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Remote Sensing of Precipitation
A Look at Radar Now and in the Future
Western South Dakota Hydrology Conference
23 April 2009
Darren R. ClaboInstitute of Atmospheric Sciences, South Dakota School of
Mines and Technology
• 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 RadarsWeather Surveillance Radar – 1988 Doppler
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
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
How Does Radar Work?
Distance is function of time!
“Radar Volume”
x
r = 1 mm
How Does Radar Work?
• Two main products:
Reflectivity Doppler Velocity
And spectrum width…
How is Precipitation Measured?
• Returned power converted to total reflectivity- z (mm6 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
n
iiiDNz
1
6
)(10)( 10 zLogdBZZ
• Reflectivity is HEAVILY weighted to the LARGEST particles in the radar volume
How is Precipitation Measured?
• How does this affect precipitation measurements?
• CASE STUDY
Rain shafts have equal VOLUME of water but vastly different drop SIZE characteristics
Case Study- Radar Volumes
3
3
3
0000021.0
2100
**)3/4(*500
m
mm
rVt
Cloud BLarge Raindrops
Cloud ASmall Raindrops
3
3
3
0000021.0
2100
**)3/4(*63
m
mm
rVt
500 drops/m3
r = 1 mm 63 drops/m3
r = 2 mm
Case Study- Radar Volumes
36
366
1
6
32000
2*500
mmm
mmm
DNzn
iii
Cloud BLarge Raindrops
Cloud ASmall Raindrops
500 drops/m3
r = 1 mm 63 drops/m3
r = 2 mm
36
366
1
6
258048
4*63
mmm
mmm
DNzn
iii
Z = 45.1 dBZ Z = 54.1 dBZ
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
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
Reflectivity One Hour Precipitation
How accurate is this?What do we know about the drops themselves?And what if there are a few LARGE hailstones?
• 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!
• 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 mm 2-3 mm
• 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
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
• Contact information:– [email protected]
– Institute of Atmospheric Sciences (605) 394-2291
Questions
