Dual-wavelength In Situ Cloud Lidar

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Unlike conventional (backscatter) lidar, SPEC’s in situ cloud lidar measures the time of arrival of multiply-scattered photons within an optically thick cloud (i.e., a cloud composed of cloud drops). The principle of operation is shown in the schematic diagram below.


Schematic showing how multiply-scattered photos are measured by the dual-wavelength in situ cloud lidar installed on the SPEC Learjet.

 

 

Two high-power Nd:YAG lasers, one operating at 532 nm (green) and a second operating at 1560 nm fire pulses of light horizontally out of optical flats installed in the SPEC Learjet (see photos). Vertically-oriented detectors, a photomultiplier tube (PMT) used for the 532 nm signal, and a custom, wide-area, low-noise, high-gain avalanche photodiode (APD) used in the 1560 nm detector, measure the time of return of photons multiply scattered by cloud drops. The 532 nm photons are all scattered by the cloud drops, but a small fraction of the 1560 nm photons are absorbed. As explained in Evans et al. (2003), inversion techniques use this information to retrieve volumetric Liquid Water Content (LWC), Extinction (βext) and Effective Drop Radius (Reff) in expanding volumes away from the aircraft. The volumetric averages of LWC, βext and Reff are typically measured within volumes from 25 m to about 1 km from the aircraft. The large volumetric measurements (106 to 109 m3) within cloud not only provide measurements at large distances from the aircraft, but that are also commensurate with the scale of typical satellite retrievals.

The dual-wavelength in situ cloud lidar is currently undergoing flight tests in wave clouds in Colorado and in marine stratus off the California coast.

Applicable Publications

Evans, K. F., D. O'Connor, P. Zmarzly, and R. P. Lawson, In situ cloud sensing with multiple scattering lidar: Design and validation of an airborne sensor., 2006: J. Atmos. Ocean Technol., Vol. 23, 1068-1081Read Article

K. Franklin Evans, R. Paul Lawson, Pat Zmarzly, Darren O'Connor and Warren J. Wiscombe, In Situ Cloud Sensing with Multiple Scattering Lidar: Simulations and Demonstration., 2003: Journal of Atmospheric and Oceanic Technology, Vol. 20, No. 11, pp. 1505-1522Read Article