1. Spaceborne Radar for Snowfall Measurements
Paul Joe
Meteorological Service of Canada
Jacques Testud, Eric Caubet, Jeff Keeler

2. Outline – The EGPM Radar Story
The detection requirements
Occurrence Statistics
PDF of Vertical Profiles of Reflectivity
EGPM radar history/rationale
Then came Cloudsat…

3. POSS Occurrence (1 minute data)
YEV=Inuvik 68°N
Surface obs
Doppler XBand
Derived Snowrates
Minutely
Snow only
Typed by observer
1 winter of data
YUL=Montreal 45°N

4. PDF of Snow Vertical Profile of Reflectivity
Montreal (45oN)
4 km
5dBZ
20 dBZ

5. Snowfall Detection Requirements
Need to go down to 5 dBZ sensitivity to capture the peak in the PDF
Low reflectivities/rates: low absolute error, relative error appears similar to rain;
Very shallow systems in Arctic and Lake effect (not shown); need to measure close to the ground
Precipitation growth ~linear in dBZ with decreasing height (3.5-5dB/km)
Small dynamic range (< 10dB, <20 dB) particularly in the Arctic

6. EGPM Radar (pre-Phase A) Original Proposal
Pre-cursor to future operational precipitation sat
Ka Band, small, low wt
3 beam to match radiometer swath
Nadir to increase integration time, reduce power requirements
Pulse compression, lower power, solid state transmitter
~4 km horizontal beam resolution
250 m vertical resolution

7. Issues Pulse Compression Blind zone Sensitivity Attenuation
Range sidelobes?
Blind zone
How close to the ground do you need to sample
Sensitivity
Target sensitivity of 18 dBZ
Attenuation
Single Wavelength

8. Pulse Compression Advantages
Low peak power waveforms (SS xmtr)
Improved sensitivity
Improved range resolution
Greater independent samples
Faster scanning/moving radar
Improved inter-clutter visibility

9. Pulse Compression Disadvantages
Range time sidelobes
Weaker targets obscured in strong reflectivity gradients (ground / sea echo)
Doppler sensitivity changes response as function of Doppler velocity
Reduced sensitivity for given t
Loss of processing gain

10. Pulse Compression requirements
Precise waveform and filtering
Coherent echo over pulse length
Good dynamic range
Lots of processing power
Reasonable SNR

11. Pulse Compression “Range Side Lobe Issue”
-60 dB range side lobes are possible though sophisticated waveforms (modulators) and signal processing if on-board computer processing available and stability of electronic components can be maintained,
Major concern of instrument builders!
0.02 dB and 0.1o phase max deviations for 60 dB sidelobes; rx dynamic range > 60 dB;
Also an issue for filter design since phase noise added by target!

12. Compression Requirement 1
Scattering target (weather and ground) must be coherent
Tcoh = l/4pW < T sec
Beam spread ~ deg
Satellite velocity component = m/s
Tcoh = .008 / (12*70) = 10 usec !!!!
Whereas T = 270 usec!!!
This appears to be the major limitation of PC at 35 GHz!!

13. Then came Cloudsat… W band radar (94 GHz) High sensitivity -28 dBZ
Using CPI klystron transmitter
Convert to 35 GHz
Pulse system!
Expect to get better than 5dBZ sensitivity, 250 m vertical resolution

14. The EGPM 35 GHz rain radar Attenuation Compensated (Testud)
Dynamics of the rain radar, for the purpose of calibration of the radiometer, is better than expected:
0.2 to 15 mm/h in the tropics
0.2 to 25 mm/h at mid –latitude
N0* retrieval possible for R>2mm/h

15. Reflectivity – Snowfall Rate Relationships
Small dynamic range!
Rasmussen et al Marshall and Gunn 1952

16. Conclusions for Spaceborne Snowfall Measurements
High sensitivity, high vertical resolution required
Use Cloudsat heritage for pulsed high sensitive radar
Some novel developments in PC with pre-phase A radar
Range sidelobes can be overcome!
Component stability in space
Coherence issue for Ka band
Attenuation
Need for snowfall measurements?
Active area of research
Need for dual wavelength to reduce the scatter?
beam matching, independent or correlated measurements