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

(F.Tridon@opgc.univ-bpclermont.fr)

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

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

X-band radar (9.41GHz) Elevation: 5° High resolution:

Time: 30 s Azimuth: 2° Range: 60 m

Max range: 20 km

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

Derivation of rainparameters: Attenuation coefficient

Iterative attenuation correction

Reflectivity factor

Rain rate

dDDDvDNR 3)()(6

dDDDNZ 6)(

dDDDNk e )()(

June 15, 07 (IOP 3b) Synoptic-scale through

moving northeastwardly giving stratiform precipitation with weak showers over the COPS area

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

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

Anomaly less visible near the ground MRR measurement issue due to:

strong updraft heavy attenuation bad noise level estimation

Z-R relationships

Global Specific

1 2 3

Corresponding rainfall

Rainrate Cumulative rainfall

MRR estimation Global Z-R Specific Z-R

Total rainfall (mm)

2.81 2.53 2.50

August 8-9, 07 (IOP 14b) An intense large-scale

precipitating system spread over the COPS area during all the night

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

Z-R relationships

Global Specific

1 2 3 4

Corresponding rainfall

Rainrate Cumulative rainfall

MRR estimation Global Z-R Specific Z-R

Total rainfall (mm)

10.87 10.74 10.80

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

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

Thanks for your attention

Questions ?