Anomalous Propagation

1. Anomalous Propagation Greater density slows the waves more. Less dense air does not slow the waves as much. Since density normally decreases with height, the radar beam is refracted toward the surface of the Earth.

2. Refraction

3. Subrefraction If the decrease in density with height is more than normal, then the beam bends less than normal and this is called subrefraction. In this case the beam might shoot over the target and miss the precipitation.

4. Subrefraction (Cont.)

5. Superrefraction If the decrease in density with height is less than normal, then the beam bends more than normal and this is called superrefraction. In this case the beam bends more toward the surface of the Earth, and it may undershoot the target.

6. Superrefraction (Cont.)

7. Ducting If the decrease in density with height is much less than normal, the beam may bend down to the surface of the Earth in a process called ducting. If the beam is backscattered to the receiver, it may result in Anomalous Propagation (AP) or “false echoes”.

8. Ducting (Cont.)

9. How to read the intensity scale Extreme Intense Severe Heavy Moderate Light Very light Light Precipitation Very light precipitation Fog, Clouds, Smoke Dust (?? Skeptic) , Insects, Birds • Units are decibels of Z (reflectivity). Precipitation Mode Scale Clear-Air Scale

10. Hail Detection • Returns > 55 dBz usually indicate hail. • However, the probability of hail reaching the ground depends on the freezing altitude. • Usually, a freezing level above 4300 m (14,000 feet) will not support much hail. • This is because the hail melts before reaching the ground. • Freezing level can be determined from an upper air sounding.

11. Hail? Max return of 60 dbZ Max return of 65 dbZ Freezing level was 2100 m Freezing level was 5200 m (7,000 feet) (17,000 feet) Produced golfball sized hail Produced no hail Hence, hail production depends directly on freezing level.

12. Vertically Integrated Liquid (VIL) • Take a vertical column of the atmosphere: estimate the amount of liquid water in it. • High VIL values are a good indication of hail. • The white pixel indicates a VIL of 70. • This storm produced golfball size hail. • Drawback: complete scan required (time)

13. The Hail Spike Also called Three-Body Scattering • A dense core of wet hail will reflect part of the beam to the ground, which then scatters back into the cloud, and is bounced back to the antenna. • The delayed returns trick the radar into displaying a spike past the core. • Usually, will only result from hail 1 inch in diameter or larger (quarter size).

14. Echo Tops Fairly accurate at depicting height of storm tops Inaccurate data close to radar because there is no beam angle high enough to see tops. Often has stair-stepped appearance due to uneven sampling of data between elevation scans.

15. Precipitation Estimates Storm TotalPrecipitation • Total estimated accumulation for a set amount of time. • Totals are in inches • Time range is sometimes listed on image. • Resets storm total whenever there is no rain detected for an hour.

16. One Hour Precipitation Total • -Updated once per volume scan. • Shows accumulated rainfall for the last hour. • Useful for determining rainfall rate of ongoing convection.

17. Precipitation Estimate Advantages and Limitations • Great for scattered areas of rain where no rain gauges are located • Has helped issue flash flood warnings more efficiently • Helps fill in the holes where ground truth information is not available • Much better lead time for warnings • Provides a graphical ‘map’ of rainfall for an entire region • Data can be overlaid with terrain and watersheds to predict reservoir and waterway crests • Estimates based on cloud water levels and not ground level rainfall • ‘Hail Contamination’ causes highly inflated values • High terrain causes underestimates • Lower resolution than reflectivity images • Useful as a supplement, not replacement for ground truth information

20. How Doppler Wind Is Displayed Inbound velocities (towards the radar) are shaded blue, with pale shades for light winds and dark shades for strong winds. Outbound velocities (away from the radar) are shaded orange with pale shades (yellow) for light winds and dark shades (red) for strong winds.

21. Velocity Imagery Warm colors are winds moving away from radome (reds, +) Cool colors are winds moving toward radome (greens, -) Wind speed is in knots Tight area of opposing winds (+ and -) can indicate convergence or rotation. Circled area called a couplet. Indicates a possible tornado.

22. Detecting Rotation A velocity couplet may indicate rotation. + (radar site) outound radial velocities inbound radial velocities X

23. Detection Rotation (Cont.) Not all velocity couplets indicate rotation. inbound radial velocities + (radar site) outbound radial velocities Linear (straight line) flow over the radar site.

24. Bragg Scatter

25. Bragg Scatter

26. Clear-Air Turbulence

27. Clear-Air Wind Profilers

29. Radial Velocity Positive Toward Surface

30. Signal Power

31. NWS-NOAA Profiler Network

32. SODAR

33. Performances of ARPL SODAR SODAR

34. Radio-Acoustic Sounding (RASS)

35. 94 GHz 35 GHz Maximum Propagation Distance Energy Absorbed by Atmosphere 10-15 km 20-30 km 3.2 mm 8 mm Radar Wavelength

36. The DOE Cloud Radars

37. Cloud Radar Data from Southern Great Plains 20-km Black Dots: Laser Measurements Of Cloud Base Height 10-km Surface time 7:00 pm 7:00 am 7:00 pm Small Cloud Particles Typical Cloud Particles Very Light Precipitation

38. Cloud Radar Data from Southern Great Plains 20-km Black Dots: Laser Measurements Of Cloud Base Height 10-km Insects Thin Clouds Surface time 7:00 pm 7:00 am 7:00 pm Small Cloud Particles Typical Cloud Particles Very Light Precipitation