Directional visibility measurements with the Vaisala Ceilometer

Vaisala's CT25K ceilometer internally calculates the extinction coefficient over the range of interest. From this, a value for visibility (Meteorological Optical Range) can be retrieved. Pointing the beam vertical or near-vertical provides a vertical visibility value. Tilting the instrument allows measuring directional visibility.

Inverting a backscatter profile for obtaining an extinction coefficient profile is a classical problem, and particularly difficult in the close range where the beam overlap and multiple scattering characteristics of a typical bistatic lidar are uncertain. We studied the accuracy of our monostatic coaxial ceilometer lidar and its visibility retrieval scheme in the case of a nearly horizontal pointing direction. Performance was validated against reference transmissometer measurements. Results will be presented and discussed.