Spaceborne Radar for Snowfall Measurements

Paul JoeMeteorological Service of Canada

Jacques Testud, Eric Caubet, Jeff Keeler

Outline – The EGPM Radar Story The detection requirements

Occurrence Statistics PDF of Vertical Profiles of Reflectivity

EGPM radar history/rationale Then came Cloudsat…

POSS Occurrence(1 minute data)

•Surface obs•Doppler XBand•Derived Snowrates•Minutely•Snow only•Typed by observer•1 winter of data

YUL=Montreal 45°N

YEV=Inuvik 68°N

PDF of SnowVertical Profile of Reflectivity

4 km

20 dBZ5dBZ

Montreal (45oN)

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

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

Issues

Pulse Compression Range sidelobes?

Blind zone How close to the ground do you need to sample

Sensitivity Target sensitivity of 18 dBZ

Attenuation Single Wavelength

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

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 Loss of processing gain

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

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!

Compression Requirement 1

Scattering target (weather and ground) must be coherent

Tcoh = /4W < T sec Beam spread ~ +- 0.3 deg Satellite velocity component = +- 35 m/s Tcoh = .008 / (12*70) = 10 usec !!!! Whereas T = 270 usec!!!

This appears to be the major limitation of PC at 35 GHz!!

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

The EGPM 35 GHz rain radarAttenuation 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

Reflectivity – Snowfall Rate Relationships

Rasmussen et al 2003 Marshall and Gunn 1952

Small dynamic range!

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