1. Radar Detection of Shallow Weather and Orographic Phenomena Paul Joe EUMETCAL Weather Radar 20130605

2. Who am I? a research scientist specializing in developing applications (severe weather, aviation, quantitative precipitation). I work for Environment Canada and live in Toronto. the chair of the WMO/Working Group on Nowcasting Research and have had a lot fun demonstrating the use of radar for nowcasting at the Sydney (2000), Beijing (2008), Vancouver (2010) and Sochi (2014) Olympics. contribute to several Expert Teams in the WMO/Commission of Instruments, Methods and Observations (Upper Air and Remote Sensing and Surface Observations). Beijing Vancouver Sochi Photography Skiing

3. This module briefly explores “radar meteorological” issues of low level weather detection in a generic way. Module Objective

4. Outline Some “radar calculations”: –Typical reflectivities of rain, drizzle, fog, snow (detection issue) –Beam height (detection issue) –Beam width (quantitative and detection issues) –Sensitivity (detection issue) Impact on Meteorology –Drizzle –Lake Effect Snow –Clear Air Echoes –Orographic Precipitation

5. Low Level Phenomena Detectable by Radar Meteorological Targets –Precipitation (Drizzle, Rain, Snow, Hail) –Lake Breezes, Convergence Lines, Gust fronts, cold pools Biological Targets (example at the end) –Insects, birds, bats Electro-magnetic Targets (examples) –Other radars, RLANs, Sun, second trip echoes Other (not discussed) –Building, Mountains, Forests –For calibration of humidity retrieval –Turbulence (Bragg scattering) Hard Targets (not discussed here) –Wind turbines, Cars, ships, space debris –Forest fires –Sea Clutter Romanian Gust Front

6. General Comments – Low Scanning Wide variety of phenomena and intensity of targets –Turbulence (too weak) to Mountains (very intense) –From very weak to very strong (-30 dBZ to 95 dBZ) Different Doppler signatures –Some have 0 velocity –Some have aliased velocity (> Nyquist) Advanced uses of weather radar –VDRAS – variational doppler radar assimilation system –Refractivity retrieval – use of ground clutter echoes Commonality –Limited range! –Low echo strength (generally), –Low height of weather, –Radar sensitivity and scanning are issues

7. Detection vs Measurement Glossary Detection – can see Measurement – can quantitatively measure

8. Drizzle Some Radar Examples

9. Drizzle reported in surface observations but no radar echoes. Drizzle in surface observations BUT NO/Little RADAR DATA Germany Example 1 Lang, DWD

10. Drizzle (mm/h) but very few echoes Germany Example 2 Lang, DWD

11. Drizzle in Finland! Saltikoff, FMI 1.Why was drizzle observed in Finland but not Germany? 2.Why is the drizzle observed only around the radar? 3.Why is the reflectivity pattern stronger near the radar and decreases away from the radar? 4.Why is there a range limit to see drizzle?

12. Minimum Detectable Signal Concept

13. Minimum Detectable Signal The detection threshold (as a function of range). Range [km] Reflectivity [dBZ] Color is Probability Distribution of Reflectivity with Range (not important for this discussion). Function of Wx. Minimum Detectable Signal (constant power) P = C Z r 2 The Radar Equation MDS can expressed as a noise temperature or a power measurement but for meteorologists it more useful to express as reflectivity at a particular range. Typically, -1 dBZ at 50 km.

14. Some Radar Considerations P = C Z r 2 P = power, C = radar constant, r = range Z = N D 6 [Z] = mm 6 /m -3 dBZ = 10 log Z Reflectivity Factor - Linear

15. Radar Equation and MDS P min = C Z min (r) r 2 The Radar measures “P” – power received The Radar Equation converts P to Z for a given range (r) –Radar Equation accounts for expanding beam with range (1 /r 2 ) Sensitivity (or MDS) is a certain power level –Just above the noise (hsssssss) level –In terms of P (power), it is a constant –In terms of Z (reflectivity), it is a function of range (1 /r 2 ) A limitation for long range detection of weak echoes is the radar sensitivity! –If the reflectivity of the target is below MDS then the radar does not detect it! artificial MDS –Beware of artificial MDS! The display of the radar data may be thresholded! Some data may not be displayed! Range Reflectivy Range Power

16. Homework Question 1 If the MDS is -10 dBZ at 10 km What is the MDS at 100 km?

17. Homework Answer 1

18. Homework Answer 2

19. Homework Answer 3

20. Rain Rate Note that if N is number concentration and D is particle size and uniform Z = N D 6

21. Terminal Velocity of Hydrometeors Gunn-Kinzer 1949 Beard and Pruppacher, 1969 Radius [microns] Velocity [cm/s]

22. Homework Answer 3

24. So, can your radar see drizzle of a reflectivity of say -5 dBZ?

25. So, how far can you see drizzle (-5dBZ)? Or anything else? P = C Z r 2 Minimum Detectable Signal (power) ~ 25km -5dBZ

26. Can you see drizzle – part 2? The Artificial MDS Situation 7dBZ Data in this shaded area is thresholded (not displayed)! ~ 25km Typical Drizzle reflectivity

27. Reflectivity vs Range for Constant Power (1/r 2 ) Where does your radar fit on this diagram? Go ask your radar engineers. Typical Radars

28. Beamheight Considerations Do you know what your minimum elevation angle is?

29. Minimum Elevation Angle and Beamwidth Impact 0.5 o Beam totally overshoots the weather beyond this range! No detection at all! Shallow Weather The weather is detected but the beam is not filled beyond this range, so reflectivities are quantitatively underestimated from this range and beyond Note: the lower the beam the longer the range for detection ability! 1 o beamwidth

30. Drizzle Drizzle is due to warm rain process. Slow growth which results in small drops (0.1 mm, 1 mm/h) Note: Colour scales are different! dBZ ZDR Saltikoff, FMI Drizzle is round! 1 km

31. Summary: Drizzle in Finland! Saltikoff, FMI 1.Why was drizzle observed in Finland but not Germany? Thresholded! 2.Why is the drizzle observed only around the radar? Sensitivity 3.Why is the reflectivity pattern stronger near the radar and decreases away from the radar? Beamfilling 4.Why is there a range limit to see drizzle? ~80-100km, function of sensitivity, beamfilling, depth of the drizzle!

32. 5-6°C Drizzle,, Unusual widespread drizzle from cloud echoes aloft. At surface only few echoes above 1dBZ. Note: change in threshold for DWD, see more drizzle! Hamburg Germany Example 3 Lang, DWD

33. Homework Answer 4

34. Homework Answer 5

35. Homework Answer 6

39. Major Factors for Detection Radar Sensitivity –Target Reflectivity/Radar MDS combination Overshoot –Lowest Angle of Radar/Height of weather / Earth Curvature combination Beam filling (quantitative) –Weather is too shallow or too low –Beam is very broad Thresholding –Artificial MDS = Minimum Displayed Signal* Use laser to select the most significant for your radar system/display! * Saltikoff

40. FOG Can the radar see fog?

41. Fog Special Cloud/Fog Radar (35 GHz or Ka Band) Fog has drop sizes from 10 to 30 microns, so very low reflectivities. An operational radar has a sensitivity as -8 dBZ at 50 km. What is the controlling factor of detecting fog for this radar? Drop Size Distributions dBZ 10 km Non-operational

42. Beamheight Again Quantitative measurements

43. Partial Beam Filling Range bins that are partially beamfilled, decreasing reflectivity with range! 0.5 degree

44. Vertical Profile of Snow Function of Range 1. Snow originates aloft but grows as it falls. 2. The same vertical profile as observed by radar at increasing range due to beam filling, beam broadening (smoothing) and Earth curvature (can’t see lowest levels)!

45. Quantitative Impact of Beamfilling Michelson, SMHI Note the fall off of values with range. This is NOT attenuation to which this is commonly attributed. It is a beam filling effect! Effective Range of a Radar for QPE before Vertical Profile Adjustments

46. Impact of Beamwidth / Beamfilling 30 day Accumulation Example of the impact of beamwidth or beamfilling on quantitative precipitation estimation. One radar is 0.65 o and the rest are 1.1 o beamwidth radars. Smaller beamwidth means less beamfilling problems with range and farther quantitative reflectivity information. 0.65 o (no blue) Patrick, EC 1.0 o (blue)

47. Applying the Correction aka Vertical Profile Correction aka Range Correction Koistinen, FMI

48. Other Shallow Features

49. Atmosphere is very layered You want to see low levels! 3 flow regimes evident but really 5? 5 layers? Virga 1234512345 Vertical profile of reflectivity

50. Radars in the Mountains Often on top for surveillance reasons Germann, MCH Can’t see low levels K e = 5/4

51. Precipitation is on the flow when the flow is blocked. Radial velocity Reflectivity

53. You want to see low levels and non-precipitating echoes Lake Breeze TStorm Outflow Lake Breeze Explosive Growth

54. Reflectivity [dBZ] 10 Hour Reflectivity Accumulation [dBZ] Airplanes as points Airplanes fly along prescribed tracks Different Color Tables

55. Summary Focused on drizzle to illustrate the impact of the radar sensitivity, lowest elevation angle on detection Understand why you can detect or not certain phenomena Do not “threshold data out”, it is all useful You should find out how low your radar scans and the impact on your forecasting –Sensitivity (actual and threshold) –Lowest elevation scan –Beamwidth –How far are your radars apart

56. Thank You Questions? paul.joe@ec.gc.ca