Estimates J. E.Bla ck Brock Buffalo Base Reflectivity May 1999 How do we interpret this image?
Mar 30, 2015
Estimates J. E.Black
Brock
Buffalo Base Reflectivity May 1999
How do we interpret this image?
Estimates J. E.Black
Interpreting NEXRAD Base Reflectivity Images
John BlackBrock University
• Two kinds of bird densities• Volume density from NEXRAD base reflectivity images• Total bird numbers • Bird current density (flow)• Bird Current Density from
NEXRAD Doppler images• A Turbine Scenario• Spring Migration
Estimates J. E.Black
Two Kinds of Bird Density
• In working with birds on radar there are two sorts of density to consider
• Bird volume density, the number of birds in a cubic kilometer of space
and
• Bird current density, the number of birds crossing a square kilometer per hour
Estimates J. E.Black
Some Weather Radar Migration Facts
• Nocturnal passerine migrants do not flock. On the Buffalo weather radar at night I have only once seen evidence of a flock of birds, perhaps shorebirds or ducks heading north.
• I have heard of diurnal hawk migration being seen on a radar on the Texas Gulf coast.
Estimates J. E.Black
Volume Density
• The base reflectivity DBZ provides a measure of the bird volume density, that is how many birds are found in a cubic kilometer.
• As a rough guide
• volume density = 3* 10(DBZ/10)
Estimates J. E.Black
Brock
Buffalo Base Reflectivity May 1999
Estimates J. E.Black
Effects of Earth Curvature
Estimates J. E.Black
Estimate of Bird Numbers in Radar Image
Density of 10 cm2 Birds = 3.0*Z
20km 75km 160km30-360m 360-1600m 1600-4300m
Add these densities(weighted) to get total of 140 birds in a column 1 km square which extends from 30 to 4300 m (can average density at each range)
Estimates J. E.Black
140 birds
140 birds/km**2 ground
1 km
4.27 km
1 km
What if all the birds aloft landed
Estimates J. E.Black
A Bird Landing Scenario
What if the 140 birds /km2 were to land peacefully?A little thought shows there would be about one bird on the ground every 84 meters, almost only one bird if you walked the length of a football field in any direction, not a lot of birds.
Estimates J. E.Black
Brock
Buffalo Base Reflectivity May 1999
Estimates J. E.Black
Total birds in radar range May 15/16, 1999.
Total Birds/km2 = 140
Approximate Radius of Echoes = 150 km
Area Covered by Echoes = 70,000 km2
Total birds in radar range = 9,900,000
(Birds Contributing to Echoes in Image)
Estimates J. E.Black
Bird Current Densityor Flow
• The product of the bird speed (km/hour) and the bird volume density(birds/km3) gives the bird current density in units of birds crossing a km2 per hour.
• This density is what we need to look at if we are considering the impact of wind turbines on birds in flight
Estimates J. E.Black
Buffalo Doppler Velocity Fall
Average velocity at each range (height)
Estimates J. E.Black
Turbine Scenarios.• The number of birds crossing
the area swept out by the wind turbine per hour is given by the current density at the height of the turbine times the area swept out by the turbine in km2.
• The number of birds actually hit by the turbine blades is then given by this number times the probability a bird passing through the sweep area will be hit by the blades
Estimates J. E.Black
Estimate
What if 100 birds per cubic kilometer approach a turbine at 50 km/hour. Then there is a current density of 5000 birds/km squared/hour. For a turbine of radius 27 meters the area swept out by the turbine is approximately 0.0023 km squared so about 11 birds cross the turbine area per hour.
What we do not know is what percentage of these birds would suffer a fatal collision with the blades.
Estimates J. E.Black
Flight density or Station Passage Rate
• In studying migration the number of birds passing overhead is of interest. This means adding up the birds in a column, as we did before, to get birds per kilometer squared. Then the product of this with the speed give the total birds crossing a kilometer through the observer per hour.
Estimates J. E.Black
FlowMay 15/16, 1999.
Total Birds/km2 = 140
Approximate Speed of Birds = 50 km / hr
Flight Density =140*50=7000 birds per km/hr
Station Passage Rate (10 hr night) = 7000*10 =70,000 birds/km/night
Birds crossing through the entire radar range over the night = 70000*300 = 21 million!
Birds Contributing to Echoes in Image
Estimates J. E.Black
Flow Scenario
What if migrating birds are stopped from crossing from the shore to the lake for 1 hour? This could happen if bad weather conditions were present over the lake such as rain. Then, from the flow arguments, we see about 7000 birds could pile up along a km of Lakeshore in one hour. Now we have about 7 birds every meter we walk along the shore.
Under these conditions the suitability of the shore for birds becomes crucial. Often the birds are exhausted and hungry when landing. The same sort of arguments apply to birds forced by weather to land after crossing the lake, or even just after a full nights migration..
Estimates J. E.Black
Worst case turbine scenario
If all birds in the vertical column were forced down to heights intercepted by the wind turbinesThe width of a typical wind turbine is about 0.045 kilometers. So if 7000 birds cross 1 km in an hour 315 will cross 0.045 km in an hour. What is not known is what percentage of these would have fatal collisions with the blades?
Estimates J. E.Black
0
0.2
0.4
0.6
0.8
1
1.21-M
ay-99
3-May
-99
5-May
-99
7-May
-99
9-May
-99
11-M
ay-99
13-M
ay-99
15-M
ay-99
17-M
ay-99
19-M
ay-99
21-M
ay-99
23-M
ay-99
25-M
ay-99
27-M
ay-99
Start Date of Night
Norm
alize
d NM
T an
d Co
unts
Warblers
NMT >350mSpring 1999
Estimates J. E.Black
NMT and Warblers, Spring 1999
0
5
10
15
20
25
30
Start Date of Night
War
bler
s/vi
sit a
nd N
MT/
1000
0
5
10
15
20
25
30Warblers/Visit
NMT/1000 > 350 m
Estimates J. E.Black
Vertical Radars
• Vertical radars measure the bird current density. If the speed is known then the volume density can be calculated from the formula
vol.density = current density/speed
• With a vertical radar adding the
current density(weighted) at the various heights gives the total flow or flight density.
Estimates J. E.Black
Volume density from Z
• One can show that the• average volume density of
birds(birds/km3)*average radar cross-section of the birds (cm2) = 28*Z
• Bird cross-sections range from 10 to 20 cm2, they depend on bird size, aspect and radar wavelength in a complicated way.
• For 10 cm2 birds we have
bird volume density = 3*Z
Estimates J. E.Black
Velocity from Weather Radar Doppler Images
• Often the images are corrupted.
• Not sure why
• If not corrupted then one can do a fit at a given range to get the velocity average at that range (and the velocity for the corresponding height) VAD