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Birds, Insects, and Refractive Index Gradients as the Source of Clear-Air Return for Meteorological Radars William Martin November 9, 2005

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Birds, Insects, and Refractive Index Gradients as the Source of Clear-Air Return for Meteorological Radars William Martin November 9, 2005. Weather radar can provide valuable wind field information for studying atmospheric phenomena and for use in numerical forecast models

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slide1

Birds, Insects, and Refractive Index Gradients as the Source of Clear-Air Return for Meteorological RadarsWilliam MartinNovember 9, 2005

slide2
Weather radar can provide valuable wind field information for studying atmospheric phenomena and for use in numerical forecast models
  • NEXRADs commonly see significant and wide spread clear-air return in the lower several kilometers of the atmosphere
  • However, if the signal is caused by migrating birds, then a significant bias may be present
typical nocturnal clear air return
Typical Nocturnal Clear-Air Return

Radar antenna tilt=1.5o

Range rings are every 400 m

in the vertical, or every 15 km

in the horizontal

This image shows strong

reflectivity to a depth of about

4 km.

Velocity is determined by

the Doppler shift of the

signal

But what is reflecting

radar energy?

diurnal cycle of reflectivity is interesting
Diurnal Cycle of Reflectivity is Interesting
  • 24 hours of KTLX NEXRAD clear-air data from August, 2002
  • Show Animation
llj t z section obtained from wsr 88d radar
LLJ T-Z Section obtained from WSR-88D radar

KTLX August 14, 2002 beginning at 16Z

Sunset near 9 hours and sunrise near 19 hours

t z sections of dbz and v
T-Z sections of dBZ and V

rapid change in dBZ

less rapid change in V

slide8
Drop in signal at sunrise and sunset suggests a change in target at those times. For example, from one daytime species of insect into a nighttime species.
  • Reflectivity changes more rapidly at sunrise and sunset than velocity, suggesting a non-meteorological target source.
clear air targets are generally either
Clear-Air Targets are Generally Either:
  • Birds
  • Insects
  • Refractive index inhomogeneities

Dust and dirt particles tend to be too small to provide much signal

slide10
Historically, insects were identified as the most common source of clear-air return: Hardy and Katz (1969).
  • Recent work has identified migratory birds as an important source of nocturnal return which seriously biases wind measurements: Wilczak et al. (1995), O’Bannon (1995), Jungbluth et al. (1995), Zrnic & Ryzhkov (1998), Gauthreaux and Belser (1998). These studies were driven by observations of occasionally large differences between rawinsondes and radar wind profiles.
  • But the extent of the problem is still unclear. Wilson et al. (1994) found mostly insects in a detailed study of clear-air return in a variety of circumstances.
some radar equations
Some Radar Equations

Reflectivity factor

Backscatter cross-section

slide12

MIE SCATTER AT 3 WAVELENGTHS FOR WATER SPHERES

optical limit

exact for water spheres

of any size

Rayleigh limit

refractive index inhomogeneities reflections and bragg scatter
Refractive Index InhomogeneitiesReflections and Bragg Scatter

Simultaneous radar and refractivity soundings, Lane and Meadows (1963).

slide15
Theory for planar reflections from refractive index gradients and discontinuities predicts reflectivities much weaker than observed (or requires very high N-gradients). For Lane and Meadows, this requires that the entire N-change occur in 8 cm or less.
  • Bragg scatter is a theory which explains enhanced reflectivity from refractive index gradients as being caused by turbulent mixing. It dates back to the 1950’s where it was first invented to account for beyond the horizon propagation of radio waves.
  • Bragg Scatter makes use of electro-magnetic theory and Obukov/Kolmogorov scaling to arrive at an equation for reflectivity in terms of radar wavelength and refractivity structure parameter:
case 1 gust front with dow3
Case #1 Gust Front with DOW3
  • Clear-air thunderstorm outflow seen by DOW3 June 11, 2000 in the afternoon
  • DOW3 is a 3 cm mobile radar
slide18

RHI scan of dBZ

Range rings are 500 m

probable birds

reflectivity ~15 dBZ

or a backscatter cross-

section of 10 cm2

ground clutter

slide19

RHI scan of

radial velocity

white shades are flow

towards radar

Targets differ in radial velocity

from surroundings by ~15 m/s

some second trip echo

and aliasing

slide20

But what are the targets

ahead of and behind the

density current?

slide22

These values are quite close to those observed, though typical

Cn2 values reported in literature are 10-13 to 10-16

slide23

birds or large insects

birds

small insects or

Bragg scatter

insects or Bragg

scatter

case 2 dow3 and kgld near midnight
Case #2 DOW3 and KGLD near Midnight
  • DOW3 co-located with the KGLD WSR-88D on May 30, 2000 near midnight in clear air
  • Both scanned the same air at the same time
  • DOW3 is a 3 cm radar and KGLD is 10 cm
  • Because of the wavelength difference, Bragg scatter should be 18 dBZ stronger with KGLD.
  • From Mie scattering calculations, signals from KGLD should be from 0 dBZ stronger than DOW3 for small insects to 20 dBZ stronger for large birds
slide25

Both scans have same tilt (2.5o) and range, but dBZ scales are different

Reflectivity in box is observed to be about 11 +- 4 dBZ stronger in KGLD

slide26

12 m gate spacing

scan from DOW3

at 10 degrees of tilt.

Shows lowest 500 m.

Echo resolved as

numerous point

targets with radar

cross-sections

of .06 to .32 cm2.

Passerines typically

have cross-sections of

10 cm2.

slide27

RHI scan showing

point targets up to 2 km

above the ground

range rings are every 200m

nearly continuous signal

in layer below 200 m.

Bragg scatter?

weak echo region about

-12 dBZ, or .002 cm2 of

cross-section.

estimate of target density
Estimate of Target Density
  • By counting targets in a sector, a target density of 5.0X10-3 per m3 is estimated from the previous figure. This would be roughly one target every 60 m, or about 1 billion birds flying over all of Kansas.
  • For comparison, bird migration traffic rates (MTR) are reported by ornithologists with units of bird crossings per mile of front per hour.
  • The DOW3 data imply an MTR of 730000.
  • Ornithologists report “heavy” MTR values of 5000 calculated from moon-watching data.
  • Gauthreaux (1998) reports MTR along Gulf Coast as high as 200000, with 20000 typical.
dow3 has revealed the targets to be most probably insects
DOW3 has revealed the targets to be most probably insects
  • Low radar cross-section inconsistent with passerine birds.
  • High target density inconsistent with all but the most extreme migrations.
case 3 ktlx and umass near midnight
Case #3 KTLX and UMASS near Midnight
  • UMASS 3 mm radar located at Max Westheimer field, about 15 miles from KTLX WSR-88D
  • Strong nocturnal clear-air signal, to 25 dBZ
  • Large numbers of moths had been noticed anecdotally
slide31

PPI scan from KTLX

shown dBZ to 25

spatially continuous

signal implies high

target density

much stronger

than KGLD echo

from case #2

slide32

T-Z section from UMASS

radar

vertical lines are every

10 sec.

horizontal lines are

every 200 m

3

2

Antenna pointed straight

up and targets pass

through the beam.

1

slide33

Wind profile

obtained by

VAD analysis

of KTLX data

slide34

UMASS has revealed the targets

to be almost certainly insects:

  • Radar cross-sections of targets at all levels are, for the strongest reflectors, .2 to .5 cm2. Too small to be birds.
  • Target density is estimate to be 10-4 per m3, twenty times that seen by KGLD from case #2.
  • Calculated KTLX reflectivity based on this cross-section and number density gives a reflectivity factor of 25 dBZ, close to that observed.
summary
Summary
  • High resolution radars for two cases of nocturnal clear-air return strongly implies that the targets were mostly, if not entirely insects.
  • This agrees with Wilson et al. (1994).
  • This disagrees with Gauthreaux et al. (1998) and Wilczak et al. (1995).
discrimination of birds and insects as radar targets
Discrimination of Birds and Insects as Radar Targets
  • Bird behavior is too variable for patterns to be reliable. Though there are definite patterns of behavior, birds can migrate at any time of the day or night, in any direction, against the wind, with the wind, ahead of and behind fronts, and on any day of the year.
slide37
Radar cross-section. Birds can be ruled out if radar cross-section is below 10 cm2 as passerines have cross-sections of 10 to 30 cm2.
  • Spatial granularity. Granular signal implies point targets. Combined with cross-section measurements, it can confirm birds.
  • Number Density. A high number density of targets could exclude birds.
slide38
Symmetry of PPI echo. Bilateral symmetry in PPI pattern is sometimes seen, and ought to be present whenever the targets are aligned. Birds may be more likely to do this. Usually this pattern is not seen at night, which is consistent with an insect explanation.
  • Use of polarization information (Zrnić & Ryzhkov, 1998). This needs more research; will be available in the future on WSR-88Ds.
  • Height of echo above ground. Perhaps insects can not reach the same altitudes attained by birds.
slide39

KTLX dBZ PPI scan

at 15 Z, Aug. 15, 2001

Shows bilateral-symmetric

echo pattern probably due

to either bird or insect

alignment

migrating birds and precipitation
Migrating Birds and Precipitation?

From Radar Ornithology Lab website, Clemson University

llj t z section obtained from nexrad radar
LLJ T-Z Section obtained from NEXRAD radar

KTLX August 14, 2002 beginning at 16Z

Sunset near 9 hours and sunrise near 19 hours

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