Journal article

Characteristics of CALIOP attenuated backscatter noise: Implication for cloud/aerosol detection

Wu D, Chae J, Lambert A, Zhang F ...see all

Atmospheric Chemistry and Physics, vol. 11, issue 6 (2011) pp. 2641-2654

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A research algorithm is developed for noise eval-uation and feature detection of the CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) Level 1 (L1) backscatter data with an emphasis on cloud/aerosol features in the upper troposphere and lower stratosphere (UT/LS). CALIOP mea-surement noise of the version v2.01 and v2.02 L1 backscatter data aggregated to (5 km) horizontal resolution is analyzed with two approaches in this study. One is to compare the observed and modeled molecular scatter profiles by scaling the modeled profile (with a fitted scaling factor α) to the ob-served clear-sky backscatter profiles. This scaling α value is sensitive to errors in the calibrated backscatter and the atmo-spheric model used. Most of the nighttime 532-nm α values are close to unity, as expected, but an abrupt drop occurred in October 2008 in the daytime 532-nm α, which is likely indicative of a problem in the v2.02 daytime calibrated data. The 1064-nm night α is generally close to 2 while its day α is ∼3. The other approach to evaluate the lidar measure-ment noise is to use the calibrated lidar backscatter data at altitudes above 19 km. With this method, the 532-nm and 1064-nm measurement noises are analyzed and character-ized individually for each profile in terms of the mean (µ) and standard deviation (σ), showing larger σ values in gen-eral over landmasses or bright surfaces during day and in radiation-hard regions during night. A significant increas-ing trend is evident in the nighttime 1064-nm σ , which is likely responsible for the increasing difference between the feature occurrence frequencies (532-nm vs. 1064-nm) de-rived from this study. For feature detection with the re-search algorithm, we apply a σ –based method to the aggre-Correspondence to: D. L. Wu ( gated L1 data. The derived morphology of feature occur-rence frequency is in general agreement with that obtained from the Level 2 (L2) 05 km CLAY+05 km ALAY products at 5 km horizontal resolution. Finally, a normalized proba-bility density function (PDF) method is employed to eval-uate the day-night backscatter data in which noise levels are largely different. CALIOP observations reveal a higher prob-ability of daytime cloud/aerosol occurrence than nighttime in the tropical UT/LS region for 532-nm total backscatters >0.01 km −1 sr −1 .

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  • Dong WuNASA Goddard Space Flight Center

  • J. H. Chae

  • A. Lambert

  • F. F. Zhang

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