Cloud Base Heights from Lidar, Rotating Beam Ceilometer, and Pilot Reports Compared by Experiment and Theory

摘要

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 degrees 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 lidar's signal profile by applying reasonable approximations to the cloud structure (with or without precipitation). The view angle of the pilot, the lidar's pointing direction, and the vertical distribution of the obscuring particles control the relationship. The relationship is recommended as the foundation of a signal-processing algorithm for better ceiling measurements.