Observation operator

for weather-radar

refractivity

Olivier Caumont1, Lucas Besson2, Laurent Goulet3, Sophie Bastin2, Jacques Parent du Châtelet2,4, Laurent Menut5, Frédéric Fabry6

1 CNRM-GAME (Météo-France, CNRS) – 2 LATMOS– 3 DIRSE (Météo-France) – 4 Observing Systems Department (Météo-France) – 5 LMD – 6 McGill University

IODA-MED meeting

16 May 2014

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IODA-MED deliverables

Talk by Clotilde Augros

No update since last year’s meeting

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What is refractivity?

Refractivity: N = (n-1) x 106, where n = index of refraction of air.

Refractivity may be expressed as (Smith and Weintraub 1953):

T²

e+

T

P=N 37300077.6

P: pressure (hPa)e: partial pressure of water vapour (hPa)T: temperature (K)

Refractivity mainly depends on moisture when temperature is high (at constant pressure): 1 N unit ~ 1 % relative humidity at 20°C

At constant pressure:High N = moist and/or coldLow N = dry and/or warm

(Fabry et al. 1997)

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Principle of refractivity measurement by weather radar

Measurement by radar based on radar pulse’s propagation time through the atmosphere, which depends on refractivity.

Phase change between radar and target or between 2 targets depends on refractivity averaged over radar ray path (Fabry et al. 1997), i.e. ~ less than a few hundred metres above ground.

In practice, measurement of time phase change. Need for initial values, usually interpolated from automatic weather stations (AWSs) in homogeneous situation.

Technique initially for klystron (= stable-frequency) transmitters. Adaptation for magnetron (= drifting-frequency) transmitters (Parent du Châtelet et al. 2012).

radarr1 r2

target #1 target #2

radarbeam

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Summary of endeavour related to radar refractivity

Work done so far:

Formulation for magnetron transmitters (Parent du Châtelet et al. 2012)

Link between refractivity and atmospheric phenomena (Besson et al. 2012)

Technical proposals for improved-quality refractivity retrievals (Besson and Parent du Châtelet 2013)

o Definition of quality index for target selectiono Investigation of the use of faster antenna rotation speeds, additional elevations and dual-

polarization returns

Observation operator for refractivity (Caumont et al. 2013):o Sensitivity study to formulation of observation operatoro Long-term comparisons of radar observations vs. Arome

Comparison of radar refractivity with automatic weather stations and numerical simulations during HyMeX SOP1 (Besson et al., in prep. for HyMeX special issue)

o Use of refractivity retrievals produced in real time during HyMeX SOP1o Cross-validation with independent observations and modelso First attempt to relate real refractivity data with Mediterranean meteorological processes

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Available observations

3 operational radars: Nîmes, Bollène, Opoul

7 automatic weather stations (AWS): Nîmes-Garons, Nîmes-Courbessac, Tarascon (Nîmes radar)Visan (Bollène radar)Perpignan, Leucate, Durban-Corbière (Opoul radar)

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Available models

NCEP analysis NCEP forecasts

WRF simulation

D-1, 00 UTCDate, Time D-1, 12 UTC D+3, 18 UTC

WRF: Initial & boundary conditions: nudging from NCEP global model

2 nested domains: 54- and 9-km horizontal resolutions N at 2 m AGL from innermost domain

AROME-WMED: Initial & boundary conditions: Arpege global model Horizontal resolution: 2.5 km 3-h forecasts from a 3DVar assimilation cycle N at 10 m AGL

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8 August – 30 November 2012

High correlation coefficients between radar refractivity and other data:

Radar vs AWS = 0.89Radar vs Arome-WMED analysis = 0.90Radar vs Arome-WMED forecast = 0.84Radar vs WRF analysis = 0.83Radar vs WRF forecast = 0.79

Similar results at other AWS locations

Large differences at times: - between WRF and other data on 18, 19, and 20 October- diurnal cycle poorly simulated on 8, 9, and 10 September (needs further investigation)

Refractivity time series @ Nîmes-Courbessac

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13

2

2

1: Convection in the vicinity of Bollène:- precipitation- humidity increases while

temperature decreases - refractivity increases

2: Convection in the vicinity of Nîmes:- precipitation- humidity alreeady close to

100%- refractivity remains constant

3: Convection in the vicinity of Bollène:- precipitation- humidity already close to

100%- refractivity remains constant

4: Front passage:- humidity decreases markedly- refractivity decreases

markedly

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4

4

4

IOP6 (24 September 2012) – Time series

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Refractivity from Nîmes and Bollène radars – Front passage

IOP6 (24 September 2012) – Front passage

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Good agreement between Nîmes radar and models

Less agreement between Bollène radar and models:

- correct magnitude near the radar

- large discrepancy at far range

Large discrepancies probably caused by mountains (Massif Central to the west and Alps to the east) which have a double impact on radar retrievals:

- lower-quality targets- calibration of retrieval

algorithm

IOP6 (24 September 2012) – Radars vs. models

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On-going and future activities

On-going work:

Investigate the relationship with near-ground turbulence (PhD thesis of R. Hallali @ LATMOS – off HyMeX),

Improve calibration

Perspectives:

Further assessment of usefulness in process studies (cold pool, valley effects, breeze, low-level flow feeding HPEs, etc.)

Model validation in AWS-sparse areas

Data assimilation (coordinate with ZAMG/University of Vienna effort to assimilate 3D GPS-tomography refractivity data?)

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References

Besson, L., J. Parent du Châtelet, 2013: Solutions for improving the radar refractivity measurement by taking operational constraints into account. J. Atmos. Oceanic Technol., 30, 1730–1742. DOI: 10.1175/JTECH-D-12-00167.1

Besson, L., C. Boudjabi, O. Caumont, J. Parent du Châtelet, 2012: Links between weather phenomena and characteristics of refractivity measured by precipitation radar. Bound.-Lay. Meteor., 143(1), 77–95, DOI: 10.1007/s10546-011-9656-7.

Besson, L. et al.: Comparison of refractivity measurement by radar with automatic weather stations, AROME-WMED and WRF forecasts simulations during the SOP1 of HyMeX campaign. In prep. for HyMeX special issue of QJRMS.

Caumont, O., A. Foray, L. Besson, J. Parent du Châtelet, 2013: A radar refractivity change observation operator for convective-scale models: Comparison of observations and simulations. Bound.-Lay. Meteorol., 148(2), 379–397, DOI: 10.1007/s10546-013-9820-3.

Parent du Châtelet, J., C. Boudjabi, L. Besson, O. Caumont, 2012: Errors caused by long-term drifts of magnetron frequencies for refractivity measurement with a radar: Theoretical formulation and initial validation. J. Atmos. Oceanic Technol., 29(10), 1428–1434, DOI: 10.1175/JTECH-D-12-00070.1.

Thank you for your attention!