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Turbulence layers detection from ground-based Rayleigh lidar

Turbulence layers detection from ground-based Rayleigh lidar. Alain Hauchecorne 1 , and the MMEDTAC Team

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Turbulence layers detection from ground-based Rayleigh lidar

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  1. Turbulence layers detection from ground-based Rayleigh lidar Alain Hauchecorne1, and the MMEDTAC Team 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 1LATMOS/IPSL, UVSQ, CNRS-INSU, Guyancourt, France, alain.hauchecorne@latmos.ipsl.fr 2ONERA/DOTA, Palaiseau, France 3OHP, CNRS-INSU, Saint-Michel l’Observatoire, France

  2. 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 safety EU-FP7 DELICAT (2009-2012), coord. Thales: to develop a lidar prototype to detect CAT on board aircrafts

  3. 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.

  4. Turbulence generation • 3 main causes • Wind shear • Convection • Orographic waves

  5. MMEDTAC ANR Project (2006-2009) 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. 2 methods proposed • Monostatic Rayleigh lidar: density fluctuations in aerosol-free atmosphere, implemented within MMEDTAC • Doppler Wind Rayleigh lidar: wind fluctuations

  6. Rayleigh density lidar Accuracy Advantages Easy to realise and operate Limitations Aerosol scattering must be negligible or need high resolution spectral filter

  7. Detection of turbulent fluctuations For isotropic fluctuations Troposphere: g/N=1000ms-1 Dr/r=1% ~ DV=10ms-1 Stratosphere g/N=500ms-1 Dr/r=1% ~ DV=5ms-1

  8. 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

  9. 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)

  10. Estimated signal with OHP Rayleigh lidar Laser Nd-Yag - 15W @ 532 nm E=4. 1019 ph/s t2=0.5 Qlid=0.01 to 0.1 A=0.5 m2 (80 cm diameter) z=10000 m br=4. 10-7 m-1sr-1 N=12000/s to 120000/s

  11. Detectivity limit with Rayleigh OHP lidarfor 10km altitude

  12. Observed variance Lidar one hour average Nearby ST radar

  13. Observed variance averaged during one hour MMEDTAC campaign –23/06/2009 23 Jun 2009 22h-23h

  14. Estimation turbulence parameters Radar PROUST 11.5 à 15 km, Dole et al., 2001 : CT2 = 0.3 à 0.6.10-3

  15. 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

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