Tropical and Midlatitude Cirrus Cloud Extinction and Backscatter From Multiyear Raman Lidar Measurements.

Wednesday, 17 December 2014
Tyler James Thorsen, University of Washington, Seattle, WA, United States and Qiang Fu, Univ Washington, Seattle, WA, United States
Lidars have the capability to provide unparalleled range-resolved
observations of particulate extinction. However, lidars fundamentally measure
backscattered energy, not extinction, and for widely prevalent single-channel
elastic backscatter lidars extinction must be obtained by assuming a
backscatter-extinction relationship. Our knowledge of this relationship,
known as the lidar ratio, mainly consists values determined via the
transmission-loss method: which can only provide layer-averaged values and is
only applicable to a subset of all cloud layers. Directly-retrieved,
vertically resolved extinction coefficients and lidar ratios are obtainable
through the use of more advance high spectral resolution lidars (HSRL) or
Raman lidars (RL). However, the complexity of operating a HSRL or RL has
limited their use for cloud observations to very limited time periods:
typical only a few months or less. In this work, we present a newly developed
retrieval for the Atmospheric Radiation Measurement (ARM) program's Raman
lidars for Feature detection and EXtinction retrieval (FEX). FEX improves
upon existing ARM products by using multiple, complimentary quantities to
identify both clouds and aerosols and retrieve their extinction and
backscatter profiles. Multiple years of data are examined at both the Lamont,
Oklahoma and Darwin, Australia ARM sites; providing the most comprehensive
climatology to date of cirrus extinction and lidar ratios. Variations in
these optical properties with classification of the synoptic state and their
relationship with microphysical parameters (temperature, relative humidity
and depolarization) are examined.