CALIOP V4 cloud thermodynamic phase assignment and the impact of near-nadir viewing angles
Authors: Avery Melody A. , Ryan Robert A. , Getzewich Brian J. , Vaughan Mark A. , Winker David M. , Hu Yongxiang , Garnier Anne , Pelon Jacques , Verhappen Carolus A. 
Published: August 2020

Abstract. Accurate determination of thermodynamic cloud phase is critical for
establishing the radiative impact of clouds on climate and weather.
Depolarization of the Cloud-Aerosol Lidar with Orthogonal Polarization
(CALIOP) 532 nm signal provides a useful addition to other methods of
thermodynamic phase discrimination that rely on temperature, cloud top
altitude or a temperature-based cloud phase climatology. Active detection of
the thermodynamic phase of multiple cloud layers in a vertical column using
cloud layer-integrated depolarization and backscatter also alleviates
ambiguities in cloud phase determination by passive radiometers. The CALIOP
phase algorithm primarily uses vertically integrated cloud layer
depolarization and attenuated backscatter to determine the dominant
thermodynamic phase of hydrometeors present in a cloud layer segment, at
horizontal resolutions for cloud layer detection varying between 333 m and
80 km, with cloud layer vertical resolutions between 60 m and 8 km. CALIOP ice cloud backscatter observations taken with a 0.3∘ near-nadir view
between June 2006 and November 2007 include a significant amount of specular
reflection from hexagonal smooth crystal faces that are oriented
perpendicularly to the incident lidar beam (horizontally oriented ice – HOI). These specular reflections from HOI are shown here to occur between 0 and −40 ∘C, with a
peak in the CALIOP distribution observed globally at −15 ∘C.
Recent viewing angle testing occurring during 2017 at 1,
1.5 and 2∘ and reported here quantifies the impact of changing the viewing angle on these specular reflections and verifies earlier
observations by POLDER. These viewing angle tests show that at the −15 ∘C peak of the HOI distribution the mean backscatter from all ice
clouds decreases by 50 % and depolarization increases by a factor of 5 as
the viewing angle increases from 0.3 to 3∘. To avoid these specular reflections, the CALIOP viewing angle was changed from 0.3 to
3∘ in November 2007, and since then CALIOP has been observing clouds almost continuously for 12–13 more years. This has provided more data
for a thorough re-evaluation of phase determination and has motivated
changes to the CALIOP cloud phase algorithm for Version 4 (V4). The V4
algorithm now excludes over-identification of HOI at 3∘,
particularly in cold clouds. The V4 algorithm also considers cloud layer
temperature at the 532 nm centroid and has been streamlined for more
consistent identification of water and ice clouds. In V4 some cloud layer
boundaries have changed because 532 nm layer-integrated attenuated
backscatter in V4 has increased due to improved calibration and extended
layer boundaries, while the corresponding depolarization has stayed about
the same. There are more V4 cloud layers detected and, combined with increasing cloud edges, the V4 total atmospheric cloud volume increases by
6 %–9 % over V3 for high-confidence cloud phases and by 1 %–2 % for all
cloudy bins. Collocated CALIPSO Imaging Infrared Radiometer (IIR)
observations of ice and water cloud particle microphysical indices
complement the CALIOP ice and water cloud phase determinations.