Birds, Insects, and Refractive Index Gradients as the Source of Clear-Air Return for Meteorological Radars William Marti

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

2. 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

3. Small groups of migrants like these are not a problem

4. 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?

5. Diurnal Cycle of Reflectivity is Interesting • 24 hours of KTLX NEXRAD clear-air data from August, 2002 • Show Animation

6. 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

7. T-Z sections of dBZ and V rapid change in dBZ less rapid change in V

8. 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.

9. Clear-Air Targets are Generally Either: • Birds • Insects • Refractive index inhomogeneities Dust and dirt particles tend to be too small to provide much signal

10. 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.

11. Some Radar Equations Reflectivity factor Backscatter cross-section

12. MIE SCATTER AT 3 WAVELENGTHS FOR WATER SPHERES optical limit exact for water spheres of any size Rayleigh limit

13. Radar Cross-Sections of Some Common Targets for 3 cm Radar

14. Refractive Index InhomogeneitiesReflections and Bragg Scatter Simultaneous radar and refractivity soundings, Lane and Meadows (1963).

15. 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:

16. Conceptual Diagram from Glover et al., 1968

17. 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

18. RHI scan of dBZ Range rings are 500 m probable birds reflectivity ~15 dBZ or a backscatter cross- section of 10 cm2 ground clutter

19. 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

20. But what are the targets ahead of and behind the density current?

21. estimate Cn2 from scaling arguments as: for use in:

22. These values are quite close to those observed, though typical Cn2 values reported in literature are 10-13 to 10-16

23. birds or large insects birds small insects or Bragg scatter insects or Bragg scatter

24. 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

25. 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

26. 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.

27. 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.

28. 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.

29. 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.

30. 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

31. PPI scan from KTLX shown dBZ to 25 spatially continuous signal implies high target density much stronger than KGLD echo from case #2

32. 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

33. Wind profile obtained by VAD analysis of KTLX data

34. 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.

35. 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).

36. 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.

37. 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.

38. 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.

39. KTLX dBZ PPI scan at 15 Z, Aug. 15, 2001 Shows bilateral-symmetric echo pattern probably due to either bird or insect alignment

40. END

41. Migrating Birds and Precipitation? From Radar Ornithology Lab website, Clemson University

42. LLJ T-Z Section obtained from NEXRAD radar KTLX August 14, 2002 beginning at 16Z Sunset near 9 hours and sunrise near 19 hours