measuring snowfall with a low-power k-band radar (micro ...measuring snowfall with a low-power...

Measuring Snowfall with a Low-Power K- Band Radar (Micro Rain Radar) in Polar Regions

University of Cologne

Institute for Geophysics and Meteorology

Maximilian Maahn, [email protected]

Maximilian MaahnP. Kollias, S. Kneifel, I. Gorodetskaya, G. Peters, C. Simmer

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Micro Rain Radar (MRR)

• Vertically pointing• 24 GHz (12.4 mm)• 30 range gates• 30...200 m vertical



Maximilian Maahn, [email protected] University of Cologne

• 30...200 m vertical resolution

• FM-CW• 50 mW, 25 W, 20 kg

• Developed for rain2

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Can we use MRRs for snowfall?



Maximilian Maahn, [email protected] University of Cologne

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Get Ze from MRR data

Good performance > 3 dBz

Must not be used for snow, because Z is derived from DSD assuming only rain

Integrated Spectra (Z, W…)




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Good performance > 3 dBz Incomplete noise removal disturbs smaller reflectivities. Correction of aliasing effects needed (Kneifel et al.,2011)

Gives best results for snow with proposed routine→ Sensitivity ~ -5 dBz

Raw Doppler Spectra

Noise & Height corrected Doppler Spectra

New routine based on Raw Doppler Spectra

1. Determine the most significant peak including its borders (modified Hildebrand and Sekhon, 1974)

2. Define remaining spectrum as noise and remove noise from peak

19/0

1/20

12, 2

3:12

Standard scheme New scheme

Hei

ght [

m]




5

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remove noise from peak3. Dealiase the spectrum,

because Nyquist range is small (using Ze - v relation by Atlas et al., 1973)

4. Calculate Z e, SNR, W

� Independent on type of hydrometeor!

Doppler velocity [m/s]

Hei

ght [

m]

New MRR routine is available as open source at http://gop.meteo.uni-koeln.de/software/

Comparison with Cloud Radar

• Reference: MIRA35 35.2 GHz cloud radar

• Collocated measurements January-April 2012




April 2012 • at UFS

Schneefernerhaus(German Alps, 2650m)

• ∆t = 60s

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MIRA vs. MRR: Z eStandard Product Proposed Routine

Offset due to different Frequencies

MR

R Z

e [d

Bz]

MR

R Z

[dB

z]

high spread




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MIRA 35 Ze [dBz]MIRA 35 Ze [dBz]Incomplete noise removal

MR

R Z

e [d

Bz]

MR

R Z

[dB

z]

MIRA vs. MRR: Frequency by altitude

MIRA35 MRR (new routine)

Hei

ght [

m]

Hei

ght [

m]




Ze [dBz]

-5 dBz � 0.01 - 0.1 mm/h

Ze [dBz]

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Hei

ght [

m]

Hei

ght [

m]

MIRA vs MRR: WStandard Product Proposed Routine

Noise

MR

R W

[m/s

]

MR

R W

[m/s

]




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MIRA35 W [m/s] MIRA35 W [m/s]

MR

R W

[m/s

]

MR

R W

[m/s

]

Outlook: Application to MRR measurements

• Ny-Ålesund (79 °°°°N): winter 2010/11

• Longyearbyen, (78 °°°°N): winter 2010/11

• Princess Elisabeth station (72 °°°°S): summer seasons 2010,11 and 12




2010,11 and 12

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(NASA)

Frequency by altitude diagrams

Ze [dBz]

Hei

ght [

m]

Spitsbergen

Antarctica




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Ze [dBz] Ze [dBz]

Hei

ght [

m]

Hei

ght [

m]

Conclusion

1) MRR standard products highly biased for snow.

2) But, snow can be observed (> -5 dBz) if the new scheme is used.




if the new scheme is used.3) MRR has potential for snowfall

climatologies

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Thank you for your attention




Thanks to ERAD for covering travel expenses!

Maahn and Kollias (2012): Improved Micro Rain Radar Snow Measurements Using Doppler Spectra Post-Processing, submitted to AMT

New MRR routine is available at http://gop.meteo.un i-koeln.de/software/

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Back-up slides




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Comparison of sigma




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Comparison with Cloud Radar

• Reference: MIRA35 35.2 GHz cloud radar

• January-April 2012 at UFS Schneefernerhaus




Schneefernerhaus (German Alps)

• >15% of data had to be discarded because of attenuation due to wet snow on dish

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Metek's (indirect) approach for Z

Recorded Doppler Power Spectrum Z ~ D6, R ~ D3

P v( ) → P D( ) →N D( ) → Z, R

Assume Doppler velocity = fall velocity

Convert to DSD using Mie Theory




• Works good for rain! Easy to compare with Z from C-band radar, Parsivel etc.

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Assume Doppler velocity = fall velocityand convert to drop size usingAtlas et al. (1973)

Why does it not work for snow?Size – fall velocity different for snow and has much higher uncertainty

Z ~ D6, R ~ D3

not feasible for snow

P v( ) → P D( ) →N D( ) → Z, R




�Z, R are biased for snow!�Instead: calculate Z e from Doppler

spectrum P(v)

Backscattering cross section differentfor snow and depends heavily on particle type

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Doppler Spectrum cannot be used19

/01/

2012

, 23:

12

Noise is not properly removed from the spectra




Sensitivity is lost due to contamination with noise

Aliasing due to limited unambiguous Doppler range

Z [dBz]

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Flow chart diagram




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Get Zere

flect

ivity

[dB

]




05/07/12

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IRTG first year evaluation

V [m/s]

Spe

ctra

lref

lect

ivity

Get Zere

flect

ivity

[dB

]




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V [m/s]

Spe

ctra

lref

lect

ivity

Ze

Get Zere

flect

ivity

[dB

]




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V [m/s]

Spe

ctra

lref

lect

ivity

Ze

noise noise

Remove clutter




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Resulting Spectra




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Aliasing

Aliasing




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Virga




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measuring snowfall with a low-power k-band radar (micro ...measuring snowfall with a low-power...

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