Aerosols and clouds observations using active lidar remote sensing

Transcription

1 ÓPTICA PURA Y APLICADA Vol. 37, núm Aerosols and clouds observations using active lidar remote sensing C. Pietras (1), Y. Morille (1), B. Romand (1), F. Lapouge (1), C. Boitel (1), M. Grall (1), C. Goukenleuque (1), M. Haeffelin (1) 1. Institut Pierre Simon Laplace Laboratoire de Météorologie Dynamique, École Polytechnique, Palaiseau ABSTRACT: The lidar for clouds and aerosols (Lidar Nuage Aerosol, LNA) is operated at Palaiseau (25km south of Paris in France) on the Site Instrumental de Recherche par Télédetection Atmosphérique (SIRTA [1] ). The instrument is developed to retrieve the optical and microphysical properties of clouds and aerosol in the boundary layer and troposphere between 0.1 and 15 kilometers. The LNA is operating since 2001 to observe routinely backscattered vertical profiles of the atmosphere. Many applications take advantage of lidar-radar synergy to analyze clouds microphysical properties, cloud dynamics and cloud macrophysics. Lidarradiometer synergy are used to retrieve cloud microphysical properties in preparation of the CALIPSO mission. A description of the instrument and its performance is provided and the algorithm developed to retrieve the macrophysical structure of the atmosphere is presented. Key words: Lidar, backscattering, optical properties, microphysical properties, clouds REFERENCES AND WEB LINKS. [1] Haeffelin M., C. Boitel, C. Goukenleuque, M. Grall, F. Lapouge, Y. Morille, C. Pietras, B. Romand et le groupe de travail scientifique SIRUS, Observatoire SIRUS pour l étude des nuages et de leur impact radiatif, Atelier Expérimentation et Instrumentation, Paris, mars 2004, [2] Mathieu A., JM. Piriou, M. Haeffelin, F. Vinit, P. Drobinksi, "Identification of error sources in planetary boundary layer cloud forecast using SIRTA observations", Geophys. Res. Let., Submitted in 2004 Letters [3] Protat, A., V. Noel, H. Chepfer, J. Delanoe, M. Haeffelin, "On the retrieval of dynamic properties of clouds from 94 GHz Doppler radar observations", J. Atmos. and Ocean. Tech., Submitted in 2004 [4] Chiriaco M., H. Chepfer, V. Noel, A. Delaval, P. Dubuisson, M. Haeffelin, P. Yang, "Effective size retrieval using lidar and infrared radiometer observations", Mon. Weath. Rev., (in press 2004) [5] Noel V., H. Chepfer, G. Ledanois, A. Delaval, P.H. Flamant, "Classification of effective shape ratios in cirrus clouds based on lidar depolarization ratio", Appl. Opt., 41 (2), (2002) Recibido: 6 - july

2 [6] Chiriaco M., H. Chepfer, V. Noel, M. Haeffelin, A. Delaval, P. Drobinski, "Dual lidar observations at 10.6µm and 532nm for retrieving semi-transparent cirrus cloud properties", J. Atm. Met. (in press 2004) [7] Naud, N., M. Haeffelin, P. Muller, Y. Morille, A. Delaval, "Assessment of MISR and MODIS cloud top heights through comparison with a back-scattering lidar at SIRTA. Geophys. Res. Let., in press. [8] Poole, L. R., D. M. Winker, J. R. Pelon and M. P. McCormick, CALIPSO: Global aerosol and cloud observations from lidar and passive instruments, SPIE, Crete, Greece (22-27 September 2002). [9] Introduction. The lidar Nuages Aérosols (LNA) operates routinely at Palaiseau (25 km south of Paris) since 2001 to observe the structure of the atmosphere above the site. The LNA is used to restitute the backscatter vertical profiles and to retrieve the optical and microphysical properties of clouds and aerosols in the boundary layer and the troposphere between 0.1 and 15 km. It was involved in the European lidar network (Earlinet) from 2001 to 2003, in the cloud observatory network (Cloudnet) from 2002 to 2005 and in the development of the SIRUS observatory ([1]) since The LNA is used and maintained by the SIRTA, site developed for the Institut Pierre Simon Laplace (IPSL), to promote the synergy between instruments. The SIRTA, involved in many research programs, deploys a pool of multiple instruments (Table 1) to explore the atmosphere and to retrieve geophysical parameters for clouds properties studies and for validation of space observations. Measurements are acquired, automatically processed and archived in a database managed by the SIRTA and accessible through the web. ( Many applications use the synergy lidar-radar to study the microphysical properties, clouds dynamic and macrophysical properties [2,3]. The synergy lidar-radiometer is applied to derive the clouds microphysical properties [4,5,6], to validate observations from space [7] and to prepare the CALIPSO mission [8]. The LNA system and its performance are presented and data processing architecture is described. Two representative cases illustrate the performance of the algorithm developed to retrieve geophysical parameters of the atmosphere. TABLE I List of operational Instruments deployed by the SIRTA near Paris. LIDAR RADAR RADIOMETER IN-SITU LNA Clouds, aerosols LMD 1999 TELEMETER Clouds, Boudary KNMI 2003 RASTA Clouds CETP 2002 RONSARD Precipitation, wind 3D CETP 1999 FLUXMETERS Flux(BSRN) LMD 2003 PHOTOMETERS Aerosols, water AERON vapor ET 2002 HIGH-FREQ Water vapor + liquid CETP 1999 METEO Pressure, temp, LMD 1999 SPEC-PLUVIO Diameter + speed CETP 2004 RADIOSOUNDING Vertical profiles METEO- PTH FRANCE 1999 ANEMOMETER Turbulent Flux SA

3 2.- Description of the LNA. Emission Telescop e WFOV Telescope NFOV Ø=60 cm Detectio Nd-Yag Detection Figure 1 : Picture of Lidar LNA, its optical system, and the backscatter signals measured on March with the NFOV telescope (top) and the WFOV telescope (bottom) - Laser Nd-YAG 1064nm pulsed, doubled at 532nm and linearly polarized - 2 telescopes, narrow field of view (NFOV) and wide field of view (WFOV) - A detection system for each telescope - Detection of the depolarized signal at 532nm (crossed polarized direction) and the component linearly polarized - Data acquisition and data transfer are managed by a PC data transfer every 30 minutes, in real-time visualization on the web («data» menu) hours after acquisition, level 1 data processed and accessible from the database. - Combination of the two telescopes is recommended to exploit entirely the lidar signals along the vertical path from ground to 15 km. 3.- Data Processing architecture. Data Processing architecture RAW SIGNAL LEVEL 0 NOISE DETECTION RSB THRESHOLD CORRECTED SIGNAL LEVEL 1 MOLECULES DETECTION SLOPE MINIMIZATION BL DETECTION Pr 2 RATIO PARTICLES DETECTION WAVELET TECHNIQUE LEVEL 2: FLAG CLASSIFICATION OPTICAL THICKNESS APPRENT BACKSCATTERING CLOUD THERMODYNAMIQUE PHASE CLOUD BASE HEIGHTS CLOUD TOP HEIGHTS BOUNDARY LAYER HEIGHTS

4 - Corrected backscatter signal - Boundary Layer detection April Lidar cloud and aerosol mask - Radar and Lidar cloud mask April : Cloud mask 6 Cloudy Lidar & Radar 5 Cloudy Radar alone 4 Cloudy Lidar alone 3 Drizzle or unknown 2 Clear PBL or Rayleigh 1 No clouds 0 No data - Apparent backscattering and optical thickness τ a =0.06 τ c =0.5 - Cloud thermodynamic phase April Conclusions. Operational system to explore the atmosphere from 0 to 15 km Acquisition and archive of backscatter profiles at 532nm, linearly polarized, cross polarized and at 1064nm Acquisition and archive of the structure of the atmosphere, including aerosols and clouds discrimination Synergy lidar-radar to distribute cloud masks product. Instrument Continuous operation of the system to increase observation periods especially during night Implementation of the Raman channel at 607 m for operational acquisition - Products Improve the classification technique for Radar and Lidar data. - Processing Development of algorithms to retrieve easily physical parameters from Level 2 data, accessible to the community

5 Acknowledgments. The development and daily operation of the LNA instrument are supported by CNES (Centre National de la Recherche Spatiale), INSU (Institut National des Sciences de l Univers), and IPSL (Institut Pierre Simon Laplace). The required equipment used by the SIRTA are implemented at the Polytechnic School in Palaiseau. The support provided by the School is vital for the development of the project. We wish to thank Juan Cuesta, Pierre Flamant and Claude Loth for fruitful discussions and expertise and William O'Hirok, UCSB, California for the radar cloud mask algorithm development