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An Investigation of Bird Radar Echoes Detected by the WSR-88D Doppler Radar at WFO Binghamton, NY
Michael B. Sporer, National Weather Service Binghamton, NY
Kirk A Lombardy, National Weather Service, Binghamton, NY
Joseph Conklin, Jersey Weather Service, Atlantic Highlands, NJ
1. Introduction
Radar detection of non-meteorological echoes is not a recently discovered application
(Eastwood 1967, Edwards 1959). However, with the proliferation of high quality doppler
radars in association with the National Weather Service (NWS) modernization and
restructuring, many new occurrences of this phenomenon have been reported. Shortly after
the WSR-74S s-band radar was replaced with the more powerful and sensitive WSR-88D s-band
doppler radar at the NWS office in Binghamton, NY in September of 1993, the periodic
appearance of a ring shaped echo configuration was noticed during the late summer and
early fall near the time of sunrise. The primary purpose of this investigation was to
determine the exact source of the echo using ground truth verification and to understand
how the targets were creating this unique return.
2. Background
Observations by the NWS Binghamton WSR-88D show that the echo usually first
appears during mid August and persists into October. The first returns are observed
around the time of sunrise as a point source that subsequently expands outward
in all directions to form a ring (Fig. 1). This expansion
is typically well depicted by the radar velocity data as a divergence signature
(Fig. 2) but very strong nocturnal inversions occasionally
lead to loss of data due to range folding. There is variability in the exact
echo configuration with some mornings exhibiting a nearly symmetric ring while
others show more of an amorphous area with little or no symmetry. Meteorologically,
the best time to observe the radar echo is the morning after good radiational
cooling with little fog and/or stratus development. These synoptic conditions
can lead to some anomalous propagation (AP) but the quiet weather associated
with these conditions allow the radar to operate in its most sensitive clear-air
mode. The radar echo in question appears to be relatively shallow since it is
only detected by the lowest radar elevation scan (0.5 degrees).
Of critical importance to this investigation will be real time data gathered
in the field from near the point where the echo appears to originate. After
examining several occurrences, a point of origin was determined to be at an
azimuth of approximately 325o and a range of approximately 60 nautical
miles (nm) from the radar. This position is at the north end of Cayuga Lake
which is one of several long and narrow lakes formed by glaciers that make up
the Finger Lakes region of central New York. The area our team focused on was
in the vicinity of a large marsh, a portion of which is designated as the Montezuma
National Wildlife Refuge (NWR) (Fig. 3). This wetland
location, combined with the time of year for the echo occurrence, lead to speculation
that the reflectors in question were composed of migrating birds, initially
thought to be some sort of water fowl. Other theories involved insects and deformation
of the nocturnal inversion caused by the boundary layer over the marsh.
After coordinating with the personnel at the refuge, three field trips were arranged
with one outing involving a special permit for an overnight stay. The outings were
scheduled for 4-5 September, 6 September, and 14 September 1997. 35mm photographs and
video were used to document any possible radar reflectors.
3. Observations
The overnight trip on 4-5 September got off to a promising start. The synoptic
conditions allowed for radiational cooling with a clear and calm night but no
fog had formed so our view would be unobstructed. We took up a position along
the eastern side of the Main Pool in the refuge (Location 1 in Fig.
4) at approximately 0830Z with sunrise occurring at approximately 1033Z.
Surface obstructions were not present within approximately 200 feet of our observing
point. After waiting patiently until 1100Z, no significant reflectors were observed
to serve as a source for a radar echo. The radar did detect a significant radar
echo but it appeared to be centered approximately 4km further south in the refuge
and thus out of our view. The refuge biologist indicated his belief that the
echo may be related to the departure of blackbirds from their nightly roost
in the many cattails of the marsh. Thus, we attempted to secure a position closer
to such wetland vegetation on our next attempt.
On 6 September, a point further south along a lock on the north end of Cayuga
Lake was selected for a point of observation (Location 2 in Fig.
4). However, sky conditions deteriorated faster than anticipated with a
solid overcast of stratocumulus clouds, intermittent showers, and approximately
a 15 mph south wind blowing up the axis of the lake. These adverse weather conditions
prohibited direct observations and it was necessary to postpone future attempts
until the synoptic situation became more favorable and time constraints were
not as great.
The morning of 14 September appeared to offer one of the last good opportunities
of the season to document the source of the radar echo. The synoptic pattern
featured a large area of high pressure building into the region with the center
of the high very close to central New York. This allowed for radiational cooling
with clear skies and light winds. However, dew points were relatively high so
some fog formation was expected. After coordinating with the personnel at Montezuma
NWR and discussing our previous trips, it was suggested that a location closer
to the Tschachie Pool may yield better results (Location 3 in Fig.
4). This location also offered an observation tower with a platform approximately
20 feet above the ground.
Upon our pre-dawn arrival at the observation tower we found patches of dense ground fog
throughout the refuge. The platform offered a position above most of the vegetation but
fog occasionally limited the visibility to less than one half mile. As sunrise approached,
we began to hear wildlife stirring in the brush. The initial activity appeared to be
limited to small numbers of assorted water fowl. However as the sky continued to brighten
large numbers of other birds joined the activity and eventually produced a volume of
squawking and cawing comparable to standing near a waterfall.
After several minutes of waiting in the first few minutes of early morning
sun, with nothing more than small, isolated flocks of birds flitting about,
we observed an enormous flock of blackbirds lifting up from the brush near Tschachie
Pool and flying to the southwest (Fig. 5). Several minutes
later, another flock containing what appeared to be thousands of birds lifted
up and flew in a southerly direction, followed by yet another large flock minutes
later. During this period of bird activity, the Binghamton doppler radar detected
an expanding, a-symmetric echo configuration over Montezuma NWR (Figs.
6 and 7). The radar data was consistent with past
observations and correlated well with the visual sighting of the birds.
In all, we observed five large flocks of blackbirds lift off and fly away from
Montezuma NWR in a southerly direction. As we left the refuge, it still sounded as if
there were a fair number of birds remaining in the marsh but the majority appeared to have
departed. The birds in each flock remained in narrow corridors within a few feet of each
other as they went off to feed.
4. Analysis
There are several factors that contribute to the detection of these birds by the NWS
Binghamton WSR-88D, the two most important being: (1) blackbird behavior patterns; and,
(2) basic principles governing radar detection of non-meteorological echoes. A detailed
discussion of these factors follows:
4.1 Blackbird Behavior Patterns
4.1.1 Migration (Nero 1984)
Each major migratory path, known as a flyway, remains the same for fall and spring
migration. The blackbirds studied at the Montezuma NWR follow the Atlantic Flyway that
originates in Canada and follows the eastern seaboard of the United States southward. The
birds usually remain in a tight formation separated by no more than 4 to 8 inches as they
cruise at altitudes normally 200 ft to 900 ft above ground level (AGL). Since the average
blackbird may live to the age of 10 years, it is likely that individual birds follow the
same flyway each year and frequent the same feeding and roosting areas along the path.
4.1.2 Blackbird Roosting (Orians 1985)
Migrating blackbirds tend to live in roosting groups of 200 or more, and are commonly
found across central New York during the summer and fall migratory seasons. Although
blackbirds are highly territorial during the breeding season, they become more tolerant of
other species during migration. This allows large collections of groups, known as
colonies, to roost together at night during the migratory season. The major benefit they
receive from roosting together is protection. However, this protection is primarily
derived from their preferred roost location (marshes) and not necessarily due to the size
of the colony. Major roosts, such as the one investigated at the Montezuma NWR, have been
known to host anywhere from one hundred thousand to over one million birds.
4.1.3 Blackbird Feeding Groups (Orians 1980)
During the early morning hours, individual feeding groups break from the colony and
take off from their roosts. Visible light is the mechanism that triggers the birds to take
flight. Blackbirds partially depend on the sun for orientation, and rely on this
orientation to return to the same feeding areas each day. If the birds are forced to
remain in their roosting place due to inclement weather, they feed on local insects and
shellfish that live within the marsh.
The members of the feeding groups are determined by bird species and sex. Adult males
form their own groups while females and young form others. These pairings are not
exclusive however, and groups may occasionally contain both sexes and all ages. Although
group species was very difficult to ascertain during the field trips to Montezuma NWR, the
two primary marsh-dwelling species of blackbirds are red-wing and yellow-headed, with only
red-wings being indigenous to central New York. There is no evidence to suggest that there
is an individual bird who leads the feeding group in flight. However, there is a
definitive dominance hierarchy regarding feeding rights once the group has arrived at its
destination.
The feeding groups have distinct feeding areas and typically follow a similar route
from their nightly roosting place. Blackbirds may travel over 100 km day to find a
suitable feeding area. The feeding areas remain the same for a period of several days
which allows each group of the colony to return to the same roosting place each night. The
feeding areas change as the groups search for more productive grounds and eventually lead
the colony to a new roosting place along the flyway.
4.2 Basic Principles Governing Radar Detection of Non-Meteorological Echoes
The WSR-88D doppler radar located at Binghamton, NY, represents a substantial
improvement over previous weather radars. However, the WSR-88D is still bound by the same
physical laws that affect the propagation of electromagnetic waves through the atmosphere.
In order to create uniformity among the various types of radar data, radar equations
always assume that the atmosphere is in a "standard" state regarding variables
such as temperature, pressure, humidity, etc (Green 1990). Unfortunately, this can lead to
inaccurate radar information during cases when the atmosphere varies from this
"standard" state, which can happen on a daily or even hourly basis. The times
when the atmosphere is in a "non-standard" state is when the majority of
non-meteorological echoes are detected.
The WSR-88D functions by emitting pulses of microwave energy in a narrow, conical
shaped beam from its antenna. The beam pulses interact with objects in the atmosphere,
reflecting some of the energy back to the radar. The radar uses this returned
energy to calculate data regarding the character of any reflectors encountered.
Under "standard" atmospheric conditions, the beam travels out from
the antenna and is refracted downward by the air at a presumed index of refraction
(Fig. 8). At the same time, due to the curvature of
the earth, the center of the beam becomes higher and higher above the ground.
The result is that under "standard" atmospheric conditions, even at
its lowest elevation angle of 0.5o the center of the radar beam is
actually more than 7,400 feet AGL at a distance of only 60 nm (U.S. Dept. of
Commerce 1991). The Montezuma NWR is located approximately 60 nm from the radar
at Binghamton so at this range the radar estimates the center of the beam to
be approximately 7,400 feet AGL over the refuge.
It is also important to note that the conical shape of the beam spreads out as the
distance from the radar increases. At the 60nm range of the refuge, the beam width of 0.95o
has expanded to a diameter of approximately 5,700 feet (Green 1990). Since the center of
the beam is approximately 7,400 feet above the refuge with a diameter of 5,700 feet, the
beam may detect reflectors between 4,550 feet and 10,250 feet AGL. If this is indeed the
case, it initially does not seem possible for the radar to detect the birds at all, since
blackbirds fly between 200 feet and 900 feet AGL. This is explained by the physics of
radar beam propagation through the atmosphere as well as the condition of the atmosphere
itself.
One type of "non-standard" atmosphere results in what is called super-refraction
of the radar beam. This condition is caused by a low level temperature inversion
which causes the index of refraction of the atmosphere to increase with increasing
altitude (Green 1990). The increasing index of refraction causes a corresponding
increase in the downward curve of the radar beam as it moves away from the radar
(Fig. 8). In some cases when the inversion is particularly
strong, the beam will be curved to such an extent that it encounters the ground.
The resulting strong returns are commonly referred to as anomalous propagation
(AP) returns.
During super-refractive conditions the actual radar beam will be considerably
lower in altitude at any given distance from the radar than the radar display
indicates. This will lead to the radar assigning higher altitudes to echoes
at a given range than are actually present. On the morning of 14 September,
the upper air soundings from both NWSFO Albany, NY and Buffalo, NY indicated
that the atmosphere was likely in a super-refractive state (Fig.
9) with inversions present near the surface and between 5000 and 10,000
ft. Thus, at an elevation angle of 0.5o and a range of 60 nm, it
can be surmised that the actual center of the radar beam when detecting the
bird echoes was much lower than the 7400 feet AGL indicated by the radar display.
As previously mentioned, the spreading of the conical radar beam also must be taken
into account. At a distance of 60 nm the beam has expanded to an approximate diameter of
5700 feet. This would mean that if the birds were flying between 200 feet and 900 feet
AGL, the center of the beam would have to be at an elevation of approximately 3000 feet
to3800 feet AGL for the birds to intersect the bottom of the radar beam at this range.
Given the fact that the atmosphere was super-refractive and that the radar did indeed
detect the birds, the data suggest that the center of the beam was no greater than 3800
feet AGL over Montezuma NWR on the morning of 14 September. It is possible that the beam
was also "deformed" by the non-standard conditions but a detailed examination of
radar beam structure in a non-standard environment is beyond the scope of this paper.
It is also important to realize that the WSR-88D doppler radar was primarily designed
to detect liquid precipitation targets which reflect, absorb, transmit, and re-radiate the
energy from the radar beam in many directions. This results in only a fraction of the beam
energy being returned to the radar for analysis. When the beam encounters an object which
returns more energy to the radar than a liquid precipitation target, it still depicts the
relative strength of the echo according to a returned power to intensity relationship that
assumes a liquid target. The echo intensity is then converted to a relative color scale
for the display screen. Also, since radar reflectivity is related to the 6th
power of the target diameter (U. S. Dept. of Commerce 1991), one large target can return
as much energy as many smaller ones. This being the case, one would expect that a bird in
flight, being much larger than a liquid droplet, would return a much stronger echo than a
raindrop. However, the radar display from 14 September shows the bird echo with returns
normally associated with very light precipitation or clouds. The explanation for the
weaker than expected echoes lies in the problems associated with radar beam spreading.
With a beam width of more than one mile over Montezuma NWR, it is highly unlikely that
the birds flying in a narrow flight zone completely fill the radar beam. This partial beam
filling has a direct impact on how strongly the birds are displayed on the radar screen.
If a target (i.e. flock of birds) does not completely fill the radar beam only a portion
of the available transmitted energy will be returned to the radar. The radar functions
under the assumption that whatever reflectors are encountered completely fill the radar
beam. So if a highly reflective target was encountered but only filled a small portion of
the beam, even if it returned almost all of the energy it encountered, the returned energy
would be only a small part of the total transmitted energy available and would cause the
target to be displayed weaker than anticipated. The assumption of complete beam filling is
not accurate for the blackbirds which fly in a narrow band only a couple of hundred feet
thick and is a major contributor to the echo being depicted much weaker than otherwise
expected.
5. Conclusion
Although there was much speculation as to the composition of the reflectors in the ring
shaped echo seen by the WSR-88D located at the NWS office in Binghamton, NY, our field
verification study indicated the echo to be comprised almost exclusively of several
species of blackbirds. A large proportion of red-winged blackbirds is suspected but could
not be verified due to low light conditions and reduced visibility in ground fog. The
appearance of the echoes during the late summer and early fall corresponds to the
southerly migration of the birds along the Atlantic Flyway. The echo originated from the
Montezuma NWR, a wetland habitat consistent with the known roosting preferences of
blackbirds. The biological habits of blackbirds allow the echo to appear around the time
of sunrise on relatively clear mornings during the migratory period. The echo dissipates
quickly due to the dissipation of super-refractive atmospheric conditions after sunrise
and to the decrease in bird density corresponding to the arrival of the birds at their
feeding grounds. The birds exhibited flight patterns consistent with the known feeding
behavior of blackbirds and correlated well with radar velocity data associated with the
echo configuration.
The radar beam propagation through a super-refractive atmosphere in conjunction with
beam width spreading allowed for the detection of the birds at their natural flight level.
This is a much lower altitude than was estimated by the radar, which does not account for
the super-refractive conditions present on the morning of 14 September. Beam width
spreading also resulted in a weaker echo return than expected because the narrow flight
zone of the birds (200 feet AGL to 900 feet AGL) returned only a small portion of the
available transmitted energy along the 5,700 foot beam width over Montezuma NWR .
The detection of migrating blackbirds by the WSR-88D in Binghamton, NY, has the
potential to provide very useful data to external users outside of the meteorological
community. The radar data provides a very good idea of where the birds roost and when they
become active in the morning. Their flight paths can be tracked to locate their feeding
grounds. The arrival and departure of the birds can be documented annually to study
migratory trends. All of this data can be used by an ornithologist to assess the status of
the flock and possibly the health of the ecosystem as a whole.
Aviation interests can also benefit from the knowledge of a possible low level bird
hazard near the time of sunrise. Bird flights of this type have also been known to
contaminate wind profiler observations and WSR-88D VAD winds (Jungbluth et al 1995, Miller
et al 1997, Wilczak et al 1995). This contamination could lead to poor numerical model
initiation and subsequent model performance. We are hopeful that this study has
illustrated the potential utility of radar data beyond the traditional scope of severe
weather warnings.
Weather radars are extremely useful tools for analyzing remote portions of the
atmosphere. It is important for meteorologists to be familiar with atmospheric conditions
and their impact on radar performance to avoid misinterpretation of radar data. This is
the challenge facing all weather radar operators as they attempt to warn of hazardous and
life threatening conditions associated with meteorological and non-meteorological
phenomena alike.
Acknowledgments:
The authors would like to thank the employees of the U.S. Fish and Wildlife Service at
Montezuma NWR for their extensive co-operation during the period of study. Their vast
knowledge of the refuge ecosystem was a valuable asset to our research.
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