Turbulence layers detection from ground-based Rayleigh lidar Alain Hauchecorne 1, and the MMEDTAC Team Charles Cot 1, Francis Dalaudier 1, Jacques Porteneuve 1, Thierry Gaudo 2, Richard Wilson 1, Claire Cénac 1, Christian Laqui 1, Philippe Keckhut 1, Jean-Marie Perrin 3, Agnès Dolfi 2, Nicolas Cézard 2, Laurent Lombard 2, Claudine Besson 2 1 LATMOS/IPSL, UVSQ, CNRS-INSU, Guyancourt, France, 2 ONERA/DOTA, Palaiseau, France 3 OHP, 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 ( ), coord. Thales: to propose new methods to improve aircraft safety EU-FP7 DELICAT ( ), coord. Thales: to develop a lidar prototype to detect CAT on board aircrafts

TAC Cases reported From 1981 through 1997 there were 342 reports of turbulence affecting major air carriers Three passengers died, two of these fatalities were not wearing their seat belt while the sign was on 80 suffered serious injuries, 73 of these passengers were also not wearing their seat belts.

Turbulence generation 3 main causes - Wind shear - Convection - Orographic waves

MMEDTAC ANR Project ( ) 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 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 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

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

Laser Nd-Yag nm E= ph/s  2 =0.5 Q lid =0.01 to 0.1 A=0.5 m 2 (80 cm diameter) z=10000 m  r = m -1 sr -1 N=12000/s to /s Estimated signal with OHP Rayleigh lidar

Detectivity limit with Rayleigh OHP lidar for 10km altitude

Observed variance Lidar one hour averageNearby ST radar

23 Jun h-23h Observed variance averaged during one hour MMEDTAC campaign –23/06/2009

Radar PROUST 11.5 à 15 km, Dole et al., 2001 : C T 2 = 0.3 à 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 offer a new tool for atmospheric studies