1. Remote Sensing of Precipitation A Look at Radar Now and in the Future Western South Dakota Hydrology Conference 23 April 2009 Darren R. Clabo Institute of Atmospheric Sciences, South Dakota School of Mines and Technology

2. What is weather radar? How does it work? How is precipitation measured? –Case study Limitations of radar The future Outline http://radar.weather.gov/index.htm NWS WSR-88D Radars Weather Surveillance Radar – 1988 Doppler

3. What IS Weather Radar? Radar- Radio Detection and Ranging Radio spectrum of electromagnetic (EM) radiation –Wavelength ~ 10 cm Coherent –Known signal phase and amplitude EM wave polarized in the horizontal

4. How Does Radar Work? Emits timed pulses (~2 μs) of EM energy in a beam Scattered by dust, planes, telephone poles, birds, insects, you, precipitation, etc. Some energy returned to radar- backscatter –Retrieve FREQUENCY- Note Doppler shift (tornado detection) AMPLITUDE- Strength of signal, power returned

5. How Does Radar Work? Distance is function of time! “Radar Volume” x r = 1 mm

6. How Does Radar Work? Two main products: Reflectivity Doppler Velocity And spectrum width…

7. How is Precipitation Measured? Returned power converted to total reflectivity- z (mm 6 m -3 ) –Drop number concentration times sixth power of the particle diameter –Radar Reflectivity Factor (Z) in units of [dBZ] as displayed on weather radar

8. Reflectivity is HEAVILY weighted to the LARGEST particles in the radar volume How is Precipitation Measured? How does this affect precipitation measurements? CASE STUDY

9. Rain shafts have equal VOLUME of water but vastly different drop SIZE characteristics Case Study- Radar Volumes Cloud B Large Raindrops Cloud A Small Raindrops 500 drops/m 3 r = 1 mm 63 drops/m 3 r = 2 mm

10. Case Study- Radar Volumes Cloud B Large Raindrops Cloud A Small Raindrops 500 drops/m 3 r = 1 mm 63 drops/m 3 r = 2 mm Z = 45.1 dBZ Z = 54.1 dBZ

11. How is Precipitation Measured? Total reflectivity used to estimate precipitation –R-z Relationships (Marshall-Palmer) –R is rainfall rate (mm/hr), z is total reflectivity Cool stratiform- z = 130*R^2 Summer Convective- z = 300*R^1.4

12. How is Precipitation Measured? Our case, If z = 300*R^1.4 –Small drop cloud (z = 32000), R = 5.51 mm/hr –Large drop cloud (z = 258048), R = 24.5 mm/hr 4.5X difference in radar derived rainfall rate for SAME VOLUME of water

13. ReflectivityOne Hour Precipitation How accurate is this? What do we know about the drops themselves? And what if there are a few LARGE hailstones?

14. Knowledge of Drop Size Distribution (DSD) is critical Limitations of Current Radar http://ga.water.usgs.gov/edu/raindropshape.html “Drippy” the unofficial USGS water icon!

15. What can give us insight into the DSD? –Dual-Polarimetric radar (NEW) TWO orthogonal EM waves “Two-Dimensional” view of particle Limitations of Current Radar x y x y http://cimms.ou.edu/~schuur/radar.html 1 mm2-3 mm

16. Other problems –Beam blockage (those mountains are in the way!) –Attenuation (“rain fade”) –Curvature Effects –Beam broadening/Non-uniform beam filling Dual-Polarimetric Radar can help with these too! –Slated for upgrades in 2010-2012 Limitations of Current Radar

17. Conclusions Conventional radar has inherent problems –Hail identification –Knowledge of DSDs –“Beam Effects” Dual-Polarimetric radar solves many of these problems –Sold on premise of better QPE

18. Contact information: –Darren.Clabo@mines.sdsmt.edu –Institute of Atmospheric Sciences (605) 394-2291 Questions