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Lidar Intercomparion Study with Airborne Aerosol Optical Measurements near Tokyo on April 23, 2001

1. Research Objective

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Lidar Intercomparion Study with Airborne Aerosol Optical Measurements near Tokyo on April 23, 2001

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  1. 1. Research Objective • Lidar technique is widely used in the atmospheric science for obtaining the vertical distribution of aerosols and clouds. Mie lidar which detects the elastically backscattered lights by scatters is relatively ease to construct. However, we need to assume the extinction-to-backscatter ratio of aerosols to retrieve the extinction profile that is important to calculate the radiative effect due to aerosols [Fernald, 1984]. Such information can obtained by the scanning lidar or constrained by simultaneous sun photometer measurement. However, we need a sensitive test to assure whether the lidar retrieval works well and see the consistency with in-situ airborne physical and optical measurements. • During the ACE-Asia intensive field campaign, such a lidar intercomparison study was conducted on April 23, 2001 off the coast of Tokyo. Due the sever air-traffic control, we had to do it apart from several tens km to 100 km. But from our lidar-lidar intercomparison between TUMM and NIES, we knew that the lidar data were similar although both locations are apart from 60km. • Thus we expect that we might do a significant intercomparison at least in the free troposphere. Lidar Intercomparion Study with Airborne Aerosol Optical Measurements near Tokyo on April 23, 2001 Toshiyuki Murayama Tokyo University of Mercantile Marine, Tokyo, JAPAN E-mail: murayama@ipc.tosho-u.ac.jp Atsushi Shimizu and Nobuo Sugimoto National Institute of Environmental Studies, Tsukuba, Japan Masanori Yabuki and Hiroaki Kuze CEReS, Chiba University, Chiba, Japan John M. Livingston SRI International, Menlo Park, CA, USA Jens Redemann and Beat Schmid Bay Area Environmental Research Institute, Sonoma, CA, USA Philip B. Russell NASA Ames Research Center, Moffett Field, CA, USA Theodore L. Anderson and Sarah J. Masonis Dept. of Atmospheric Science, University of Washington, Seattle, WA, USA Steven G. Howell, Cameron S. McNaughton, and Antony Clarke Dept. of Oceanography, University of Hawaii, Honolulu, HI, USA

  2. 2. Participated Lidars Following five lidars were participated the intercomparison: TUMM (Koto, Tokyo: 35.66N, 139.80E) NIES (Tsukuba, Ibaraki: 36.05N, 140.12E) CEReS (Inage, Chiba: 35.58N, 140.10E) CREPI (Komae, Tokyo: ??, ??) TMU (Hachiohji, Tokyo: 35.62N, 139.38E) The operated wavelengths are as follows: TUMM: 532nm(Pol.), 1064nm NIES: 532nm(Pol.) CEReS: 355, 532, 756, 1064nm CREPI: 301nm TMU: 532nm(Pol.) Here we compare the extinction profiles at 532nm obtained at TUMM, NIES and CEReS with those from the tracking sun photometer AATS-6 and in-situ optical measurements by nephelometers and particle soot absorption photometer (PSAP) on the C-130. Lidar legs were done at six altitudes during 0530-0620. Fig. 1

  3. 3. Results A. Extinction profile Lidar retrieval methods: [TUMM] Observed optical depth at 532nm by the co-located sun photometer was 0.3466+-0.031. However, a thin cirrus appeared nearby and the optical thickness was estimated to be 0.11+-0.04 from the lidar profile itself by Young’s method [1995]. The optical thickness before the appearance of cirrus was 0.31. From these reasons, we assumed the aerosol optical thickness was 0.31±0.05 here. After the correction of the overlapping to the normalized backscatter, we searched the uniform lidar ratio by an iterative use of Fernald’s method [Welton et al., 2000]; these were 33.87, 44.44 and 57.19sr for AOD=0.26, 0.31 and 0.36, respectively. Fig. 2 [NIES] They also used Fernald’s method but using the constant lidar ratio of 50sr and the constant boundary condition; the scattering ratio is 1.0 at 6km. [CEReS] They also used Fernald’s method but with the lidar ratio of 47.90sr, which is based the Mie calculation for urban aerosol model.

  4. B. Sun photometer AATS-6 Indicated extinction was derived from the smoothed curve of the AOD profile at 525nm. From the extinction profiles, We can see less wavelength dependence (dust like) above 3km but a high wavelength dependence (fine particle like). A relatively high AOD was seen even at 7.5km. Fig. 3

  5. C. Nephelometers+PSAP Extinction coefficient at 550nm at ambient relative humidity was obtained by the scattering coefficient corrected by the simultaneously measured humidification factor plus the absorption coefficient measured by the PSAP. At the same time, we obtained vertical profiles of the single scattering albedo, fine mode fraction (FMF) of light scattering at 550nm. Furthermore, we obtained the lidar ratio by specially design backscatter/total Nephelometer at 532nm [Doherty, et al., 1999]. These data are shown with the ambient RH and lidar depolarization ratio. Fig. 4

  6. 4. Discussions 1) Behaviors of profiles of the depolarization ratio, RH and FMF are significantly changed. These can be explained by increasing of fine mode particles as increasing RH because the fine particles tend to cause a small depolarization in contrast to the super-micron dust [Murayama, et al, 2000]. It is mostly clear in the boundary layer where the depolarization ratio took a minimum. 2) Features and magnitude of the extinction coefficient in the free troposphere were retrieved by the lidars within the error of the lidar ratio. We can reach a reasonable agreement of extinction coefficient with in-situ measured lidar ratio by the nephelometer. 3) Due to a large variety of the boundary layer height, large fluctuations were seen in airborne measurements. It is out of discussion to compare the extinction between lidar measurements and air-borne ones below the boundary layer because of the a large horizontal distance. 4) A large discrepancy among the lidar-retrieved extinction profiles below 1km. We need to investigate the origins of the discrepancy although partly expected from the difference of the locations. 5. Conclusions 1) We successfully accomplished a lidar intercomparison experiment in the free troposphere even several tens km apart. It is an important trial of the column closure issues, e.g., cross check of the lidar ratio. 2) Effects of the humidification and vertical gradient of the fine and dust particles was seen.I t become clear that the depolarization ratio related with the aerosol size distribution. 6. Acknowledgements We thank to all crews of NCAR C-130. References: S. J. Doherty, T. L. Anderson, and R. J. Charlson, App. Opt., 38, 1823-1832, 1999. F. G. Fernald,Appl.Opt., 23, 652-653, 1984. T. Murayama et al., in Advances in Laser Remote Sensing(Ecloe Polyrtechnique), 2001 E. J. Welton, et al., Tellus, 52B, 636-651, 2000. S. A. Young, Appl. Opt., 34, 7019-7031, 1995.

  7. Time-height cross section of backscatter and Depolarization ratio over TUMM (Far-field receiver only)

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