Avery, MA; Ryan, RA; Getzewich, BJ; Vaughan, MA; Winker, DM; Hu, YX; Garnier, A; Pelon, J; Verhappen, CA (2020). CALIOP V4 cloud thermodynamic phase assignment and the impact of near-nadir viewing angles. ATMOSPHERIC MEASUREMENT TECHNIQUES, 13(8), 4539-4563.

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 333m and 80 km, with cloud layer vertical resolutions between 60m and 8 km. CALIOP ice cloud backscatter observations taken with a 0.3 degrees 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 degrees C, with a peak in the CALIOP distribution observed globally at -15 degrees C. Recent viewing angle testing occurring during 2017 at 1, 1.5 and 2 degrees 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 degrees 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 degrees. To avoid these specular reflections, the CALIOP viewing angle was changed from 0.3 to 3 degrees 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 degrees, 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.