S, and WSR-74C is believed to be a combination of three
factors: (1) display thresholding and large display quantizing intervals; (2) low reflectivity at the top of the thin line combining
with partial beam filling; and (3) absence of clutter filters resulting in the loss of the thin line in clutter.
References Consulted: Battan, 1973; Brown et al., 1978; Burgess and Lemon, 1990; Burgess et al., 1993; Burpee, 1986;
Carbone et al., 1990; Fabry et al., 1992; Fujita, 1981; Hjelmfelt, 1988; Joss and Waldvogel, 1990; Klingle et al., 1987;
Kingsmill and Wakimoto, 1991; Mahoney, 1988; Marks, 1990; Mueller and Carbone, 1987; Pettit, 1990; Wakimoto, 1982;
Wakimoto and Wilson, 1989; Weekwerth and Wakimoto, 1992; Wilson, 1975; Wilson and Brandes, 1979; Wilson et al., 1984;
Wilson et al., 1994.
18
Assessment of NEXRAD Coverage
.
Experience indicates that for all radars the maximum range for detecting the bright band
(the horizontally stratified region of enhanced reflectivity) is considerably greater than
the calculated value. This is due to the assumption that the bright band could be detected
if the half-power beam width were less than the typical 300-m thickness of the bright
band. But since the bright band has a peak reflectivity about 9 dB (Fabry and
Zawadzky, 1995) or nearly 10 times greater than Me echoes above and below the band,
it would still be detected when the beam width is larger than 300 m, although it would
appear to be of lesser intensity.)
While not specifically mentioned in Tables 2-! and 2-2, radar estimates of precipitation rate and
amount are extremely important for flash-flood forecasting, heavy-snow advisories, heavy-rain
advisories, and water management of reservoirs and streams. Precipitation rate is directly related to
the intensity of the radar returned power; however, We relationship is not absolute and varies in time
and space. In addition, the vertical profile of radar reflectivity is affected by precipitation growth,
evaporation, and type; thus, the precipitation observed at radar beam height may not be representative
of that reaching the ground (Wilson and Brandes, 1979~. Consequently, the maximum ranges for
estimating precipitation rate or amount will be less than those given for detection in Table 2-2.
Estimates of Me maximum range that precipitation can be measured quantitatively are convective
rain, 100 to 200 km; stratiform rain, 75 to iSO km; and lake-effect snow, 40-100 km. Provided that
suitable precipitation-accumulation software was present on an old network radar, possible degradation
in precipitation measurement could occur for locations more than 100 km to 150 km from a NEXRAD
that replaced a radar at a nearby location. For snow and rain events with very low melting levels, this
range could be less than 100 km. The NEXRAD has automated algorithms for routinely estimating
precipitation accumulation. These algorithms represent a major improvement over the rudimentary
techniques used with the WSR-57s and WSR-74s.
Hail detection is not specifically listed in Tables 2-l and 2-2. Although the NEXRAD algorithms
show skill in had! detection, little is known about how this type of detection varies with range. Hail
could be detected out to ranges of 250 to 300 km; however, it cannot be distinguished from heavy
convective rainfall reliably.
COMPARISON OF DETECTION COVERAGE
BY PRE NEXRAD AND NEXRAD NETWORKS
The median values of the maximum detection range for each radar under the "Experience"
column of Table 2-2 were used to prepare maps showing national detection coverage for both the pre-
NEXRAD and NEXRAD networks for selected weather phenomena. Maps were also prepared
indicating those areas where the pre-NEXRAD network afforded better coverage for each phenomenon.
The pane! refers to the latter as "difference maps.") The phenomena shown in these coverage and
difference maps are hurricanes (Figures 2-2a and 2-2b), supercelis (Figures 2-3a and 2-3b), mini-super-
cells (Figures 2-4a and 2-4b), macrobursts (Figures 2-5a and 2-5b), lake-effect snow (Figures 2-6a and
2-6b), and stratiform snow (Figures 2-7a and 2-7b). Note that boundaries are added to hurricane, lake-
effect snow, and stratiform-snow maps to mask-out areas with negligible risk of occurrence of these
~ An extensive analysis of bright bands in the vicinity of Montreal by Fabry and Zawadzky (1995) showed an
average value of about 9 dB. This value is similar to that shown by Battan (1973~.
~ ~_57, ~-7" 9U
O ~-74C SllE
+ NWS WSR-MID SllE
x DeD W5R-~) 9U
NEXRAD
NE`RAD AND PRE-NOMAD
~3 PRE-NEXRAO
Not -
0 100 NAUTICAL UlE5 (nml)
IDSI
O 185 AL - AS (an)
. s3 _~
Figure 2-Sa Areas of macroburst detection (reflectivity signature) by the NEXRAD network and the pre-NEXRAD network. These areas are shown
in dotted-red and striped-blue backgrounds, respectively. The coverage extent of each individual radar in the two networks is taken from the median
values of the "experience" maximum range values from Table 2-2. The pie chart shows the percentage of vulnerable areas (contiguous United States
+ 230 km across borders or oceans) where only the NEXRAD network provides this coverage (dotted red), where both networks provide coverage
(combined red/blue), where only the pre-NEXRAD network provides coverage (striped blue), and where neither network provides coverage (white)
for this weather phenomenon. Courtesy of SRI International.
(~ ~ 1~ ~d
A+\ 1+
a WS8-S7, WSR-74S snE
O W5R-7U) 511E
+ NWS WSR-8BD D
X OoD WSR-8. ~m
~ AH OoD
1~3 without Don
O COD NAUTICAL UI~S (nml)
kit
~ 1= Anal - S (en)
.:
Figure 2-Sb Areas with potentially degraded detection coverage for macrobursts (reflectivity signature) by NEXRAD with DoD (red) and without
DoD (blue) NEXRADs compared with the pre-NEXRAD radars. Courtesy of SRI International.
~ A: ~
I~ +1~
0 100 NAIJllCAL lAIIES Annul)
~ ~155 KILO - TERS (an)
~ do_
0~=- ~, ,. ~
O W9~-74C9lE _< ~OX o ~0 ~ ~ O Xi X O]
~ NWS W5R-88D SITE: aid, ~a ~ X ~ , ~ ~)t
x OoO WSR-800 SITE ~of 1. ° dim
LAO OX O +
@~ N3~AO ANO FRE-ND
O WSR-57, WSR-74S ~U
O WSR-74C SITE
+ HWS W5R-ID 51'1E
X ow ~-~0
NEXRAD
NEXRAD AND PRE-NE) - D
~3 PRE-NE)~Ao
1=1 NEllH13t
0 100 NAUnCAL L"S (~1)
O 1 - IGLOUC~S (ten)
~ ~> ~
,
v.
~q/3o
Figure 2-7a Areas of stratiform snow detection by the NEXRAD-network and the pre-NEXRAD network. These areas are shown in dotted-red and
striped-blue backgrounds, respectively. The coverage extent of each individual radar in the two networks is taken from He median values of the
Inexperienced maximum range values from Table 2-2. The pie chart shows the percentage of vulnerable areas (contiguous United States + 230 km
across borders or oceans within boundary lines shown) where only the NEXRAD network provides this coverage (dotted red), where both networks
provide coverage (combined red/blue), where only the pre-NEXRAD~network provides coverage (striped blue), and where neither network provides
coverage (white) for this weather phenomenon. Courtesy of SRI International.
~ '! i ~4 ;
~;~N
O WSR~ SR-74S
O ~R-74C SITE
+ NWS 1~-BJD 511E
X OoD WSR-C8D Silt
~3 PITH OoD
~3 outpour coo
O 100 NAUlU:A1 DINS (my)
1~1
. .0 1BS HI-IEl-S (km)
~ ~ J °d~ ~
it'
Figure 2-7b Areas with potentially degraded detection coverage for stratiforrn snow by NEXRAD with DoD (red) and without DoD (blue)
NEXRADs compared with the pre-NEXRAD radars. The solid black line marks Be boundaries on each map for the areas with a significant risk of
stratiform snow occurring. Courtesy of SRI International.
Network Configuration and Detection
31
phenomena. With the exception of convective rain, stratiform rain, and melting lever, the other
phenomena in Table 2-2 either were not detectable by the old network or were only marginally
detectable. In the case of stratiform and convective rain, the detection (not quantitative precipitation
measurement) range for all He radars is so large that there are no locations over land where the old
network had better detection capability.
It is quite apparent from a cursory visual scan of these 12 maps that the NEXRAD network
greatly increases the total area over which these phenomena can be detected. The hurricane coverage
maps show full coverage by NEXRAD for areas of risk. However, for the rest of the significant
weather phenomena, there are some areas where the pre-NEXRAD network had better coverage. A
few such areas appear in Figure 2-3b, which shows supercell detection. There are more areas where
the old network could detect mini-supercelis (Figure 2-4b) or macrobursts (Figure 2-5b) and the
NEXRAD network will not. These are mostly locations that were near pre-NEXRAD radars but are
more distant from the nearest NEXRAD or planned NEXRAD. For example, Table 2-2 indicates that
a mini-supercell that is more than 100 km from a NEXRAD but within 45 km of a WSR-57 would be
more likely to be detected by the WSR-57 (beam resolution and smoothing of Doppler velocity
structures are key factors). It is this type of location that is evident in Figure 2-4b. Unfortunately, no
definitive experiments have been carried out to indicate that this actually would be the case. Because
of the limitations in using the bow-echo feature for macroburst detection, as discussed earlier, it is not
clear whether detection by the pre-NEXRAD radar in these cases gave better service, the detection
often was not timely enough to give adequate warning.
The coverage areas in the figures are based on estimated median maximum range values from
Table 2-2. It should again be emphasized that there is considerable variability in these maximum range
values, as indicated by the numbers within the brackets in Table 2-2. Small changes in the maximum
range values used to prepare Figure 2-4 and the other maps couic! easily eliminate or acid areas where
the pre-NEXRAD network had better coverage. An example is provided by Figure 2-8a, which shows
(in comparison with Figure 2-3a) how radar-detection coverage changes when maximum detection
range (i.e., "experience") values are used for supercell detection. The corresponding difference chart
in Figure 2-8b shows that the potentially degraded areas of supercell detection coverage by NEXRAD
for the contiguous United States are reduced substantially when the maximum NEXRAD capability
threshold is used. Rather than using any of these figures to label areas where the pre-NEXRAD net-
work provided better coverage, the pane! believes that it is preferable to use these figures in a relative
sense. Thus, those areas identified in the figures as having detection coverage by the pre-NEXRAD
network and not by NEXRAD are areas where radar-detection coverage is more likely to be degraded
than in other areas.
Where there is a one-for-one replacement of an old radar by a NEXRAD, radar-detection
capability will unquestionably be improved. Degradation of detection capability might occur where the
pre-NEXRAD radar is decommissioned and coverage is provided by a NEXRAD located some distance
away. Appendix F shows the distance of the nearest NEXRAD from each of the pre-NEXRAD radars.
Following are examples of areas shown in Figures 2-2 through 2-7 in which raclar-detection
capability for one or more phenomena is likely to be degraded with the NEXRAD network. The order
of locations discussed below is alphabetic by state. The order does not imply priority.
Northern Alabama (e.g., Huntsville vicinity)
This area currently has a WSR-74C Doppler weather radar ant! is slated to be covered by a
NEXRAD located 164 km away. As a consequence of the large increase in distance of this area from
~ Altitude at which ice/snow melts.
32
Assessment of NEXRAD Coverage
the new NEXRAD radar, radar detection of smaller-scale, low-altitude weather phenomena (i.e., mini-
supercelIs, tornado vortices, macrobursts, and microbursts) and of precipitation reaching the ground
(important for flash-flood warnings) may be significantly degraded.
Northern Indiana (e.g., Fort Wayne, South Bend vicinity)
A DoD NEXRAD at Grissom Air Force Base was planned, but this NEXRAD was eliminated
when the base closed. As a result, radar-detection capability for a number of weather phenomena that
occur in this region (e.g., supercelis, mini-supercelis, lake-effect snow, macrobursts, and stratiform
snow) may be degraded when existing radars are decommissioned.
Northwest North Dakota (e.~., Williston vicinitY)
The existing radar site for this area is the most distant (207 km) from a NEXRAD installation
of any of the pre-NEXRAD radars that will be decommissioned. The locally occurring weather
phenomena whose radar-detection capability may be degraded include stratiform snow, supercelis, and
macrobursts. The state of North Dakota is currently sponsoring work in hail suppression in a portion
of the potentially degraded radar-detection coverage area which is identified in Figure 2-3b (top center
of map).
Northwest PennsYlvania (e.~., Erie vicinity)
This area has a region-specific, low-altitude weather phenomenon (lake-effect snow) that is
difficult to detect with NEXRADs located at Cleveland, Ohio; Buffalo, New York; and Pittsburgh,
Pennsylvania. The pre-NEXRAD radar in this region was able to detect lake-effect snow within
approximately ~10 km of the existing radar site. Other locally occurring low-altitude phenomena that
may have degraded radar-detection coverage include mini-supercelIs and macrobursts.
Southeastern Tennessee (e.g., Chattanooga vicinity)
Radar-detection capability in this area for low-altitude phenomena (e.g., mini-supercelis and
macrobursts) and even supercelIs is difficult to achieve at even moderate ranges from a NEXRAD due
to the complex terrain features. Figures 2-3 through 2-5 show that the area covered by the existing pre-
NEXRAD radar in this location is, itself, significantly limited by the topography.
The difference maps also make it evident that there are 15 DoD NEXRAD sites (the striped blue
areas in Figures 2-2b through 2-7b) that cannot be considered "supplemental." These sites provide the
only detection coverage of certain phenomena in their area of coverage. Without these 15 radars, Here
would be more geographic regions in Be contiguous United States where radar-detection capability may
be degraded.
~ These 15 NEXRAD sites are East Alabama, Alabama; Beale and March, California; Dover, Delaware; NW
Florida, Florida; Moody and Robins, Georgia; Ft Pole, Louisiana; Columbus, Mississippi; Minot, North Dakota;
Frederick and Vance, Oklahoma; Central Texas, Dyess, and Laughlin, Texas. There are five additional DoD
NEXRAD sites that provide sole coverage for areas that were not covered by the pre-NEXRAD network. These
are Edwards and Vandenberg, California; Cannon and Holloman, New Mexico; and Griffins, New York.
l
O W5R-74C SITE
+ NW5 ~iR-8230 RU
X 060 WSR-880 SITE
~'~40
AND If_
Pee-NE)tRAD
WITHER
- 35/3y
O 300 HAlJllCAL AlIIES tnml)
. ~
0 lBS KILO - TERS (km)
Figure 2-8a Maximum range detection coverage of supercells for the pre-NEXRAD (blue) and NE=XRAD (red) networks. The coverage extent of
each individual radar in the two networks is taken from He higher values of Be ~experience" median maximum range values from Table 2-2. The
pie chart shows the percentage of vulnerable areas (contiguous United States + 230~ km across borders or oceans) where only the NEXRAD network
provides this coverage (dotted red), where both networks provide coverage (combined red/blue), where only the pre-NEXRAD network provides
coverage (striped blue), and where neither network provides coverage (white) for this weather phenomenon. Courtesy of SRI International.
~ '
~r ~
o
:+(~ ~+_~~-~-~6,.+ot:~.O-o~
X sol) W511-850 SITE
YITH Doe
~3 WITHOUT OoD
| O 100 NAUTK AL 1111~; (nml)
0 1BS KILOS (all)
O~V5R_57.~74S~QF | X
O ~ 743 SllE -; ~ 1 1 1 1 0+ x\) 0<
+ NYS WSR-60D SllE i ~OX O + 0
t~J6
Figure 2-8b Areas with potentially degraded detection coverage for supercells by NEXRAD with (red) and without (blue) DoD NEXRADs compared
with the pre-NEXRAD radars. Courtesy of SRI International.
Network Configuration and Detection
35
IMPACT OF NEXRAD COMMUNICATIONS LINKS
AND FORECAST OFFICE STAFFING
In generating NEXRAD radar-detection coverage information for Figures 2-2 through 2-7, He
pane! assumed that data from the closest NEXRAD radar to each location are available to the cognizant
WFO. When the cognizant WFO is not located near the NEXRAD radar product generator (RPG), the
products to be displayed on the WFO's principal user processor (PUP) must be provided over a
telephone line. Full-capability access is provided by a 56-kbs communications line. Full-capability
access is also needed for the four forecast offices that will provide service using two NWS NEXRADs
to cover an extended service area.)
Coverage by the 15 DoD NEXRADs referred to previously is needed as part of Be overall
national detection coverage for significant weather. Currently, these NEXRADs are slated to have only
a 9.6-kbs communications capability to the area WFO that provides warnings to the public. Additional-
ly, there will be delays in providing certain products to a remote user. The consequence is that the
effective radar-detection capability during significant weather, as perceived by the cognizant WFO,
may be degraded due to inadequate communications and back-up capabilities. The pane} has not
quantified this potential effective degradation in racIar-detection capability. However, this is an
important issue that needs to be addressed immediately by the NWS through tests at a WSFO in con-
junction wig the supporting DoD radar (e.g. Norman, Oklahoma, operating in conjunction with the
DoD NEXRAD at Frederick, Oklahoma).
The effective radar-detection capability of a NEXRAD depends on the skill and attentiveness
of the forecaster using the PUP. Therefore, it is essential that WFOs using two NEXRADs to cover
their extended service area have adequate staffing during significant weather to analyze the data from
both radars, and from neighboring radars, and to issue timely forecasts and warnings for their entire
extended area of responsibility. This issue needs to be addressed by the NWS (e.g., as a part of He
Modernization and Associated Restructuring Demonstration (MARD) activity) Trough their use of
assessment criteria to resolve degradation-of-service issues in areas of public concern.
~ These offices are Phoenix for Yuma, Arizona, NEXRAD; Miami for Key West, Florida, NEXRAD; and
Portland for Caribou, Maine, NEXRAD, which are a consolidation of two former service areas; and Salt Lake City
for Cedar City, Utah, NEXRAD, whose service area does not change.
