COMPARISON OF LIDAR AND TRANSMISSOMETER MEASUREMENTS

A lidar-transmissometer intercomparison was made during an internation al experiment held in the German Alps to characterize the vertical str ucture of aerosols and clouds. The transmission path was 2325-m long a nd inclined at 30 deg along the slope of a steep mountain ridge. The t ransmissometer consisted of a Nd:YAG and a CO2 laser located in the va lley and a large-mirror receiver that captured the full beams on the m ountain top. Two lidars, one at 1.06 mu m and one at 1.054 mu m, were operated with their axes approximately parallel to the transmissometer axis but separated by a horizontal distance of the order of 20 to 40 m. The first lidar was operated in retroreflector mode and the relativ e transmittance was determined from the reflection off the mountain ri dge above the cloud layer. The second lidar had a special receiver des igned to make simultaneous recordings at four fields of view. The rang e-resolved scattering coefficient and effective cloud droplet radius a re calculated from these four-field-of-view measurements by solving a simplified model (Bissonnette and Hutt, 1995) of the multiply scattere d returns, The two simultaneous solutions for the scattering coefficie nt and effective droplet size make possible extrapolation at wavelengt hs other than the lidar wavelength of 1.054 mu m. The main measurement event analyzed lasted 1.5 h and produced transmittances ranging from less than 5% to more than 90%. The comparisons show good correlation b etween the transmissometer data and all lidar solutions including extr apolation at 10.59 mu m. The experiment also indicates the possibility of doing active imaging through optically thick clouds and demonstrat es that lidars can measure turbulence structures in the atmosphere. (C ) 1997 Society of Photo-Optical Instrumentation Engineers.