1. Where (not) to measure rainfall Neil I. Fox University of Missouri - Columbia

2. Three parts  One problem with radar and three (possible) solutions –Microwave links –TV station radar –Corrections  Using gauges?

3. Problems with radar  Lots of problems with getting accurate estimates of precipitation rate  Using dual-polarization should make estimates a lot better but…  We are observing precip above the surface

4. Many things can happen  Between the height of the beam and the surface  Precip can evaporate  It can grow  It can be advected  We may even be above the precip and not see it at all

5. Dealing with this  Near to surface instruments  More radars  Radar correction

6. Nexrad coverage

7. Illustration of Wind-drift  1-dimensional view with constant shear profile in u-direction  Notice that when wind- drift is applied the original pixel is spread to 2 neighboring pixels  In the example none of the original rainfall contributes to the pixel directly below it

8. Data Sets Used for Case Study  From the Sydney 2000 Games C-POL radar  Wind Component Retrieval Methods from Sun and Crook (1994).  CAPPI height 1500m, 45x45 grid, 2.5km grid spacing

9. U and V wind components

10. Results from Single Time Step  The following images are taken from the midpoint of the event near Sydney.  The comparison between the given reflectivity and the wind drift correction will be shown.  There is a noticeable dispersion of reflectivity in the correction scheme

11. Reflectivity vs. Correction

12. Corrected Reflectivity Movie

13. Accumulation vs. Correction

14. Accumulation Error  There is a noticeable area of underestimation on the western edge of storm as it tracks northeastward, coupled with an area of overestimation, on the order of 5-10km

15. Accumulation Error Movie

16. Introduce Topography

17. Wind drift plus topography  Incorporating topography into the wind-drift scheme simply reduces the error between the original and wind drift correction, esp. in higher elevation  Reduces the distance the wind impacts falling drops

18. Conclusions of this  Errors predicted by this study are significant –Up to 30 millimeters in this case over the span of 6 hours (~100% error in some regions on the edge of the precipitation) –Convective cases result in higher accumulation errors than stratiform –Higher elevation reduces these errors  Implications for assimilation into hydrological and meteorological models

19. Getting closer to the ground  More radars  Microwave attenuation  Gauges

20. Use of broadcast radars  Regular low-level PPI  Flexibility when desired –But must put broadcasters’ needs first

21. Relative Beam heights St Louis: KLSX Fulton Pleasant Hill, KEAX

22. Benefits to be gained  Low-level cold season precip  Low-level modification of warm season precipitation  Low-level and small-scale (high-resolution) rotation  Boundaries

23. Limitations  Quality Control  Calibration  Staffing considerations  Only PPI

24. Benefit to Broadcasters  Publicity  Community service  Radar is not really used most of the time

25.  S-band  1  Beamwidth  250kW  25m tower

26. Example from the tornado outbreak of 4 -10 May 2003  Numerous rotations spotted (and shown on TV) using the radar, but not detected by NWS radars  Some (small) hook echo features clearly observable were not so clear with NWS radar imagery

27. Chariton County is roughly 105 km from the radar site in Fulton and the beam height is roughly 1550 m. KEAX is 150 km: 2750 m. KLSX is 210 km away: 4300 m.

28. The Northern Audrain County Cell: From Fulton : 56 km @ 670 m From KEAX: 200km @ 4150 m From KLSX: 135 km @ 2300 m. The Boone County Cell: From Fulton: 48 km @ 550m. From KEAX: 165 km @ 3100 m. From KLSX: 160 km @ 2850 m

29. Making a problem useful: Microwave attenuation  Microwave signals at the frequencies typically used for cellphones are strongly attenuated by precipitation

30. Bolton Project

31. Transmitter Receiver

32. Transmitter Receiver Effect of dumb clipart rain: differential attenuation

33. From Rahimi et al. submitted to JGR

34. Using gauges  Gauges give us a measure of surface precipitation –At a point –They have bigger errors than you think

35. Especially when installed by council workmen

36. Using gauges 2  If we’re careful  If we understand the errors  If we recognize that we’re using gauges!

39. No cheese today Gromit

40. Poor Gromit – didn’t check the forecast