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THOR: Cloud Thickness from Off beam Lidar ReturnsConventional wisdom is that lidar pulses do not significantly penetrate clouds having optical thickness exceeding about tau = 2, and that no returns are detectable from more than a shallow skin depth. Yet optically thicker clouds of tau much greater than 2 reflect a larger fraction of visible photons, and account for much of Earth s global average albedo. As cloud layer thickness grows, an increasing fraction of reflected photons are scattered multiple times within the cloud, and return from a diffuse concentric halo that grows around the incident pulse, increasing in horizontal area with layer physical thickness. The reflected halo is largely undetected by narrow field-of-view (FoV) receivers commonly used in lidar applications. THOR - Thickness from Off-beam Returns - is an airborne wide-angle detection system with multiple FoVs, capable of observing the diffuse halo, detecting wide-angle signal from which physical thickness of optically thick clouds can be retrieved. In this paper we describe the THOR system, demonstrate that the halo signal is stronger for thicker clouds, and validate physical thickness retrievals for clouds having z > 20, from NASA P-3B flights over the Department of Energy/Atmospheric Radiation Measurement/Southern Great Plains site, using the lidar, radar and other ancillary ground-based data.
Document ID
Document Type
Preprint (Draft being sent to journal)
Cahalan, Robert F.
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
McGill, Matthew
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Kolasinski, John
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Varnai, Tamas
(Maryland Univ. Baltimore County Catonsville, MD, United States)
Yetzer, Ken
(Raytheon Information Technology and Scientific Services Lanham, MD, United States)
Date Acquired
September 7, 2013
Publication Date
June 1, 2004
Subject Category
Meteorology And Climatology
Distribution Limits
Work of the US Gov. Public Use Permitted.

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