Temperature‐Independent Cloud Phase Retrieval From Shortwave‐Infrared Measurement of GCOM‐C/SGLI With Comparison to CALIPSO

• Published in: Earth and space science (Hoboken, N.J.) Vol. 8; no. 11
• Main Authors: Nagao, Takashi M., Suzuki, Kentaroh
• Format: Journal Article
• Language: English
• Published: Hoboken John Wiley & Sons, Inc 01.11.2021; American Geophysical Union (AGU)
• Subjects: 3360; Algorithms; Classification; Clouds; Ice; Meteorological satellites; Oceans; Phase transitions; Remote sensing
• ISSN: 2333-5084; 2333-5084
• DOI: 10.1029/2021EA001912

The shortwave infrared (SWIR) channels commonly accommodated in satellite‐borne passive sensors contain information on cloud thermodynamic phase as well as cloud optical thickness (COT) and cloud effective radius (CER). This study develops algorithms for simultaneous retrieval of COT, CER, and cloud thermodynamic phase to estimate the fractional probability of the cloud phase as an alternative to discrete discrimination into liquid and ice typical of operational cloud retrievals. Two algorithms were developed and applied to the SWIR channels centered at 1.05, 1.63, and 2.21 μm of Second‐generation Global Imager (SGLI). The first algorithm retrieves COT, CER, and ice COT fractions relative to the total COT, which is a continuous quantity representing the partitioning into liquid and ice phases. The second represents the cloud phase partitioning in the form of differences between radiances observed and simulated under the assumptions of either liquid or ice clouds when retrieving COT and CER. The cloud phases from these algorithms agreed quantitatively with each other, and were able to characterize the phase occurrence on a global scale. The two types of cloud phase characterization were further compared against CALIPSO to find that the zonal‐mean occurrences of the cloud phase from the first algorithm were broadly consistent with those from CALIPSO, except for significantly smaller occurrences of supercooled water clouds in SGLI than in CALIPSO over middle‐to‐high latitude oceans. The cloud phase occurrences were also found to systematically vary with CER and cloud‐top temperature on a global scale in a manner significantly different between SGLI and CALIPSO. Plain Language Summary Remote sensing from space is the only way to observe the cloud thermodynamic phase on a global scale. This study introduces algorithms to identify the cloud phase using short‐wavelength infrared channels commonly used in Earth‐observing satellites. The algorithms developed have the unique feature that they represent the cloud phase continuously in the form of the fractional probability of liquid and ice phases determined independent of temperature, contrary to traditional discrete discriminations into fixed categories of, “liquid,” “ice” and “mixed‐phase,” often relying on temperature information. The cloud phase characterization obtained by applying the algorithms to sensors onboard polar‐orbiting satellites provides fundamental data for the occurrence of different cloud phases on a global scale that can be utilized for a meaningful analysis of the temperature‐dependent phase transition in the context of climate. A remarkable finding of this study is that occurrences of supercooled water and their relationship to temperature obtained by our methodologies are different from those obtained by another type of satellite measurement technique with lidar over mid‐to‐high latitude ocean. This difference highlights the uncertainty and potential use of satellite‐based cloud thermodynamic phase information from various measurement wavelengths to better characterize the microphysical structures of mixed‐phase clouds. Key Points Two quasi‐analytical cloud phase retrieval algorithms using shortwave infrared measurements from space are introduced The cloud phase is retrieved as a continuous, not discrete, quantity correlated with the partitioning of cloud water into liquid and ice The occurrence of cloud phase was found to vary systematically with cloud effective radius and cloud‐top temperature on a global scale