Cloud base heights from lidar, rotating beam ceilometer, and pilot reports compared by experiment and theory
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Cloud base heights from lidar, rotating beam ceilometer, and pilot reports compared by experiment and theory

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  • Personal Author:
    Eberhard, W. L.
  • Corporate Authors:
    Wave Propagation Laboratory
  • NOAA Program & Office:
    OAR (Oceanic and Atmospheric Research)
  • Description:
    An experiment is described, wherein cloud signals from a range-corrected ruby lidar and a rotating beam ceilometer showed excellent agreement in height at which peak signal occurred. However, pilot reports of ceiling when viewing at 3° below horizontal were at significantly lower altitude. To develop an understanding of the relationships between ceilometer signals and pilot visibility, the physical bases of these signals and of pilot’s perceptions are analyzed. A relationship is derived that connects pilot optical depth with an idealized lidarfs signal profile by applying reasonable approximations to the cloud structure (with or without precipitation). The view angle of the pilot, the lidarfs pointing direction, and the vertical distribution of the obscuring particles control the relationship. This relationship is recommended as the foundation of a signal-processing algorithm for better ceiling measurements. Multiple scattering and a ceilometer’s range dependence and resolution are complicating factors. Accurate cloud base predictions should also account for horizontal inhomogeneities at the cloud’s bottom as shown by simple examples. All of these concepts are also pertinent to measurement of slant visual range. A monitoring program to study the variation of optical density near cloud bottom, especially during low ceiling or when accompanied by precipitation, is needed to improve ceilometer practice.
  • Keywords:
  • Series:
    NOAA technical memorandum ERL WPL ; 129
  • Document Type:
    Technical Memorandum
  • License:
  • Rights Information:
    CC0 Public Domain
  • Compliance:
    Library
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