Set-up of a ground-based Rayleigh lidar to detect clear air

turbulence

Alain Hauchecorne1, Charles Cot1, Francis Dalaudier1, Jacques Porteneuve1, Thierry Gaudo2, Richard Wilson1, Claire Cénac1,

Christian Laqui1, Philippe Keckhut1, Jean-Marie Perrin3, Agnès Dolfi2, Nicolas Cézard2, Laurent

Lombard2, Claudine Besson2

1 LATMOS/IPSL, UVSQ, CNRS-INSU, Guyancourt, France, alain.hauchecorne@latmos.ipsl.fr

2ONERA/DOTA, Palaiseau, France

3OHP, CNRS-INSU, Saint-Michel l’Observatoire, France

Clear air turbulence

Clear air turbulence (CAT) is an important problem for safety of commercial airplanes:• It can cause severe passenger injuries and material damages • It is not easy to detect in advance on-board by radar or other methods

Cleair air turbulence is related to small-scale wind and air density fluctuations but its characteristics and mechanisms of formation are not well known.

European projects EU-FP6 Flysafe (2005-2009), coord. Thales: to propose new methods to improve aircraft safetyEU-FP7 DELICAT (2009-2012), coord. Thales: to develop a lidar prototype to detect CAT on board aircrafts

MMEDTAC ANR Project (2006-2009)

2 methods proposed - Monostatic Rayleigh lidar: density

fluctuations in aerosol-free atmosphere, implemented within MMEDTAC

- Doppler Wind Rayleigh lidar: wind fluctuations

Objectives- To set-up a ground-based lidar system to detect CAT

- To improve and test algorithms developed in EU-DELICAT for the detection of CAT using lidar signals.

Rayleigh density lidar

Accuracy

phN

1

Advantages

Easy to realise and operate

Limitations

Aerosol scattering must be negligible or need high resolution spectral filter

Detection of turbulent fluctuations

For isotropic fluctuations

NgV

TT

/

Troposphere: g/N=1000ms-1

=1% ~ V=10ms-1

Stratosphere g/N=500ms-1

=1% ~ V=5ms-1

Detection based on variance of density fluctuations

Background removal (average signal from high altitude)

: integrated signal in time slice i and altitude layer j

Perturbation

Variance

€

Pi, j =Si, j −

1

2(Si, j−1 + Si, j+1)

1

2Si, j +

1

4(Si, j−1 + Si, j+1)€

Si, j

€

V j =1

NiPi, j −

1

2(Pi−1 + Pi+1)

⎛

⎝ ⎜

⎞

⎠ ⎟2

i

∑

Laser Nd-Yag - 15W @ 532 nm

E=4. 1019 ph/s

2=0.5

Qlid=0.01 to 0.1

A=0.5 m2 (80 cm diameter)

z=10000 m

r=4. 10-7 m-1sr-1

N=12000/s to 120000/s

Estimated signal with OHP Rayleigh lidar

Detectivity limit with Rayleigh OHP lidarfor 10km altitude

Field campaign at Observatoire de Haute-Provence (OHP)

Use of NDACC Rayleigh temperature lidar at OHP

•Dedicated reception telescope (53 cm diameter)

•2 channels at 532 nm (parallel and perpendicular polarizations (detection solid particles)

•Distance emission-reception 6m to avoid PM saturation at low altitude

•Dedicated data acquisition chain

• Shot by shot acquisition at 50 Hz

• Detection in analogic mode

• Sampling 100 ns (15 m), resolution 37.5m (4 MHz)

23 Jun 2009 22h-23h

Variance du signal sur une heure Campagne MMEDTAC OHP – 23/06/2009

Radar PROUST 11.5 à 15 km, Dole et al., 2001 :CT

2 = 0.3 à 0.6.10-3

Estimation turbulence parameters

Conclusion

• A new lidar system has been set-up at OHP to detect clear air turbulence from Rayleigh scattering fluctuations

• Analysis of the results indicate the probable detection of CAT layers

• Derived turbulent parameters in the same range than ST radar estimations

• This technique can be applied to any powerful Rayleigh lidar.