The in situ cloud lidar

The in situ cloud lidar is designed to measure cloud volumes of millions of cubic meters to overcome the sampling limitations of traditional cloud probes in inhomogeneous clouds. This technique sends laser pulses horizontally from an aircraft inside an optically thick cloud, and measures the time series of the multiply scattered light with wide-field-of-view detectors viewing upwards and downwards. Diffusion theory shows that the extinction in liquid clouds, averaged over tens to hundreds of meters, and the distance to cloud boundaries can be retrieved with a single wavelength in situ lidar. A dual wavelength (532 and 1560 nm) in situ lidar could accurately determine the volume average cloud liquid water content and effective radius.

The presentation will describe the design and operation of an in situ cloud lidar. A laser in the aircraft cabin outputs 532 nm wavelength pulses at 10 Hz, which are sent through beam expanding optics for eye safety. The upward and downward viewing detectors use photomultiplier tubes and operate with either daytime (3 degree half angle FOV and 0.37 nm solar blocking filter) or nighttime (30 degree FOV) optics. Example daytime lidar signals in dense cloud have a dynamic range greater than 1000 after solar background subtraction. Results from a nighttime flight in marine stratus are analyzed in detail. The variations in the lidar signals with aircraft location are much smoother for the longer photon travel times, indicating that the later photon times sample volumes hundreds of meters in size. Retrievals are performed using neural networks trained on simulations of in situ lidar time series made with a Monte Carlo radiative transfer model in stochastically generated inhomogeneous stratocumulus clouds. Extinction retrievals for 25 m radius volumes have high correlation (R²=0.84) with FSSP derived extinction, while the correlation is relatively low (R²=0.40) for 200 m volumes due to cloud inhomogeneity. Lidar retrievals of cloud base and top height from inside the cloud are consistent with cloud boundaries obtained from aircraft penetrations on ascents and decents.