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Towards improved QPE with a local area X-band radar in the framework of COPS
F. Tridon, J. Van Baelen and Y. Pointin
Laboratoire de Météorologie Physique, UMR CNRS/UBP 601624, avenue des Landais, 63177 Aubière Cedex, France
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Introduction (1)
Radar technology: tool for quantitative rainfall measurements
Main parameters: Reflectivity factor : Z (mm6.m-3) Rainrate: R (mm.h-1) Drop Size Distribution: DSD (l-1.mm-1)
Power law relationship: Z = aRb Variability of DSD Use of unique Z-R relationship for one precipitating event
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Introduction (2)
Objective: Quantitative precipitation estimation on a small catchment basin with a simple scanning X-band radar
Use of a nearby vertically looking MicroRain Radar to: study the properties of precipitation (DSD) with a high resolution do a classification of different rain regimes within the same
precipitating event derive specific Z-R relationships for these regimes
Check the efficiency of these specific Z-R relationships against a single one
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X-band radar (9.41GHz) Elevation: 5° High resolution:
Time: 30 s Azimuth: 2° Range: 60 m
Max range: 20 km
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Micro Rain Radar (K-band, 24.1GHz)
Doppler spectra of 63 bins (0 to 12 m.s-1) over 32 range gates every 10 s
Relation between drop diameter and terminal fall velocity (Atlas et al. 1973):
Profile of DSD
mmD
eDv D
61.0
3.1065.9)( 6.0
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Derivation of rainparameters: Attenuation coefficient
Iterative attenuation correction
Reflectivity factor
Rain rate
dDDDvDNR 3)()(6
dDDDNZ 6)(
dDDDNk e )()(
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June 15, 07 (IOP 3b) Synoptic-scale through
moving northeastwardly giving stratiform precipitation with weak showers over the COPS area
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DSD temporal evolution Grayscale: Number of raindrops in size interval Bold solid line: Median-volume diameter Thin solid lines: 10th and 90th percentiles of distribution of
liquid rain water content over raindrop diameters measure of the width of the raindrop size distribution
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DSD temporal evolution 1st period: small drops, narrow spectra, high variability 2nd period: large drops, wide spectra, medium variability 3rd period: low variability
anomaly
1 2 3
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Anomaly less visible near the ground MRR measurement issue due to:
strong updraft heavy attenuation bad noise level estimation
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Z-R relationships
Global Specific
1 2 3
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Corresponding rainfall
Rainrate Cumulative rainfall
MRR estimation Global Z-R Specific Z-R
Total rainfall (mm)
2.81 2.53 2.50
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August 8-9, 07 (IOP 14b) An intense large-scale
precipitating system spread over the COPS area during all the night
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DSD temporal evolution 1st period: medium drops, medium variability 2nd period: small drops, narrow spectra, low variability 3rd period: small drops, narrow spectra, medium variability 4th period: high variability
1 2 3 4
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Z-R relationships
Global Specific
1 2 3 4
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Corresponding rainfall
Rainrate Cumulative rainfall
MRR estimation Global Z-R Specific Z-R
Total rainfall (mm)
10.87 10.74 10.80
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Conclusions and perspectives High variability of rain even within a stratiform precipitation
system Detection of temporally stable regimes of precipitation
with significantly different Z-R relationships
Derivation of the equivalent exponential or gamma distribution to explain the difference between the Z-R relationships
Next step: apply these specific Z-R relationships on the X-band reflectivity
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Problem: How to detect these regimes with a single parameter X-band radar ?
Periods of increasing, stagnating and decreasing intensity can be part of the same temporally stable regime of precipitation
Conclusions and perspectives
1 2 3 42 31
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Thanks for your attention
Questions ?